Blood islands are structures around the developing

embryo An embryo ( ) is the initial stage of development for 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 sp ...

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

allantois The allantois ( ; : allantoides or allantoises) is one of the extraembryonic membranes arising from the yolk sac. It is a hollow sac-like structure filled with clear fluid that forms part of the developing conceptus in an amniote that helps the ...

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 li ...

and

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

. They are also known as Pander's islands or Wolff's islands, after

Heinz Christian Pander Heinz Christian Pander, also Christian Heinrich Pander ( Russian: Христиан Иванович Пандер; – ) was a Russian biologist and embryologist of Baltic German origin. Biography In 1817 he received his doctorate from the U ...

Caspar Friedrich Wolff Caspar Friedrich Wolff (18 January 1733 – 22 February 1794) was a German physiologist and embryologist who is widely regarded as one of the pioneers of modern embryology. Life Wolff was born in Berlin, Brandenburg. In 1759 he graduated as an ...

Development

In humans, the formation of extraembryonic blood vessels starts at the beginning of the third week after

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

Vasculogenesis Vasculogenesis is the process of blood vessel formation, occurring by a ''De novo synthesis, de novo'' production of endothelial cells. It is the first stage of the formation of the vascular network, closely followed by angiogenesis. Process ...

begins as

mesodermal The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embr ...

cells differentiate into

hemangioblasts Hemangioblasts are the multipotent precursor cells that can differentiate into both hematopoietic and endothelial cells. In the mouse embryo, the emergence of blood islands in the yolk sac at embryonic day 7 marks the onset of hematopoiesis. From ...

, 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 The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the r ...

. Mesenchymal cells exterior to this form the muscular and connective tissue components of blood vessels. Roughly 3 weeks after

red blood cells Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

, 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 Nucleu ...

, and

blood plasma Blood plasma is a light Amber (color), amber-colored liquid component of blood in which blood cells are absent, but which contains Blood protein, proteins and other constituents of whole blood in Suspension (chemistry), suspension. It makes up ...

develop outside the embryo. They develop from undifferentiated

in blood vessels in the walls of the umbilical vesicle,

and

. 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 The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

, and at the twelfth week in the

spleen The spleen (, from Ancient Greek '' σπλήν'', splḗn) is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The spleen plays important roles in reg ...

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 c ...

and

thymus The thymus (: thymuses or thymi) is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. The thymus ...

. Blood islands have been seen in the area vasculosa in the

vitelline veins The vitelline veins are veins that drain blood from the yolk sac and the gut tube during gestation. Path They run upward at first in front, and subsequently on either side of the intestinal canal. They unite on the ventral aspect of the canal ...

and

arteries An artery () is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in ...

, and in the

dorsal aorta The dorsal aortae are paired (left and right) embryological vessels which progress to form the descending aorta. The paired dorsal aortae arise from aortic arches that in turn arise from the aortic sac. The primary dorsal aorta is located dee ...

References

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