Oogenesis & Follicular Development
Oocyte and follicular development
Functional units of the ovary
- Oocytes
- 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
Developmental processes:
Embryologic origins
- 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.
Primordial follicle
- After meiosis I is initiated, the primoridial follicle forms around the oocyte.
- Comprises a single layer of flat granulosa cells, which interact to guide follicular maturation.
- Basement membrane (aka, lamina) surrounds the follicle.
- After the primordial follicle forms, meiosis I arrests in prophase (dictyone phase).
Puberty
- 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
- Primary oocyte, which has grown larger.
- 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
- 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.
- Thecal 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.
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.
- Ovulated secondary oocyte takes the corona radiata with it.
- Ruptured follicle transitions physiologically and morphologically to become the corpus luteum, which acts as a temporary endocrine gland.
- If the oocyte is not fertilized, it will be discharged with the menstrual fluid:
- In this case, the corpus luteum will involute and become the corpus albicans ("white body"); it will no longer produce hormones.
- If fertilization does occur, the secondary oocyte will resume and complete meiosis II, and a another polar body will be formed.
- If the fertilized oocyte successfully implants, the corpus luteum will persist until the placenta can perform its hormonal duties.