Blood islands are structures around the developing

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

which lead to many different parts of the circulatory system. Blood islands arise external to the developing embryo on the

umbilical vesicle The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...

allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...

connecting stalk The connecting stalk, or body stalk is an embryonic structure that is formed by the third week of development and connects the embryo to its shell of trophoblasts. The connecting stalk is derived from the extraembryonic mesoderm. Initially it lie ...

and chorion. They are also known as Pander's islands or Wolff's islands, after Heinz Christian Pander or Caspar Friedrich Wolff.

Development

In humans, the formation of extraembryonic blood vessels starts at the beginning of the third week after fertilization. Vasculogenesis begins as mesodermal cells differentiate into hemangioblasts, which in turn differentiate into angioblasts. Clusters of angioblasts make up the blood islands. Within the blood islands, lumens begin to appear by the growth of intercellular clefts. The flattened cells at the periphery form the endothelium. Mesenchymal cells exterior to this form the muscular and connective tissue components of blood vessels. Roughly 3 weeks after fertilization,

red blood cells Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

, still with a

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

, and blood plasma develop outside the embryo. They develop from undifferentiated hemangioblasts in blood vessels in the walls of the

and chorion. In either case the mass thus formed projects from and is attached to the wall of the vessel. Later the cells on the surface round-up, giving the mass a mulberry-like appearance. Then the red blood cells break loose and are carried away in the plasma. Such free blood cells continue to divide. Blood islands will sprout endothelial projections and these projections will fuse. This forms a plexiform network of blood vessels. Over time, the network will continue to grow, and will eventually penetrate the embryo proper and fuse with the concurrently developing embryonic circulation. Consequently, some blood islands end up inside the embryo proper, while others remain in the annexes.

Sources

The blood islands and vessels ''outside'' of the embryo is initially the sole source of blood cells and plasma, beginning 3 weeks after fertilization. Blood formation inside the embryo proper begins around 5 weeks after fertilization in the liver, and at the twelfth week in the spleen,

red bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...

and thymus. Blood islands have been seen in the area vasculosa in the vitelline veins and arteries, and in the dorsal aorta.

References

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