Human embryogenesis is the process of cell division and cellular
differentiation of the human embryo that occurs during early stages
of this development. To be exact, it spans from the moment when a
spermatozoon meets and fuses with an ovum which is called
fertilization to the end of the 8th week of development. The most
spectacular changes occur in this first two months during which the
developing baby acquires its main organs and just begins to be
recognizable as human. This process represents a masterpiece of
spatial and temporal control of gene expression. Most of the genes
governing human development also operate in other animals across a
wide range of species. From the ninth week until birth the term
Human fertilization is the union of a human egg and sperm, usually
occurring in the ampulla of the fallopian tube to form the zygote.
It constitutes the penetration of the oocyte (egg) by a sperm
succeeded by fusion of their genetic material. This genetic material
consists of the 23 chromosomes contained in the nucleus of the ovum
and 23 chromosomes from the nucleus of the sperm. The 46 chromosomes
undergo changes prior to the mitotic division which leads to the
formation of an embryo having two cells. The fertilized ovum thus
begins to divide into several cells i.e. it starts to undergo
cleavage. The two daughter cells are still surrounded by zona
As already mentioned a human develops from a single cell called a
zygote, which results from the fusion of two reproductive cells; an
ovum (egg) being fertilized by a single spermatozoon (sperm). The
cell is surrounded by a strong membrane of glycoproteins called the
zona pellucida (membrane derived from the ovum) which the successful
sperm has managed to penetrate.
The zygote undergoes cleavage, increasing the number of cells within
the zona pellucida. After the 8-cell stage, embryos undergo what is
called compactation, where the cells bind tightly to each other,
forming a compact sphere. After compactation, the embryo is in the
morula stage (16 cells). Cavitation occurs next, where the outermost
layer of cells - the trophoblast - secrete water into the morula. As
a consequence of this when the number of cells reaches 40 to 150, a
central, fluid-filled cavity (blastocoel) has been formed. The zona
pellucida begins to degenerate, allowing the embryo to increase its
volume. This stage in the developing embryo, reached after four to
six days, is the blastocyst (akin to the blastula stage), and lasts
approximately until the implantation in the uterus, and is referred
to as the preimplantation phase of development.
Each cell of the preimplantation embryo is totipotent. That is, each
cell has the potential to form all of the different cell types in
the developing embryo. This totipotency means that some cells can be
removed from the preimplantation embryo and the remaining cells will
compensate for their absence. This has allowed the development of a
technique known as preimplantation genetic diagnosis (PGD), whereby
a small number of cells from the preimplantation embryo created by
IVF, can be removed by biopsy and subjected to genetic diagnosis.
This allows embryos that are not affected by defined genetic
diseases to be selected and then transferred to the mother's uterus.
The blastocyst is characterized by a group of cells, called the
inner cell mass (also called embryoblast) and the mentioned
trophoblast (the outer cells).
The inner cell mass gives rise to the embryo proper, the amnion,
yolk sac and allantois, while the trophoblast will eventually form
the placenta. The blastocyst can be thought of as a ball of a
(mostly single) layer of trophoblast cells, with the inner cell mass
attached to this ball's inner wall. The embryo plus its membranes is
called the conceptus. By this stage the conceptus is in the uterus.
The zona pellucida ultimately disappears completely, allowing the
blastocyst to invade the endometrium, performing implantation.
The trophoblast then differentiates into two distinct layers: the
inner is the cytotrophoblast consisting of cuboidal cells that are
the source of dividing cells, and the outer is the
The syncytiotrophoblast implants the blastocyst in the endometrium
(innermost epithelial lining) of the uterus by forming finger-like
projections called chorionic villi that make their way into the
uterus, and spaces called lacunae that fill up with the mother's
blood. This is assisted by hydrolytic enzymes that erode the
epithelium. The syncytiotrophoblast also produces human chorionic
gonadotropin (hCG), a hormone that "notifies" the mother's body that
she is pregnant, preventing menstruation by sustaining the function
of the corpus luteum. The villi begin to branch, and contain blood
vessels of the fetus that allow gas exchange between mother and
Inner cell mass differentiation
While the syncytiotrophoblast starts to penetrate into the wall of
the uterus, the inner cell mass (embryoblast) also develops.
The embryoblast forms a bilaminar (two layered) embryo, composed of
the epiblast and the hypoblast. The epiblast is adjacent to the
trophoblast and made of columnar cells; the hypoblast is closest to
the blastocyst cavity, and made of cuboidal cells. The epiblast, now
called primitive ectoderm will perform gastrulation, approximately
at day 16 after fertilization. In this process, it gives rise to all
three germ layers of the embryo: ectoderm, mesoderm, and endoderm.
The hypoblast, or primitive endoderm, will give rise to
extraembryonic structures only, such as the lining of the primary
By separating from the trophoblast, the epiblast forms a new cavity,
the amniotic cavity. This is lined by the amnionic membrane, with
cells that come from the epiblast (called amnioblasts). Some
hypoblast cells migrate along the inner cytotrophoblast lining of
the blastocoel, secreting an extracellular matrix along the way.
These hypoblast cells and extracellular matrix are called Heuser's
membrane (or exocoelomic membrane), and the blastocoel is now called
the primary yolk sac (or exocoelomic cavity).
Cytotrophoblast cells and cells of Heuser's membrane continue
secreting extracellular matrix between them. This matrix is called
the extraembryonic reticulum. Cells of the epiblast migrate along
the outer edges of this reticulum and form the extraembryonic
mesoderm, which makes it difficult to maintain the extraembryonic
reticulum. Soon pockets form in the reticulum, which ultimately
coalesce to form the chorionic cavity or extraembryonic coelom.
Another layer of cells leaves the hypoblast and migrates along the
inside of the primary yolk sac. The primary yolk sac is pushed to
the opposite side of the embryo (the abembryonic pole), while a new
cavity forms, the secondary or definitive yolk sac. The remnants of
the primary yolk sac are called exocoelomic vesicles.