Mesodermal derivativesAs 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 CoelomThe 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