Ovarian follicle

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Ovarian follicle


Section of vesicular ovarian follicle of cat. X 50.
Latin	folliculi ovarici primarii, folliculi ovarici vesiculosi
Gray's	subject #266 1256
MeSH	Ovarian+Follicle
Dorlands/Elsevier	f_11/12372180

Ovarian follicles are the basic unit of female reproductive biology, they are roughly spherical aggregations of cells found in the ovary. They contain a single oocyte (aka ovum or egg). These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single competent oocyte. Oocytes are swaddled in a layer of granulosa cells that are together enclosed in a thin layer of Extracellular matrix – the follicular basement membrane or basal lamina - to constitute the ovarian follicle.

As ovarian follicles grow larger they become discernible to the naked eye, these large ovarian follicles are sometimes called Graafian follicles (after Regnier de Graaf).

In humans, oocytes are established in the ovary before birth, and may lay dormant awaiting initiation for up to 50 years ^[1].

The maturation of the follicle is called folliculogenesis.

1 Development of oocytes in ovarian follicles
2 References
3 Additional images
4 External links

[edit] Development of oocytes in ovarian follicles

Gametes (sperm and oocytes) are produced by a special cellular division process called meiosis. Oogenesis is the process in which oocytes are formed. During early embryonic development, somatic cells from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. These primordial germ cells (PGCs) multiply by mitosis and once they have reached the gonadal ridge they are called oogonia (diploid stem cells of the ovary). The gonadal ridge consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells. Development proceeds and the oogonia become fully surrounded by a layer of cells (pre-granulosa cells).

Oogonia continue to multiply by dividing mitotically; this proliferation ends when the oogonia enter meiosis. The amount of time that oogonia multiply by mitosis is species specific. In the human fetus, cells undergoing mitosis are seen until the second and third trimester of pregnancy ^[2]; ^[3]. After beginning the meiotic process, the oogonia (now called primary oocytes) can no longer replicate. Therefore the total number of gametes is established at this time. Once the primary oocytes stop dividing the cells enter a prolonged ‘resting phase’. This ‘resting phase’ or dictyate stage can last anywhere up to fifty years in the human.

For each primary oocyte that undergoes meiosis, only one functional oocyte is produced. The other two cells produced are called polar bodies. Polar bodies have no function and eventually deteriorate.

A mature ovarian follicle actually contains a secondary oocyte. Unlike the sperm, the egg is arrested in the secondary stage of meiosis until fertilization.

Upon fertilization by sperm, the secondary oocyte continues the second part of meiosis and becomes a zygote.

[edit] References

^ McGee, E. A., and Hsueh, A. J. (2000). Initial and cyclic recruitment of ovarian follicles. Endocrine Reviews 21, 200-14.
^ Baker, T. G. (1982). Oogenesis and ovulation. In "Book 1: Germ cells and fertilization" (C. R. Austin and R. V. Short, Eds.), pp. 17-45. Cambridge University Press, Cambridge.
^ Byskov, A. G., and Hoyer, P. E. (1988). Embryology of mammalian gonads and ducts. In "The physiology of reproduction" (E. Knobil and J. Neill, Eds.), pp. 265-302. Raven Press, Ltd, New York.

[edit] Additional images

Histology of the preovulatory follicle.	Section of the ovary of a newly born child.	Pre-antral follicle	Graafian follicles
Histological presentation of early folliculogenesis	Diagram of folliculogenesis	Order of changes in ovary