Primary Follicle Flashcards
Oogenesis from the oogonial stage until the formation of the early primary follicle is
endocrine independent.
Selection mechanisms determining which particular follicle is selected to grow
remain unknown.
Emerging evidence points to a role for the BMPs, the PTEN/protein kinase B (Akt) pathway, PDK1 and FOXO 3a in the regulation of
primordial follicle activation
AMH, another member of the TGF beta family, may act as a modified of
the rate of primordial follicle activation although the mechanism by which it achieves this role is unclear.
Indeed, AMH functions in the ovary as either
positive or negative modulators.
These and other data suggest multiple activating and inhibitory pathways communicate and converge to regulate controlled
primordial follicle activation and concurrent maintenance of the quiescent pool of primordial oocytes.
Primordial follicle activation is thought to be irreversible with most follicles eventually undergoing
atresia.
Ovarian cortical tissue pressure has also been proposed as a physical
non-hormonal regulator of both fetal ovary oocyte arrest and adult oocyte recruitment.
The primordial to primary follicle transition can be seen as
“initial recruitment” and is distinct from “cyclic recruitment” of more developmentally advanced follicles that occurs during the menstrual cycle of adult females.
Once the primordial follicle is activated to grow the oocyte
orchestrates and coordinates its development.
The remaining stages of oogenesis and meiosis are dependent upon
FSH and LH from the pituitary and estrogen from the ovarian follicle as well as a myriad of regulatory substrates that may act differently and/or specifically on the oocyte itself or the surrounding cells of the follicle.
Increases in systemic FSH and LH levels stimulate an
increase in the volume of ooplasm and secretion of the zona pellucida (ZP).
The ZP is a
protective acellular glycoprotein layer that surrounds the oocyte.
It is composed of 4 glycoproteins in the human;
ZP1, ZP2, ZP3, ZP4
The site(s) of ZP glycoprotein secretion in the human follicle remain to be determined but
may include both the oocyte and GC.
Elevated FSH and LH levels also stimulate GC
mitoses leading to the formation of several layers of GC.
The ovarian stromal cells adjacent to the GC differentiate into
a thin layer of thecal cells that become endocrinologically active.
Folliculogenesis is a very protracted process that takes approximately a year for a recruited
primordial follicle to develop to the pre-ovulatory stage.
The first 9-10 months (about 290 days) are required to transition through
the primary stage to a pre-antral follicle and a further two months (about 65 days) to develop into a follicle with a fully formed antrum.
A high concentration of androgen receptors are expressed at
pre-antral to antral stages declining thereafter.
Androgens primarily affect
granulosa cells and appear to act synergistically with FSH.
