Chapter 113. Cardiovascular Circulatory and Lymphatic Systems: Early Development - Review of Medical Embryology Book

Introduction: the blood and cardiovascular system are derived from mesoderm. They both develop at the same time, beginning about the middle of week 3, when the embryo can no longer satisfy its nutritional needs by diffusion alone. Their development is correlated with the absence of a significant amount of nutritive yolk in the oocyte and yolk sac. The first blood and vascular elements appear near the exterior of the embryo in the mesenchyme lining the yolk sac. This extraembryonic network rapidly blends with the intraembryonic circulation which appears a little later, beginning week 4. As the yolk sac regresses, near the end of month 2, so do the blood-forming islands, and the hematopoietic function is then assumed by the liver
The primitive cardiovascular system and blood islands

EXTRAEMBRYONIC BLOOD VESSELS: angiogenesis begins in the extraembryonic mesoderm as clusters of mesenchymal cells which differentiate in the chorion, the connecting stalk, and the yolk sac wall, toward days 13 to 15

These angiogenetic clusters, made up of mesenchyme angioblasts, give rise to blood and vascular-forming structures called the blood islands of Wolff and Pander (about 2 days later) when a lumen is formed in the clusters by the appearance and confluence of intercellular clefts
Cells on the periphery of an island flatten and form endothelial cells which outline the vessels. Mesenchymal cells surrounding the primitive endothelial vessels differentiate into the muscular and connective tissue elements of the vessels
The central cells of the islands become free and give rise to the blood cells. These early "parent" cells are called hemocytoblasts and represent the origin of 3 lines of blood cells. However, at this stage, they give rise essentially to nucleated red cells called megaloblasts

Blood formation does not begin in the embryo until week It is seen first in the various mesenchymal areas, namely, in the liver and later in the spleen, the bone marrow, and the lymph nodes

The blood islands approach each other by sprouting endothelial cells, fuse, and form a plexiform network. The latter is transformed into small blood vessels under the influence of hemodynamic factors

With continuous budding, the extraembryonic vessels gradually penetrate the embryo proper

INTRAEMBRYONIC BLOOD VESSELS develop independently from angiogenetic cell clusters that appear in the splanchnic mesoderm layer of the late presomite embryo

The clusters, at first, are seen on the lateral sides of the embryo, but soon spread rapidly in a cephalic direction

In time, they acquire a lumen, unite, and form a plexus of small blood vessels which gradually becomes horseshoe-shaped
In addition, other clusters of angiogenetic cells appear bilaterally, parallel and close to the midline of the embryonic shield

These also acquire a lumen and form a pair of longitudinal vessels, the dorsal aortae. These, at a later date, connect up with the horseshoe-shaped plexus (the latter becomes the heart tube)

The intraembryonic coelomic cavity found over the central portion of the horseshoe-shaped plexus later develops into the pericardial cavity. Thus, the pericardial cavity lies anterior to the prochordal plate