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Samenvatting Decentrale selectie geneeskunde embryologie
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Samenvatting embryologie decentrale geneeskunde Utrecht. Zelf selectie gedaan in 2017, hierbij binnengekomen met een mooie plek.
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Summary Human Development (AB_1140) partial exam 1+2+3
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Lecture 7 -- Human Development -- Formation of the placenta
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Cleavage subsides zygote without increasing its sizeIn the first 24 hours after fertilization, the zygote initiates a rapid series of mitotic cell divisions called
cleavage. Because no cell growth occurs, the zygote does not get larger and the divisions divide the
zygote into smaller daughter cells called blastomeres. The embryo remains enclosed in the zona
pellucida. The first cleavage division divides the zygote in two daughter cells. After about 40 hours,
the second division has completed and there are four blastomeres. By three days, there are six to
twelve cells, and by four days there are sixteen to thirty-two cells. At this point, the embryo is called
a morula.
Segregation of blastomeres into the embryoblast and
trophoblast
The cells of the morula will give rise to the embryo proper and its extraembryonic membranes, and
to part of the placenta and related structures. During cleavage, the cells that will follow these
developmental paths become segregated. Starting at the eight-cell stage, the blastomeres flatten,
developing an inside-outside polarity that maximizes cell-to-cell contact. As differential adhesion
develops, the outer surfaces of the cells become convex and their inner surfaces become concave.
The reorganisation, called compaction, also involves changes in the blastomere cytoskeleton. Some
blastomeres segregate to the center of the morula and other to the outside. The center is the inner
cell mass or embryoblast, since it gives rise to the embryo proper. The blastomeres at the periphery
are the trophoblast. The trophoblast is the source of the fetal component of the placenta.
, Morula develops fluid-filled cavity and is transformed
into blastocyst
By four days of development, the morula begins to absorb fluid. First, as the trophoblast
differentiates, it assembles into an epithelium in which cells are stacked tightly. This adhesion of cells
results from the deposition on lateral cell surfaces of E-CAHDERIN, a calcium-dependent cell
adhesion molecule, and the formation of intercellular junctions, specifically tight functions, gap
junctions, adherens junctions, and desmosomes. Second, forming trophoblast cells express a basally
polarized membrane sodium/potassium ATPase, allowing them to transport and regulate the
exchange of metabolites between the outside and inside of the morula. The sodium/potassium
ATPase pumps sodium into the interior of the morula, and water follows through osmosis to become
blastocoelic fluid. As the hydrostatic pressure of the fluid increases, a large cavity called the
blastocyst cavity (blastocoel) forms within the morula. The embryoblast cells then form a compact
mass at one side of the cavity, and the trophoblast organizes into a thin, single-layered epithelium.
The embryo is now a blastocyst. The side of the blastocyst containing the inner cell mass is the
embryonic pole, and the opposite side is the abembryonic pole.
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