Human prenatal development can be divided into 3 stages: pre-embryonic,
embryonic, and fetal development. Fertilization usually occurs in the
ampulla of fallopian tube. The fertilized egg, referred to as the conceptus at this stage,
immediately travels toward the uterus. During the journey,
it undergoes several mitotic cell divisions, producing daughter cells, called blastomeres.
This process is known as cleavage, because the cells divide without growing in volume.
After about 3 days, the conceptus arrives in the uterus:
it now contains about 16 cells and is called a morula. Cells of the morula are totipotent,
they are capable of differentiating into all cell types, both embryonic and extra-embryonic.
The morula continues to divide while floating freely in the uterus for several more days.
During this time, it consumes nutrients stored in the egg cytoplasm, and “uterine
milk” secreted by the endometrium. When there are about 100 cells,
the cells start to arrange themselves around a fluid-filled cavity, forming a blastocyst.
The blastocyst consists of a pluripotent inner cell mass, called the embryoblast,
destined to be the embryo; and an outer shell, called the trophoblast, which nourishes the
embryo. The trophoblast later becomes the chorion - the fetal portion of the placenta.
The trophoblast secretes an enzyme that dissolves the membrane surrounding the conceptus, enabling
it to “hatch”, ready to be implanted. At the end of the first week,
the blastocyst becomes attached to the endometrium. At the site of contact,
superficial cells of the trophoblast fuse together to form the syncytiotrophoblast, which grows into
the endometrium and derives nutrition from it. The endometrium responds by growing over
and eventually enveloping the blastocyst. The syncytiotrophoblast secretes the hormone
named “human chorionic gonadotropin”, HCG, to instruct the corpus luteum to continue the
production of progesterone. Progesterone is the hormone that maintains pregnancy.
Among its many functions, progesterone stimulates the growth of nutrient-rich
decidual cells to feed the early embryo. Around the middle of week 2, the embryoblast
starts to form a two-layered embryonic disc - hypoblast and epiblast, at the mid-plane
of blastocyst, with a cavity on either side. This bilaminar disc sets the dorsal/ventral axis of the
embryo, with the epiblast being the dorsal side. By the end of second week, the 2 layers grow
around the cavity on their respective side and form the yolk sac and amnion, respectively.
The yolk sac provides nutrients absorbed from the trophoblast, as well as gas exchange for
the embryo during weeks 2 and 3, before the placenta takes over these functions at week
4. It is also the first site of embryonic blood cell production, before the liver.
In week 3, the two-layered disc transforms into 3 germ layers,
through the process of gastrulation, during which the cells lose their pluripotency and become
committed to a smaller number of cell types. Gastrulation begins with the emergence of the
primitive streak on epiblast surface. This event sets the cranial/caudal,
or head/tail axis, of the embryo. A so-called primitive node at the cranial
end of primitive streak produces signaling factors that control the movements of cells.
Epiblast cells migrate toward and through the primitive streak into the embryo. The
first set of cells displaces the hypoblast and forms the endoderm. The second set of cells
fills in the space between the endoderm and epiblast, creating the mesoderm. The
remaining epiblast cells form the ectoderm. Each of the 3 germ layers differentiates to
become different organs and tissues. All organ systems are present in their primitive form by the
end of the embryonic period, but most of them are not yet functional, or only partially functional.
The yolk sac gives rise to the allantois, a primitive urinary
bladder and part of the umbilical cord. The amnion grows to surround the embryo
by week 4. It is filled with amniotic fluid which protects the embryo from trauma and temperature
fluctuations. The fluid also allows the fetus to move freely and prevents adhesion of body parts.
The yolk sac, amnion, allantois and chorion are the 4 embryonic membranes.