Development of Haemopoiesis in animals
The first site of blood formation is in the area vasculosa of the wall of the yolk sac. Here between the extraembryonic splanchnic-mesoderm and endoderm the blood islands of Pander develop. These are solid masses of mesenchymal cells united by cellular stands to form a network.
As they become differentiated the outer cells join to form the tubular wall of blood vessels, while the inner cells are separated by the secretion of plasma between them and become primitive blood cells, which multiply rapidly by mitosis. Formation of primitive blood cells also occurs for some time by the detachment of cells from the endothelial wall.
These cells die out during embryonic life and do not form definitive erythrocytes. Other primitive blood cells remain as colourless proliferating hemocytoblast. Blood formation is soon transferred from the yolk sac to the body of the embryo-where it is continued by the body mesenchyme, endothelium liver, spleen, thymus, lymph nodes and bone marrow. The haemocytoblasts or stem cells are large amoeboid cells with large pale spherical nucleus and a basophilic cytoplasm. All the definitive blood cells develop from this stem or blast cell.
According to monophyletic theory, a common stem cell gives rise to all the types of blood elements, both red and white. The polyphyletic theory holds that there are two stem cells-one stem cell for erythrocytes and granular leukocytes and the other for non-granular leucocytes.
The hemocytoblast differentiates by the gradual elaboration of haemoglobin consequent to transformation of basophilic cytoplasm to the acidophilic cytoplasm, nuclear condensation and reduction in size of the cell. The successive stages are proerythroblast, erythroblasts, and normoblast. The normoblast (which have a size about equal to the erythrocytes, a small dense nucleus and an eosinophilic cytoplasm) lose their nuclei by extrusion and become erythrocytes.
The haemocytoblast also gives origin to the differentiating granular leucocyte termed myelocytes. These elaborate within their cytoplasm specific kinds of granules, giving rise to neutrophilic, acidophilic and basophilic myelocytes. After a number of mitotic divisions, these myelocytes lose the capacity for division, the nucleus becomes indented and these cells enter the blood stream, as the different types or granular leucocyte.
The lymphocytes are produced in the lymphatic tissue by differentiation of haemocytoblasts into large lymphocytes. The large lymphocytes give rise to small lymphocytes.
As believed by some, the monocytes arise by the transformation of lymphocytes in the sinusoids of spleen, liver and bone marrow. Others believed that they are derived from the lining cells of sinusoids or from monoblasts in the bone marrow.
The megakaryocytes are giant cells derived from haemocytoblast and occur in the embryo, in the hemopoietic organs (red marrow, liver spleen) and their processes extend through the walls of sinuses and by constriction and segmentation of these, platelets are formed.