Additional Notes on Female Reproduction Endocrinology

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Notes on Menstrual Cycle

• Hormones secreted by the hypothalamus, anterior pituitary, and ovaries control the main events.

• The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of an oocyte.

• Steroid hormones control the uterine cycle, a cycle that prepares the endometrium of uterus for arrival of the fertilized ovum.

• If the ovum doesn't arrive, the endometrium sloughs off.

• The female reproductive cycle is the ovarian and uterine cycle.

Regulation of the Menstrual Cycle

• The hypothalamus secretes GnRH, which stimulates the release of FSH and LH from the anterior pituitary.

• FSH and LH stimulate the growth of follicles and secretion of estrogen by growing follicles.

• LH also promotes the formation of corpus luteum.

• At lease six different estrogens exist.

• Estrogen, secreted by follicular cells, has many important and complex functions.

• Progesterone, secreted by cells of the corpus luteum, acts synergistically with estrogens to prepare and then maintain the endometrium for implantation of a fertilized ovum.

• Progesterone in high levels inhibits the secretion of GnRH and LH.

• Relaxin produced by the corpus luteum relaxes the uterus by inhibiting the contractions of the myometrium.

• Inhibin is secreted by granulose cells of growing follicles and by corpus luteum after ovulation; it inhibits the secretion of FSH and, to a lesser extent, LH.

Phases of Female Reproductive Menstrual Cycle

Days 1-5: Menstrual Phase:

Menstruation: Growth of primary/secondary follicles (Follicular phase):

• In the ovaries, secondary follicles begin to enlarge; granulosa cells secrete follicular fluid.

• In the uterus, menstrual flow occurs, declining levels of ovarian hormones (especially progesterone) stimulates release of prostaglandins, causing uttering spiral arteries to constrict > cells they supply die > entire endometrium sloughs off.

Days 6-13: Preovulatory Phase:

Proliferative phase: Maturation of one dominant follicle (Follicular phase):

• In the ovaries, FSH causes follicles to grow and release estrogens and inhibin. Soon, a single follicle becomes the dominant follicle, which secretes estrogens and inhibin (with the help of LH) that cause decrease in FSH secretion. Decrease in FSH kills all other growing follicles.

• In the uterus, all the extra estrogen causes repair of the endometrium. The thickness of the endometrium increases and all the arteries redevelop. Endometrium is proliferating.

Days 14-14: Ovulation:

• Mature follicle is ruptured and secondary oocyte is released into the pelvic cavity.

• Development of the secondary follicle takes a total of 20 days.

• Secondary oocyte halted in meiosis II metaphase.

• High levels of estrogens stimulate release of GnRH and LH.

• LH causes the rupture and expulsion. LH levels can be detected by home pregnancy to predict the day of ovulation in advance.

Days 15-24: Postovulatory Phase: Secretory Phase

• In the ovaries, mature follicle collapses and becomes corpus hemorrhagicum, and then to corpus luteum under the influence of LH. Corpus luteum secretes progesterone, estrogen, relaxin, and inhibin. Luteal cells absorb the blood clot.

• If oocyte is not fertilized, in the ovaries, corpus luteum survives 2 more weeks and then becomes corpus albicans. Levels of progesterone, estrogen, and inhibin decrease, while levels of GnRH, FSH, and LH rise. A new cycle begins.

• If oocyte is fertilized, corpus luteum lives the usual 2 weeks, and is then rescued by hCG, produced by the embryo. Presence of hCG in urine or blood indicates pregnancy.

• In the uterus, progesterone and estrogen from corpus luteum promote growth and thickening of the endometrium. If oocyte is not fertilized, progesterone levels drop causing menstruation.

More Female Reproductive Cycle Facts:

• The entire menstrual and ovarian cycle are carried out by ovarian hormones.

• The ovarian follicle nurtures the resident oocyte, and releases it at the right time.

Puberty:

• Puberty is the period when secondary sexual characteristics begin to develop.

• There are pulses or burses of LH and FSH triggered by GnRH.

• Before puberty, the release of GnRH is more sensitive to negative feedback action of gonadal steroids.

• The cause of GnRH triggering is unknown, but the hormone leptin may play a role.

• Leptin receptors are present in the hypothalamus and anterior pituitary.

• The rate of decline of melatonin may also be a normal regulator of the onset of puberty.

Also See: FAQ on Male Reproduction

• Organs: breasts, Fallopian tubes, ovaries, uterus, vagina, and external organs.

Ovaries:

• Ovaries produce gametes and hormones: progesterone, estrogens, inhibin, and relaxin.

• The primary follicles are stuck in meiosis I.

• The corpus luteum produces: progesterone, estrogens, relaxin, and inhibin.

• Spermatogenesis begins in males at puberty; oogenesis begins in females before they are even born.

• Each 28 days after puberty, gonadotropins secreted by the anterior pituitary stimulate the resumption of oogenesis.

• At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the first polar body and corona radiata; these cells are swept into the uterine tube.

• If fertilization doesn't occur, the cells degenerate.

• If sperm are present, one penetrates the secondary oocyte and meiosis II resumes.

• Peristalic contractions of the muscularis and ciliary action of the mucosal cells help move the oocyte or fertilized ovum toward the uterus.

• The outer layer of the uterine tubes is a serous membrane, the serosa.

• Fertilization can occur anytime upto 24 hours after ovulation.

Uterus:

• Blood supply comes from uterine arteries, and drained from uterine veins.

• Cervical mucous is produced by secretory cells of the mucosa.

• Females secrete 20-60 mL of mucous per day.

• Cervical mucous is hospitable to sperm at or near the time of ovulation, and supplies the energy to sperm, and also play a role in capacitation.

Breasts:

• The two mammary glands are modified sudoriferous (sweat) glands that produce milk.

• Cooper's ligaments become looser with agem resulting in drooping breasts.

Events during Fertilization:

• Pregnancy is most likely to occur during a 3-day window from 2 days before ovulation to 1 day after ovulation.

• Prostaglandins in sperm stimulate uterine motility.

• Sperm immediately reaching the oocyte are not capable of fertilizing until after seven hours. They undergo capacitation during this time.

• Once sperm enters, the oocyte becomes depolarized, and a depolarized oocyte cannot allow entry of another sperm.

Secretions of the Trophoblast:

• hCG: rescues the corpus luteum from degeneration and sustains its secretions of progesterone and estrogen.

• These two hormones prevent further menstruation.

Additional Readings:

Basic Endocrinology

1. Introduction to Endocrinology
2. Hypothalamic-pituitary system
3. Adrenal Hormones
4. Antidiuretic Hormone (ADH) and ECF Regulation
5. Endocrine Pancreas
6. Growth Hormone
7. Adrenal Medulla
8. Hormonal Control of Calcium and Phosphate
9. Thyroid Hormones
10. Hormones of Male Reproduction
11. Hormones of Female Reproduction
12. Fluid Compartments of the Body
13. Notes on Hypothalamus Anterior Pituitary and Thyroid
14. Additional Notes on Female Reproduction
15. Hormonal Signaling Pathways
16. FAQ on Adrenal Hormones
17. FAQ on Male Reproduction
18. Synthesis and Deficiencies of Adrenal Hormones
19. Significance of Glycosylated Hemoblogin (HbA1c)
20. Significance of Measuring Albumin while with Calcium Levels
21. Stepwise Approach to Treatment of Ascites
22. How to differentiate between Diabetes Insipidus vs Psychogenic Polydipsia

Related Topics

1. Histology of the Endocrine System
2. Histology of the Male Reproductive System
3. Histology of the Female Reproductive System

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