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Summary Embryology Week 1 to 4
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Medicine Decentralized Selection Summary from the book of “Human Embryology” van Larsen Embryology week 1 to 4
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Geneeskunde EmbryologieChapter 1: Gametogenesis, fertilization and first week
Cleavage subdivides zygote without increasing its size
Within 24hours after fertilization, the zygote (bevruchte eicel) initiates a rapid series of
mitotic cell divisions (mitotische delingen) called cleavage. These divisions are not
accompanied by cell growth, so they subdivide the large zygote into many smaller daughter
cells called blastomeres.
Thus the embryo as a whole does not increase in size during cleavage and remains enclosed
in the zona pellucida!
The first cleavage division divides the zygote to produce two daughter cells. The second
division, which is complete at about 40 hours after fertilization, produces four equal
blastomeres.
By three days, the embryo consists of 6-12 cells, and by fours days, it consists of 16-32 cells.
The embryo at this stage (32 cells) is called a morula (from Latin morum, mulberry).
Morula is formed at fallopian tube, after 3-4 days of fertilization.
Segregation of blastomeres into embryoblast and trophoblast precursors
The cells of the morula will form the embryo proper and its associated extra embryonic
membranes and part of the placenta and related structures. The cells that will follow these
different developmental paths becomes segregated (onderverdeeld) during cleavage (dus
tijdens klieving vindt er al enkele onderverdeling plaats, wat bepaalt welke cellen welke
“bestemming” zullen krijgen).
, Starting at the 8-cell (dus dag 3) stage of
development, the originally round and
loosely adherent blastomeres begin to
flatten, developing an inside-outside
polarity that maximizes cell-to-cell
contact among adjacent blastomeres.
As differential adhesion develops, the
outer surfaces of the cells become convex
and their inner surfaces become concave.
This reorganization, called compaction,
also involves changes in the blastomere
cytoskeleton.
Soon after development of the 8-cell or
16-cell embryo (depending on the
species), the blastomeres begin to form
tight junctions with one another, leading
to deformation of their round shape and
formation of a mulberry-shaped mass of
cells called a morula. This change in
shape of the embryo is called compaction.
With compaction, some blastomeres segregate to the center of the morula and others to the
outside. Differentiation (onderscheiden) of blastomeres is based on their position in either
inner cell mass or trophoblast — more central cells of the morula become inner cell
mass, and cells on the outside of the morula become trophoblast.
— The inner cell mass gives rise to the embryo proper, and therefore it is also called the
embryoblast
— The trophoblast is the primary source of the fetal component of the placenta
Morula develops fluid-filled cavity and is transformed into blastocyst
By 4 days of development, the morula, consisting now of about 30 cells, begins to absorb
fluid. Different processes are involved:
1. As the trophoblast differentiates, it assembles into an epithelium in which adjacent
cells are tightly adherent to one another. This adhesion results from the formation of
E-CADHERIN (a calcium-dependent cell adhesion molecule) on the lateral cell
surfaces and the formation of intercellular junctions, specifically, tight junctions,
gap junctions, adherens junctions, and desmosomes
2. Forming trophoblast cells express a basally (bottom layer) polarized membrane
sodium/potassium ATPase, allowing them to transport and regulate the exchange of
metabolites between the outside of the morula (i.e., the maternal environment of the
oviduct) and the inside of the morula (i.e., toward the inner cell mass). The sodium/
potassium ATPase pumps sodium (natrium) 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.
Sodium/potassium (Na/K-pomp) ATPase is an energy-dependent ion-exchange pump
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