Control of the Female Reproductive Cycle: Hormonal and Neural Regulation - Prof. Robert E. | Study notes Biology

BIOL 1120 REEDER

CONTROL OF THE REPRODUCTIVE PROCESS

I. Control of the Female Reproductive Cycle

A. Neural and Hormonal Control

l. Hypothalamus (neurosecretory tissue deep in the brain) secretes a releasing hormone (initially at

puberty, and then, cyclic throughout female's reproduction life) that stimulates the pituitary gland

(branches off the hypothalamus deep in the brain) to secrete FSH (follicle stimulating hormone)

initially, and LH (luteinizing hormone) later in stages of the menstrual cycle

a. FSH: induces one or more primary follicles of both ovaries to begin maturing (oogenesis: meiotic

development of potential ova) and stimulates follicular cells of the ovaries to secrete estrogens.

l) Oogenesis:

a) Diploid oogonia (egg mother cell) mitotically proliferate during prenatal development; at

about the third month, oogonia develop into larger diploid cells called primary oocytes

located within primary follicles; (500,000 total at birth; arrested in Prophase I at birth).

b) With beginning (puberty) and continuing of each menstrual cycle, Meiosis is stimulated

resulting in the eventual singling out of one of several secondary follicles from one ovary

to reach maturity due to hormonal influence; usually several follicles are stimulated from

both ovaries, but only one attains maturity (ovulates).

** c) Normally, only one functional cell (potentially fertilizable) out of four of one ovary will

be ovulated per menstrual cycle period of a female: primary follicles become secondary

follicles of which one becomes a Graafian follicle that ovulates.

d) Reproductive capability ends relatively early in female's life (average age in the 40's and

termed menopause due to the absence of hormonal stimulation).

e) The human egg is approximately 0.1mm in diameter and barely visible to the unaided

eye.

2) Estrogens will be significant in the first half of the menstrual cycle (and other features

mentioned later in pregnancy and birth) to influence the females uterine lining (endometrium)

to begin its increased thickening and vascularization.

b. LH: its increased release results in ovulation (secondary oocyte and polar body release from a

Graafian follicle) and formation of the corpus luteum (ruptured follicle of one ovary becomes a

large yellow body capable of secreting progesterone).

1) Progesterone (gestatory hormone) will be significant in the latter half of the menstrual cycle

(as well as during pregnancy) to continue estrogens preparation and maintenance of the

endometrial lining in anticipation of receiving for implantation a fertilized egg (pregnancy).

2. Placental Hormones: includes progesterone and estrogens.

a. Normal qualities of these hormones are secreted by the follicle cells and the corpus luteum and

are adequate to initiate implantation (zygote to endometrium) if fertilization occurs (sperm + egg

= single-celled zygote).

b. Once implantation has occurred, there is a need for these hormones in much higher

concentrations.

c. The placenta once formed begins secreting large quantities of estrogens, as much as 300 times the

quantity supplied by follicle cells; the estrogens continue to influence the endometrium and more

importantly aids in the enlargement of the breasts and the external genitalia.

d. Progesterone secretion is also carried on at an elevated rate by the placenta; it aids continuing

endometrium development during early stages of pregnancy, and also inhibits uterine contractions

that might otherwise expel the fetus (inhibits oxytocin from posterior pituitary) prematurely.

e. At birth, the sudden drop in estrogen and progesterone concentrations in the maternal

bloodstream removes the inhibiting affect on the hormone prolactin and permits its secretion from

the anterior pituitary (thereby stimulating the production of milk by the mammary gland:

(lactation).

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BIOL 1120 REEDER

CONTROL OF THE REPRODUCTIVE PROCESS

I. Control of the Female Reproductive Cycle A. Neural and Hormonal Control l. Hypothalamus (neurosecretory tissue deep in the brain) secretes a releasing hormone (initially at puberty, and then, cyclic throughout female's reproduction life) that stimulates the pituitary gland (branches off the hypothalamus deep in the brain) to secrete FSH (follicle stimulating hormone) initially, and LH (luteinizing hormone) later in stages of the menstrual cycle a. FSH: induces one or more primary follicles of both ovaries to begin maturing (oogenesis: meiotic development of potential ova) and stimulates follicular cells of the ovaries to secrete estrogens. l) Oogenesis: a) Diploid oogonia (egg mother cell) mitotically proliferate during prenatal development; at about the third month, oogonia develop into larger diploid cells called primary oocytes located within primary follicles; (500,000 total at birth; arrested in Prophase I at birth). b) With beginning (puberty) and continuing of each menstrual cycle, Meiosis is stimulated resulting in the eventual singling out of one of several secondary follicles from one ovary to reach maturity due to hormonal influence; usually several follicles are stimulated from both ovaries, but only one attains maturity (ovulates). ** c) Normally, only one functional cell (potentially fertilizable) out of four of one ovary will be ovulated per menstrual cycle period of a female: primary follicles become secondary follicles of which one becomes a Graafian follicle that ovulates. d) Reproductive capability ends relatively early in female's life (average age in the 40's and termed menopause due to the absence of hormonal stimulation). e) The human egg is approximately 0.1mm in diameter and barely visible to the unaided eye.

Estrogens will be significant in the first half of the menstrual cycle (and other features mentioned later in pregnancy and birth) to influence the females uterine lining (endometrium) to begin its increased thickening and vascularization. b. LH: its increased release results in ovulation (secondary oocyte and polar body release from a Graafian follicle) and formation of the corpus luteum (ruptured follicle of one ovary becomes a large yellow body capable of secreting progesterone).
Progesterone (gestatory hormone) will be significant in the latter half of the menstrual cycle (as well as during pregnancy) to continue estrogens preparation and maintenance of the endometrial lining in anticipation of receiving for implantation a fertilized egg (pregnancy).

Placental Hormones: includes progesterone and estrogens. a. Normal qualities of these hormones are secreted by the follicle cells and the corpus luteum and are adequate to initiate implantation (zygote to endometrium) if fertilization occurs (sperm + egg = single-celled zygote). b. Once implantation has occurred, there is a need for these hormones in much higher concentrations. c. The placenta once formed begins secreting large quantities of estrogens, as much as 300 times the quantity supplied by follicle cells; the estrogens continue to influence the endometrium and more importantly aids in the enlargement of the breasts and the external genitalia. d. Progesterone secretion is also carried on at an elevated rate by the placenta; it aids continuing endometrium development during early stages of pregnancy, and also inhibits uterine contractions that might otherwise expel the fetus (inhibits oxytocin from posterior pituitary) prematurely. e. At birth, the sudden drop in estrogen and progesterone concentrations in the maternal bloodstream removes the inhibiting affect on the hormone prolactin and permits its secretion from the anterior pituitary (thereby stimulating the production of milk by the mammary gland: (lactation).

B. The Menstrual Cycle l. The fluctuations of individual hormone concentrations in the bloodstream produces cyclic, structural and functional changes in the female reproductive organs (pituitary and ovarian hormones affect endometrium)--referred to as the menstrual cycle.

Normal series of this cycle begins at puberty at intervals of 25 to 35 days, except when pregnancy and lactation intervene; it continues for 35 to 40 years until there is cessation of reproductive capability (menopause); cycle is approximately 28 days duration.
Uterus is not in perpetual readiness to receive a fertilized egg, but must undergo structural changes so that it can effectively accept the fertilized egg and nourish the fetus.
First day of the cycle is usually timed with beginning of menstruation and continues until beginning of next menstrual period; menstruation refers to endometrium deterioration and the bleeding that occurs.
Essential purpose of the menstrual cycle is to bring about the release of ova and to prepare the uterus for its acceptance of a fertilized egg; if fertilized egg is not available after the preparation phase, reduced hormone levels cause the endometrium to degrade and then preparation stages all over again; if pregnancy occurs, the endometrium is influenced by hormones to maintain its state of preparedness, culminating in the formation of placental tissues that nourish the fetus.
A typical menstrual cycle can be described in 3 broad phases: Menstrual phase, proliferative or follicular phase, and the luteal or secretory phase. a. Menstrual Phase l) Characterized shedding of the endometrium resulting in the discharge of bloody fluid; discharge consists of epithelial cells, mucus, interstitial fluid, and about 25 to 65 ml. of blood, and the microscopic, unfertilized (now dead) ovum.
- 1. Although menstruation actually is the terminal event of each cycle, it is the time most easily fixed by any woman; thus the first day of the menstrual flow is said to be the first day of the menstrual cycle; flow may last one to eight days with average duration four to six days; the degeneration of the endometrium is a progressive one involving only one small area at a time.
  2. Ovarian activity in this phase involves beginning growth and development of several primary follicles (20-25 primary follicles are stimulated each time of both ovaries); under influence of FSH: usually only one follicle is selected for ultimate maturation --- rising blood concentration of estrogen & progesterone further inhibit maturation of other follicles. b. Proliferative Phase l) Associated with a rapidly growing ovarian follicle and the production of estrogen by cells surrounding the follicle.
- 1. Activities depend on a mixture of FSH and LH, where FSH predominates initially.
  2. Estrogen induces endometrium to become highly vascular and thickened.
  3. These activities occur from fourth to thirteenth day of a 28 day cycle.
  4. The rising estrogen causes a sharp shift in the FSH-LH mixture, inhibiting FSH secretion and increasing LH secretion.
  5. The end of the proliferative phase coincides with the release of egg (immature ova or secondary oocyte) from the Graafian follicle (Ovulation) at about the l4th day of the cycle ( hours after surge of LH, ovulation occurs). a) Normally, only one secondary follicle of one ovary is singled out by the estrogens for ultimate ovulation and eventual maturing (if fertilized). b) Rising blood concentrations of estrogen and progesterone will further inhibit the maturation of other follicles. c. Secretory Phase
  6. Following ovulation, LH stimulates development of the corpus luteum and the secretion of progesterone (progesterone continues and accelerates the work begun by estrogen-continued thickening of endometrium, becoming more vascular and developing specialized glandular cells that provide nutrient substances in anticipation for a fertilized egg); these changes are maximal about one week after ovulation.
  7. If fertilization does not occur in the fallopian tubes, the rising progesterone and estrogen tide serves to inhibit the secretion of LH (negative feedback).

e. The placenta receives nutrients and oxygen from maternal blood coursing through the uterus and transmits these substances by way of the umbilical cord to the embryo; also excretions of fetus; placenta is completely developed in 10 weeks.

The placenta serves as the embryonic lung, intestine and kidney.
The umbilical cord consists of: a) Umbilical vein: carries nutrients to fetus via the liver; oxygenated blood. b) Umbilical arteries: carries wastes out of fetus; deoxygenated blood. f. Specialized membranes termed extra-embryonic, are associated with the developing human embryo, although their functions may differ from that as associated with lower vertebrates (reptiles, birds, and other mammals).
The allantois is no longer a urinary bladder, but becomes the stalk, or umbilical cord.
The chorion, which is the outermost membrane, forms most of the placenta.
The yolk sac is incorporated into the umbilical cord, and serves as the lymphocyte source early in development: these cells (stem cells) ultimately migrate to the spleen, liver, bone marrow and thymus to originate the body's immune system.
The amnion remains unchanged: a protective water jacket in which the embryo floats. g. The further division of the cells results in germ layers in a process called gastrulation in the gastrula stage (2-3 weeks); these layers, the ectoderm, mesoderm, and endoderm, will later differentiate into specific tissues, organs and systems with specific tasks to perform; first system to develop is the nerve; heart beats in 3-5 weeks.
Ectoderm differentiation--into outer body tissues as skin, hair, nails, skin glands, and the entire nervous system; teeth enamel; *the nerve system is the first system to begin development; glandular epithelium (pineal gland, pituitary and adrenal medulla).
Mesoderm--muscle, bones, cardiovascular system, internal make-up of the internal organs; blood vessel and lymphatic linings, reproductive and excretory system; dermis of skin; middle ear.
Endoderm--lining of gastrointestinal tract and urinary bladder, urogenital linings; glandular epithelium (thyroid, parathyroid, pancreas, thymus glands); respiratory system (lungs, tracheal system); liver and pancreas. h. Mitosis and differentiation proceed at an accelerated rate during the first month of human fetal development: Tissue organs, and systems are formed to such an extent that by end of first month of pregnancy, the fetus has recognizable human features (morphogenesis). i. Embryo: first two months of development; rudiments of all principle adult organs will be formed, as well as the embryonic membranes; organogenesis occurs to transform the structurally simple three-layered embryo (gastrula) into an individual with distinct organs (brain, notochord, and spinal cord are among the first); the 8 week old unborn child looks quite human (1 inch long weighing 1/30 of an ounce). j. Fetus: from the third month through the ninth, developmental events occurring; approximately 266 days after fertilization, a baby is ready to be born (about 21 inches long weighing 6 to 10 pounds).

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BIOL 1120 REEDER

CONTROL OF THE REPRODUCTIVE PROCESS