Menstrual cycle 2

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.

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?

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

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