Male and Female Anatomy
Males: The gonads in males are the testes. Inside the testes, sperm is made in the seminiferous tubules. These convoluted tubules are associated with two really important types of cells - Leydig and Sertoli cells. Leydig cells are stimulated by leutinizing hormone (LH) and convert cholesterol into testosterone. Testosterone is required for male secondary sex characteristics like pubic hair, lowered voice, growth of the larynx, and fusion of the epiphyseal plate (growth plate). Testosterone also acts on adjacent cells called Sertoli cells. Testosterone acts to encourage sperm production. In the Sertoli cells, testosterone is converted to dihydrotestosterone. Secondly, Sertoli cells are induced by Follicle Stimulating Hormone (FSH) which is required for sperm production. Negative feedback is used by both Leydig cells and Sertoli cells. If too much testosterone is produced, it can act to inhibit GnRH production by the hypothalamus as well as LH production in the anterior pituitary. Similarly, Sertoli cells can inhibit FSH production of the Anterior Pituitary by releasing a peptide hormone called “Inhibin.”
Females: The gonads in females are the ovaries. By birth, all of the ova that a female will produce are made. During puberty, these ova will be released into the fallopian tubes (ovaducts) and try to get fertilized. In order for all this to happen, lets back up a little. A primary oocyte is surrounded by granulosa cells which together are called a primary follicle. Thecal cells now surround this primary follicle and create what is called a secondary follicle. Theca and granulosa cells are similar to Leydig and Sertoli cells in men. Theca cells are stimulated by LH and make testosterone from cholesterol. The testosterone is then converted to estrodiol (a form of estrogen) in Granulosa cells. Similar to Sertoli cells, Granulosa cells are stimulated by FSH which help mature primary oocytes.
The female menstrual cycle can be broken down into an average of 28 days. During the first 14 days, the primary oocyte has matured to a secondary oocyte. On the 14th day, there is a surge of estrogen which causes a subsequent surge of LH. This LH surge (known as the luteal surge) causes the secondary follicle to burst, releasing the secondary oocyte into the ovaduct.The left over follicle is now called the Corpus Luteum and begins to secrete both estrogen and progesterone. In combination, these two steroid hormones inhibit LH secretion from the anterior pituitary. This allows for the endometrium to grow and thicken. This occurs for 2 weeks after ovulation. If fertilization does not occur, the endometrium will be shed and the Corpus Luteum will degrade into the Corpus Albicans.
The period of 14 days up till ovulation is known as the Follicular Phase. The period from ovulation until degredation of the Corpus Luteum is known as the Luteal Phase. The shedding of the uterine lining is known as “flow.” Flow represents the last 5 days of the cycle and is observed by vaginal bleeding.
