1 Invertebrate Olfaction

Tom Finger

EVOLUTION OF TASTE
Thomas E. Finger
Dept. Cell & Developmental Biology
Rocky Mountain Taste & Smell Center
Univ. Colorado Medical School
Aurora CO 80045 USA
Contact:
Thomas Finger
Univ. Colorado Health Sciences Center
Room L18-11118, RC-1
12801 E. 17th Ave. MS 8108
PO Box 6511
Aurora CO 80045
Tom.finger@uchsc.edu
KEYWORDS:
Amphid
Amphioxus
Cell type
Chemoreception
Drosophila
Fish
Insect
Labellum
Leech
Mammal
Nematode
Octopus
Solitary Chemoreceptor Cell
Taste Bud
Taste receptor
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SYNOPSIS: The sense of taste is well-defined only for vertebrates where there are readily
identified taste buds. These endorgans always are innervated by particular cranial nerves and
always serve in the regulation of feeding and food intake. Since invertebrates, including
amphioxus, have no such endorgans, comparisons across groups must be made on the basis of
analogy rather than homology. I adopt the term “taste” only for chemosensory endorgans
devoted to feeding-related behaviors without regard to endorgan structure, expression of
molecular receptors, chemical nature of the stimulus (e.g. volatility), or vehicle in which a
stimulus is presented (e.g. air vs water). In that context, nearly all metazoans have a sense of
taste, i.e. perioral or intraoral chemoreceptors that regulate feeding. This chapter explores the
diversity of such endorgans in both vertebrates and invertebrates. For the vertebrate lineage, this
modality is mediated by taste buds innervated only by the facial, glossopharyngeal or vagus
nerves. Taste buds in all vertebrates share many common features but two basic forms of
organization can be recognized. In non-mammalian vertebrates, taste buds consist of elongate
taste cells and Merkel-like basal cells. In contrast, taste buds in mammals contain only elongate
cells, but one of these (Type III taste cells) shares many characteristics with the Merkel-like
basal cells of non-mammalian vertebrates.
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01.1 INTRODUCTION
We humans recognize the sense of taste as those sensations arising from the oral cavity
and indicating information about the chemical quality of potential foodstuffs. The sense of taste
uniquely arises from the specialized sensory endorgans for this system: taste buds. In humans,
taste sensations are only those which we describe as salty, sweet, sour, bitter, and “umami” (the
taste of glutamate). All other oral sensations, e.g. the coolness of mint, the smoothness of fats or
the hotness of pepper, arise from the general cutaneous innervation of the epithelium and should
not be considered to be “taste”. So for humans and other vertebrates, “taste” is a system defined
by the sensory endorgans mediating the sensibility. For humans, and by extension, other
vertebrates, taste can be defined by the use of taste buds in the context of food selection.
When examining the evolution of the sense of taste, we are met with several difficulties.
First, how can the sense of taste be defined for organisms lacking taste buds and second, is the
sense of taste evolutionarily conserved across species, and if so, across what range of species?
This second question leads directly to the issue of where and at what point in phylogeny did taste
buds evolve, and what tissues may they have evolved from? I will consider each of these points
in the following chapter leading to the conclusion that taste is a well-defined sense only in
vertebrates, where taste buds are a clearly recognizable feature. Related chemical senses in other
taxa may have arisen independently and are not “taste” other than by having an analogous
function.
01.1.1. What Is Taste?
For humans and other vertebrates, taste is a sensory system that starts with taste buds as
the specialized sensory endorgan and deals with information concerning the chemical
composition of food in contact with mouthparts. Primarily, the sense of taste is used across taxa
to distinguish the edible from the inedible (Glendinning et al., 2000). The important features in
the above definition are that taste is a chemical sense associated with mouthparts and utilized in
control of feeding. Note that the definition says nothing about the medium conveying the
stimulus (e.g. air vs. water), nor does it include any description of molecular features or
transduction mechanisms. Neither is a defining feature of the taste system. For the vertebrate
clade, taste is defined by the sensory endorgan; for invertebrates, this definition fails since taste
buds exist only in vertebrates.
Single-cell organisms may show positive chemotaxis toward a food source by following a
concentration gradient of an attractive substance (Van Houten, 2000). Although this behavior
shares some aspects of taste-mediated behaviors in more complex organisms, it is not taste.
Single-cell organisms have no oral cavity and have no specialized sensory endorgans. To
include the positive chemotaxis of single celled organisms under the rubric of “taste” would
necessitate extending the abilities of taste and smell to plants which exhibit positive and negative
growth in response to chemical signals in the environment (Filleur et al., 2005).
A more difficult situation arises when examining the invertebrates. Complex
invertebrates such as crustaceans and mollusks (Ache, 1987) have specialized chemoreceptors
associated with well-defined mouth parts. These chemoreceptive endorgans are not homologous
to taste buds although they share several features with taste buds e.g. multicellular aggregates
specialized for the detection of a limited variety of chemical substances. The presence of such
specialized chemoreceptor organs on mouthparts certainly makes these endorgans similar to taste
4
buds in terms of function and behavior. Yet are they taste? The difficulty in drawing a
conclusion about this depends on the context in which one wishes to use the comparison. For
example, if one wishes to compare the behavior of a fly to the behavior of a rat, then referring to
the feeding-related peri-oral chemosensors of these animals as “taste” has some utility.
However, calling both of these systems “taste” is misleading when considering the detailed
molecular or cellular features of the sensory endorgans, i.e. the labellar sensillae of a fly are
entirely different from the taste bud of a rat. The sensory cells in flies are bipolar neurons
extending an axon into the central nervous system; the sensory cells of taste buds are axonless,
modified epithelial cells that synapse onto the peripheral process of a cranial nerve ganglion cell.
These systems are analogous, but clearly not derived of a common ancestral condition, i.e. they
are not homologous.
Even for vertebrates, including humans, the word “taste” is confounded by common
usage meaning sensations arising from the mouth. Conversationally, we use the word “taste” to
include many aspects of flavor other than salt, sweet, sour, bitter, and umami. The confusion
arises because of the nasopharynx connecting our oral and nasal cavities. Vapors from food in
the oral cavity passes retronasally through the nasopharynx to reach the olfactory epithelium .
Thus food in our mouth stimulates not only taste buds, but chemoreceptors of the olfactory and
trigeminal systems. A further confound is that even among vertebrates, taste buds are not always
confined to the oral cavity. Catfish, for example, have plentiful taste buds scattered across the
body surface, being especially densely distributed on the barbels and leading edges of the fins.
Despite their location, these oddly-situated taste buds are innervated by a gustatory nerve (facial
N.) and are used in the context of finding foods (Bardach et al., 1967).
01.2 TASTE IN INVERTEBRATES
As mentioned above, taste, when applied to invertebrates, is not as clearly defined as for
vertebrates. Following the definition above, I will consider the sensory endorgans used by
different invertebrates in detecting nutritive substances and toxins in potential food items. By
definition, the sensory endorgans for taste must be associated with mouthparts or other
appendages used in feeding. However, in many segmentally-organized invertebrates, similar
endorgans often occur on mouthparts and legs. This may, in part, be due to the fact that
mouthparts and legs are serial homologues in many segmented invertebrates. Even in nonsegmented
invertebrates, e.g. octopus, apparent taste endorgans occur on the legs as well as
mouthparts. In these cases, the anatomical distinctions are blurred and one must rely more on the
context in which the endorgan is used to define the system. By analogy to vertebrates, for
invertebrates, we can then extend the definition of taste to include contact, or near-range (i.e.
high threshold) chemoreceptors used in a feeding context and which are similar to the
chemoreceptors of the mouthparts.
The invertebrate clade includes relatively primitive, radially symmetric groups, e.g.
Cnidaria and Porifera, and the Bilateria including the Protostomes (Holland, 2000). While the
more basal group of animals clearly respond to a variety of chemicals (presumably via
specialized chemoreceptors), it is difficult to draw distinctions between various modes of
chemoreception. Also, comparatively little is known about the nature of chemosensory cells in
these basal forms.
The Protostomes fall into two large groups: Ecdysozoa (including nematodes and
arthropods) and Lophotrochozoa (including flatworms, annelids and molluscs) (Holland, 2000).
In both groups, taste as a feeding-related sense, can be distinguished from other well-developed
5
chemosensory modalities. This chapter will describe aspects of the “taste” systems in
representatives of each of these major groups; it is not meant to be a comprehensive review.
01.2.1 Ecdysozoa
Many ecdysozoa have a relatively impermeable cuticle covering the outside of the body.
Hence exteroceptive endorgans including chemoreceptors must have sensory processes
extending beyond the cuticle or else have openings in the cuticle to permit access to the external
stimuli. Sensory endorgans of this clade have a common general structure in which the cell
bodies of the sensory neurons lie beneath the surface cuticle and extend dendrites to reach
through or near the cuticle. The apical dendrites of the sensory cells are usually associated with
one or more non-neuronal accessory cells designated by a variety of names, e.g. sheath cells,
socket cells, auxiliary cells, tormogen cell, thecogen cell.
The overall organizational scheme of taste-like sensory organs in ecdysozoa is similar in
many respects to vertebrate taste systems. Yet any similarity must be attributed to convergence
rather than common origin. In both major groups, each taste organ comprises a variety of
sensory cells “tuned” to different chemical stimuli. That is, although each endorgan responds to
many different chemical cues, the individual sensory cells within the endorgan are tuned fairly
narrowly. In the ecdysozoa, each receptor cell responds either to appetitive or to aversive
substances, but never both. This dichotomy is reflected in the non-overlapping central
connectivity of the receptor cells and the behaviors driven by their stimulation (Wang et al.,
2004).
01.2.1.1 Nematodes
Chemoreceptor cells and their molecular receptors are well studied in C. elegans.
Unfortunately, the literature in this field is confounded by the tendency to refer to
chemoresponses to water-soluble compounds as “taste” while chemoresponses to volatiles is
termed “olfaction” although the same endorgan (amphid chemoreceptors) is used to mediate both
responses. As discussed above, the separation of taste and smell according to chemical nature of
the stimulus is not generally useful (e.g. compare taste and olfaction in catfish). The wellstudied
chemoreceptor of the nematode C. elegans consists of paired amphid organs each
innervated by 12 neurons (Ward et al., 1975). Of these, 11 are chemosensory, the other being
thermoceptive (Bargmann and Mori, 1997). Seven of the chemosensory neurons (ADF, ADL,
ASE, ASG, ASH, ASI, ASJ, ASK) extend dendrites through the amphid pore in the cuticle to be
in fairly direct contact with the environment. These cells respond to water-soluble substances.
Three other amphid chemosensory neurons (AWA, AWB, AWC) have dendrites extending near
the amphid pore, but are encapsulated by the “sheath” or “wing” cell and thus do not have direct
contact with the environment. The AWA, AWB and AWC cells respond to volatile substances,
presumably those capable of diffusing through or being transported across the sheath cell. The
commonly-studied chemotaxic behaviors are driven almost entirely by these amphid
chemoreceptors (Bargmann & Mori, 1997). As described above, the nematode chemotaxic
behaviors are commonly divided into “taste” and “smell” according to the nature of the chemical
stimulus (water-soluble or volatile, respectively). I suggest that all behaviors mediated by the
amphids should more properly be considered “smell” since none of the measured behaviors is
concerned with palatability of a suspected food object. Rather the amphid drives locomotor
behaviors, just as the olfactory sense in vertebrates drives approach/avoidance locomotor
responses.
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Figure 1: Chemoreceptor organs on the head of nematodes.
A: Diagram showing the location of the head sensilla of
Pratylenchus sp. reprinted with permission from Trett and
Perry, “Functional and evolutionary implications of the
anterior sensory anatomy of species of root-lesion nematode
(genus Pratylenchus)”. Revue de Nematologie 8(4):341-355
OSTROM 1985. CS, cephalic sensillum; ILS, inner labial
sensillum; OLS, outer labial sensillum. B: Diagram of a inner
labial sensillum (rendered in red in panel A). IL2, whose
dendritic tip is exposed to the outside milieu, is a
chemosensory neuron while IL1, whose tip is not exposed, is
reported to be mechanosensory (see
http://www.wormatlas.org). C. The amphid contains
numerous chemosensory neurons which detect either soluble
(ASE, ASG, ASH, ADF, ADL, ASI, ASJ, ASK) or volatile
(AWA, AWB, AWC) substances. It is not clear whether the
amphid chemoreceptors should be considered “taste”
according to the definition in this chapter since stimulation of
these receptor cells results in chemotaxis rather than feeding.
From Farbman (2000); © 2000 Reprinted with permission of
J. Wiley & Sons, Inc.
Nematodes, including C.
elegans, have a set of lesserknown
chemoreceptors, the inner
labial neurons, which are situated
more within the oral cavity and
appear likely to mediate taste-like
behaviors (Tabish et al., 1995).
Yet little is known of the function
or responses of these perioral
presumed chemoreceptors. Trett
& Perry (1985) suggest on the
basis of structure, that the IL2
neuron of inner labial sensilla
serve as contact chemoreceptors
(just as taste buds have been
described) but this speculation has
yet to be confirmed by functional
or behavioral studies. Nematodes
studied to date possess six
radially-symmetric paired inner
labial sensilla with cuticular
openings facing the inner side of
the rostral end of the oral cavity in
many species (See Fig. 1). Each
sensillum is innervated by 2
neurons (IL1 and IL2) one of
which (IL2) extends a process to
reach the outer environment; the
other sensory dendrite terminates
just below the surface beneath the
opening in the cuticle (Ward et al.,
1975); http://www.wormatlas.org).
In parasitic species, however, the
inner labial sensilla may be purely
mechanosensory in that their
sensory processes do not have
access to the surface (Fine et al., 1997) but this arrangement would not preclude detection of
volatile substances like the AWA, AWB and AWC amphid neurons.
01.2.1.2 Arthropods
Arthropods, including insects, arachnids and crustaceans, rely on chemosensory sensilla to
detect chemicals in the environment. The best characterized system is that of the fruitfly
Drosophila but other arthropods appear to have receptors of similar ilk. Chemosensory sensilla
are present not only on the mouthparts but also on the wing margins, tarsi (feet) and some other
appendages likely to contact potential foodstuffs (e.g. Dethier, 1962). The chemosensory
7
Fig. 2: Taste receptors on the fly Drosophila. Left: Drawing
of a sagittal section through the head of the fly showing the
location of the major taste organs: labellum, labral sense
organ, and cibarial sense organs. Center: Labial palp wholemount
preparation showing the aboral surface of left palp.
Anterior is left and dorsal top. Sensilla marked with stars are
purely mechanosensory and the remaining are taste bristles.
Taste sensilla are divided into three sub-types: short (small
arrowheads); intermediate (arrows) and large (large
arrowheads). Only some sensilla of each sub-type are
marked. Bar 50 μm. Right: Drawing of a single sensillum
showing the receptor cells, and auxiliary cells. Center panel
reprinted with kind permission of Springer Science and
Business Media from: Fig. 1B in Shanbhag et al., (2001). Left
and Right panels reprinted from Finger and Simon, (2000)
and modified from the original work of Singh, (1997) © J.
Wiley & Sons, Inc.
sensilla in the perioral region and upper alimentary canal apparently mediate feeding behavior
and therefore fit into the definition of a sense of taste. The chemosensory sensilla on the other
appendages are structurally and molecularly similar to the oral ones and are usually used in the
context of food detection. So it is not reasonable to exclude these from the taste system merely
because of their location on the body. The endorgan structure is right and the behavioral context
is right. Including the tarsal chemoreceptors as part of the taste system is analogous to including
the taste buds on the barbels and body of fishes in their taste system. In the case of the external
taste buds of fishes, they are clearly part of the taste system (based on endorgan structure,
innervation and behavioral context). By analogy, we should then accept the tarsal
chemoreceptors of arthropods as being part of the taste system. A similar argument can be made
for the chemoreceptors on the wing margins, although their behavioral context is less well
studied. In contrast, the chemosensory sensilla of other body parts, e.g. antenna or ovipositor,
should not be included in the taste
system since they are used in other
behavioral contexts, e.g.
navigation or egg-laying .
The basic structure of the
chemosensory sensilla is similar
whether the endorgan be on a
mouthpart, wing, or leg. These
endorgans contain one
mechanoreceptor cell and several
(2-4), physiologically distinct
chemosensory cells with an apical
process (outer dendritic segment)
extending into the sensilla proper
which is a thin, hair-like
protrusion of the cuticle (Shanbhag
et al., 2001). One or more pores
lies at the apex of the sensilla
thereby permitting substances in
the outside medium to come into
contact with the fluid (sensillum
lymph) filling the space around the
dendrites within the sensillum.
Potential tastants then must
traverse the fluid-filled space to
activate receptors on the dendrites
of the sensory cells. As is typical
of invertebrate sensory cells, each
receptor cell of the chemosensory
sensilla contributes an axon to the peripheral nerves which then enter the central nervous system.
In addition, there are numerous taste “pegs”, which are smaller sensory sensilla that protrude
little from the surface of the epithelium and which bear only one chemosensory cell along with a
mechanosensory cell (Shanbhag et al., 2001).
8
In Drosophila, chemosensory sensilla are especially dense on the labellum and to a lesser
extent on the labrum, which sits at the entrance to the oral cavity. Intraoral chemosensory
sensilla also are present in the cibarial sense organs. Both the intraoral and oral chemosensory
endorgans form nerves that terminate within the subesophageal ganglion, in contradistinction to
the olfactory (antennal) receptors that project to the antennal lobes of the supraesophageal
ganglion.
The labellar chemosensory sensilla are divisible into 3 morphological types according to
the length of the sensillum: short (s-type), intermediate (I-type) and long (l-type) (Shanbhag et
al., 2001). The I-type sensilla possess only two chemosensory cells, whereas the s- and l-types
have 4 chemosensory cells. The chemosensory cells fall into 4 broad functional classes
according to chemoresponsiveness. The w-cells respond to water, s-cells respond to sugars, L1-
cells respond to low concentrations of salt and L2 cells to high concentrations of salt and to
various bitter substances. But this formulation may be overly simple (e.g. see Hiroi et al., 2002).
The two chemosensory cells of the I-type sensilla consist of one cell with L2-type responses
(bitter, high salt) and the other cell with a combination of S and L1 properties (Hiroi et al., 2004).
Water responsive units are present only in the s-type and l-type sensilla. In summary, the
sensory cells of Drosophila gustatory sensilla fall into one of two groups according to the
behavior elicited by their activation: one group (e.g. s-units, w-units and L1-units) drive
appetitive behaviors under the right motivational conditions, while the other group (L2-units
responsive to high salt and bitter substances) drive aversive behaviors.
The dichotomy in driven behaviors of the different types of receptor cells coupled with the
presence of an axon extending directly from the receptor cell to the CNS, permits direct
assessment of the pattern of projection into the brain of these functionally different types of
receptor cells (Inoshita and Tanimura, 2006; Wang et al., 2004). Gustatory information in the
CNS of Drosophila is organized first, according to gustatory endorgan, and secondly according
to behavior driven – appetitive or aversive. Thus, the taste sensilla on the labellum project to a
different part of the subesophageal ganglion than do the taste organs within the oral cavity proper
(Stocker and Schorderet, 1981); (Wang et al., 2004). Within the subesophageal ganglion, bitterresponsive
cells (L2-type) map dorsomedial to the sugar-responsive (L1-type) neurons. Waterresponsive
receptor cells also project to the lateral neuropil of the subesophageal ganglion,
perhaps overlapping or slightly lateral to the sugar-responsive group (Inoshita and Tanimura,
2006).
Some interesting similarities exist between the insect and mammalian gustatory systems.
First, the gustatory endorgans comprise multiple sensory cells exhibiting a limited range of
chemoresponsiveness. That is, each endorgan responds to a spectrum of tastants although each
sensory cell within that endorgan is more limited. Second, the fundamental organizational plan
in the CNS is one of organotopy, i.e. each part of the body is represented in a unique part of the
CNS, suggesting that the location of a chemical cue is key to gustatory-mediated behavior.
Finally, within each organ-specific zone of the CNS, quality may be encoded by position within
the somatotopically-delineated field of neuropil. Just as different areas of neuropil are
implicated in appetitive vs. aversive cues in the subesophageal ganglion of the fly, different areas
of neuropil appear activated by different tastants in the gustatory centers of mammals (Harrer
and Travers, 1996; Sugita and Shiba, 2005)
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01.2.2 Lophotrochozoa
01.2.2.1.Annelids:
The annelids, as represented by earthworms and leeches, have widespread
chemoreceptors scattered across their body surface, but a set of these, associated with the lips
(labia) control feeding behavior (Elliott, 1987). These are relatively poorly characterized except
for the labial chemoreceptors of leech which were studied both anatomically and physiologically
by Ellen Elliot (1986,1987).
The medicinal leech, Hirudo medicinalis, will initiate a full sequence of feeding behavior
in response to human blood or plasma whether presented at room or body temperature (Elliott,
1986). The essential components of blood appear to be NaCl and arginine, which together
provoke the full feeding behavior. The sensory region crucial to this behavior is the dorsal lip
whose ablation results in loss of the feeding sequence in response to chemical stimulation.
Likewise, in Haemopsis marmorata, a carnivorous leech that eats and trails earthworms, ablation
of the dorsal lip abolishes their ability to track earthworm trails (Simon & Barnes, 1996).
The dorsal lips of leeches contain large and small sensilla containing unique ciliated
sensory cells. As is typical of invertebrates, the sensory cells are bipolar neurons with a centrally
directed axon and a dendritic process that extends to the surface of the epithelium. The sensory
cells are grouped together into sensilla of two different sizes. The approximately 150 larger
sensilla are arrayed roughly 125 μm apart in a band across the dorsal lip. Each sensillum forms a
raised papilla of about 35 μm in diameter with an apical opening of about 20 μm through which
extend the cilia of the underlying sensory cells (Elliott, 1987). Each sensillum contains multiple
sensory cells, but the number is not specified. The 250 small lip sensilla are 8-10 μm in diameter
and lie along the edges of the stripe of large sensilla. The smaller sensilla which sit flush to the
surface of the surrounding epithelium can be recognized by the collection of cilia protruding
from the surface. Since each sensory cell possesses a small number of cilia, a small sensillum is
likely to comprise a dozen or so sensory cells (Elliott, 1987).
The nerves formed by the axons of the sensory cells assemble mostly into the dorsal
cephalic nerves (Perruccio and Kleinhaus, 1996) to reach the cerebral ganglia. Stimulation of the
lip region with either NaCl or arginine evokes robust neural activity (Li et al., 2001).
Interestingly, simultaneous stimulation with quinine or denatonium, both of which are feeding
deterrents in these animals, reduces peripheral afferent activity. These findings suggest that
feeding deterrents may act, at least in part, by inhibiting the neural response to appetitive cues
(Li et al., 2001).
01.2.2.2. Molluscs
The taste-related chemoreceptors of molluscs have been characterized in both gastropods
and cephalopods. In gastropods, as typified by Aplysia, feeding-related chemoreceptors are
present on the lips and anterior tentacles (Jahan-Parwar, 1972). Likewise, cephalopods,
especially well-studied in octopus, have chemoreceptors on the tentacles as well as in the
perioral region. Those associated with the suckers on the tentacles were well described in an
elegant series of papers by Graziadei (Graziadei, 1964a,b; 1965; Graziadei and Gagne, 1976)
following studies by Emery (Emery, 1975a,b) on ciliated sensory cells (assumed to be
chemoreceptors) on the lips of squid and octopus. A brief summary of Graziadei’s findings
follows, but the reader should refer to the original papers for a complete description of the
sensory apparatus of the tentacles.
10
Fig. 3: Diagram of sensory neurons in the rim of a
sucker on the arm of an octopus. Based on structural
considerations, Type 2 receptors are likely to be
chemoreceptors as are the Type 4 cells which look
similar to olfactory receptor cells in squid. Type 3 cells
appear to be mechanoreceptors. The clusters of Type
2 cells superficially resemble vertebrate taste buds, but
obvious structural differences exist. The occasional
contacts between some Type 2 cells and basal
interneurons is reminiscent of the relationship between
elongate taste cells and Merkel-like basal cells in nonmammalian
vertebrates. Reprinted from Graziadei and
Gagne © 1976. Reprinted with permission of Wiley-Liss
Inc., a subsidiary of John Wiley & Sons, Inc.
Sensilla of the molluscs are
similar in many ways to the sensilla of
leeches. The sensory cells are bipolar
neurons with an apical dendrite that
extends to the surface of the epithelium,
and a basal axonal process that
contributes to nerves coursing to the
central nervous system. The likely
chemoreceptors of the sucker are
elongate epithelial cells (Termed Type 2
cells by Graziadei, & Gagne (Graziadei
and Gagne, 1976) which are collected
into small “apical clusters” (5-10 cells),
superficially similar to taste buds in
vertebrates (See Fig. 3). The most
common form of sensory cell in the
apical cluster is a narrow elongate cell
(Type 2a of Graziadei & Gagne) which
extends a small number (e.g. 3-8) cilia
above the surface of the surrounding
epithelium. The apical clusters may
contain a second elongate cell type
(Type 2b) which is larger than the 2a
cells and which has somewhat different
cytological features. The apical clusters
also may be associated with a
horizontally-oriented “basal interneuron” lying between the apical cluster and the basal lamina of
the epithelium. These basal interneurons as well as “encapsulated” interneurons apparently
receive synaptic contacts from the Type 2 cells of the apical cluster. The situation is reminiscent
to the organization of taste buds in bony fishes where the elongate sensory cells synapse onto a
Merkel-like basal cell (see below). Of course the tentacle sensilla of octopus is not homologous
to taste buds in vertebrates, hence similarities in organization must be due to convergence rather
than phyletic continuity.
0.1.3.TASTE IN VERTEBRATES & CHORDATES
Taste buds are recognizable throughout the vertebrate lineage – from lampreys to teleosts
to mammals. Although structural details can be quite varied across species, taste buds retain a
host of key features that distinguish them from other endorgans. The common features of taste
buds include: 1) aggregates of specialized epithelial cells including both receptor and supporting
cells; since the cells are epithelial, they have a limited lifespan and are continuously replaced
throughout the lifespan of the animal, 2) more than one type of sensory cell reaching the
epithelial surface via an opening (taste pore) in the surrounding epithelial covering, 3) sensory
(afferent) innervation from facial, glossopharyngeal or vagus nerves which project to the
viscerosensory column of the medulla. Taste buds in diverse vertebrates share other features but
it is unclear whether such features are necessary as defining features, or are rather elements in
common to a subset of vertebrates. Such common features include: 1) a cell type capable of
11
Fig. 4: Schematic drawing of a Type II
sensory cell from Amphioxus. These
receptor cells bear a long central cilium
surrounded by a ruffle of microvilli. The
numerous microvilli, which serve to
expand the surface area of the cell,
coupled with the lack of a 9 + 2
microtubule arrangement in the cilium
are consonant with a chemosensory
function. These sensory cells often
extend a short process, sometimes
through the basal lamina, to synapse on
nearby nerve fibers. Based on
descriptions and figures in Lacalli &
Hou, 1999.
concentrating and releasing serotonin (Kim and Roper, 1995; Nada and Hirata, 1977), 2) one or
more cells that manifest a neuron-like phenotype (e.g. expressing NCAM: (Nelson and Finger,
1993; Smith et al., 1993), neuron-specific enolase (NSE) (Toyoshima et al., 1991; Yoshie et al.,
1989), or neural differentiation markers such as Mash-1 (Kusakabe et al., 2002)., and 3) strong
ectoATPase activity (Iwayama, & Nada, 1967; Barry, 1992) perhaps because ATP is a requisite
neurotransmitter in this system (Finger et al., 2005).
01.3.1 Epithelial Chemoreceptors in chordates:
The chordate lineage includes the invertebrate
cephalochordates (e.g. Amphioxus) and craniates. The
craniates can be subdivided into 2 groups: 1) hagfish
and their relatives, and 2) true vertebrates, including
both agnathan (lamprey) and gnathostome lineages.
All extant vertebrates, from lampreys to amniotes,
have clearly recognizable taste buds innervated by
branches of the facial (CN VII), glossopharyngeal (CN
IX) or vagus (CN X) nerves. The cells of taste buds
are modified epithelial cells and, unlike most
invertebrate receptors, do not possess an axon or any
process extending below the basal lamina. While taste
buds are clear in all vertebrates, the evolutionary
origins of these endorgans is obscure.
Sensory cells in nearly all invertebrates are
primary sensory neurons, also called Type I receptors,
complete with both a sensory dendrite extending to the
epithelial surface and an axon connecting to the
central nervous system (see above). The amphioxus
has many such epithelial sensory cells including Type
I Cells of Lacalli (Lacalli and Hou, 1999). But
secondary sensory neurons first make a substantial
appearance in this group of organisms. The epithelial
secondary sensory cells of Amphioxus (Type II
receptors, Holland and Yu, 2002) extend immotile
cilia to the epithelial surface. These cilia are contain
numerous microtubules (Lacalli and Hou, 1999) rather
than the more standard 9 + 2 arrangement for cilia.
The apical morphology of the Type II receptors is
striking in that a ruff or collar of microvilli surround a
central elongate cilium (Fig. 4). This feature is
commensurate with a chemosensory rather than mechanosensory function. The Type II
epithelial receptors extend 2 or 3 basal processes a short distance within the epithelium to
synapse onto neural processes (Lacalli and Hou, 1999).
01.3.2. Solitary Chemosensory Cells& Schreiner Organs
All craniates, including hagfishes, possess solitary chemosensory cells scattered within
the epithelium of the gut, respiratory tract and even across the body surface (Whitear, 1992;
12
Fig. 5: Schematic drawing comparing a Schreiner Organ in
a hagfish (A) to solitary chemosensory cells (B) and a taste
bud in a typical teleost fish (C). (A) The receptor cells (R)
do not extend to the basal lamina and are flanked by
various supporting or secretory cells. Reprinted with
permission from Georgieva et al., (1979), Zoologica Scripta
© Blackwell Publishing. (B) Schematic diagram of solitary
chemosensory cells (SCCs) in a typical teleost. The SCCs
are isolated in the epithelium appearing without associated
supporting or secretory cells. (C) Schematic diagram of a
taste bud from a typical teleost. Multiple types of receptor
cells are surrounded by flattened edge cells (ec). The
receptor cells reach nearly to the basal lamina where they
form synapses with both nerve processes and Merkel-like
basal cells. B = basal cell; Bm = Basal membrane; ec =
edge cell; MB = Merkel-like Basal Cell; N = nerve fiber; Np
= nerve plexus; R = receptor cell; Sz = mucous cell; Sg =
glandular supporting cell; St = Type II supporting cell.
Finger, 1997; Sbarbati and Osculati,
2003). Solitary Chemosensory Cells
(SCCs) resemble the Type II sensory
cells of amphioxus as well as the
individual cells of taste buds in
terms of being elongate, columnar
epithelial cells which synapse onto
cranial nerve sensory processes.
The SCCs differ from taste buds in
that they can be innervated by any
cutaneous or visceral nerve. For
example, SCCs scattered across the
surface of the body of fishes are
innervated by the local cutaneous
nerve – either spinal or trigeminal
according to location. In contrast,
taste buds on the body are
innervated by a recurrent branch of
the facial nerve, not by the local
spinal nerve (Herrick, 1901).
Hence taste buds always have a
unique relationship with the cranial
nerves associated with epibranchial
placodes (Northcutt and Barlow,
1998).
Hagfish (chordates, but not vertebrates) lack taste buds as defined above, although they
do possess Schreiner organs, which are multicellular aggregates of presumed chemoreceptor
cells. These may simply be aggregations of SCCs, but the cells of the Schreiner organ are not
identical to SCCs. Schreiner Organs also have several features similar to taste buds, but do not
share all of the features of taste buds, e.g. Schreiner organs do not span the full thickness of the
epithelium and do not possess 3 cytologically distinct cell types. The relationship between
Schreiner organs, SCCs and taste buds remains enigmatic (see Braun & Northcutt 1998 for a nice
discussion of this issue).
The ultrastructure of the Schreiner organs has been described by Georgieva et al (1979)
who found there to be one type of sensory cell (Type I) replete with microvilli, a likely
supporting cell and an associated secretory cell similar to mucus-cells elsewhere in the
epithelium. The sensory cells of Schreiner organs appear identical to the SCCs in the same
species . Further ultrastructural studies are necessary in order to determine the degree of
similarity between the Schreiner organ cell types and those of taste buds. For example, the
supporting cells (Type II cells) of Schreiner organs are similar to Type I (glial-like) cells of taste
buds in that they wrap around the sensory cells. Our preliminary data indicate that Schreiner
organs are not associated with high levels of ectoATPase, which is a key feature of the Type I
(glial-like) cells of vertebrate taste buds in which ATP serves as a neurotransmitter (Finger et al.,
2005; Kirino et al., 2006). Thus Schreiner organs and tastes buds are further distinguished in
terms of utilizing different neurotransmitter systems.
13
Fig. 6: Photomicrograph of a Schreiner Organ
for the hagfish, Eptatretus. Note that the
sensory organ (arrowheads) lies well above
the basal lamina. Photomicrograph courtesy
of Dr. C. Braun, Hunter College, New York,
NY).
Whatever the similarities of Schreiner
organs and taste buds, it is noteworthy that SCCs
themselves and taste buds share several features.
Both comprise modified epithelial cells that
undergo continuous replacement during the life
of the animal. In the catfish, Ictalurus punctatus,
the SCCs and taste buds react similarly to the
PHA-E lectin (Phaseolus vulgaris agglutinin)
which reacts with the arginine-binding taste
receptor protein (Finger et al., 1996). Thus in
these fish, it appears that SCCs and taste buds
may utilize a common receptor mechanism.
Similarly, in mammals, nasal and gut SCCs, like
taste buds, express T2R (bitter) and T1R
receptors and their associated downstream
signaling components (Finger et al., 2003;
Sbarbati and Osculati, 2003). Thus in both
teleosts and mammals, SCCs and taste buds may
utilize common receptor mechanisms.
Nonetheless, differences do exist. Whereas
SCCs form clear synapses with nerve fibers, the
cells of taste buds that share biochemical
features with SCCs (Type II cells – see below)
do not. Further studies are needed to understand
the evolutionary relationships between these
cutaneous chemoreceptor systems.
01.4. TASTE BUDS IN VERTEBRATES
In this chapter I present an overview of some of the different appearances of taste buds, but
this is not meant to be comprehensive. An excellent comparative view of taste buds can be
found in the work of Reutter & Witt (Reutter and Witt, 1993).
Taste bud structure varies considerably across vertebrates (see Fig . 7) but several consistent
features emerge when comparing across species as described above. These include: 1)
aggregates of 50-150 specialized epithelial cells including both receptor cells and glial-like
supporting cells, 2) multiple types of elongate cells reaching an opening in the epithelial surface,
and 3) innervation by one of the 3 gustatory nerves: facial, glossopharyngeal or vagus.
Categorization of cell types within taste buds is complicated by the fact that taste buds consist
not only of different functional types of cells, but also cells of different ages within each
functional class. Taste buds are surrounded by specialized epithelial cells, “edge”, “marginal” or
ciliated cells (in frog), which form the outer boundary of the taste bud proper. In addition, all
taste buds are closely associated with proliferative basal cells which divide to replace the aging
and apoptotic cells of the taste bud. The literature on the types of cells in taste buds is extensive
and complex (Reviewed in: (Yee et al., 2001). Rather than reviewing the vagaries of this
literature, I will present a summary of our current understanding of the organization and structure
of taste buds.
14
Fig. 7: Schematic drawings of taste buds from various
vertebrates. The area over which receptor cells gain
access to taste substances (receptor area) is relatively
broad in aquatic species, but narrows to a “taste pore” in
mammals and birds. Taste buds in all species contain
different types of elongate cells indicated by the varied
shading. Also in all species, taste buds are bounded by
specialized epithelial cells termed “Edge” cells or
“marginal” cells (mc). In all species, taste buds contain a
serotonergic cell type – Merkel-like Basal cells (MBC) in
non-mammalian forms, and Type III taste cells (III) in
mammals. All taste buds also are associated with a
population of proliferative basal cells PBC) which
undergo continuing cell division to replace the taste bud
cells throughout the lifespan of the animal. Copyright
1993 from “Morphology of Vertebrate Taste Organs and
Their Nerve Supply” by Reutter and Witt. Reproduced
and modified by permission of Routledge/Talor & Francis
Group, LLC.
Some groups, such as frogs, have distinctive apomorphic characteristics, where taste buds
take on a broad cylindrical form of large taste “disks” spanning 100 μm. Most vertebrates have
more compact taste buds organized in
an onion-like configuration with an
apical pore only tens of micra across.
These more compact taste buds, found
in all vertebrate groups, have 2
different plans of organization,
typified in the descriptions below as
the non-mammalian and mammalian
schemes (the situation in birds is not
clear). Whether these differences in
morphology are more related to
phylogeny or to habitat is unknown.
Historically, elongate taste cells in
taste buds have been categorized
according to their propensity to stain
with acidophilic dyes or degree of
osmiophilia in preparations for
electron microscopy. This has led to
descriptions of cells as being either
“light” or “dark” but these descriptors
may vary according to preparatory
technique and particular stain utilized.
Some authors have extended this
classification system to imply
function, characterizing the elongate
taste cells as being “sustentacular” (or
“supporting”) versus “sensory”
(“receptor”) cells. More careful
ultrastructural analysis leads to a
characterization according to
structural features such as size and
shape of apical specialization,
presence of distinctive granules, or
size and shape of the nucleus.
Nonetheless, the mixed nomenclature
remains in the current literature.
01.4.1. Taste Buds in Non-mammalian
vertebrates
Taste buds in these aquatic forms
have been described in many teleosts,
a few elasmobranchs and urodeles
(reviewed in: Reutter and Witt, 1993)
as well as in a lamprey, where the
15
Fig. 8: Electron micrograph of a taste bud from
a zebrafish (courtesy of Dr. Anne Hansen,
Univ. Colorado). Even at this low magnification
the different sizes of microvilli within the
receptor area (taste pore) are evident. The
large microvillus belongs to a “Light Cell” while
the smaller microvilli originate from “Dark
Cells”. Inset (upper right) shows an
enlargement of the receptor area.
endorgans have been called “terminal buds” (Baatrup, 1983). The detailed structure of taste buds
can vary substantially between species, or even within a species, between taste buds situated in
different locations, e.g. oral compared to extraoral (Reutter and Witt, 1993). Nonetheless, a
common organizational plan can be abstracted.
01.4.1.1 Cell types
Taste buds in this group are distinguished by containing not only elongate (columnar)
spindle-shaped cells, but also a small number (e.g. 5) of non-proliferative, “Merkel-like” basal
cells, lying in the lower half of the taste bud and which do not extend to the apical surface of the
epithelium. Like cutaneous Merkel cells, the Merkel-like basal cells of taste buds concentrate
biogenic amines including serotonin and are immunoreactive for neuron-specific enolase
(Reutter and Witt, 1993). Also like cutaneous Merkel cells, the Merkel-like cells of taste buds
extend numerous spine-like processes from their cell body to form synapses on nerve fibers as
well as on the elongate taste cells in the taste bud. It is likely that these Merkel-like basal cells,
like cutaneous Merkel cells, serve as mechanoreceptors or perhaps in the taste bud, as integrative
elements (Ewald and Roper, 1994). It is unfortunate that these Merkel-like basal cells are
sometimes referred to simply as “basal cells” in that this causes confusion with the proliferative
basal cells associated with taste buds of both aquatic and terrestrial species.
The non-mammalian type of taste bud also
possesses several types of elongate modified
epithelial cells that extend an apical process into
the region of the taste pore. In aquatic forms,
including fishes and aquatic amphibians, the
apex of the taste bud is a substantial opening –
10-20 μm or larger — in the surrounding
epithelium through which extend the apices of
the elongate taste cells. This opening in the
epithelium is much larger than the equivalent
“taste pore” present in mammals or birds.
Whether this difference in the size of the taste
pore is characteristic of the clade of vertebrates
(e.g. poikilothermic vs. homothermic) or of the
habitat (aquatic-terrestrial) is unclear.
Elongate cells in fish and amphibia
usually are characterized as being “light” or
“dark”. These two descriptors are undoubtedly
inadequate to fully characterize all of the
different types of elongate cell present in these
taste buds. The light cells are spindle-shaped
cells with a single, large apical microvillous
extending into the taste pore. Light cells extend
short branches from their base to synapse with
the Merkel-like basal cells and with nerve
fibers. Dark taste cells are irregular in crosssectional
form and may envelop or extend
interdigitating processes between the light cells
16
Fig. 9: Drawing of the principal cell types of a frog taste
disk. Ciliated cells (cil) surround the receptor surface of the
taste organ. The superficial third of the organ is occupied
by mucus (mc) and wing cells (wc) which are probably
involved in maintenance of the mucus layer covering the
taste disk. In the middle layer of the disk lie the cell bodies
of the elongate receptor (Type II and Type III) cells which
extend an apical process penetrating the surface layer.
Both Type II and Type III cells contact afferent nerve fibers,
although distinct synaptic complexes occur only between
the Type III cells and the nerve fibers. Sustentacular (sus =
Type Ic) cells wrap the other cell types and nerve fibers
Finally, Merkel-like basal cells (MBC) lie in the deepest
portion of the taste disk. Reprinted from Prog. Neurobiol.
46, Osculati and Sbarbati, “The frog taste disc: a prototype
of the vertebrate gustatory organ.” 351-399 (1995) with
permission of Elsevier.
(Reutter and Witt, 1993). At its apex, a dark cell extends numerous (10-25) small microvilli into
the taste pore. Although dark cells apparently form synaptic contacts with the Merkel-like basal
cells, they rarely do so with nerve fibers. In Necturus, light cells constitute only about 25% of
the elongate cells within the taste bud, the remainder being dark cells. Taste buds in fish and
Necturus also contain a less common, third cell type with a brush-like or bushy microvillous
apex.
01.4.1,2, Proliferative cells
In non-mammalian vertebrates, the taste bud is closely associated with a small number
(e.g. 5) of proliferative basal or marginal cells that apparently generate daughter cells which
enter into the taste bud and differentiate into the various mature cell types. These proliferative
cells do not sit directly below the taste bud, where the Merkel-like basal cells reside, but rather
around the basal circumference of the bud (Raderman-Little, 1979).
01.4.2 The specialized taste organ of frogs
The taste organs of frogs,
called “taste disks” are highly
derived compared to other
anamniote vertebrates (Osculati and
Sbarbati, 1995) although many
commonalities can be observed. In
frogs, the apical opening is an
expansive disk over 100μm in
diameter. The taste disk is
surrounded by specialized, ciliated
cells. Inside this ring is a floor
largely consisting of short, broad
mucous cells each surrounded by the
apical processes of “wing” cells,
thought to be supporting cells (Fig.
9). The elongate taste (sensory)
cells have their nucleus situated
deeper in the taste disk than the wing
and mucous cells, but extend a thin
apical process to the surface of the
taste organ. These elongate taste
cells are divided into two forms:
Type II cells and Type III cells.
Although Type II cells have
substantial contacts with basallysituated
axons, no obvious synaptic
junctions occur. This situation
appears similar to the Type II taste
cells of mammals (see below). The
Type III cells of the frog taste disk
do exhibit clear synaptic contacts
17
with nerve and are similar in that respect to Type III cells of mammals. Glial-like sustentacular
cells embrace and separate the different cells and nerve fibers in the lower half of the taste disk.
This relationship is similar to the Type I cells in mammalian taste buds. In addition, frogs have
serotonergic Merkel-like basal cells characteristic of non-mammalian taste buds. The presence
of both these Merkel-like basal cells and the Type III sensory cells suggests that the transition
from a non-mammalian-type of taste bud to a mammalian-type of taste bud is not simply the
migration and transformation of the Merkel-like basal cells to an elongate morphology.
01.4.3. Mammalian Taste Buds
Taste buds in amniotes differ from anamniote taste buds in 2 respects. First, the taste
pore is considerably narrower (about 10 μm or less). Whether this is attributable to a drier,
terrestrial lifestyle or to phylogenetic factors is unclear. Second, mammalian taste buds lack the
Merkel-like basal cell characteristic of non-mammalian taste buds. The taste buds of mammals
do, however, possess a type of elongate cell which, like the Merkel-like basal cells, concentrates
serotonin and forms distinctive synapses with the afferent nerve fibers. This has led many
authors to speculate that the serotonin-containing elongate cells of amniote taste buds are
homologous, if not functionally-equivalent, to the Merkel-like basal cells (e.g. (Ewald and
Roper, 1994).
01.4.3.1. Taste cells
Taste buds in mammals comprise 3 distinct morphological types of elongate cells (Type
I, II and III taste cells). These are defined according to ultrastructural criteria following the
original descriptions of taste cells in rabbit foliate papillae by Murray (Murray, 1986). Although
the different types of taste cells are fairly distinct in rabbit foliate papillae, the morphological
distinctions are less clear in other species. This has led to a great deal of confusion in the
literature as to the equivalencies and distinctions between taste cell types in various mammals,
especially rats and mice. In reviewing past literature on this subject, it is important to keep in
mind that one author’s “Type II” cell may not be the same as another author’s cell of the same
name. To further complicate matters, some authors have retained the older light microscopic
terms: dark cell and light cell (originally based on staining properties of aniline dyes). The lightdark
cell descriptors are only loosely equivalent to the morphological types as defined by
electron microscopy. That is, Type I cells nearly always have an electron dense cytoplasm and
thus are called dark cells. Unfortunately, Type III cells are more variable in staining
characteristics and have been grouped by various authors into the category of “light cell”, “dark
cell” or “intermediate cell” thereby seriously confusing the literature. With the advent of
immunocytochemistry, it is possible to recognize the 3 distinct cytological and functional classes
as originally defined by Murray.
01.4.3.1.1. Type I taste cell
The Type I taste cells constitute over 50% of the total cells within a mature taste bud. As
described by Murray and others, this cell often wraps around other taste cell types and nerve
fibers. The cytoplasm is electron dense and stains heavily with acidophilic dyes, giving the cell a
dark appearance in both light and electron microscopy. The nucleus is elongate with an
irregular, indented nuclear membrane and substantial amounts of heterochromatin along the
inner leaflet. These cells usually contain large apical granules roughly 100nm in diameter and
extend long, slender microvilli into the taste pore.
18
Fig. 10: Fluorescence micrographs of
immunocytochemically-reacted taste buds of the
circumvallate papilla from rodents. LEFT: Reactivity
for the inositol-trisphosphate receptor 3 (IP3R3) in a rat
circumvallate papilla shows the morphology of typical
Type II taste receptor cells: broad, triangular cell body
with a prominent, large, round nucleus. This section is
also reacted for synaptobrevin revealing the numerous
afferent nerve fibers and a rare taste cell (arrow), most
likely a Type III taste cell near the edge of the taste
bud. Photo courtesy of Drs. JC Kinnamon & R Yang,
Denver Univ., Denver CO. RIGHT: A taste bud in the
circumvallate papilla of a mouse, reacted for serotonin
to reveal a population of Type III taste cells. Note that
they are more slender and less regular in shape than
the Type II cells illustrated in the left hand panel. The
approximate boundary of the taste bud is indicated by
the dashed line.
In many ways,, the Type I cells are similar to glia of the central nervous system. They
express GLAST, a glial glutamate transporter (Lawton et al., 2000) and NTPDase2, an astrocytic
ectoATPase (Bartel et al., 2006; Wink et al., 2006). The processes of Type I cells insinuate
themselves between the other cell types and often cover a point of contact between other taste
cells and nerve fibers, just as astrocyte processes embrace synapses in the central nervous
system. Since ATP is a crucial neurotransmitter between taste cells and the afferent nerve fibers
(Finger et al., 2005), these Type I cell processes may serve to restrict cross-talk between cells
within the taste bud by diffusion of ATP away from points of functional contact between taste
cells and nerve fibers.
01.4.3.1.2. Type II taste cell
Type II cells represent about 25-30% of the cells in each taste bud and are responsible
for transduction of many tastants. These cells are elongate, spindle-shape cells with short, thick
apical microvilli. The cell is typified by a large, round, clear nucleus and by pale cytoplasm.
Type II cells express the bevy of receptor and second-messenger proteins implicated in
transduction of bitter, sweet or umami (glutamate) stimuli. These include the known T1R and
T2R families of taste receptors, gustducin (G protein), PLCβ2, and IP3R3 (Yang et al., 2000b;
Miyoshi et al., 2001; Kusakabe et al., 2002; Clapp et al., 2004). Thus Type II cells mediate
detection of these classes of tastants.
Curiously, although Type II cells closely
contact afferent nerve fibers within taste
buds (e.g. (Kinnamon et al., 1985; Yang
et al., 2000a), synapses between these
elements are rare. Rather, subsurface
cisternae appear at points of contact
between afferent fibers and Type II cells
(Clapp et al., 2004).
Each Type II taste cell is
specified for detection of one class of
taste substance and therefore expresses
only one class of taste receptor, although
multiple members of a class may be
expressed in a single taste cell
(Chandrashekar et al., 2000; Zhang et
al., 2003). For example, a taste cell that
expresses one member of the T2R
family of receptors (for detecting bitter
substances) will express many members
of this same family (Adler et al., 2000).
Since each receptor molecule is
responsive to only a small set of bitter
substances, by expressing multiple
members of the T2R family, a taste cell
then exhibits broad responsiveness to
many different bitter compounds.
Whether Type II cells also are the
19
transduction elements for detection of sour and salty stimuli is unclear. At least some evidence
implicates Type III cells in these processes.
01.4.3.1.3. Type III taste cell
The Type III cell is relatively scarce in taste buds, comprising only 10-15% of the total
population. Type III cells are sometimes called “intermediate cells” since they share features
with both Type I and Type II cells. What distinguishes Type III cells from the others is the
presence of well-formed synapses from the taste cells onto the afferent nerve fibers. Type III
cells also exhibit some distinctive histochemical features which can be used to distinguish them
from the other cell types. A subset of Type III cells concentrates biogenic amines, including
serotonin. This property has led to their being likened to the Merkel-like basal cells of
anamniote vertebrates (v.s.).
Type III taste cells are narrow, spindle-shape cells that extend a single, thick apical
process into the taste pore. The nucleus is more elongate than that of Type II cells and exhibits
some degree of indentation. The cytoplasm is variable in staining density, ranging from light to
dark. These features then are intermediate between the Type I an II cells and have led some
authors to consider Type III cells to be a stage in the maturation of Type II cells or to combine
these cells into a single class (e.g. (Delay et al., 1986; Pumplin et al., 1997). Recent studies
suggest that Type II and Type III cells may arise from a common lineage (Finger, 2005;
Kusakabe et al., 2002; Miura et al., 2005).
The function of Type III cells is not established, but two possibilities are clear. First, the
Type III cells, being the only taste cells with prominent synapses, may serve as the only output
cells of the taste bud, receiving input from the transducing, Type II taste cells and integrating this
information before transmitting a signal to the afferent nerve fibers (Roper, 1992). The other
possibility is that both Type II cells and Type III cells transmit information to nerve fibers, but
that Type II cells do so using a mechanism that does not require a conventional-looking synapse.
Since Type III cells exhibit voltage-gated ion channels (Medler et al., 2003) and such cells
respond to acidification (Richter et al., 2003), then it is likely Type III cells are capable of
directly transducing sour information and passing this along too the nerve fibers.
01.4.3.1.4. Other cells
In addition to the 3 elongate types of taste cells described above, the taste buds of mammals, like
those of non-mammalian vertebrates, are associated with proliferative basal cells and edge or
marginal cells. Indirect evidence indicates that the taste bud progenitor cells of the basal
epithelium are different than the basal cells of the general epithelium. When gustatory nerves are
directed to grow into lingual epithelium that does not normally produce taste buds, taste buds do
not form despite the abundance of gustatory nerve fibers (Krimm et al., 2001). The fact that taste
nerves are unable to induce taste buds in anything but taste epithelium suggests that the epithelial
cells in taste-bud bearing regions have a special capacity to generate these endorgans.
Conversely, when taste epithelia are innervated only by non-gustatory nerves, then production of
taste buds is limited at best (Farbman, 1971). Together, these studies indicate that basal cells of
taste epithelia have a unique capacity to produce taste buds under the influence of gustatory
innervation.
20
01.5. DETECTION AND REPRESENTATION OF DIFFERENT TASTES
The taste systems in vertebrates has the ability to respond to a variety of stimuli
according to the habitat and nutritional needs of the organism. These taste stimuli are varied in
chemical properties including size, charge, hydrophobicity and pH. Despite the diverse array of
vertebrates and habitats, the taste system has a remarkable consistency in the types of
compounds it can respond to. This may be due to the fact that many substances, e.g. plant
alkaloids, are toxic to most vertebrates and therefore all vertebrates require a food monitoring
system capable of detecting potential toxins in the food supply. Conversely, different vertebrates
have different nutritional needs and drives, so somewhat more divergence exists in terms of what
substances can drive appetitive behaviors. Looking across all organisms, the taste system serves
two primary functions: avoiding toxins and driving ingestion for nutritive substances. This
means that the responses of the taste system should vary according to the diet of the particular
organism. For example, the taste system of carnivores should not be driven by sugars, whereas
the taste system of herbivores should be highly responsive to sugar. In contrast, most species
should respond to amino acids.
Different cells in taste buds respond optimally to different taste qualities. It is interesting
to note that this principle of cellular coding also occurs in taste organs of invertebrates. Since
taste buds and “taste” organs of invertebrates are not homologous, this property of encoding taste
information should be viewed as convergent rather than evolutionarily conserved. Indeed, the
chemosensory cells of the invertebrates seem more organized according to the behaviors they
induce rather than the nature of the chemical stimulus detected. For example, a single
chemosensory cell (ASE) in the amphid of C. elegans may respond to cAMP, biotin and lysine
despite their diverse chemical structures. But all of these substances are attractants. So
stimulation of the ASE cell will produce attraction. The taste systems in more complex
organisms, e.g. flies or fish or mammals, have more complexity. Several substances may drive
ingestion, but may be detected by different receptor cells. For example, both alanine and
arginine drive appetitive behavior in catfish, but these amino acids appear to be detected by
different receptors expressed in different taste cells (Finger et al., 1996). Similarly, different taste
cells in mice express receptors for glutamate and sweeteners although both drive food intake
(Zhang et al., 2003).
01.6. EVOLUTION OF TASTE PREFERENCE AND TASTE RECEPTORS
In order for a species to adapt to a new habitat or feeding strategy, the spectrum of
substances to which its taste organs respond must change. For example, for terrestrial animals,
especially those with a purely vegetarian diet, sodium is a crucial nutrient. Salt-deprived
amniotes have a drive to seek out and ingest salt (Schulkin, 1991). Their taste system carries
unique information about the sodium content of potential foodstuffs and detection of the sodium
is regulated in part by circulating hormones that alter the sensitivity of the sodium-detecting
channels of the taste buds (Herness, 1992; Lin et al., 1999). Yet, sodium is not a crucial nutrient
for aquatic anamniotes, so their taste systems are not particularly responsive to sodium content of
food (Caprio et al., 1993). Thus, the responsiveness of taste buds had to change when
vertebrates made the transition from water to land. In frogs, the entire epithelium is sensitive to
sodium levels, perhaps via the same ion channel (AsNaC) used in sodium detection in many
mammals (Nagai et al., 1999). Thus transduction of sodium by taste may have evolved from a
general epithelial property of regulated sodium transport.
21
The receptors for tastes can be ion channels themselves (as in the case of sodium (salty)
or protons (sour), may be ligand-gated ion channels (e.g. for arginine-detection in catfish; (Brand
et al., 1991) or G-protein coupled receptors (Brand et al., 1991; Adler et al., 2000;
Chandrashekar et al., 2000; Zhang et al., 2003; Ishimaru et al., 2005). The G-protein coupled
receptors, T1R and T2R families, are phylogenetically old since family members have been
identified in fish as well as in mammals. Yet in mammals, some of these receptors respond to
sweeteners whereas in fish, sweeteners are not effective taste stimuli. Accordingly, evolutionary
change of the receptor molecules is likely to correspond to evolutionary changes in the spectrum
of substances to which the taste system can respond. This can be seen in the evolution of felines
which are insensitive to sugars and other sweeteners unlike other carnivores. The taste system of
basal carnivores most likely responds well to sweet substances since many contemporary
carnivores, e.g. dogs, bears, are strongly attracted to sweet, whereas cats are not (Li et al., 2005).
One of the genes encoding the sweet taste receptor is non-functional in cats thereby rendering
them insensitive to sugar and other sweeteners. Thus a simple mutation in a single taste receptor
gene is capable of altering the diet of a species.
Useful URLs:
http://www.wormatlas.org
http://flybase.bio.indiana.edu/
ACKNOWLEDGEMENTS
This chapter is dedicated to the memory of Theodore H. Bullock whose boundless
enthusiasm and insights served as an inspiration to us all. The author thanks Drs. Anne Hansen
and Linda Barlow for their comments and improvements on various drafts of this work. This
work was supported by NIH Grant s to T. Finger and D. Restrepo.
22
GLOSSARY
Amphid – A paired chemosensory organ at the anterior end of nematodes including
Caenorhabditis elegans. The amphid is innervated by 11 chemosensory neurons and one
associated mechanosensory neuron.
Auxiliary Cells – Non-sensory cells of invertebrate sensilla. This class of cells includes socket
cells, sheath cells, tormogen cells and thecogen cells.
Ecdysozoa – One of two major groups of the protostome division of the animal kingdom (c.f.
Lophotrochozoa). Members of this group, including arthropods and nematodes, possess an outer
cuticle rather than an internal skeleton.
Ecto-ATPase – An enzyme that breaks down extracellular ATP. The ecto-ATPases can be
divided into several molecular families.
Inner Labial Sensillum – One of eight chemosensory organs around the mouth (stoma) of
nematodes. This organ contains only two sensory cells only one of which has free access to the
external surface of the worm.
Labellum – A fleshy ovoid pad at the end of the probiscus of a fly used as a taste organ. The
labellum houses numerous taste and tactile sensilla.
Labrum – An chemosensory organ in flies situated at the anterior end of the oral cavity.
Lophotrochozoa – One of two major groups of the protostome division of the animal kingdom
(c.f Ecdysozoa). Members of this group, including annelids and mollusks, share common
developmental forms although the adults appear quite diverse.
Merkel Cell – A specialized epithelial cell found in vertebrates and which participates in
mechanoreception. Upon stimulation, Merkel cells release serotonin and ATP to activate a
closely-associated sensory nerve fiber.
Merkel-Like Basal Cell – A basal cell of non-mammalian taste buds named because of their
similarity with epithelial Merkel cells in terms of structure and neurotransmitter contents
Schreiner Organ – a presumed epithelial chemosensory organ of hagfish. This multicellular
endorgan is superficial similar to, but not homologous to, taste buds. Unlike taste buds,
Schreiner organs can be innervated by any epithelial nerve.
Sensillum – a sensory organ of invertebrates in which the sensory cells extend a hair-like
process out of the cuticle.
Sheath Cells – The inner non-neuronal cell of a sensory endorgan of invertebrates, esp. C.
elegans, closely surrounding the sensory neurons.
Socket Cells – The outer non-neuronal cell of an invertebrate sensory organ.
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Solitary Chemoreceptor Cell (SCC) – scattered specialized chemosensory cells in the
epidermis of aquatic vertebrates also found in the gut and airways of terrestrial vertebrates.
T1R – a family of mammalian taste receptors that includes three members which heterodimerize
to form either sweet or umami receptors
T2R – a large family of mammalian taste receptors that form bitter-sensitive taste receptor
molecules.
Taste Cell – an elongate specialized epithelial cell of vertebrate taste buds.
Thecogen Cell. – the inner auxiliary (non-sensory) cell of a sensillum, c.f. sheath cell.
Tormogen Cell – the outer n auxiliary (non-sensory) cell of a sensillum, c.f. socket cell.