After completing this study unit you will be able to:
- Understand the histological structure of the ovary at different stages of its development.
- Identify its parts under the microscope.
The ovaries are small almond shaped structures in which ooctyes develop and female sex hormones are produced.
The ovaries are externally ensheathed by a layer of simple squamous epithelium known as ovarian mesothelium (an extension of the peritoneum), except at the attachment site of the mesovarium which suspends the ovary. The ovary itself is enclosed by a thick capsule of dense fibrous connective tissue, the tunica albuginea.
The ovary has a cortex, in which ovarian follicles can be found, as well as a highly vascular medulla.
During embryonic/fetal development, germ cells start their differentiation into oogonia, undergo cell division (mitosis) before finally entering the prophase of meiosis I, without completing it. At this stage, the cells become known as primary oocytes and are typically formed by the fifth month of fetal development and remain dormant in this meiotic division until just before ovulation (thereby remaining dormant up to fifty years).
The cortex of the ovary is populated by ovarian follicles which may be seen in various stages of development:
- The most numerous of these are primordial follicles which contain a single, small primary oocyte (as described above) surrounded by a single layer of squamous follicular epithelial cells, as
- Primordial follicles transition into primary follicles through oocyte enlargement and changes to the follicular epithelium, which becomes cuboidal in appearance. A glycogen rich extracellular coat, known as the zona pellucida, develops around the oocyte,
- The follicular cells become known as granulosa cells and then undergo stratification, at which point the follicle becomes known as a secondary follicle.
- Once the granulosa cells reach a thickness of 6-12 cell layers, small cavities containing follicular liquid appear among them; these cavities eventually coalesce to form a large single cavity known as a follicular antrum, at which point the follicle becomes known as a tertiary (vesicular/antral) follicle. Stromal cells surrounding the granulosa cells form a connective tissue sheath around the follicle known as the theca folliculi.
- Continued enlargement of the follicular antrum, as well proliferation of the granulosa cells cause the follicle to increase in size, until it eventually becomes known as a mature (preovulatory) follicle. At this stage, the granulosa cells form a mound (cumulus oophorus) upon which the oocyte rests, as well as a single cell layer around the oocyte (corona radiata). In the mature follicle, the primary oocyte completes its first mitotic division, after which it is known as a secondary oocyte.
Post ovulation, the ruptured mature follicle is initally known as a corpus rubrum (due to the presence of blood clots) before eventually becoming a transitory endocrine organ known as the corpus luteum. In the case of no fertilization/implantation, or in the latter half of pregnancy, the corpus luteum collapses and is eventually replaced by hyaline connective tissue to form the corpus albicans.
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