Download NEURAL TUBE

• 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.