Transcript 07 - mesodermal

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.
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.
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 embryo’s age.
35
42 - 44
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.
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
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.
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.
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
-
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.
Bone; cartilage and ligaments