Oogenesis & Follicular Development
Oogenesis & Folliculogenesis
Ovarian follicles are the functional units of the ovary.
Oocytes are female sex cells (gametes).
The ovary houses millions of oocytes. However, only a very small percentage of these will reach maturity; most will undergo atresia (aka, degeneration).
Follicular cells support the oocyte and produce hormones that regulate ovarian and uterine events (the events of the ovarian and uterine cycles are discussed in detail, elsewhere).
Oocytes arise from primordial germ cells (undifferentiated stem cells) that migrate to the gonadal ridge early in fetal development (approximately 3-6 weeks gestation).
Germ cells proliferate via mitosis; once they reach the gonadal ridge, they become oogonia, which reside in clusters (aka nests).
The majority of oogonia undergo atresia (aka, degeneration); the minority enter meiosis I.
Overview
Embryologic origins
Preantral follicles
Primordial follicle
After meiosis I is initiated, the primoridial follicle forms around the oocyte; it comprises a single layer of flat granulosa cells, which interact to guide follicular maturation. A basement membrane (aka, lamina) surrounds the follicle.
After the primordial follicle forms, meiosis I arrests in prophase (dictyone phase).
Following puberty, follicles are cyclically "recruited" for further development; at any given time after this, more than 90% of follicles present in the ovary are arrested in the primordial stage.
Despite meiotic arrest, the oocyte and follicle may continue to grow.
Primary follicle
Contains the primary oocyte, which has grown larger, and is now surrounded by the zona pellucida.
The zona pellucida is thick a-cellular coat that covers the oocyte; the zona pellucida enables fertilization. It displays sperm receptors and facilitates the acrosome reaction; after fertilization, the zona pellucida prevents additional sperm from joining with the oocyte.
Granulosa cells proliferate via mitosis and change from flat to cuboidal, which reflects their greater activity.
Secondary follicle
The primary oocyte achieves meiotic and developmental competence (it is capable of completing meiosis and preparing for implantation). Cuboid granulosa cell layers have multiplied from 1 to now 6-9 layers (typically) by the end of the secondary follicular stage.
Theca cells, which arise from the ovarian interstitium, begin to accumulate around the basement membrane of the secondary follicle.
Of the secondary follicles, some will be "recruited" by follicle-stimulating hormone (FSH) to become tertiary follicles.
Antral follicles
Early Tertiary Follicle
Granulosa cells are separated by an antrum, which is a fluid-filled cavity within the follicle; notice that it "pushes" the oocyte to one side of the follicle.
Thecal cells differentiate into two layers:
- The theca externa, aka, external thecal cells, form the outermost layer of the tertiary follicle; this layer will form the follicular capsule.
- The theca interna, aka, internal thecal cells, form the inner layer; these cells will have key roles in hormonal communication and vascularization of the follicle.
Late tertiary stage (aka, Graafian, pre-ovulatory stage):
- Cumulos oophorus is the collection of granulosa cells that support the secondary oocyte and polar body.
- Corona radiata is a subset of the cumulus oophorus that directly surrounds the zona pellucida.
- In this case, the corpus luteum will involute and become the corpus albicans ("white body"); it will no longer produce hormones.