ch.9 Female reproductive cyclicity - Luteal Phase

3 major processes in the luteal phase

1. luteinization

2. synthesis and secretion of large quantities of progesterone

3. luteolysis

length of luteal phase

from the time of ovulation until luteolysis, near the end of the estrous cycle

2 stages in luteal phase

metestrus and diestrus

dominant ovarian hormone during luteal phase

progesterone

corpus hemorrhagicum life span

from ovulation until day 1-3 of estrous cycle

corpus albicans 

a regressed corpus luteum that can be observed several estrous cycles after luteolysis

connective tissue remains and the glandular tissue degrades

cells that undergo luteinization after ovulation

theca interna and granulosal cells

hormone that governs luteinization

LH

large luteal cells

derived from granulosal cells and secrete oxytocin (cycle) and relaxin (during pregnancy), produce progesterone

small luteal cells

derived from thecal cells, posses numerous lipid droplets in cytoplasm, do not contain secretory granules like the large luteal cells, produce progesterone

process that the large luteal cells undergo as the CL develops

hypertrophy (increase in size)

process that small luteal cells undergo as the CL develops

hyperplasia (increase in cell numbers)

2 things the “vigor” of the CL depends on

• the number of luteal cells 

• the degree to which the CL becomes vascularized

primary target organs for progesterone

hypothalamus, uterus, and mammary glands

what progesterone stimulates in the uterus

maximal secretion by endometrial glands and inhibits motility of the myometrium

requirements for progesterone secretion by luteal cells

basal (tonic) levels of LH and cholesterol

effect of progesterone on the hypothalamus 

negative feedback —> reduces pulse frequency of GnRH by the tonic center 

processes progesterone inhibits

• reduces GnRH pulse frequency

• prevents behavioral estrus

• stops the preovulatory LH surge

• reduces myometrial tone

duration of luteolysis

1-3 days at the end of luteal phase

hormone that induces luteolysis

prostaglandin F2-alpha secreted by the uterine endometrium

the uterus’s role in luteolysis

• responsible for luteolysis (produces the hormone that induces it) 

• uterus must be near the ovary (or else the lifespan of the CL is prolonged)

how PGF2-alpha gets from the uterus to the ovary

through a vascular countercurrent exchange mechanism involving two closely associate blood vessels in which blood the blood draining the uterus flows in adjacent to the artery supplying the ovary

importance of the vascular countercurrent exchange mechanism in luteolysis

ensures the CL receives PGF2-alpha from the endometrium directly and without dilution by the systemic circulation (PGF2-alpha has a short half life)

when does PGF2-alpha exert its most potent effect

after day six of the cycle

requirements for luteolysis

• presence of oxytocin receptors on endometrial cells

• presence of a critical level of oxytocin

• PGF2-alpha synthesis by the endometrium

fashion in which PGF2-alpha is released

PGF2-alpha is released in a pulsatile fashion

stimulus that initiates PGF2-alpha secretion

in the first half of the estrous cycle, progesterone blocks the formation of oxytocin receptors in the uterus, inhibiting PGF2-alpha synthesis 

after 10-12 days, progesterone loses its ability to block the receptors, and oxytocin and PGF2-alpha work in a positive feedback loop wh

what species is the vascular countercurrent exchange mechanism present in 

ewe, cow, sow