The Role of Reproduction (Part 5)

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The Role of Reproduction (Part 5)

Control of the Ovary

The control systems in male and female are fundamentally similar,utilizing the same hypothalamic and pituitary hormones. Things

work differently for two reasons:

o The ovary produces few, large gameteso The principal ovarian hormones are not androgens, but

estradiol and progesterone

The Ovarian Cycle

The ovary produces relatively few, large gametes (ova, or eggs).Reproductive success depends on everything begin prepared so that

these few ova have the best chance of survival and development to

an embryo.

For this reason, females cycle each cycle including a sequence ofevents designed to stimulate coordinated development of both the

eggs and the reproductive tract.

The First Step: Oogenesis

In the female fetus, the primordial germ cells migrate from the yolksac to the developing ovaries

A single layer of follicular cells surrounds each primordial germ cell The germ cell is called the oogonium Starts meiosis

Meiosis and the Primary Follicle


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Two cell divisions where the chromosome number is halved The oogonium stops meiosis at the first prophase primary

oocyte

Thousands of these primary follicles are present at birth Each primary oocyte results in one mature ovum and three

rudimentary cells (polar bodies)


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Sexual Maturity

In mammals, it is essential that the development of the eggs andthe development of the reproductive tract be precisely synchronized

to make sure that if fertilization occurs, pregnancy can occur

successfully.

This means that the brain, pituitary, and reproductive tract mustrespond in an appropriately timed fashion to the hormonal signals

from the ovary

Reproduction is initiated in females in exactly the same way asmales hypothalamic GnRH release is turned on.

The resultant rise in FSH stimulates a group of follicles to startdeveloping.

GnRH-LH/FSH-gonadal axis


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Inhibitory Feedback

As the follicles start to develop, they produce estradiol and inhibino Both act back to inhibit FSH secretion

This produces a limiting situation in the ovaryo Only the healthiest follicles survive the resultant FSH

starvation

o In large mammals, this is used to limit ovarian developmentto the single healthiest follicle for each cycle


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Ovulation

A sustained rise in estradiol causes the brain to override theinhibition of the pituitary, causing the entire pituitary store of LH to

be suddenly released This surge of LH causes ovulation

Ovulation at the Cellular Level

The primary oocyte undergoes the first meiotic division to producethe secondary oocyte (and first polar body)

o In the mare this happens just after ovulationAfter Ovulation

Formation of a corpus luteum from the ovulated follicle

o Now starts to secrete progesterone as well as estradiol The cells in the corpus luteum have a short life if pregnancy does

not occur (~7 days in a woman)

After ovulation, progesterone from the CL induces uterinedifferentiation, preparing for pregnancy

In the absence of more LH, the luteinized cells in the corpus luteumdie and a new cycle begins

The only way that the corpus luteum can be rescued is if pregnancyhappens and the new embryo secretes hormones that will stop

the cells of the corpus luteum from dying


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Uterus

In all mammals, the loss of both estradiol and progesterone at theend of the cycle results in regression of the uterus. In primates, this

is associated with shedding of the lining of the uterus(menstruation)

If pregnancy happens, menstruation is prevented because thecorpus luteum is rescued from death by the embryo so it

continues to secrete estradiol and progesterone, maintaining the

uterus

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The Role of Reproduction (Part 5)