The blastocyst ( ancient Greek. Βλάστη Blaster "sprout", "seed"; κύστις kystis "bubble";, blastocyst), Germanized also blastocyst , in most mammals is ( marsupials and higher mammals ) that stage of development of embryogenesis , the formation of the Morula follows. The development into a blastocyst is called blastogenesis or blastulation .
In human embryogenesis, fluid-filled intercellular spaces develop from the intercellular gaps within the initially compact cell structure of the morula , which finally flow together approximately three to four days after fertilization to form the blastocyst cavity and thus form the eponymous bladder. The filling of the intercellular spaces with fluid is brought about by a differentiation of the inner and outer cells of the morula. While the inner cells are connected to one another via gap junctions , the outer cells form a stable and flat demarcation to the outside via tight junctions . Furthermore, microvilli can only develop on the outer cells .
This polarization of the cells triggers an active flow of fluid through the outer cells into the interior of the morula. As a result, the blastocyst forms, the volume inside increases and it emerges from the zona pellucida (egg shell). This process is supported by an enzymatic decomposition of the zona pellucida. The inner cells of the morula - the embryoblast or the inner cell mass (IZM) - are only preserved on one side (embryonic pole), while the trophoblast forms from the enveloping cells - a protective covering, from which the placenta and membranes ultimately develop become. In the blastocyst stage, mammals, including humans, implant (nidation) .
There are various types of cells in blastocysts , e.g. B. embryonic stem cells (ES cells) or various pluripotent stem cells . Among other things, the ES cells can be obtained from this. This destroys the blastocyst. This type of stem cell production is therefore ethically controversial.
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