3.1.15 Oogenesis & Spermatogenesis

True or False: Spermatogenesis is a continuous and prolific process in the adult male

True or False: Spermatogenesis is a continuous and prolific process in the adult male

True

Where does spermatogenesis occur?

The seminiferous tubules of the testes

How does oogenesis differ from spermatogenesis?

Cytokinesis is unequal, with almost all the cytoplasm monopolized by a single daughter cell, the secondary oocyte (goes on to become the ovum). The smaller cells are called polar bodies. Spermatogenesis results in 4 mature sperm.
Oogenesis has long resting periods

True or false: the female reproductive cycle is one integrated cycle involving 2 organs, the uterus and the ovaries

Tru

Oogenesis

1. Primary germ cells in embryo differentiates into oogonia (ovary-specific stem cells(
2. Oogonium multiplies by mitosis and begins at meiosis but stops at Prophase I
3. The cells suspended in Prophase I are called primary oocytes; they remain here until puberty
4. At puberty, FSH periodically stimulates a follicle to grow and induces its primary oocyte (2n) to complete meiosis I and start meiosis II
5. Meiosis then arrests the SECONDARY oocytes (n) at metaphase II; they are released at ovulation when the follicle breaks open
6. Only one oocyte matures and is released each month; meiosis continues if a sperm penetrates the oocyte (only then is oogenesis completed to produce an ovum)
7. After meiosis is completed, the haploid nuclei of the sperm and the mature ovum fuse (fertilization)

What happens if the released oocyte is not fertilized?

The ruptured follicle left behind after ovulation develops into the corpus luteum. If no fertilization, the corpus luteum degenerates.

Spermatogenesis

1. Primordial germ cells differentiate into spermatogonia (stem cells that give rise to sperm)
2. As spermatogonia differentiate into spermatocytes and then into spermatids, meiosis reduces the chromosome number from diploid (2n = 46) to haploid (n = 23). Start meiosis I as Primary spermatocyte (2n) and start meiosis II as secondary spermatocyte (n), end meiosis II as spermatids, mature into sperm cells
3. The developing cells are pushed from a location near the outer wall of the seminiferous tubule toward the lumen and then to the epidymis, where they become motile

Ovarian Cycle: Follicular Phase

1. cycle begins with release of GnRH (from hypothal) -- stimulating anterior pituitary to secrete FSH and LH
2. FSH stimulates follicle growth, aided by LH
3. cells of growing follicles start to make estrogen. low levels of estrogen inhibit secretion of pituitary hormones, keeping levels of FSH and LH relatively low
4. levels of FSH and LH shoot up when secretion of estrogen by growing follicle begins to rise steeply (<b>WHEREAS A LOW LEVEL OF ESTROGEN INHIBITS THE SECRETION OF PITUITARY GONADOTROPINS, A HIGH CONCENTRATION HAS THE OPPOSITE EFFECT: IT STIMULATES THE SECRETION OF GONADOTROPINS BY ACTING ON THE HYPOTHALAMUS TO INCREASE OUTPUT OF GnRH</b>*)
5. high concentration of estrogen increases sensitivity of LH-releasing cells in the pituitary to GnRH. by now, the follicles can respond more strongly to LH bc more of their cells have receptors for this hormone. (increase in LH caused by increased estrogen = pos feedback)
6. LH induces final maturation of follicle

Ovarian Cycle: Ovulation

about a day after LH surge, ovulation occurs (the follicle and adjacent wall of the ovary rupture, releasing the secondary oocyte

Ovarian Cycle: Luteal Phase

1. LH stimulates formation of corpus luteum
2. under continued stimulation by LH, the corpus luteum secretes progesterone and estrogen
3. as levels of progesterone and estrogen rise, the combination of these hormones exerts negative feedback on the hypothalamus and pituitary, inhibting secretion of LH and FSH
4. Near end of luteal phase, corpus luteum disintegrates, causing concentrations of estrogen and progesterone to decline sharply
5. dropped level of ovarian hormones liberate hypothal and pit from inhibitory effects of hormones
6. Pit then secretes enough FSH to stimulate growth of new follicle in ovary for new cycle

Menstrual cycle (uterine cycle)

1. estrogen secreted in increasing amounts signals endometrium to thicken
2. rapid drop in level of ovarian hormones when corpus luteum degenerates causes spasms of arteries in the uterine lining that deprive it of blood. the upper 2/3 of endometrium disintegrates, resulting in menstruation