Collaborative Resources for
Learning Developmental Biology
Collaborative Resources for Learning Developmental Biology
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Ovarian Follicle Development
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Emely Larios

Additional Author(s): Alejandra E. Ontiveros, and Richard R. Behringer

Published on SDB CoRe: Mar 11 2020

The Germline: Oogenesis
Organism: Bat
Stage of Development: Oogenesis

Object Description

Cross sections of Carollia perspicillata (short-tailed fruit bat) ovary showing follicle development. (A) primordial follicles (arrowheads); (B) four primary follicles; (C) secondary follicle; (D) antral follicle, asterisk - antrum; (E) Graafian follicle, black arrowhead - cumulus cells, red arrowhead - mural granulosa cells; (F) ovary. Bouin's fix, paraffin-section, hematoxylin and eosin stain. Bars (A-E) = 50 μm, (F) = 200 μm.

Ovarian Follicle Development

In mammals, oocytes mature within structures called follicles before they are released from the ovary during ovulation. Follicles are assigned developmental stages based on oocyte diameter and on the morphology of the associated somatic granulosa cells.  These stages are described further in [1]. Initially, primordial follicles (A, arrowheads) are comprised of the oocyte surrounded by a single layer of squamous (flat) granulosa cells [2]. Once primordial follicles are activated (B) [3], the oocyte increases in diameter and the granulosa cell layer proliferates and changes from a squamous epithelial layer to a single layered cuboidal epithelium, generating a primary follicle [1]. As granulosa cells continue to proliferate, the single layer of granulosa cells becomes multi-layered or stratified. Follicles with this structure are called secondary follicles (C) [1]. As the follicle matures (D), fluid is secreted from the granulosa cells.  This fluid coalesces to form a cavity called the antrum (asterisk). Follicles at this stage are called antral follicles [1].  The granulosa cells around the antrum then differentiate into two populations: cumulus cells (black arrowhead, E) adjacent to the oocyte, and mural cells (red arrowhead, E) lining the antrum [1]. The follicle at this stage is called a preovulatory or Graafian follicle (E). It is ready for ovulation and fertilization [1].


1. Clarke, H., Control of Mammalian Oocyte Development by Interactions with the Maternal Follicular Environment, in Oocytes: Maternal Information and Functions, M. Kloc, Editor. 2017, Springer International Publishing: Cham. 17-41.

2. Ren Y., Suzuki H., Jagarlamudi K., Golnoski K., McGuire M., Lopes R., Pachnis V., Rajkovic A., Lhx8 regulates primordial follicle activation and postnatal folliculogenesis. BMC Biol, 2015, 13:39.

3. Hsueh A.J., Kawamura K., Cheng Y., Fauser B.C., Intraovarian control of early folliculogenesis. Endocr Rev, 2015, 36(1):1-24.

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