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Embryology
Dr.Maan Lec.5
By :Qasim M.Al-hussainy
Dr.Maan Alkhalisy
Embryology
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The Gut tube and th:e Body cavities
During the 3rd & 4th wk, the neural plate forms the neural tube dorsally,
while endoderm folds ventrally forming gut tube. In between t~se two
tubes, the mesoderm holds these .two tubes.
Lateral to this midline mesoderm, there is lateral plate component of the
mesoderm which will split into visceral (splanchnic) layer & parietal
(somatic) layer.
The visceral layer covers intimately the gut tube while the parietal layer
lines the ectoderm.
The parietal mesoderm, together with the overlying ectoderm, forms the
lateral body wall fold. This fold, from each side, will curved ventrally to
close the ventral body wall.
The space between visceral & parietal layer of lateral plate of mesoderm
is called primitive body cavity, which is a continuous cavity (since not
subdivided to thoracic & abdominal cavity).
With the folding of the fetus (from head to tail region) the amniotic cavity
will surround the fetus while the yolk sac (as lateral body walLfolded
ventrally) will be incorporated with body cavity of the embryo leaving
vitelline duct connecting the remaining part of yolk sac with the midgut.
Later on, the yolk sac disappear (between the 2nd & 3rd month of ·
gestation) & vitelline duct become narrower & remain one of the
structures of the umbilical cord.
The parietal layer of the lateral plate will be differentiated into
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peritoneal, pleural & pericardial parietal membranes lining the
outside of the corresponding cavities.
Dr.Maan Alkhalisy
Embryology
Paraxial
mesoderm
Parietal
mesoderm
Intermediate
A
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of
yolk sac
B
Amniotic cavity
Viseral
mesoderm
c
Yolk sac
Dorsal
mesentery
Viseral
mesoderm
Parietal
Connection
between
gut and yolk sac
D
Gut
r::
In a similar manner, some cells of the visceral layer of the lateral plate
form the visceral layer of the serous membrane & cover the lung, heart &
abdominal organs.
Visceral & parietal layers are continuous dorsally, forming the dorsal
mesentery which extends from caudal limit of foregut to the end of the
hindgut.
While ventral mesentery exists only from caudal foregut to the upper
portion of duodenum. This results from thining mesoderm of septum
transversum which is a block of mesoderm forms connective tissue in
the liver& central tendon of diaphragm.
Diaphragmatic & thoracic cavities:
The septum transversum is thick plate of mesoderm occupying the space
between thoracic cavity & the stalk of the yolk sac.
Septum transversum is derived from visceral (splanchnic) mesoderm
surrounding the heart, therefore; it assumed its position between primitive
thoracic & abdominal cavities.
Embcyology
Septum transve.tsum does- not separate abdominal& tftoracic cavities
~siJce· it leaves large openings wlllch.are peril:al".._
peritoneal canals on each side offoregu.L
W'hen the fun_g buds be_gfu to grow.\ tTley expand" caudalf.ywitlt tile
pericardio-petitooea\: canals: Pis a resuh of this expansion, pericardiop,eritom:al canalbecome smaDer 8i mesenchyme .o fflle ~ -wan
expands laterallyTdorsally.& ventrally, fomrlrrgpleura-pericard"w ·
This fold sbares in reduction of space lbat ce~ .between th(')facic &
peritoneal cavili.ti..,
Later on, sequence or events continue to occur u1lfonning P'~
perieardillA:_peritmeai t::aVilics..
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AlthoUjh:~al;,perieardful & _
p eritoneal cavities .are
fOrmed., yet no
complete separation between thor.aoic. .&. abdominal cavities occurs.
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Dr.Maan Alkhalisy
Lec.S
Embryology
Later on, the fold fuses with the mesentery of the esophagus & septum
transversum. Therefore; the connection
between abdominal & thoracic
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cavities will be disappeared.
Myoblasts originate from somite (c3, 4, 5) will form the muscular part of
the diaphragm.
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Therefore the diaphragm is derived from the following structures:
1. Septum transversum (which later form central tendon of diaphragm).
2. The two pleura-peritoneal membranes.
3. Muscular component from cerv_ical somite (C3-C5).
4. Mesentery of esophagus (which forms the future crura of the
diaphragm).
The nerve supply of the diaphragm is the phrenic nerve whose component
is C3, C4 (mainly) & C5 for sensory & motor innervations .
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