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The ovary is enclosed by a clear membrane, the ovarian bursa, and consists of a poorly defined central medulla and outer cortex covered by the germinal epithelium, a single layer of cuboidal to columnar cells. The medulla comprises vascular, dense fibrous connective tissue stroma, while the ovarian cortex consists of primordial and maturing follicles and corpora lutea separated by loose fibrous stroma and groups of ovarian interstitial cells. Primordial follicles contain oocytes, clear round cells, surrounded by a single layer of flat cells. The majority of primordial follicles eventually undergoes atresia, but each estrus several primordial follicles mature to larger follicles (primary unilaminar follicle, secondary multilaminar follicle) and the preovulatory Graafian follicles. In the maturing follicles, many layers of small basophilic granulosa cells have replaced the single layer of flat cells, a vascular theca layer has formed from stromal cells, and the oocyte is surrounded by the acellular zona pellucida and the cumulus oophorus, a cluster of follicle cells. The maturing follicles cause the germinal epithelium to bulge, which ruptures at ovulation, releasing the oocyte with its cluster of cells from the Graafian follicle. Next, the granulosa and theca cells change the follicle to a corpus luteum, which consists of lutein cells, large cells with a clear eosinophilic cytoplasm and a large nucleus, organized in cords around sinusoids.
The 4X micrograph is a cross section of an ovary, depicting (part of) the ovarian bursa, the germinal epithelium, and the ovarian cortex with follicles at different stages of maturation. The 10X micrograph offers a closer view of several of those follicles: a primordial follicle, a primary unilaminar follicle, a secondary multilaminar follicle, and a corpus luteum. The 20X micrograph presents a secondary multilaminar follicle, showing the vascular theca layer and the centrally located oocyte surrounded by the zona pellucida and cumulus oophorus. The 40X micrograph displays in detail the ovarian cortex, including primordial follicles, ovarian interstitial cells, and stroma, and the single layer of cuboidal cells forming the germinal epithelium.
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