Oocyte Maturation
Oocyte Maturation
During oogenesis, described in the preceding section, an egg prepares itself for fertilization, and for the beginning of development. Whereas a sperm eliminates all of its cytoplasm and condenses its nucleus to the smallest possible dimensions, an egg grows in size by accumulating yolk reserves to support future growth. The egg cytoplasm also contains vast amounts of messenger RNA, ribosomes, transfer RNA, and other elements that will be required for protein synthesis. In addition, eggs of most species contain morphogenetic determinants that will direct the activation and repression of specific genes later in postfertilization development. The nucleus also grows rapidly in size during egg maturation, becoming bloated with RNA and so changed in appearance that it is given a special name, the germinal vesicle.
Most of this intense preparation occurs during the prolonged prophase of the first meiotic division. The oocyte is now poised to resume meiotic divisions that are essential to produce a haploid female pronucleus that will join a male haploid pronucleus at fertilization. After resumption of meiosis, the egg rids itself of excess chromosomal material in the form of polar bodies (described in The Reproductive Process). A vast amount of synthetic activity has preceded this stage. The oocyte is now a highly structured system, provided with a dowry which, after fertilization, will support the nutritional requirements of the embryo and direct its development through cleavage.
During oogenesis, described in the preceding section, an egg prepares itself for fertilization, and for the beginning of development. Whereas a sperm eliminates all of its cytoplasm and condenses its nucleus to the smallest possible dimensions, an egg grows in size by accumulating yolk reserves to support future growth. The egg cytoplasm also contains vast amounts of messenger RNA, ribosomes, transfer RNA, and other elements that will be required for protein synthesis. In addition, eggs of most species contain morphogenetic determinants that will direct the activation and repression of specific genes later in postfertilization development. The nucleus also grows rapidly in size during egg maturation, becoming bloated with RNA and so changed in appearance that it is given a special name, the germinal vesicle.
Most of this intense preparation occurs during the prolonged prophase of the first meiotic division. The oocyte is now poised to resume meiotic divisions that are essential to produce a haploid female pronucleus that will join a male haploid pronucleus at fertilization. After resumption of meiosis, the egg rids itself of excess chromosomal material in the form of polar bodies (described in The Reproductive Process). A vast amount of synthetic activity has preceded this stage. The oocyte is now a highly structured system, provided with a dowry which, after fertilization, will support the nutritional requirements of the embryo and direct its development through cleavage.
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