Blastocoel

The blastocoel ( ), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals the blastula is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel. It develops following cleavage of the zygote after fertilization. It is the first fluid-filled cavity or lumen formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. In the Xenopus a very small cavity has been described in the two-cell stage of development.

The blastocoel ( ), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals the blastula is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel. It develops following cleavage of the zygote after fertilization. It is the first fluid-filled cavity or lumen formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. In the Xenopus a very small cavity has been described in the two-cell stage of development. == In mammals== After fertilization, the zygote undergoes several rounds of cleavage divisions forming daughter cells known as blastomeres. At the 8- or 16-cell stage, the embryo undergoes compaction and forms the morula. Eventually, the morula is a solid ball of cells that has a small group of internal cells surrounded by a larger group of external cells. Then blastomeres undergo cellular differentiation with internal cells adopting the inner cell mass fate and the external layer becoming trophectoderm. The inner cell mass will go on to become the actual embryo. The external, surrounding cells develop into trophoblast cells, which only contribute to extra-embryonic tissues. At this stage there is no lumen within the embryo. In a process called cavitation, trophectoderm cells transport fluid into the embryo to create a blastocoel, the fluid-filled lumen. The membranes of the trophectoderm cells contain sodium (Na + ) pumps, Na + /K + - ATPase and Na + /H + exchangers, that pump sodium into the embryo. The oviduct cells stimulate these trophoblast sodium pumps as the fertilized egg travels down the fallopian tube towards the uterus. The accumulation of sodium pulls in water through osmosis. The accumulation of water breaks open cell-cell contacts via hydraulic fracturing. To form a single lumen, the fluid from multiple water pockets collects into a single entity in process akin to Ostwald ripening. The blastocoel further expands and the inner cell mass becomes positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. The axis formed by the inner cell mass and the blastocoel is the first axis of symmetry of mammalian embryo and determines its attachment point to the uterus. ==References== ==Further reading== Dorlands Staff (2004). "blastocoel [distionary entry]". Dorland's Illustrated Medical Dictionary (online). Amsterdam, NDE: Elsevier-Saunders. Retrieved 30 January 2016. "blastocoel...[blaso- + -coele] the fluid-filled cavity of the mass of cells (blastula) produced by cleavage of fertilized ovum. Sometimes spelled...[c]alled...'Also' blastocoelic ...pertaining to the blastocoele."; Dorlands.com

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