Ovarian follicles or Graafian follicles (after Regnier de Graaf) are the roughly spherical cell aggregations in the ovary containing an ovum and from which the egg is released during ovulation. It consists of an external fibro-vascular coat, an internal coat of nucleated cells, and a transparent, albuminous fluid in which the ovum is suspended. The maturation of the follicle is called folliculogenesis.

Formation of egg cells in follicles

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Gametes, sperm and egg cells, are produced by the cellular division process of meiosis. Oogenesis is the process of meiosis for egg cell formation. It begins in the ovaries and is regulated by follicle stimulating hormone (FSH). This hormone initiates the growth of ovarian follicles. FSH also stimulates the secretion of estrogen from the follicle cells, promoting the maturation of the ovum. For each primary oocyte that undergoes meiosis, only one functional egg cell is produced. The other three cells produced are called polar bodies. Polar bodies have no function and eventually deteriorate. A mature ovarian follicle actually contains the secondary oocyte. Unlike the sperm, the egg is arrested in the secondary oocyte stage due to oocyte maturation inhibitor (OMI) fertilization. Upon fertilization by sperm, the secondary oocyte undergoes second meiosis and becomes a zygote.

Female reproductive system

Ovarialfollikel | Follicule ovarien | Grafo folikulas | 卵胞 | äggblåsa