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The blastula (from Greek ''βλαστός'' (blastos), meaning "sprout") is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form. A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoele.〔Forgács & Newman, 2005: (p. 27 )〕 In mammals the blastula is referred to as a blastocyst. The blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and the trophoblast, which goes on to form the extra-embryonic tissues.〔 During the blastula stage of development, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, and regulate gene expression.〔 In many animals such as ''Drosophila'' and ''Xenopus'', the mid blastula transition (MBT) is a crucial step in development during which the maternal mRNA is degraded and control over development is passed to the embryo. Many of the interactions between blastomeres are dependent on cadherin expression, particularly E-cadherin in mammals and EP-cadherin in amphibians. The study of the blastula and of cell specification has many implications on the field of stem cell research as well as the continued improvement of fertility treatments.〔 Embryonic stem cells are a field which, though controversial, have tremendous potential for treating disease. In ''Xenopus'', blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling. By manipulating the cell signals during the blastula stage of development, various tissues can be formed. This potential can be instrumental in regenerative medicine for disease and injury cases. In vitro fertilisation involves implantation of a blastula into a mother’s uterus. Blastula cell implantation could potentially serve to eliminate infertility. == Development == The blastula stage of early embryo development begins with the appearance of the blastocoele. The origin of the blastocoele in ''Xenopus'' has been shown to be from the first cleavage furrow, which is widened and sealed with tight junctions to create a cavity. In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「blastula」の詳細全文を読む スポンサード リンク
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