Oogenesis

Sherwood’s Human Physiology 20-12 5th Ed. & 20-13 6th Ed.

Ovulation

• For a 28 day ovarian cycle, ovulation occurs 14 days after the onset of menstruation.

• Right before ovulation the outer wall of the follicle swells and the follicle ruptures resulting in the oozing out of fluid and the ovum surrounded by thousands of granulosa cells together called the _corona radiata .

Sherwood’s Human Physiology 20-13 5th Ed. & 20-14 6th Ed.

Ovulation• LH is absolutely necessary for final follicular growth

and ovulation. • 2 days before ovulation the anterior pituitary

increases secretion of LH (up to 10–fold) peaking 16 hrs prior to ovulation.

• FSH also increases (up to 2-fold).• LH and FSH act synergistically to cause the swelling

of the follicle before ovulation.• LH converts granulosa & theca cells from secreting

estrogen to secreting progesterone.• As a result the levels of estrogen secretion start to

decrease about 1 day before ovulation.• At this time the levels of progesterone begin to

increase.

Guyton’s Textbook of Medical Physiology 81-3

Guyton’s Textbook of Medical Physiology 81-3

The Environment of Ovulation

• Rapid growth of follicle.

• Excessive estrogen levels begin to decrease.

• Increasing levels of _progesterone_.

• Without surge of LH secretion ovulation would not take place.

Mechanism of Ovulation• LH induces the granulosa & theca cells to secrete

progesterone.• The theca externa releases proteolytic enzymes which

weakens and begins to dissolve the outer capsule.• Simultaneously, there is rapid vascularization in the follicle

wall and secretion of prostaglandins which cause plasma to leak out into the follicle leading to follicle swelling.

• The combination of weakening of the follicle wall and swelling of the follicle result in follicle rupture and ovum discharge.

Guyton’s Textbook of Medical Physiology 81-5

Mechanism of OvulationLH

Progesterone

Follicular hyperemiaProstaglandin secretion

Plasma leaks into the follicle

Follicle swells

Follicle rupture

Proteolytic enzymes

Weekend follicle wall

Degeneration

Evagination of ovum

Luteal Phase

• During the first few hours following the expulsion of the ovum from the follicle the granulosa cells and the theca cells differentiate into lutein cells.– This differentiation is due to LH.– The cells enlarge and fill with lipid inclusions

becoming yellowish in color.– The granulosa derived lutein cells produce

progesterone and estrogen.– The theca derived lutein cells produce

androstenedione and testosterone Which get converted into progesterone and estrogen by the granulosa derived cells.

Sherwood’s Human Physiology 20-13 5th Ed. & 20-14 6th Ed.

Luteal Phase• The high levels of progesterone

and estrogen have a negative feedback effect on the secretion of FSH and LH from the anterior pituitary gland.

• In addition, the lutein cells secrete small amounts of inhibin (remember from male physiology?) which also inhibits LH and FSH secretion.

Guyton’s Textbook of Medical Physiology 81-3

Corpus Luteum• The lutein cells are now called the corpus

luteum.

• The corpus luteum continues to grow in size until about 7-8 days after ovulation, then it begins to involute.

• During its involution it looses its secretory ability and yellowish color around day 12 post ovulation and is called a corpus albicans.

• Over time the corpus albicans is replaced by connective tissue and absorbed.

Sherwood’s Human Physiology 20-13 5th Ed. & 20-14 6th Ed.

Corpus Luteum

• The abrupt cessation of progesterone, estrogen, and inhibin secretion by the former corpus luteum removes the inhibition of anterior pituitary secretion of FSH and LH.– This results in the growth

of new follicles and the beginning of a new cycle.

Guyton’s Textbook of Medical Physiology 81-3