Implantation is the very early stage of pregnancy at which the embryo adheres to
the wall of the uterus. At this stage of prenatal development, the embryo is a
blastocyst. It is by this adhesion that the fetus receives oxygen and nutrients
from the mother to be able to grow.
In humans, implantation of a fertilized ovum is most likely to occur about 9 days
after ovulation, ranging between 6 to 12 days.
The amnion is a membrane building the amniotic sac that surrounds and
protects an embryo. The primary role of this is the protection of the embryo for
its development. It stems from the extraembryonic somatic mesoderm on the
outer side and the extraembryonic ectoderm on the inner side.
When first formed, the amnion is in contact with the body of the embryo, but
about the fourth or fifth week fluid (liquor amnii) begins to accumulate within it.
This fluid increases in quantity and causes the amnion to expand and ultimately
to adhere to the inner surface of the chorion, so that the extra-embryonic part of
the coelom is obliterated. The liquor amnii increases in quantity up to the sixth
or seventh month of pregnancy, after which it diminishes somewhat; at the end
of pregnancy it amounts to about 1 liter.
The amniotic fluid allows the free movements of the fetus during the later stages
of pregnancy, and also protects it by diminishing the risk of injury from without.
It contains less than two percent solids, consisting of urea and other extractives,
inorganic salts, a small amount of protein, and frequently a trace of sugar.
The yolk sac is a membranous sac attached to an embryo, providing early
nourishment in the form of yolk. It functions as the developmental circulatory
system of the human embryo, before internal circulation begins.
It is the first element seen in the gestational sac during pregnancy, usually at 5
weeks gestation. The yolk sac is situated on the ventral aspect of the embryo; it is
lined by extra-embryonic endoderm, outside of which is a layer of extra-
embryonic mesenchyme, derived from the mesoderm. Allantois:
It is a part of a developing amniote's conceptus (which consists of all embryonic
and extra-embryonic tissues). It helps the embryo exchange gases and handle
liquid waste. This sac-like structure is primarily involved in nutrition and
excretion, and is webbed with blood vessels. The function of the allantois is to
collect liquid waste from the embryo, as well as to exchange gases used by the
In placental mammals, the allantois is part of and forms an axis for the
development of the umbilical cord. The human allantois is an endodermal
evagination of the developing hindgut, which becomes surrounded by the
mesodermal connecting stalk. The connecting stalk forms the umbilical
vasculature. The fetal bladder is connected to the allantois via the urachus, which
removes nitrogenous waste from the fetal bladder. The allantois is vestigial and
may regress; yet the homologous blood vessels persist as the umbilical arteries
and veins connecting the embryo with the placenta.
The placenta is an organ that connects the developing fetus to the uterine wall to
allow nutrient uptake, waste elimination, and gas exchange via the mother's
blood supply. The umbilical cord inserts into the chorionic plate (has an
eccentric attachment). Vessels branch out over the surface of the placenta and
further divide to form a network covered by a thin layer of cells. This results in
the formation of villous tree structures. On the maternal side, these villous tree
structures are grouped into lobules called cotyledons. In humans, the placenta
usually has a disc shape, but size varies vastly between different mammalian
The placenta begins to develop upon implantation of the blastocyst into the
maternal endometrium. The outer layer of the blastocyst becomes the
trophoblast, which forms the outer layer of the placenta. This outer layer is
divided into two further layers: the underlying cytotrophoblast layer and the
overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated
continuous cell layer that covers the surface of the placenta. It forms as a result
of differentiation and fusion of the underlying cytotrophoblast cells, a process
that continues throughout placental development. The syncytiotrophoblast
(otherwise known as syncytium), thereby contributes to the barrier function of
The placenta grows throughout pregnancy. Development of the maternal blood
supply to the placenta is complete by the end of the first trimester of pregnancy. Trophoblast:
Trophoblasts are cells forming the outer layer of a blastocyst, which provide
nutrients to the embryo and develop into a large part of the placenta. They are
formed during the first stage of pregnancy and are the first cells to differentiate
from the fertilized egg. This layer of trophoblasts is also collectively referred to
as "the trophoblast", or, after gastrulation, the trophectoderm, as it is then
contiguous with the ectoderm of the embryo.
Trophoblasts are specialized cells of the placenta that play an important role in
embryo implantation and interaction with the decidualised maternal uterus. The
core of placental villi contain mesenchymal cells and placental blood vessels that
are directly connected to the fetal circulation via the umbilical cord. This core is
surrounded by two layers of trophoblast; a single layer of mononuclear
cytotrophoblast that fuse together to form the overlying multinucleated