| Blastocyst |
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FROM ONE CELL TO BLASTOCYST A mammal develops from a single cell called a Zygote , which results from an Oocyte (egg) being Fertilized by a single Sperm . The zygote is surrounded by a strong membrane of Glycoproteins called the '' Zona Pellucida '' which the successful sperm has managed to penetrate. The zygote undergoes Cleavage , increasing the number of cells within the zona pellucida. When there are about 4 to 16 cells, the embryo is in the Morula stage. When the number of cells reaches 40 to 150, a central, fluid-filled cavity ( Blastocoel ) forms. The zona pellucida begins to degenerate. This stage in the developing embryo, reached after four to six days, is the blastocyst, and lasts approximately until the implantation in the Uterus . The outer cells develop into the Placenta . BLASTOCYST GROWS AND INVADES and Trophoblast .]] In the Blastocyst there are cells which eventually form the Placenta , and Inner Cell Mass (also called Embryoblast ) of undifferentiated cells which are a source of embryonic Stem Cells . The blastocyst can be thought of as a ball of a (mostly single) layer of Trophoblast cells, with inner cell mass attached to this ball's inner wall. The embryo plus its Membrane s is called the Conceptus . By this stage the conceptus is in the Uterus . The Zona Pellucida ultimately disappears. The consisting of Cuboidal cells that are the source of dividing Cells , and the outer is the Syncytiotrophoblast . The cytotrophoblast implants the blastocyst in the Endometrium (innermost Epithelial lining) of the uterus by forming finger-like projections called ''villi'' that make their way into the uterus, and spaces called '' Lacunae '' that fill up with the mother's blood. This is assisted by Hydrolytic Enzyme s that erode the Epithelium . The Syncytiotrophoblast also produces Human Chorionic Gonadotropin (hCG), a Hormone that "notifies" the mother's body (though not necessarily the mother) that she is Pregnant , which prevents menstruation The villi begin to branch, and contain blood vessels of the Fetus that allow gas exchange between mother and child. INNER CELL MASS DIFFERENTIATION ]] While the syncytiotrophoblast starts to penetrate into the wall of the uterus, the inner cell mass (embryoblast) also develops. The embryoblast forms a bilaminar (two layered) embryo, composed of the ''epiblast'' and the ''hypoblast''. The epiblast is adjacent to the trophoblast and made of columnar cells; the hypoblast is closest to the blastocyst cavity, and made of cuboidal cells. The epiblast, now called ''primitive ectoderm'' will give rise to all three Germ Layer s of the embryo: ectoderm, mesoderm, and endoderm. The hypoblast, or ''primitive endoderm'', will give rise to extraembryonic structures only, such as the lining of the yolk sac. CAVITY FORMATION By separating from the trophoblast, the epiblast forms a new cavity, the ''amniotic cavity''. This is lined by the amnionic membrane, with cells that come from the epiblast (called ''amnioblasts''). Some hypoblast cells migrate along the inner cytotrophoblast lining of the blastocoel, secreting an Extracellular Matrix along the way. These hypoblast cells and extracellular matrix are called '' Heuser's Membrane '' (or ''exocoelomic membrane''), and the blastocoel is now called the ''primary yolk sac'' (or ''exocoelomic cavity''). Cytotrophoblast cells and cells of Heuser's membrane continue secreting extracellular matrix between them. This matrix is called the ''extraembryonic reticulum''. Cells of the epiblast migrate along the outer edges of this reticulum and form the ''extraembryonic mesoderm'', which makes it difficult to maintain the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the ''chorionic cavity'' or ''extraembryonic coelom''. Another layer of cells leaves the hypoblast and migrates along the inside of the primary yolk sac. The primary yolk sac is pushed to the opposite side of the embryo (the ''abembryonic pole''), while a new cavity forms, the ''secondary'' or ''definitive yolk sac''. The remnants of the primary yolk sac are called ''exocoelomic vesicles''. SEE ALSO EXTERNAL LINKS |
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