Folliculogenesis/Menstrual Cycle Flashcards
At what stage does the oocyte arrest in the 3-5 mo fetus? What is this oocyte callled?
Prophase I (4C, 2N) Germinal vesicle
When does meiosis 1 resume?
LH surge
If egg unfertilized, in what stage does it arrest?
Metaphase II (2C, n)
Characteristics of the oocyte at ovulation?
1N/2C, secondary oocyte, first polar body extruded
What stage oocyte is capable of fertilization?
Metaphase II of meiosis II
What happens when sperm fuses with oocyte membrane?
Cortical reaction modifies ZP to block polyspermy by cortical granule release of enzymes that digest sperm receptor glycoproteins ZP2 and ZP3 so they can no longer bind sperm
What is a primordial follicle?
- Oocyte arrested in diplotene of mitotic prophase
- Surrounded by single layer of flattened pre-granulosa cells (prevent atresia)
How do TGF-beta family proteins (4) influence primordial follicles? (which ones promote development, which ones inhibit development?)
- Activins – promote primordial develop
- Inhibins – inhibit primordial develop
- AMH – inhibits primordial development
- BMPs – promote primordial development
How do neurotrophins (4) influence primordial follicles?
(NGF, BDNF, NT-3, NT-4/5) bind trk proto-oncogene family to promote development
How does FOXL-2 influence primordial follicles?
- Essential for granulosa cell differentiation
- Causes blepharophimosis/ptosis/epicanthus-inversus syndrome
- Affects eyelid and causes premature ovarian failure
How does kit-ligand influence primordial follicles?
- Expressed on GCs of growing follicles
- Mutations lead to failure of follicular growth beyond primary stage
General structure of primary oocyte
Oocyte surrounded by a cuboidal layer of granulosa cells
What are gap junctions in the primary oocyte?
Collections of intercellular membrane channels that allow adjacent cells to share small molecules (< 1 kDa). Gap junction channels are composed of connexins, a homologous family of more than 20 proteins. Located btwn granulosa cells and btwn granulosa cells and oocytes.
Name 2 of the most important connexins in primary oocyte and their location
Connexin-37 and connexin-43: form gap junctions with different permeability properties.
Connexin-37: predominant connexin in the oocyte
Connexin-43: predominates in granulosa cells
Layers of theca cells in secondary follicle
- Theca interna (closest to basal lamina)
- Theca externa (furthest from basal lamina)
Theca cells express genes for what hormone and which has the highest output?
Androgen production, androstenedione
What enzyme evidence in theca/granulosa cells supports the 2-cell, 2 gonadotropin explanation for estrogen production?
Presence of P450c17 (17alpha hydroxylase) only in theca and P450arom (aromatase) only in granulosa
Name 2 gonadotropin independent signals in primary follicle
GDF-9, BMP-15
What is GDF-9 and where is it expressed?
TGF-beta family protein – highly expressed in oocytes; lesser extent in GCs
• Signals via serine-threonine kinase receptors, produced by oocytes in primary and larger follicles but absent in primordial follicles
• Mutation in mice prevents follicle development beyond primary stage and absence of thecal markers, and eventual oocyte death
What does BMP-15 do and when does its action start?
- Promotes growth and maturation of follicles
- Starting in gonadotropin independent phase (primary follicle)
Characteristics of secondary/pre-antral follicle
Ongoing granulosa cell proliferation (2nd layer forms)
theca layer forms; vascular supply develops
Increase in oocyte size
Presence of FSH receptors (needs FSH to form antrum)
Hormone signaling to secondary/pre-antral follicle?
FSH: Required beyond two layers of granulosa cells
• FSH acts via FSH-R in adenylate cyclase system
• This induces aromatase and causes estrogenic microenvironment
• FSH and estrogen act synergistically to up-regulate FSH receptors
• The relative estrogen/androgen ratio determines the fate (grown vs. atresia) of the follicle
Characteristics of antral follicle
Antrum first appears as coalescence of Call-Exner bodies, cavity first filled with cellular debris, liquid accumulates from transudation of blood through avascular granulosa from theca vessels.
early - Granulosa cells surrounding the oocyte are now designated cumulus oophorus (also have corona radiate around ZP, membrana - interior to basal lamina, periantral - adjacent to the antrum)
Formation of antrum is FSH dependent
late - secrete estrogen & inhibin
Describe the two-cell, two-gonadotropin system as pertains to the antral follicle
- Granulosa cells lack c17 (similar to placenta, which lacks C17) so follicular estrogen production is dependent on aromatization by the granulosa of androgen substrate made by the theca
- LH receptors only on theca cells (in response to LH: cholesterol -> androgens)
- FSH receptors only on granulosa cells (in response to FSH: androgens -> estrogens, via aromatase)
- Only FSH is required for folliculogenesis; however, the final stages of maturation are optimized by LH, increasing the amount of androgen substrate for estrogen production and promoting growth of dominant follicle while simultaneously hastening the regression of smaller follicles
Two advantages of dominant follicle over other growing antral follicles
• Greater content of FSH receptors; because rate of granulosa cell proliferation surpasses that of its cohort
• Enhancement of FSH action
(these allow dominant follicle to remain responsive to FSH despite waning levels)
What induces LH receptor development on granulosa cells for response to LH surge?
FSH
When does ovulation occur?
34-36 hours after onset of LH surge and 24-26 hours after peak E2 levels
Molecular events following LH surge (3)
- Increase in cAMP
- Breakdown of gap junctions in cumulus
- Decrease in oocyte maturation inhibitor and luteinization inhibitor
Detail response to the LH surge
- Initiates continuation of meiosis in the oocyte, luteinization of the granulosa, and synthesis of progesterone and prostaglandin within the follicle
- Stimulates plasminogen activator -> plasmin -> activates collagenase to disrupt follicular wall
- Ovulation is the result of proteolytic digestion of the follicular apex (called the stigma)
Response of MMP enzymes and their endogenous inhibitors TIMPS to LH surge
Increase
Characteristics of the corpus luteum
o Granulosa cells hypertrophy, gradually filling in the cystic/hemorrhagic cavity of the early corpus luteum
o For the first time, granulosa becomes markedly luteinized - incorporating lipid-rich vacuoles in its cytoplasm
o A new yellow body is formed, now dominated by the enlarged, lipid-rich, fully vascularized granulosa
Estrogen feedback at pituitary and hypothalamus
Initial inhibitory role at both the hypothalamus and anterior pituitary, decreasing both GnRH pulsatile secretion and GnRH pituitary response followed by switch to positive feedback in the late follicular phase
Progesterone feedback
o When introduced after adequate estrogen priming, P4 facilitates the positive feedback response, in a direct action on the pituitary
o Both PR isoforms (A and B) are highest in the peri-ovulatory phase; PR-A is always higher than PR-B
Follicular inhibin structure differences and production location
Inhibin A and B: common alpha subunit, different beta
Inhibin A: mostly produced by CL
Inhibin B: produced by granulosa cells of small developing antral follicles
Inhibin A levels throughout life, menstrual cycle, and pregnancy? (puberty, luteal phase, postmenopause, pregnancy)
- Undetectable before puberty
- Declines in late luteal phase (resulting in FSH rise)
- Very low levels in postmenopausal state due to absent follicular secretions
- Pregnancy: Secreted by placenta. Peaks at 8-10 weeks, declines until 20 weeks, then increases to term
Inhibin B levels from puberty to menopause
- Rises to peak in early puberty; constant level thereafter.
- In early menopause, follicular phase Inhibin B declines while Inhibin A and E2 are still within normal range
Inhibin action on FSH
- Suppresses FSH by blocking synthesis/secretion, preventing GNRH R upregulation, reducing the number of GNRH receptors, promoting intracellular degradation of gonadotropins
- Inhibin A rises in luteal phase (under control of LH) to inhibit FSH in luteal phase
- Inhibin B rises in follicular phase – central inhibition (via GnRH)
Activin structure
Combination of beta subunits from inhibin A and B
Activin action on FSH
- Stimulates GnRH receptors
- Acts centrally to augment FSH secretion
Follistatin action on FSH
Binds activin and therefore inhibits FSH
FSH primary regulators
Activin (+), follistatin (- via binding activin), and inhibin (-)
LH regulation
GnRH pulses (primarily frequency)
LH regulation in PCOS
Increased GnRH pulse frequency –> increased LH
Gonadotropin regulation in hypothalamic amenorrhea
- Decreased GnRH pulse frequency (via CRH, opioids)
- increase FSH/LH ratio
Sialic acid residue action
Decrease pH, decrease excretion
Increased during follicular phase because low estrogen environment favors the production of so-called big gonadotropins, gonadotropins with an increased carbohydrate component and, as a result, decreased biologic activity. The greater bioactivity of FSH at midcycle is associated with less sialyated, shorter-lived isoforms
MOA of estrogen in abnormal uterine bleeding (3)
a. Rapid regrowth of endometrium over denuded epithelial surface
b. Stabilizes lysosomal membranes
c. Stimulates proliferation of endometrial ground substance
How progesterone influences GnRH pulse
GnRH pulse frequency determined by duration of exposure to progesterone
Long P4 exposure → decreased GnRH pulse frequency → favors FSH production
GnRH pulse amplitude determined by level of progesterone
High P4 level → increased GnRH pulse amplitude → favors FSH production
GnRH Pulse characteristics - for which hormone?
LH - inc freq (q60 min), low amplitude
FSH - dec freq (q200 min), high amplitude
LH surge time lengths
Occurs 34-36 hours before follicle rupture
LH surge lasts 48-50 hours
Usually occurs at 3AM
- Rapidly ascending limb of 14 hours
- Plateau of 14 hours
- Descending phase of 20 hours
Most important prostaglandins involved in ovulation? (2)
PGE2 is most crucial
PGF2α is also involved
Role of IGF-1 in relation to folliculogenesis
- Major mediator of growth hormone stimulated somatic growth
- IGF-I and IGF-II is produced in thecal cells, granulosa cell, and luteinized granulosa cells
- IGF-II is the most abundant IGF in follicles
- Amplifies the action of FSH/LH
- Coordinates the functions of theca and granulosa cells
- Increases the number of FSH/LH receptors
- Stimulates aromatase activity and progesterone synthesis
- Stimulates secretion of inhibin
- Stimulates granulosa cell proliferation
How does IGF-1 circulate?
98% bound to IGFBP-1 thru 6
Predominant binding protein is IGFBP-3
Length of time in preantral phase? antral phase?
preantral - 290 days
antral - 60 days
What stage follicle does ZP form?
Primary
What stage follicle are FSH receptors starting to appear?
Primary
