Luteal Phase (lec 11) Flashcards
What phase occurs after ovulation?
immediately after ovulation Luteal Phase begins.
What 3 major processes occur within the luteal phase?
- Luteinisation = the transformation of follicle cells into luteal cells after ovulation - the corpus luteum develops (Metestrus phase)
- Increase secreation of progesterone (P4) - growth and development of the corpus luteum which produces high levels of progesterone (diestrus phase)
- Luteolysis - Destruction of the corpus luteum resulting in follicular phase (diestrus phase) becoming corpus albicans
How does luteolysis occur?
luteolysis of corpora lutea occurs due to PGF 2a (prostaglandin 2 alpha) being released by the uterine endometrium in most mammals and ovary in humans.
Followed by reduction in progesterone.
Progesterone usually causes a negative feedback on the hypothalamus, thus when removed female enter follicular phase because pulse frequency and amplitude of GnRH increases, causing FSH and LH to increase
Controlled by oxytocin of CL and PGF2a
What is the dominant hormone during luteal phase?
progesterone
What cells are involved in luteinization?
in the preovulatory follicle (antral follicle)
- Granulosa cells line the antrum (large) become LLC
- theca interna (small) become SLC
Governed by LH
What is the process of luteinization?
Luteinization is the formation of the corpus luteum
- the preovulatory follicle consists of two cells types, granulosa (larger) an theca interna cells (smaller). They are divided by a basement membrane that degenerates due collagenase – this allows the cells to intermingle
- Corpus hemorrhagicum occurs during ovulation. This causes blood vessels to rupture and hemorrhage. This cells begin to mix and the basement membrane becomes the connective tissue substructure of the corpus luteum.
- Functional Corpus luteum -
The cells then become LLC - mixture of large luteal cells ( Granulosa) and SLC - small luteal cells. (tunica interna) Produce progesterone.
What hormones do LLC and SLC contain specifically?
LLC (granulosa) = posses secretory granules that contain oxytocin during oestrus cycle and relaxin during pregnancy
SLC (theca interna) = contain no secretory granules.
BOTH PRODUCE PROGESTERONE
What mode of action does progesterone have?
progesterone is an inhibitor because it; (negative feedback)
- reduces basal GnRH amplitude & frequency
progesterone produce by the CL exerts a negative feedback on the hypothalamus (tonic and surge centre). Suppressing GnRH thus FSH and LH and little estrogen is produced. - prevents behavioural oestrus
- stops the preovulatory LH surge
- reduces myometrial tone
progesterone exerts stone Positive influence on endometrium of uterus . P4 inhibits myometrium and reduces its contractility and tone.
Why does follicular development continue during the luteal phase of the cow’s
oestrous cycle, yet there is a
corresponding failure to
display oestrus and a lack of
ovulation?
ovulation has just occurs, moved into mid-estrus - diestrus during this period a CL on ovary producing P4.
Even though CL is releasing p4 negative feedback on hypothalamus reduces GnRH - but still allows tonic secretion to occur of gnRH, therefore waves with smaller amounts of FSH and LH are being produce - follicles will be recruited and selected - never will be sufficient large enough to induce ovulation
How is progesterone synthesised in a luteal cell?
- esterified cholesterol is delivered to the luteal cell by way of low density lipoprotein (LDL).
The LDL cholesterol complex binds to specific receptors in the outside plasma membrane.
The LDL cholesterol receptor complex is internalised in and the cholesterol is released from the receptor as cholesterol esters.
After LDL-cholesterol is removed the receptor is recycled and become available to transport another LDL cholesterol complex.
Esterified cholesterol is delivered to the luteal cells in the form of LDL and HDL
The LDL cholesterol complex binds to a receptor.
- LH binds to specific LH receptors (LHR) on the plasma membrane
- the LH receptor complex activates a G-protein that activate membrane bound adenylate cyclase (AC)
- adenylate cyclase promotes the conversion of ATP to cAMP the second messenger
- cAMP activates protein kinase enzymes.
a) accelerates LDL-cholesterol receptor internalisation
b) activates cholesterol-esterase that cleaves cholesterol from its ester and promotes
c) entry of cholesterol into mitochondria
- Mitochondrial enzymes are responsible for converting cholesterol to pregnenoline (PREG)
- PREG leave the mitochondria and is converted enzymatically to progesterone -
progesterone leaves the cell and enters the blood where it travels to target tissues.
What happens when luteolysis fails?
will suspend the female in a state of continual luteal phase as it inhibits the release of GnRH - retained Corpus Luteum
What is required to have a successful luteolysis?
in order to enter the follicular phase - the CL releases P4 and oxytocin.
Uterine endometrium releases PGE2a
Communication occurs between the CL and uterine endometrium occur for successfully have luteolysis
Counter exchange occurs between the uterine vein and ovarian artery - allowing transfer of PGf2a to the ovary at high enough concentrations to cause luteolysis.
Luteolysis is induced by large luteal cells that contain oxytocin, oxytocin is release when PGF2a is release.
How is progesterone decreased during luteolysis?
PGF2a is secretion by the endometrium of the uterus in the second half of the luteal phase.
P4 prevents secretion of PGF2a bu blocking formation of oxytocin receptors in the uterus - p4 then loses ability to bind to oxytocin receptors.
late stage oxytocin causes secretion of PGF2a. PGF2a pulses, increasing in frequency and amplitude - they both stimulate each other in a positive feedback manner. will eventually induce luteolysis.
- PGF2a binds to specific receptors on the plasma membrane of the luteal cells
- THe PGF2a receptor complex is believe to be Ca+ channels - Ca+ influx can occur to cause apoptotic effects.
- PGF2a receptors also activates protein Kinase-C which inhibits progesterone synthesis
How is regression of CL controlled in humans and primates?
PGF2a will induce luteolysis in primates and ovarian origin is responsible for causing luteal regression - NOT UTERUS
What is the initiator of menstruation?
Luteolysis - the endometrium proliferates after menstruation - until ovulation, The CL is formed and P4 causes continued proliferation of endometrium during secretory phase
PGF2a causes P4 and estrogen to drop causing sustained vasoconstriction in spiral arteries - causes ischemia and endometrium undergoes necrosis and sloughs into uterine lumen
