The zygote is still not an embryo because the paternal and maternal gene pools have not yet merged.
Superior izquierda: The paternal and the maternal pronuclei move towards each
other with the help of microtubules, which begin to be formed immediately after
impregnation, i.e., by the penetration of the spermatozoon. They grow in a star-like
pattern out of the paternal centrosome directly beside the forming paternal
pronucleus (= formation of an aster made of dozens of microtubules). The
microtubular proteins themselves arise from the cytoplasma of the oocyte.
Inferior derecha: While the microtubules of the aster pull the pronuclei together in
the center of the oocyte, the synthesis of the DNA is taking place in the pronuclei.
This duplication takes roughly 12 hours. The pronuclei grow in size in this time.
Superior izquierda: After the two pronuclei have come as close together as they can,
no merging of them takes place, i.e., a fitting together of the chromosomes of the
two pronuclei within a single nucleic membrane does not happen. It is much more
accurate to say that the nucleic membranes of both pronuclei dissolve and the
chromosomes of both align themselves on the spindle apparatus at the equator.
Inferior derecha: The mitotic spindle divides the chromosomes that have just been
brought together into the two first cells of the embryo. This proceeding towards the
two-cell stage occurs on average between 22 and 26 hours after fertilization.
Video: The pronuclei are still approaching each other and thereby position
themselves so they are optimally oriented for the impending division (turning of the
The pronucleus membrane dissolves and thus the pronuclei are no longer
recognizable. Now the zygote has been created. Following a phase of great
movement the division occurs and a new nucleus forms in each cell (visible towards
the end in the right cell).
On the 4th day after insemination the outermost cells of the morula that are still
enclosed within the pellucid zone begin to join up with each other (so-called
compaction). An epithelial cellular layer forms, thicker towards the outside, and its
cells flatten out and become smaller. The cells contact one another by means of tight
junctions and gap junctions. A cavity forms in the interior of the blastocyst into which
fluid flows (the so-called blastocyst cavity). The two to four innermost cells of the
preceding morula develop into the so-called inner cell mass of the blastocyst. The
actual embryo will develop solely from these cells (embryoblast). These cells are
concentrated at one pole, the embryonic pole of the blastocyst. What has thus been
formed is an outer cell mass (the trophoblast), consisting of many flat cells, and the
embryoblast, formed from just a few rounded cells. The ratio between the number of
embryoblast cells to those making up the trophoblast amounts to roughly 1:10. From
the trophoblast the infantile part of the placenta and the fetal membranes will arise.
Around the end of the fifth day the embryo frees itself from the enveloping pellucid
zone. Through a series of expansion-contraction cycles the embryo bursts the
covering. This is supported by enzymes that dissolve the pellucid zone at the
abembryonic pole. The rhythmic expansions and contractions result in the embryo
bulging out of and emerging from the rigid envelope. This "first birth" is called
After the apposition of the free blastocyst at the uterine epithelium (4)
themicrovilli on the surface of the outermost trophoblast cells interact with the
epithelial cells of the uterus. In this stage the blastocyst can no longer be eliminated
by a simple flushing out. The adhesion of the blastocyst on the endometrium arises
through cell surface glycoproteins, the specific mechanisms of which, though, are not
yet well understood.
The cytotrophoblast, deep inside, consists in an inner irregular layer of ovoid, single-
nucleus cells. This is also where intensive mitotic activity takes place.In the periphery
the syncytiotrophoblast forms a syncytium, i.e., a multi-nucleic layer without cell
boundaries that arises from the fusion of cytotrophoblast cells. The
syncytiotrophoblast produces lytic enzymes and secretes factors that cause apoptosis
of the endometrial epithelial cells. The syncytiotrophoblast also crosses the basal
lamina and penetrates into the stroma that lies below, eroding the wall of capillaries.
With the implantation of the blastocyst in the endometrium the syncytiotrophoblast
develops quickly and will entirely surround the embryo as soon as it has completely
embedded itself in the endometrium.
In the middle of the 2nd week extracellular vacuoles appear in the ST. They join
together forming lacunae. Initially these lacunae are filled with tissue fluids and
uterine secretions. Following the erosion of the maternal capillaries, their blood fills
the lacunae that later develop further into intervillous spaces. The invasive growth of
the ST ceases in the zona compacta of the endometrium. At around the 13th day
theprimitive utero-placental circulatory system arises.
The development of the bilaminar germ disk 3 and the establishment of a feto-
maternal blood circulation system were given in detail in
the placenta andimplantation modules.
In the bilaminar primordium of the embryo (hypoblast or primary endoderm and
epiblast) one recognizes in the epithelium of the epiblast a fluid-filled space, the first
primordium of theamniotic cavity 5.
Ventrally, the roof of the still incompletely uncovered primary umbilical
vesicle 5 (previously the blastocyst cavity) is formed by the hypoblast.
Schematically, amniotic cavity and primary umbilical vesicle together formtwo
hemispheres with two layers (epi- and hypoblast) lying close to one another, thus
representing the first embryonic primordium.
However, only the epiblast is responsible for forming the embryo.
Thehypoblast develops into a part of the extraembryonic appendages.