and Follicular Development AP Biology > Reproduction and Development > Reproduction and Development

OOCYTE AND FOLLICULAR DEVELOPMENT

Ovarian follicles: Functional units of the

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Female sex cells

- 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 and produce hormones that regulate ovarian and uterine events (the events of the ovarian and uterine cycles are discussed in detail, elsewhere). 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.

1 / 3 • Despite meiotic arrest, the oocyte and follicle may continue to grow.

Primary follicle

• Primary oocyte, which has grown larger.

• is thick a-cellular coat that covers the oocyte; the zona pellucida enables fertilization.

- It displays sperm receptors and facilitates the ; 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 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 , aka, external thecal cells, form the outermost layer of the tertiary follicle; this layer will form the follicular capsule.

- The , aka, internal thecal cells, form the inner layer; these cells will have key roles in hormonal communication and vascularization of the follicle.

Pre-ovulatory surge in LH initiates oocyte development resumption and Meiosis I completion; oocyte enters meiosis II, then arrests in metaphase; a Polar body forms. 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.

Of the late tertiary follicles, only one, the so-called "dominant follicle," is ovulated.

• Ovulated secondary oocyte takes the corona radiata with it.

2 / 3 • Ruptured follicle transitions physiologically and morphologically to become the , 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 ("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.

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