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Structure of a blastocyst
Blastocyst on the 5th day

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.


  • N. Schrode, P. Xenopoulos, A. Piliszek, S. Frankenberg, B. Plusa, AK Hadjantonakis: Anatomy of a blastocyst: cell behaviors driving cell fate choice and morphogenesis in the early mouse embryo. In: Genesis (New York, NY: 2000). Volume 51, Number 4, April 2013, pp. 219-233, ISSN  1526-968X . doi : 10.1002 / dvg.22368 . PMID 23349011 . PMC 3633705 (free full text).

Individual evidence

  1. SA Morris, M. Żernicka-Goetz: Formation of distinct cell types in the mouse blastocyst. In: Results and problems in cell differentiation. Volume 55, 2012, pp. 203-217, ISSN  0080-1844 . doi : 10.1007 / 978-3-642-30406-4_11 . PMID 22918808 .
  2. P. Xenopoulos, M. Kang, AK Hadjantonakis: Cell lineage allocation within the inner cell mass of the mouse blastocyst. In: Results and problems in cell differentiation. Volume 55, 2012, pp. 185-202, ISSN  0080-1844 . doi : 10.1007 / 978-3-642-30406-4_10 . PMID 22918807 . PMC 3469159 (free full text).