Animal Reproduction

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7

Negative feedback

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Slide 46

Testosterone regulates the production of GnRH, FSH, and LH through negative feedback mechanisms.

Sertoli cells secrete the hormone inhibin, which reduces FSH secretion from the anterior pituitary.

Slide 47

The Reproductive Cycles of Females

In females, the secretion of hormones and the reproductive events they regulate are cyclic.

Prior to ovulation, the endometrium = uterine lining, thickens with blood vessels in preparation for embryo implantation.

If an embryo does not implant in the endometrium, the endometrium is shed in a process called menstruation.

Slide 48

Hormones closely link the two cycles of female reproduction:

Changes in the uterus / uterine lining with blood vessels define the menstrual cycle (also called the uterine cycle).

Changes in the ovaries / follicle / egg chamber define the ovarian cycle.

Slide 49

The reproductive cycle of the human female

(a)

Control by hypothalamus

Hypothalamus

GnRH

Anterior pituitary

1

Inhibited by combination of estradiol and progesterone

Stimulated by high levels of estradiol

Inhibited by low levels of estradiol

2

FSH

LH

Pituitary gonadotropins in blood

(b)

6

FSH

LH

FSH and LH stimulate follicle to grow

LH surge triggers ovulation

3

Ovarian cycle

8

(c)

7

Growing follicle

Maturing follicle

Corpus luteum

Degenerating corpus luteum

Follicular phase

Ovulation

Luteal phase

Estradiol secreted by growing follicle in increasing amounts

Progesterone and estradiol secreted by corpus luteum

4

Ovarian hormones in blood

Peak causes LH surge

(d)

5

Estradiol

Progesterone

9

10

Estradiol level very low

Progesterone and estra- diol promote thickening of endometrium

Uterine (menstrual) cycle

Endometrium

(e)

Menstrual flow phase Proliferative phase

Secretory phase

Days

0

5

10

14

20

25

28

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15

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Slide 50

Control by hypothalamus

Inhibited by combination of estradiol and progesterone

Stimulated by high levels of estradiol

Inhibited by low levels of estradiol

Hypothalamus

GnRH

Anterior pituitary

FSH

LH

Pituitary gonadotropins in blood

LH

FSH

FSH and LH stimulate follicle to grow

LH surge triggers ovulation

Ovarian cycle

Growing follicle