Mesodermal derivatives

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Mesodermal derivatives As the notochord neural
tube form, the intraembryonic mesoderm on each
side of them proliferates to form a thick,
longitudinal column of paraxial mesoderm . It
continuous with intermediate mesoderm which
gradually thins into lateral plate mesoderm. The
lateral mesoderm is continuous with the
extraembryonic mesoderm covering the yolk sac
amnion. Toward the end of the 3rd week ( days 20
), the paraxial mesoderm differentiates and
begins to divide into paired cuboidal bodies (
somites ). So, somites are compact aggregation of
mesenchymal cells.
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The first pair of somites appear in the future
occipital region of the embryo at the end of the
3rd week ( days 20 ) a short distance caudal to
the cranial end of the notochord. So, they
develop craniocaudally .



About 38 pairs of somites are present at days 30.
By the end of the 5th week 42 to 44 pairs of
somites are present. So. The somite period of
human embryo development is days 20 30. The
somites form distinct surface elevations on the
embryo. There are 4 occipital , 8 cervical , 12
thoracic , 5 lumbar , 5 sacral and 8- 10
coccygeal. An slitlike cavity ( myocoele )
appears within each somite.
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The first occipital and the last 5-6 coccygeal
somites disappear. The remaining
divide into 1- Ventromedial part called
sclerotome. It gives rise to the bones cartilages
and ligaments of the vertebral column
ribs. So, the somites give rise to most
of the axial skeleton. 2- Middle part called
myotome. It gives rise to skeletal muscles of the
chest and abdomen. 3- Dorsolateral part
called dermatome. It gives rise to dermis and
subcutaneous tissue of the
skin. The somites are used to determine an
embryos age.
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42 - 44
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Intraembryonic Coelom The embryonic body cavity
appears as isolated coelomic spaces in the
lateral mesoderm and cardiogenic mesoderm. These
spaces coalesce to form a single horseshoe-
shaped cavity ( intraembryonic coelom ) which
divides the lateral plate mesoderm into 2 layers.
1- A somatic or parietal layer continuous with
the extraembryonic mesoderm
covering the amnion. 2- A splanchnic or
visceral layer continuous with the extraembryonic
mesoderm covering the yolk sac.
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The somatic mesoderm and overlying embryonic
ectoderm form the embryonic body wall or
somatopleure. The splanchnic ( visceral
)mesoderm and the underlying embryonic endoderm
form the embryonic gut or splanchnopleure. During
the 2nd month, the intraembryonic coelom is
divided into 3 body cavities. 1- Pericardial
2- Pleural
3- Peritoneal
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Early Development of Cardiovascular
System During the 3rd week the heart tubes
develops in the visceral ( splanchnic ) mesoderm
between the pericardial part of I.E.C. and the
roof of the secondary yolk sac ( cardiogenic area
). So, the heart tubes are paired longitudinal
endothelial lined channels. These tubes fused to
form a primordial heart tube. The tubular heart
joins with blood vessels in the embryo
connecting stalk chorion and yolk sac to form a
primordial cardiovascular system. By the 3rd
week the blood is circulating and the heart
begins to beat on the 22 day ( 5th week after
LNMP ). The cardiovascular system is the 1st
organ to reach a functional state.
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Heart beat can be detected ultrasonographically
using Doppler technology during the 5th week (
the 7th week after LNMP ). At the end of the 2nd
week, embryonic nutrition is obtained from the
maternal blood by diffusion through the
extraembryonic coelom and yolk sac. During the
3rd week a primordial uteroplacental circulation
develops.
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• Vasculogenesis Angiogenesis
• At the beginning of the 3rd week blood vessels
  formation first appear in the extraembryonic
  mesoderm of the yolk sac connecting stalk and
  chorion. They develop within the embryo about 2
  days later.
• A- Vasculogenesis ( blood vessels)
• 1- Mesenchymal cells differentiate into
  endothelial cell precursors,
  angioblasts ( vessel-forming cells )
  which aggregate to form blood islands.
• 2- Small cavities appear within the
  blood islands.
• 3- Angioblastic flatten and arrange
  themselves around the cavities to form
  endothelial cells of blood vessels.
• 4- These endothelium lined cavities soon
  fuse to form networks of
  endothelial channels

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5- Angiogenesis The formed vessels sprout into
adjacent areas by endothelial budding and fuse
with other vessels. Hematogenesis Blood cells
formation occurs on the yolk sac allantois at
the end of the 3rd week and in the embryo in the
5th week. The hemangioblasts ( blood cells ) are
formed from the endothelial cells of the blood
vessels. Blood cells formation occur first in
the mesenchyme of the embryonic liver then the
spleen bone marrow and lymph nodes. Fetal
adult erythrocytes are derived hemangioblasts (
different hematopoietic precursors or progenitor
cells ). N.B. the mesenchymal cells surroundimg
the vessels differentiate into the muscular
connective tissue elements of the vessels.
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Bone cartilage and ligaments