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The production of ova and sperm occurs via the process of meiosis (whereas somatic cells undergo division via mitosis). Oogenesis produces ova, and spermatogenesis produces sperm. One spermatogonium results in four sperm, and one oogonium results in one ovum and two polar bodies. Meiosis is a reduction division normally allowing each gamete to contain 23 chromosomes (haploid). Thus, when fertilization occurs and the two haploid gametes unite, the resulting zygote contains 46 chromosomes (diploid) under normal circumstances. Two meiotic divisions occur, and each contains several stages. . | DEVELOPMENT AND MALDEVELOPMENT GAMETOGENESIS The production of ova and sperm occurs via the process of meio-sis whereas somatic cells undergo division via mitosis . Oogenesis produces ova and spermatogenesis produces sperm. One spermatogonium results in four sperm and one oogonium results in one ovum and two polar bodies. Meiosis is a reduction division normally allowing each gamete to contain 23 chromosomes haploid . Thus when fertilization occurs and the two haploid gametes unite the resulting zygote contains 46 chromosomes diploid under normal circumstances. Two meiotic divisions occur and each contains several stages. FIRST MEIOTIC DIVISION A. Prophase I has five stages. 1. Leptotene wherein the chromatin condenses into individual elongated threadlike structures. 2. Zygotene the migration of single threadlike chromosomes toward the nuclear equatorial plate where homologous chromosomes pair to form bivalents that exchange segments at several points synapses . 3. Pachytene where chromosomes contract and thicken then split longitudinally into two chromatids attached at the centromere. 4. Diplotene is marked by crossing-over of the nonidentical chromatid constituents of homologous chromosomes at bridges or chiasms. The male sex chromatids X and Y chromatids however do not cross over. 5. Diakinesis the last stage which occurs when the bivalents contract chiasms move toward the ends of the 55 Copyright 2001 The McGraw-Hill Companies. Click Here for Terms ofUse. 56 BENSON PERNOLL S HANDBOOK OF OBSTETRICS AND GYNECOLOGY chromosome homologs pull apart and the nuclear membrane disappears. B. During metaphase I the very short and thick bivalents are aligned along the equatorial plate of the cell spindle forms. C. In anaphase I the centromeres divide so that the homologous chromatids rather than the identical sister chromatids are drawn to opposite poles of the spindle. D. Telophase I is marked by spindle breakage division of cellular cytoplasm and formation of a nuclear .
