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Oogenesis
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Oogenic meiosis
Oogenesis—the differentiation of the ovum—differs from spermatogenesis in several ways. Whereas the gamete formed by spermatogenesis is essentially a motile nucleus, the gamete formed by oogenesis contains all the materials needed to initiate and maintain metabolism and development. Therefore, in addition to forming a haploid nucleus, oogenesis also builds up astore of cytoplasmic enzymes, mRNAs, organelles, and metabolic substrates. While the sperm becomes differentiated for motility, the egg develops a remarkably complex cytoplasm.

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Figure 19.19
Changes in the number of germ cells in the human ovary (more...)
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Figure 19.19
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Changes in the number of germ cells in the human ovary over the life span. (After Baker 1970.)
Themechanisms of oogenesis vary among species more than those of spermatogenesis. This difference should not be surprising, since patterns of reproduction vary so greatly among species. In some species, such as sea urchins and frogs, the female routinely produces hundreds or thousands of eggs at a time, whereas in other species, such as humans and most mammals, only a few eggs are produced duringthe lifetime of an individual. In those species that produce thousands of ova, the oogonia are self-renewing stem cells that endure for the lifetime of the organism. In those species that produce fewer eggs, the oogonia divide to form a limited number of egg precursor cells. In the human embryo, the thousand or so oogonia divide rapidly from the second to the seventh month of gestation to formroughly 7 million germ cells
(Figure 19.19[pic]). After the seventh month of embryonic development, however, the number of germ cells drops precipitously. Most oogonia die during this period, while the remaining oogonia enter the first meiotic division (Pinkerton et al. 1961). These latter cells, called the primary oocytes, progress through the first meiotic prophase until the diplotene stage, atwhich point they are maintained until puberty. With the onset of adolescence, groups of oocytes periodically resume meiosis. Thus, in the human female, the first part of meiosis begins in the embryo, and the signal to resume meiosis is not given until roughly 12 years later. In fact, some oocytes are maintained in meiotic prophase for nearly 50 years. As Figure 19.19[pic]indicates, primary oocytescontinue to die even after birth. Of the millions of primary oocytes present at birth, only about 400 mature during a woman's lifetime.

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Figure 19.20
Polar body formation in the oocyte of the whitefish (more...)
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Figure 19.20
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Polar body formation in the oocyte of the whitefish Coregonus. (A) Anaphase of first meiotic division, showing the first polar body pinching offwith its chromosomes. (B) Metaphase (within the oocyte, arrow) of the second meiotic division, with the first polar body still in place. The first polar body may or may not divide again. (From Swanson et al. 1981; photographs courtesy of C. P. Swanson.)
Oogenic meiosis also differs from spermatogenic meiosis in its placement of the metaphase plate. When the primary oocyte divides, its nucleus,called the germinal vesicle, breaks down, and the metaphase spindle migrates to the periphery of the cell. At telophase, one of the two daughter cells contains hardly any cytoplasm, whereas the other cell has nearly the entire volume of cellular constituents (Figure 19.20[pic]). The smaller cell is called the first polar body, and the larger cell is referred to as the secondary oocyte. During thesecond division of meiosis, a similar unequal cytokinesis takes place. Most of the cytoplasm is retained by the mature egg (ovum), and a second polar body receives little more than a haploid nucleus. Thus, oogenic meiosis conserves the volume of oocyte cytoplasm in a single cell rather than splitting it equally among four progeny.
In a few species of animals, meiosis is severely modified such that...