Embryogenesis
Week Three
From now until the end of the eighth week is the embryonic period. At the beginning of the
third week the embryonic disc becomes differentiated by the formation of the prochordal plate
cranially on the ventral surface of the embryonic disc, and the primitive streak caudally on the
dorsal aspect of the embryonic disc. The three germ layers of the embryo have been established
(gastrulation). An important organizer, the notochord, develops between the ectoderm and
endoderm extending from the primitive node cranially to the prechordal plate. As the notochord
develops and extends cranially, the primitive streak shrinks caudally. Caudal to the primitive
streak the ectoderm and endoderm are fused. This is the site of the future anus. The embryonic
disc enters the third week as an oval but slowly becomes narrower and longer by the end of the
third week. The mesoderm lies between the ectoderm and endoderm except in the midline
cranially where the ectoderm and endoderm are fused at the prochordal plate; where the
notochord lies between the ectoderm and endoderm; and caudally at the cloacal membrane. The
notochord induces the overlying ectoderm to form the neural plate.
The notochord
The notochord is the structure around which the vertebral column forms. As each vertebral body
forms the notochord is replaced. It persists in the spaces between adjacent vertebrae forming the
intervertebral discs. During development the notochord develops from the notochordal process
which passes through a phase in which it forms a hollow rod which subsequently opens to both
the amniotic and yolk sac cavities, followed by reversion to a solid rod.
Neurulation
The notochord induces changes in the overlying ectoderm resulting in the formation of the neural
plate. The neural plate ectoderm forms the central nervous system. By about day 18 the neural
plate extends from the primitive node to the oropharyngeal membrane (future mouth). At this
time a groove develops in the midline running from caudal to cranial, causing the neural plate to
develop a fold on each side of the midline, the neural folds. The neural folds move together to
form the neural tube leaving the cranial and caudal ends open. As the folds move together to
form the tube, the tube sinks below the ectoderm. At this stage some of the cells which were on
the edge of each fold separate from the neural tube as the neural crest. The openings in the tube,
the cranial and caudal neuropores remain open until the fourth week when the cranial neuropore
closes followed by the caudal neuropore. The cells of the neural crest migrate laterally to form
various tissues including the peripheral ganglia, the adrenal medulla, the meninges covering the
brain (pia and arachnoid), and skeletal and muscular components of the head.
Somites
The body of the developing embryo forms structural units, the somites. The mesoderm lying
lateral to the neural tube and notochord is formed into these units, beginning at the head end and
moving caudally. Each somite will have associated with it a strip of ectoderm, a block of
mesoderm and eventually a spinal nerve. The blood vessels will come to lie between adjacent
blocks. The formation of somites starts by the end of the third week close to where the future
occipital region will be. The process continues caudally so that by the end of the fifth week there
are about 43 pairs of somites.
Body cavities
In the adult there are four separate body cavities lined by a simple squamous epithelium, the
mesothelium. These cavities are invaginated by organs. The cavity contains a small amount of
fluid to lubricate the moving surfaces and reduce friction. The cavities are, the two pleural
cavities, the pericardium (for the heart) and the peritoneal cavity (abdomen). Between the second
and third weeks of development a space expands within the mesoderm running up both sides and
joining in the middle at the cranial end of the embryo to form a U shape. As development
proceeds the bend in the U is folded underneath and comes to be related to the heart. The two
limbs of the U form the pleural cavities at the cranial end, and fuse together to form the peritoneal
cavity caudally.
The cardiovascular system
By the beginning of the third week, blood vessel formation begins in the tissue surrounding the
yolk sac. These primitive vessels are at first not connected to each other, but do start to make
primitive blood cells. In the third week the heart and large vessels form in the cardiogenic area
from mesoderm. The first step in formation of the cardiovascular system is the development of
two simple tubes lying on either side of the midline in the mesoderm. These tubes are connected
to: umbilical arteries connected to vessels in the chorion (primitive placenta): vitelline arteries to
the yolk sac; dorsal intersegmental arteries to the body of the embryo. Umbilical veins, vitelline
veins and cardinal veins return the blood to the circulation. At 21 days the two longitudinal tubes
(the dorsal aortae) fuse at their cranial ends to form the primitive heart. Heart beats can be
detected by ultrasound by the fifth week.
Return to MENU