Figure 229

Polycystic ovarian disease. This photomicrograph shows a section through the cortex of the ovary from an individual with polycystic ovarian disease. Note the unusually thick tunica albugĂ­nea (TA) that overlies numerous follicles. The thickness of the tunica albugĂ­nea prevents ovulation of the mature (Graafian) follicles. Note that one of the follicles has developed to the primary follicle stage. x45.

sibility of multiple births by causing simultaneous maturation of several follicles.

The primary oocyte is arrested for 12 to 50 years in the diplotene stage of prophase of the first meiotic division

The primary oocytes within the primordial follicles begin the first meiotic division in the embryo, but the process is arrested at the diplotene stage of meiotic prophase (see the section on meiosis in Chapter 2). The first meiotic prophase is not completed until just before ovulation. Therefore, primary oocytes remain arrested in the first meiotic prophase for 12 to 50 years. This long period of meiotic arrest exposes the primary oocyte to adverse environmental influences and may contribute to errors in meiotic division, such as nondisjunction. Such errors result in anomalies such as trisomy of chromosome 21 (Down syndrome).

As the first meiotic division (reduction division) is completed in the mature follicle (Fig. 22.10), each daughter cell of the primary oocyte receives an equal share of chromatin, but one daughter cell receives most of the cytoplasm and becomes the secondary oocyte. It measures 150

/im in diameter. The other daughter cell receives a minimal amount of cytoplasm and becomes the first polar body.

The secondary oocyte is arrested at metaphase in the second meiotic division just before ovulation

As soon as the first meiotic division is completed, the secondary oocyte begins the second meiotic division. As the secondary oocyte surrounded by the cells of the corona radiata leaves the follicle at ovulation, the second meiotic division (equatorial division) is in progress. This division is arrested at metaphase and is completed only if the secondary oocyte is penetrated by a spermatozoon. If fertilization occurs, the secondary oocyte completes the second meiotic division and forms a mature ovum with the maternal pronucleus containing a set of 23 chromosomes. The other cell produced at this division is a second polar body. In humans, the first polar body does not divide; therefore, the fertilized egg can be recognized by the presence nearby of the second polar body. The polar bodies, which are not capable of further development, degenerate.

i ! slow growth of oogonia development of primordial follicle primary oocyte fusion of

migration of nucleus to cell surface penetrating spermatozoon second polar body may divide

nonfunctional polar bodies proliferation of oogonia i ! slow growth of oogonia development of primordial follicle penetrating spermatozoon second polar body may divide

nonfunctional polar bodies primary oocyte fusion of

migration of nucleus to cell surface first meiotic division ovary)

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