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. 2014 Aug 13;12:78. doi: 10.1186/1477-7827-12-78

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© Araújo et al.; licensee BioMed Central Ltd. 2014

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Schematic sequence of complete follicular development. Preantral phase: Formation and beginning of growth and activation of primordial follicles and growth of primary and secondary follicles. Antral phase: formation of tertiary follicle (antral-filled follicular fluid cavity). Follicle growth continues through the phases of recruitment, selection, dominance, and preovulatory stage of follicular waves. Oogonia develop from a primordial germ cell and differentiates into an oocyte in the ovary. Primordial follicle has a single layer of flattened granulosa cells. Primary follicle has a single layer of cuboidal granulosa cells. Secondary follicle has two or more layers of cuboidal granulosa cells and a small number of theca cells. All the preantral follicles have a primary oocyte. Tertiary follicle has several granulosa cell layers, theca cells, and primary oocyte and is characterized by an antral cavity which contains follicular fluid. Preovulatory or also called as Graafian follicle is the last stage of follicle development; these follicles are larger, have more antral fluid and may contain a secondary oocyte. Follicular fluid is a plasma exudate conditioned by secretory products from the granulosa cells and oocyte.