Unit 17: Human Reproductive Systems — Long Questions

Spermatogenesis is controlled by hormones from the hypothalamus and pituitary gland. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH acts on the seminiferous tubules to stimulate spermatogenesis. When FSH stimulates the Sertoli cells, they release a peptide hormone called inhibin, which inhibits further FSH secretion (negative feedback).
LH stimulates the Leydig cells, located in the interstitial tissue between the seminiferous tubules, to secrete testosterone.
Testosterone stimulates development of the germinal epithelium (supporting sperm production) and maintains male secondary sexual characteristics. The release of testosterone also inhibits the release of LH and GnRH, forming a negative feedback loop that keeps hormone levels balanced.

The female reproductive system consists of the gonads and accessory organs (oviducts, uterus and vagina).
1. Ovaries (Gonads): Two oval ovaries (3-5 cm x 2-3 cm), one on each side of the uterus, contain many ovarian follicles that nurture immature eggs. Each month one ripening follicle ejects its egg (ovulation); the ruptured follicle then becomes the corpus luteum, a glandular structure that secretes hormones.
2. Oviducts (Fallopian tubes): Two tubes leading from the ovaries to the uterus. When an ovum is released, it enters an oviduct and is moved toward the uterus by ciliary movement in the lining. Fertilization occurs here, and the resulting zygote undergoes early mitotic divisions while still in the oviduct.
3. Uterus: A hollow, muscular organ with three parts - the fundus (uppermost rounded portion), the body (major portion) and the cervix (lower narrow part opening into the vagina). Its wall has three layers: the outer perimetrium, the middle muscular myometrium, and the inner endometrium (with a basal layer that regenerates a functional layer, which thickens and is shed in the menstrual cycle, or thickens further and forms the placenta if pregnancy occurs).
4. Vagina: An 8-10 cm thin-walled tube leading from the cervix to the outside; it serves as the passageway for menstrual flow, for sperm entry, and for delivery of the baby.

The menstrual cycle lasts about 28 days and has three phases:
1. Menstrual Phase (Days 1-5): Low levels of estrogen and progesterone cause the functional layer of the endometrium to be shed as blood and tissue (menstruation), lasting 3-5 days. In this phase, gonadotropins and FSH begin to rise.
2. Proliferative Phase & Ovulation (Days 6-14): Rising FSH causes several ovarian follicles to begin growing, but only one becomes the mature Graafian follicle, which rises to the ovary's surface. FSH stimulates this follicle to secrete estrogen, which causes initial vascularization and thickening of the endometrium. As estrogen rises, FSH falls by negative feedback; the falling FSH triggers the anterior pituitary to release a surge of LH around day 14. This LH surge causes the Graafian follicle to release its ovum into the oviduct - this event is ovulation.
3. Secretory/Post-Ovulatory Phase (Days 15-28): LH converts the ruptured Graafian follicle into the corpus luteum, which secretes estrogen and progesterone, making the endometrium more vascular and glandular (ready for implantation). If fertilization does not occur, the corpus luteum degenerates, LH secretion stops, and estrogen and progesterone fall abruptly, causing the thickened endometrium to be shed as menstruation on day 28, starting the next cycle.