Dorsal lip of the blastopore3/13/2024 ![]() ![]() The grey crescent is also the site of the first cleavage plane, which bisects the gray crescent in an animal-vegetal direction. Cortical rotation is necessary for establishing the dorsal-ventral axis of the egg, in the absence of rotation no dorsal structures form (muscles, nervous system, etc.), and in the case of overrotation no ventral structures form (gut, endoderm, etc.). Rotation can be measured experimentally by using dye to mark positions of the cortex, and can be inactivated by UV light. The rotation of a fertilized frog egg results in formation of the grey crescent, which will become the Nieuwkoop centre. The sperm centrosome initiates formation of the sperm aster, and cortical rotation follows via a microtubule dependant process. 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. One such candidate gene, noggin, is sufficient for rescuing development of irradiated embryos when its mRNA is injected (C).Another important model organism in developmental biology is the African clawed frog, Xenopus laevis. Establishment of cDNA libraries from extracted dorsal lip mRNA identified candidate genes that may be responsible for neural induction. Injection of extracted mRNA from the dorsal lip into irradiated Xenopus embryos rescued neural induction and overall development demonstrating there is a genetic basis of neural induction (B). ![]() Figure 1: Mangold's dorsal lip transplant experiment in Xenopus demonstrated that a transplanted dorsal lip could induce the formation of a double axis in the new host embryo, solidifying the dorsal lip's sufficiency in neural induction (A). It is particularly important for its role in neural induction through the default model, where signaling from the dorsal lip protects a region of the epiblast from becoming epidermis, thus allowing it to develop to its default neural tissue. The dorsal lip is formed during early gastrulation as folding of tissue along the involuting marginal zone of the blastocoel forms an opening known as the blastopore. The dorsal lip of the blastopore is a structure that forms during early embryonic development and is important for its role in organizing the germ layers.
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