Gonads

The gonads (ovaries in females, testes in males) are the organs responsible for gamete production. They also have endocrine functions. In response to FSH and LH from the pituitary, the gonads produce various hormones.

The ovaries produce estrogen and progesterone. These two hormones, along with FSH and LH from the anterior pituitary, control the female menstrual cycle. After ovulation occurs, the ovaries also produce inhibin, which suppresses the release of FSH. The ovaries have an additional role during pregnancy. Along with the placenta, the ovaries produce relaxin, which helps dilate the cervix during labor and delivery, and relaxes the pubic symphysis.

This graph shows the complex nature of hormones controlling the female menstrual cycle. For the time being, you do not need to understand the interaction of these hormones….we’ll leave that for something to look forward to in Unit 20.

Estrogens and progesterone also promote the development of female secondary sex characteristics at puberty such as breast development, hair growth in the pubic area and underarms, and widening of the hips.

The main hormone produced by the testes is testosterone. This hormone stimulates the testes to descend from the abdominal cavity to the scrotum before birth, controls the production of sperm, and stimulates male secondary sex characteristics, those that develop with puberty. The testes also produce inhibin, which suppresses the secretion of FSH.

Pineal Gland

The pineal gland is attached to the top of the third ventricle of the brain; it is part of the epithalamus.

The pineal gland produces and secretes a hormone called melatonin. It is theorized that melatonin assists with the setting of the daily biological clock by promoting sleepiness. More melatonin is released during darkness than light.

Sleep/wake cycles are very evident when comparing different seasons of the year. What if the doorbell rings at 8 PM during the dark winter months? People get mad because they are getting ready for bed. What if the same salesman drops by at 8 PM in the summer months? People still get mad, but it’s because he interrupted dinner on the patio.

Thymus Gland

The thymus gland is located in the mediastinum (behind the sternum and between the lungs). The thymus is important to our immune system, and is discussed in more detail in Unit 15.

Thymosin and other related hormones from the thymus gland encourage the maturation of T-lymphocytes, a specific type of white blood cell (WBC) crucial for fighting disease.