Gametogenesis Essay Example | Topics and Well Written Essays - 1000 words

Gametogenesis - Essay Example In the early stage of embryonic development, groups of cells become committed to differentiate into various cell types such as liver cells, nerve cells and muscle cells. One group of cells becomes committed to form the germ cell line, cells that eventually develop into eggs if the embryo is female or sperm if the embryo is male. These are the only cells that undergo meiosis. These committed cells, primordial germ cells increase in number through mitotic cell division. Later, they divide meiotically to produce mature sperm or eggs. The process of forming mature eggs is called oogenesis (Aberts et al, 2002; Snustad and Simmons, 2002). The primordial germ cells that migrate to the developing ovary during early embryogenesis become oogonia. They multiply rapidly, undergoing several rounds of mitotic cell division and eventually differentiate into primary oocytes. The primary oocytes begin meiotic cell division and complete the diplotene stage of prophase I and then division stops. They remain in this suspended prophase. During this period, the oocyte undergoes many changes that prepare it for the completion of meiosis and for fertilization. It acquires a special coat that protects the developing egg from mechanical damage and in many cases acts as a barrier to sperm from other species. Just beneath the membrane cortical granules develop that alter the egg coat so that only a single sperm fertilizes an egg. In addition, the primary oocyte accumulates large quantities of nutrients and other molecules that nourish the early embryo and coordinate as well as direct its early development. With the attainment of sexual mat urity, the next stage of oocyte development occurs, triggered by hormones. The oocyte completes meiosis I and two haploid nuclei are formed, each containing one member of each chromosome pair in a replicated state. But cytokines are very asymmetrical. One cell, called secondary oocyte gets virtually all the cytoplasm and is ancestral to the mature egg. The other cell, called a polar body, gets very little cytoplasm. Both of these cells , the secondary oocyte and the polar body undergo meiosis II to produce four haploid nuclei. Again cytokinesis is asymmetrical: Meiosis II in the secondary oocyte produces one large cell, the ovum, or egg, which has virtually all the cytoplasm and a small polar body with very little cytoplasm. Thus, of the four meiotic products, only one forms the mature egg. the polar bodies, which are small with little cytoplasm to support their metabolism, eventually degenerate (Aberts et al, 2002; Snustad and Simmons, 2002). Oogenesis occur in the ovaries of the f emale. Each primary oocyte is surrounded by a spherical cluster of cells in a cavity or sac called the primary or Graafian follicle. In response to hormone signals, the primary oocyte completes meiosis I to become a secondary oocyte. the follicle then ruptures, releasing the secondary oocyte into the oviduct, where it begin the second meiotic division. in humans, the oviduct is called the Fallopian tube. the release of the secondary oocyte from the Graafian follicle is called ovulation (Aberts et al, 2002; Snustad and Simmons, 2002). As the secondary ooc