| | | Baker, T.G.Oogenesis and ovulation. In Reproduction in Mammals: I. Germs Cells and Fertilization (C.R. Austin, R.V. Short, eds.), 2nd ed., pp. 17-45. Cambridge, UK: Cambridge University Press, 1982.
Browder, L.W.Oogenisis. New York: Plenum Press, 1985.
Metz, C.B.; Monroy, A., eds. Model Systems and Oogenesis. Biology of Fertilization, Vol. 1. Orlando, FL: Academic Press, 1985. |
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|  | Eggs Develop in Stages8
A developing egg is called an oocyte. Its differentiation into a mature egg (or ovum) involves a series of changes whose timing is geared to the steps of meiosis in which the germ cells go through their two final, highly specialized divisions. Oocytes have evolved special mechanisms for arresting progress through meiosis: they remain suspended in prophase I for prolonged periods while the oocyte grows in size, and in many cases they later arrest in metaphase II while awaiting fertilization.
While the details of oocyte development (oogenesis) vary in different species, the general stages are similar, as outlined in Figure 20-16. Primordial germ cells migrate to the forming gonad to become oogonia, which proliferate by ordinary cell division cycles for a period before differentiating into primary oocytes. At this stage the first meiotic division begins: the DNA replicates so that each chromosome consists of two chromatids, the homologous chromosomes pair along their long axes, and crossing-over occurs between the chromatids of these paired chromosomes. After these events the cell remains arrested in prophase of division I of meiosis (in a state equivalent, as we previously pointed out, to a G2 phase of an ordinary division cycle) for a period lasting from a few days to many years, depending on the species. During this long period (or, in some cases, at the onset of sexual maturity) the primary oocytes synthesize a coat and cortical granules and, in the case of large nonmammalian oocytes, they accumulate ribosomes, yolk, glycogen, lipid, and the mRNA that will later direct the synthesis of proteins required for early embryonic growth and the unfolding of the developmental program. In many oocytes the intensive biosynthetic activities are reflected in the structure of the chromosomes, which decondense and form lateral loops, taking on a characteristic "lampbrush" appearance, signifying that they are very busily engaged in RNA synthesis (discussed in Chapter 8).
The next phase of oocyte development is called oocyte maturation and usually does not occur until sexual maturity, when it is stimulated by hormones. Under these hormonal influences the cell resumes its progress through division I of meiosis: the chromosomes recondense, the nuclear envelope breaks down (this is generally taken to mark the beginning of maturation), and the replicated homologous chromosomes segregate at anaphase I into two daughter nuclei, each containing half the original number of chromosomes. To end division I, the cytoplasm divides asymmetrically to produce two cells that differ greatly in size: one is a small polar body, and the other is a large secondary oocyte, the precursor of the egg. At this stage each of the chromosomes is still composed of two sister chromatids. These chromatids do not separate until division II of meiosis, when they are partitioned into separate cells by a process that is identical to a normal mitosis, as previously described. After this final chromosome separation at anaphase II, the cytoplasm of the large secondary oocyte again divides asymmetrically to produce the mature egg (or ovum) and a second small polar body, each with a haploid number of single chromosomes (see Figure 20-16). Because of these two asymmetrical divisions of their cytoplasm, oocytes maintain their large size despite undergoing the two meiotic divisions. Both of the polar bodies are small, and they eventually degenerate.
In most vertebrates oocyte maturation proceeds to metaphase of meiosis II and then arrests until fertilization. At ovulation the arrested secondary oocyte is released from the ovary, and if fertilization occurs, the oocyte is stimulated to complete meiosis.
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