Ch.8 Female Reproductive Cyclicity - Follicular Phase

4 significant events that occur during follicular phase

1. gonadotropin release from the anterior pituitary

2. follicular preparation for ovulation

3. sexual receptivity (estrus)

4. ovulation

hypothalamic nuclei making up the tonic center

ventromedial nucleus and arcuate nucleus

hypothalamic nuclei making up the surge center

preoptic nucleus, anterior hypothalamic area, and suprachiasmatic nucleus 

surge center role 

secretes basal levels of GnRH until a threshold concentration of estradiol is reached in the absence of progesterone, where it then secretes a large amount of GnRH, to signal to the anterior pituitary to release a large amount of LH

tonic center role

secretes a basal amount of GnRH in a pulsatile fashion

hormone that infleunces GnRH secretion from the tonic center

progesterone

hormones that control GnRH secretion from the surge center

high levels of estradiol and low levels of progesterone during mid to late follicular phase

dynamics of antral follicles

• recruitment (emergence)

• selection

• dominance

• atresia

atresia of antral follciles

degeneration of the antral follicle when a follicle is not exposed to the right endocrine conditions

recruitment (emergence) of antral follicles 

the phase of follicular development in which a cohort (group) of small antral follicles begins to grow and secrete estradiol.

most of the recruited follicles under atresia

selection of antral follicles

recruited follicles that don’t undergo atresia

dominance of antral follicles

dominant follicles continue to secrete increasing amounts of estradiol and inhibin.

characterized by 1 or more large preovulatory follicles exerting a major inhibitory effect on other antral follicles from the recruited and selected cohort

inhibin

a protein hormone that is secreted by the antral follicle that selectively inhibits the release of FSH from the anterior pituitary.

prevents the surge in FSH to the same extent as LH

monotocous 

giving birth to a single offspring 

polytocous

litter bearers

hormones concentrations present during recrutiment

high FSH, low LH pulse frequency, low inhibin, low estradiol

hormones present during selection

low FSH, moderate LH, low inhibin

hormone concentrations during dominance

low FSH, high LH pulse frequency, high inhibin 

control of FSH

estradiol and inhibin secreted by the growing follicles have a negative effect on FSH

control of rising LH concentrations

increasing GnRH pulse frequencies

why do most follicles undergo atresia during the cycle?

because they lack sufficient numbers of LH receptors on the granulosal cells to fully repons to LH in the final stages of growth and maturation

2-cell, 2-gonadotropin model

LH binds to LH receptors of the theca interna cell of the developing follicle, leading to the conversion of cholesterole to testosterone.

Testosterone then diffuses into the granulosal cell, where FSH binds to the FSH receptor and stimulates the conversion of testosterone into estradiol.

this pathway continues to function until levels of estrogen reach the threshold to stimulate the preovulatory LH surge

major effects of estradiol on the reproductive tract

• increased blood flow

• genital swelling

• change in tissue electrical conductivity

• leukocytosis

• increased mucosal secretion

• initiation of uterine gland growth

• elevated myometrial tone

hyperemia (local elevated blood flow) to the reproductive tract control/stimulus

histamine, prostaglandin E2, and angiogenic factors of the dominant antral follicle

angiogenic factors

substances that promote the growth of new blood vessels

factors leading to ovulation

• elevated blood flow 

• breakdown of connective tissue

• ovarian contractions

progesterone secretion after LH surge but before ovulation

progesterone begins to be secreted by theca interna cells instead of testosterone at low levels, stimulating the synthesis of collagenase by theca interna cells 

role of collagenase in ovulation

collagenase breaksdown the connective tissue of the tunica albuginea (outer covering of the ovary)

stigma

apex of the follicle

hormones secreted by the ovary after the LH surge

prostaglanding E2 and prostaglandin F2-alpha

role of prostaglandin F2-alpha in ovulation

stimulates lysosomes within the granulosal cells to rupture, which releases their enzymes, causing further connective tissue degradation.

also causes contractions of the smooth muscle components of the ovary

role of prostaglandin E2 in ovulation 

stimulates the follicle in remodeling itself into a corpus luteum after ovulation by stimulating the synthesis of plasminogen, which gets converted into plasmin, which is an enzyme that participates in tissue remodeling

reflex (induced) ovulators

females that require stimulation of the vagina and/or cervix for ovulation to occur

(ex. rabbit, relids, ferret, mink)

spotaneous ovulators

females that ovulate with a regular frequency and do not require copulation to occur.

(ex. cow, sow, mare, ewe, woman)

pathway for induced ovulation

• copulation stimulates sensory nerves in the vagina and cervix

• impulses are then relayed to the spinal cord

• impulses are then relayed to the surge center in the hypothalamus

• large quantities of GnRH is secreted

• LH surge is stimulated

how to accomplish premature luteolysis to hormonally induce ovulation

administer exogenous prostaglandin F2-alpha, which allows endogenous progesterone to decline, and GnRH to be released, stimulating FSH and LH

superovulation (definition and requirements)

due to an abnormally high number of follicles that are selected, followed by ovulation.

requires administration of exogenous gonadotropins (usually include injections of equine chorionic gonadotropics (eCG) or FSH followed by LH, GnRH or human chorionic gonadotropin (hCG))

4 phases of oocyte maturation

1. mitoic division of primordial germ cells (prenatal)

2. nuclear arrest (dictyotene)

3. cytoplasmic growth

4. resumption of meiosis

nuclear arrest phase (definition, duration, purpose)

the nucleus of the oocyte becomes dormant and will remain so until stimulated by gonadotropins after puberty

occurs from late fetal life, through birth and to puberty

purpose: to inactivate the DNA so that it may not be vulnerable to insult during the lifetime of the female, which could compromise reproduction

zona pellucida

a translucent band around the cytoplasm that begins to form when primary oocytes develop into secondary oocytes and begin to accumulate larger volumes of cytoplasm

junctional complexes

allows for cell contacts between neighboring follicular cells (granulosal) and the oocyte, permiting communication of ionic and electronic coupling

resumption of meiosis

LH surges, the gap junctions between oocyte and granulosal cell deteriorate, which stimualtes meiosis 

prenatal oogenesis

primordial germ cell —> 2 oogonia —> 4 primary oocytes —(meiotic prophase)—> 4 primary oocytes —| Nuclear arrest

post pubertal oogenesis

1st meiotic division after LH surge —> secondary oocyte

2nd meiotic division (around fertilization) —> ootid

first polar body

formed when the bivalent chromosomes line up and the chromatids are separated 

occurs slightly before ovulation 

second polar body

produced after the second meiotic division, after (or slightly before) fertilization