Maturation of Oocytes Begins Before Birth
المؤلف:
T.W. Sadler
المصدر:
Langmans Medical Embryology
الجزء والصفحة:
14th, p24-26
2025-06-02
415
Once PGCs have arrived in the gonad of a genetic female, they differentiate into oogonia (Fig. .1A,B). These cells undergo a number of mitotic divisions, and by the end of the third month, they are arranged in clusters surrounded by a layer of flat epithelial cells (Figs. 2 and 3). Whereas all of the oogonia in one cluster are probably derived from a single cell, the flat epithelial cells, known as follicular cells, originate from surface epithelium covering the ovary.
Fig1. Differentiation of PGCs into oogonia begins shortly after their arrival in the ovary. By the third month of development, some oogonia give rise to primary oocytes that enter prophase of the first meiotic division. This prophase may last 40 or more years and finishes only when the cell begins its final maturation. During this period, it carries 46 double—structured chromosomes.
Fig2. Segment of the ovary at different stages of development. A. Oogonia are grouped in clusters in the cortical part of the ovary. Some show mitosis; others have differentiated into primary oocytes and entered prophase of the first meiotic division. B. Almost all oogonia are transformed into primary oocytes in prophase of the first meiotic division. C. There are no oogonia. Each primary oocyte is surrounded by a single layer of follicular cells, forming the primordial follicle. Oocytes have entered the diplotene stage of prophase, in which they remain until just before ovulation. Only then do they enter metaphase of the first meiotic division.
Fig3. A. Primordial follicle consisting of a primary oocyte surrounded by a layer of flattened epithelial cells. B. Early primary or preantral stage follicle recruited from the pool of primordial follicles. As the follicle grows, follicular cells become cuboidal and begin to secrete the zona pellucida, which is visible in irregular patches on the surface of the oocyte. C. Mature primary [preantral] follicle with follicular cells forming a stratified layer of granulosa cells around the oocyte and the presence of a well—defined zona pellucida.
The majority of oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis I and form primary oocytes (Figs. 1C and 2.A). During the next few months, oogonia increase rapidly in number, and by the fifth month of prenatal development, the total number of germ cells in the ovary reaches its maximum, estimated at 7 million. At this time, cell death begins, and many oogonia as well as primary oocytes degenerate and become atretic. By the seventh month, the majority of oogonia have degenerated except for a few near the surface. All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat follicular epithelial cells (Fig. 2B). A primary oocyte, together with its surrounding flat epithelial cells, is known as a primordial follicle (Fig. 3A).
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