42 Determining Cell Movements During the Development of the Iris Stroma

Faculty Mentor: Kristen Kwan (Human Genetics, University of Utah)

The role of the iris in the vertebrate eye is to regulate the amount of light entering the pupil, which is essential to protect the retina from light damage and enhance visual acuity. However, despite its importance, how the iris forms during embryonic development is unknown. Identifying the steps in normal iris development can provide us with a foundation to investigate congenital eye disorders affecting the iris in humans. The iris is composed of two tissue layers: the iris epithelium and the iris stroma. Both tissue layers contain pigment producing cells that give the iris its characteristic color or hue. The pigment is thought to help regulate the amount of light entering the pupil by absorbing and refracting light. Additionally, in some species the pigment provides camouflage to the eye. The iris epithelium and stroma have different embryonic origins; neural crest derived mesenchyme forms the iris stroma, while the iris epithelium is derived from the neural retina. Zebrafish eyes are anatomically very similar to those of human eyes and zebrafish possess both an iris stroma and epithelium.

Therefore, zebrafish serve as a good model for studying the development of the iris stroma and epithelium, and in the future, to understand how these processes are disrupted in diseases in which the iris does not form correctly. This study aims to determine how cells assemble to form the tissues of the iris during embryonic development, specifically focusing on the iris stroma. We hypothesize that the prospective iris stromal cells migrate through the space that separates the cornea from the anterior rim of the retina, moving towards the anterior lens periphery.

We used zebrafish embryos with fluorescently labeled nuclei (mCherry) and membranes (EGFP), Tg(bact2:H2A-mCherry;bact2:EGFP-CAAX), and performed multidimensional time-lapse confocal imaging to record the development of the iris stroma. We performed manual 4D cell tracking using LongTracker, a MATLAB-based program developed in our lab, and quantified cell speed, distance traveled, and net displacement. Using our cell tracking and live imaging data, we generated a 4D movie using FluoRender. The data tell us, for the first time, about the migration of mesenchyme cells into the iris stroma. Understanding iris stromal cell movements in wild-type embryos may provide general insight into the assembly of neural crest-derived tissues.

The cell tracking data demonstrates that the iris stromal cells move through the space between the cornea and anterior retina as predicted, however, unexpectedly, the cells move primarily dorsally rather than radially towards the lens periphery. All tracked cells move similarly along the dorsal- ventral axis, first moving dorsally but then reversing course and moving ventrally. Some, but not all, of these cells move towards the lens periphery in the manner we predicted. The tracked iris stromal cells also appear to follow consistent trends in distance traveled and net displacement, but the cell speed varied significantly between the two embryos. A possible explanation for this behavior in iris stromal cells could be due to the natural variation between zebrafish embryos. Additional embryos will be required to test this. Future directions are to examine iris stromal cell shape changes, and to track iris epithelial cells to identify potential differences in cell migration across the iris tissue layers. Next, we also plan to investigate what molecular and genetic factors are influencing the dorsal-ventral cell movement pattern we observed.

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