In the days immediately preceding ovulation, under the influence of FSH and LH, the vesicular follicle grows rapidly to a diameter of 25 mm to become a mature vesicular (graafian) follicle. Coincident with final development of the vesicular follicle, there is an abrupt increase in LH that causes the primary oocyte to complete meiosis I and the follicle to enter the preovulatory mature vesicular stage. Meiosis II is also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovulation. In the meantime, the surface of the ovary begins to bulge locally, and at the apex, an avascular spot, the stigma, ap pears. The high concentration of LH increases collagenase activity, resulting in digestion of collagen fibers surrounding the follicle. Prostaglandin levels also increase in response to the LH surge and cause local muscular contractions in the ovarian wall. Those contractions extrude the oocyte, which together with its surrounding granulosa cells from the region of the cumulus oophorus breaks free (ovulation) and floats out of the ovary (Fig. 1). Some of the cumulus oophorus cells then rearrange themselves around the zona pellucida to form the corona radiata.

Fig1. A. Mature vesicular follicle bulging at the ovarian surface. B. Ovulation. The oocyte, in meta— phase of meiosis II, is discharged from the ovary together with a large number of cumulus oophorus cells. Follicular cells remaining inside the collapsed follicle differentiate into lutein cells. C. Corpus luteum. Note the large size of the corpus luteum, caused by hypertrophy and accumulation of lipid in granulosa and theca interna cells. The remaining cavity of the follicle is filled with fibrin.

مواضيع ذات صلة

CELL SIGNALING

المواعيد السريرية والجنينية Clinical and embryological timings

طبيعة مواد التكاثر اللاجنسي

التكاثر اللاجنسي

المفاهيم الأساسية في علم الجنين

مقدمة في علم الجنين