The secondary oocyte completes meiosis at fertilization and extrudes second polar body. Haploid ovum •Fig. 44.18 Eventsinvolvedinmeioticarrestandmaturationoftheoocyte.MAPK,mitogen-activatedproteinkinase.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.) the outer-lying thecal layers. Mural granulosa cells become highly steroidogenic and remain in the ovary after ovulation to differentiate into the corpus luteum.
Cumulus cells are the inner cells that surround the oocyte (they are also referred to as the cumulus oophorus and corona radiata). The innermost layer of cumulus cells maintains gap and adhesion junctions with the oocyte. Cumulus cells are released with the oocyte (collectively referred to as the cumulus-oocyte complex) during the process of ovulation. Cumulus cells are crucial for the ability of the fimbriated end of the oviduct to “capture” and move the oocyte by a ciliary transport mechanism along the length of the oviduct to the site of fertilization (see
Early antral follicles are dependent on pituitary FSH for normal growth. Large antral follicles become highly dependent on pituitary FSH for their growth and sustained viability. As discussed later, 2to 5-mm follicles are recruited to enter a rapid growth phase via the transient increase in FSH that occurs toward the end of the previous menstrual cycle.
The Gamete
The oocyte grows rapidly in the early stages of antral follicles; growth then slows in larger follicles. During the antral stage the oocyte synthesizes sufficient amounts of cell cycle components so it becomes competent to complete meiosis I at ovulation. (Note that the human egg arrests after ovulation at a second point, metaphase II, until it is fertilized by sperm.) Thus in early primary and secondary follicles, the oocyte fails to complete meiosis I because of a dearth of specific meiosis-associated proteins. However, larger antral follicles gain meiotic competence but still maintain meiotic arrest until the midcycle LH surge. Meiotic arrest is achieved by maintenance of elevated cyclic adenosine monophosphate (cAMP) levels in the mature oocyte (
Figs.
•Fig. 44.19 ModelofhowLHsurgeleadstoresumptionofmeiosis.PDE,phosphodiesterase.Granulosa & cumulus cells ZP GPR3 AMP cAMP PKA PDE 3A MEIOTIC ARREST (–) (–) LH SURGE LHSURGEcGMPcAMPMEIOTICRESUMPTIONGsACPDE3AcGMP cGMP cGMP 44.18 44.19 ). The constitutively active (i.e., not requiring a ligand) Gs protein-coupled receptor GPR3 maintains high cAMP. The oocyte-specific phosphodiesterase PDE3A degrades cAMP to inactive AMP. Before the LH surge, PDE3A is inhibited by cGMP, which is produced within cumulus and granulosa cells and enters the oocyte via gap junctions.