Early development and implantation of the embryo.
A. The zygote stage begins upon fertilization
of the secondary oocyte by the sperm. The zygote contains both pro-nuclei and
is contained within the zona pellucida, until the blastocyst stage.
B. The morula stage. Following compaction and
symmetrical cleavage divisions of the blastomeres (the cells of the early
embryo), the embryo contains 8 (early morula) to 32 cells (morula). The inner
cells will give rise to the inner cell mass, whereas the outer cells will give
rise to the trophoblast, which forms a cavity called the blastocoele
cavity.
C. The blastocyst stage. The developing
embryo is defined as a blastocyst from the appearance of the blastocoele
cavity, and now contains two cell populations- the surrounding outer
trophoblast cells, and the inner cell mass cells, located at one side of the
inner cavity. The portion of the trophoblast nearest to the inner cell mass is
called the polar trophoblast (embryonic pole) and the portion of the
trophoblast farthest and opposing the inner cell mass is called the mural
trophoblast (abembryonic pole). In the blastocyst stage, the embryo hatches
from the zona pellucida layer and is now able to begin implantation into the
maternal endometrium.
D. The hatched human embryo makes contact
with and implants into the maternal endometrium at the polar trophectoderm.
Meanwhile, the inner cell mass differentiates to two layers collectively called
the bilaminar embryonic disc. The layer closest to the trophoblast is called
the epiblast, and will give rise to the embryo proper, and also contribute to
several extraembryonic tissues. The layer bordering the blastocoele cavity is
the hypoblast (primitive endoderm), which is a transient structure that
contributes to the development of the extraembryonic
mesoderm and yolk sac and plays a key role in signaling to
establish axial patterning in the embryo itself.
E. The embryo implants into the maternal endometrium
as the trophoblast differentiates to the cytotrophoblast and
syncytiotrophoblast layers. At the stage shown here, the embryo is fully
implanted and lacunae filled with maternal blood form in the
syncytiotrophoblast, enabling exchange of nutrients. The three embryonic
cavities (the amniotic cavity, the yolk sac cavity and the blatocoele cavity)
have already formed, and the bilaminar embryo is situated between the amnion
and the yolk sac.
by: Fuensanta
Hernández Salmerón.
