EARLY EMBRYONIC DEVELOPMENT
In humans, as in most other primates, fertilization takes place in the distal part of the uterine tube, near its fimbriated end, about 12 to 24 hours after ovulation. The fertilized ovum, or zygote, is transported to the uterus by rhythmic contractions of the tube, aided by the action of the cilia of the epithelium. During this passage down the uterine tube, which takes about 4 days, the zygote executes a number of cell divisions and, on reaching the uterus, consists of a clump of blastomeres, the morula, which has not increased appreciably in size from the zygote.
After entering the uterus, a blastocyst
is formed as a fluid-filled cavity develops in the enlarging morula. The
wall of the blastocyst consists of a single layer of flattened cells, the trophoblast,
and an eccentrically placed mass of cells, the inner cell mass or embryoblast.
The trophoblast of the blastocyst attaches to the uterine epithelium to begin
the process of implantation into the endometrium during the second week. Its
cells soon differentiate into an inner cytotrophoblast and an outer syncytiotrophoblast
(syntrophoblast) and will eventually form the outer embryonic membrane (the
chorion) and the fetal portion of the placenta. The embryo develops from
the inner cell mass, which also contributes to the formation of the amnion and
umbilical vesicle.
Soon after implantation, the cells
of the inner mass differentiate into two layers: an inner hypoblast (future
endoderm), and an outer epiblast (future ectoderm). Together, these two
layers form the embryonic disc. At the same time, a space appears within the
inner cell mass, the primitive amniotic cavity, lined by the cells that
are continuous with the epiblast cells of the embryonic disc. The cavity of the
blastocyst becomes lined by hypoblast cells from the embryonic disc migrating
along the trophoblast to form the primitive umbilical vesicle (yolk
sac). Cells of the primitive umbilical vesicle also give rise to extraembryonic
mesoderm, loose connective tissue that separates the umbilical vesicle from
the trophoblast. The embryo proper now is a disc made of two layers of cells
and from which all the intraembryonic tissues will be derived: the ectoderm,
consisting of a simple columnar epithelium that is also the floor of the
amnion, and the cuboidal endodermal cells that form the roof of the yolk
sac. Small cavities appear in the extraembryonic mesoderm and coalesce to form
the extraembryonic coelom (chorionic cavity), except for a mesodermal
stalk that connects the amnion to the trophoblast. This remaining mesodermal
stalk later forms the connecting stalk (future umbilical cord).
With the formation of the
extraembryonic coelom, the extraembryonic mesoderm becomes separated into two
thin layers, completing formation of the three extraembryonic membranes. The
trophoblast, with its inner coating of extraembryonic mesoderm, is now called
the chorion. The primitive amnion with its outer coating of
extraembryonic mesoderm is the amnion proper; and the primitive umbilical
vesicle (yolk sac) becomes the umbilical vesicle proper with its outer coating of mesoderm.
At the future caudal end of the
embryonic disc, ectodermal cells form in the midline a thickened ectodermal primitive
streak with a primitive node at the cephalic end of the streak. The
primitive streak and node give rise to a third germ layer, the intraembryonic
mesoderm, situated between the ectoderm and endoderm. This creation of
intraembryonic mesoderm is through the process of gastrulation, occurring
during the third week. The mesodermal cells migrate laterally and cranially
until ectoderm and endoderm are separated from each other by intraembryonic
mesoderm, except at the cephalic oropharyngeal membrane and the caudal cloacal
membrane, where ectoderm remains in contact with endoderm. The primitive
streak mesoderm cranial to oropharyngeal membrane in the midline is the cardiogenic mesoderm. Laterally, all along the margin of the embryonic
disc, the intraembryonic mesoderm is continuous with the extraembryonic
mesoderm. The embryo, now trilaminar, becomes more elongated and pear shaped
when viewed from its dorsal (ectodermal) or ventral (endodermal) side. The
embryo is attached at its narrow caudal end to the chorion by the connecting
stalk.
The ovulation age of the embryo at
this stage of development is about 20 days, and its length is almost 1.5 mm. The time is now rapidly approaching when
simple diffusion of oxygen and nutrients cannot adequately provide for the
greatly increasing metabolic needs of the embryo, and a functional circulatory system becomes necessary.
