Question

Distinguish structurally & functionally between primordial follicles, primary follicles, secondary follicles & a mature follicle

Answer 1

The primordial follicle consists of  a small primary oocyte (approx.25 μm in diameter) arrested in the diplotene stage of meiosis, a single layer of flattened squamous granulosa cells, and a basal lamina.They are in their resting stage.They wakeup by the process called recruitment.

The primary follicle contains one or more cuboidal granulosa cells that are arranged in a single layer surrounding the oocyte .The genome of the oocyte is activated in this stage and also develop FSH receptors.Zona pellucida( a glycoprotein capsule) is formed which contains enzymes that catalyse sperm penetration.

The secondary follicle is a preantral follicle with 2 to 10 layers of cuboidal or low columnar cells that form a stratified epithelium called stratum granulosum.The oocyte produces novel regulatory ligands,that are crucial for folliculogenesis.Theca externa and theca interna are formed from the differentiation of the basal layer.

The tertiary follicle is the mature follicle.They contain a fluid filled cavity called antrum.The granulosa cells of the tertiary follicle undergo differentiation into

• corona radiata, surrounding the zona pellucida
• membrana, interior to the basal lamina
• periantral, adjacent to the antrum
• cumulus oophorous, which connects the membrana and corona radiata granulosa cells together.

This happens under the action of an oocyte-secreted morphogenic gradient.

Androgens are produced by the theca cells under the influence of LH, which is later aromatised to estrogens by the granulosa cells.