The entry point of the sperm during fertilization in the animal hemisphere will determine the future dorsal ventral axis of the zygote. The oocyte will stay in this phase of meiosis II until fertilization when the necessary centrosome provided by the sperm will enter the egg. The process of oocyte maturation involves the completion of the first phase of meiosis: germinal vesicle breakdown (GVBD), polar body formation and arrest during the early prophase of meiosis II. The maturation of the oocyte is induced by progesterone, a hormone which is secreted in response to environmental signals and which can be induced artificially in the laboratory by the injection of pituitary hormones. Thus, the animal vegetal polarity of the oocyte can be easily determined visually. The yolk platelets accumulate in the vegetal hemisphere of the oocyte, and the pigment granules accumulate along the cortex of the animal pole along with the germinal vesicle. After being surrounded by somatic follicle cells, these lampbrush chromosomes will recompact prior to the onset of vitellogenesis, the process by which yolk is imported from the liver via the circulation into the growing oocyte. Each primary oocyte will enter meiosis, during which huge extended chromosomes, called lampbrush chromosomes will form, indicating early activation of transcription. In contrast to Drosophila, each of the 16 cells generated by four rounds of mitoses in the primary oogonium will become a primary oocyte. The oocytes are very large, and develop from the primary oogonium. Another important model organism in developmental biology is the African clawed frog, Xenopus laevis.
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