Menstrual cycle 2

Question 1

Describe preovulatory egg prior to LH surge

Answer 1

Loss of Ovarian surface epithelium & breakdown of underlying basal lamina and Granulosa cell & theca cell at apex to allow for rupture.

GC basal lamina is disrupted allowing extension of blood vessels into GC layer and for infiltration of theca cells & leukocytes into GC compartment. COC detaches from surrounding GC to expand.

Question 2

What factors are responsible for meiotic arrest

Answer 2

The oocyte is held in meiotic arrest

High cAMP → keep maturation promoting factor (MPF) inactive
cGMP enters oocytes from cumulus cells via gap junctions to inhibit oocyte cAMP phosphodiesterase PDE3A activity (PDE3A normally degrades cAMP)
H2O2/NO/calcium
other cells/ ovarian environment & integrity of the follicle?

Question 3

Effect of LH on the oocyte (on resumption of meiosis)

Answer 3

Within 3-12h of LH surge we see:
Detachment of COC from surrounding mural GC, followed by cumulus cell expansion – formation of unique extracellular matrix between cumulus cells (aka “mucification”)

Comprised of long chains of hyaluronan
Visco-elastic properties of CC matrix important for successful ovulation, ovum pick up by oviducts and penetration of sperm

↓cGMP production and closure of gap junctions
Activation of PDE3A → ↓cAMP → activation of pathways leading to breakdown of nuclear membrane in primary oocyte aka germinal vesicle breakdown (GVBD)
Resumption of meiosis in oocyte → completion of Meiosis I & release of 1st polar body
Arrests again in Metaphase II

Question 4

Describe meiosis in the oocyte

Answer 4

Early oocytes classified as immature i.e. at germinal vesicle (GV) or metaphase 1 stage
Meiosis I is completed with half chromosomes but nearly all cytoplasm remaining in the secondary oocyte
Remaining chromosomes move with small bag of cytoplasm to form discarded polar body (PB)
Why unequal division of cytoplasm?
Need to conserve for the oocyte all the material synthesised earlier → takes fertilized zygote through growth and implantation

Chromosomes of secondary oocyte immediately enter 2nd meiotic division, form the 2nd metaphase spindle and arrest
This arrest is maintained by cytostatic factor (protein complex)
Egg is ovulated in this arrested state