DORSAL LIP OF BLASTOPORE SERIES

The different regions of the blastula have different inductive capabilities. A third type of map, called a competence map, that describes the developmental potential of different regions of the blastula when developed in different developmental contexts via transplantation experiments can also be constructed. Whereas the fate maps show that under normal conditions, cells can be predicted to form the epidermis, neural tube, notochord, blood, somatic muscle, and gut. The specification map is more general and describes regions of future ectoderm, mesoderm, mesendoderm, and endoderm. Thus the fate map and specification maps differ slightly. While the fate maps show that regions of the late blastula can be predicted to follow a particular fate during normal development, those regions may not yet be committed to that exact fate if developed in isolation. Through these studies several regions were identified, the animal cap, the dorsal and ventral marginal zones (DMZ and VMZ respectively), and the vegetal cap.

DORSAL LIP OF BLASTOPORE SERIES

The specification maps were constructed by a series of explant studies that measured developmental potential of different regions of the embryo. Alternatively, a specification map can be constructed that describes which cells of the late blastula are committed to certain fates. Fate maps can be constructed that describe what various regions of the blastula will become under normal developmental circumstances. The mid-blastula transition (MBT) occurs at this stage, beginning the onset of zygotic transcription, increasing cell motility, and slowing the rate of cell division.ДЄt this stage, maps can be made that describe the developmental potential of regions of the blastula in different ways. These early cleavages occur in the absence of maternal transcription. The outer surface of the blastula forms an epithelium. During cleavage, tight junctions form and ions are pumped inside the embryo, raising osmotic pressure and causing the blastocoel to form. The early cleavages of the zygote occur very rapidly, and cells in the animal hemisphere cleave more rapidly than cells in the yolky vegetal hemisphere. 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. Another important model organism in developmental biology is the African clawed frog, Xenopus laevis.

Dorsal lip of blastopore