Embryonic development of blood cell.

There are three ill-defined phases of hemopoiesis during intrauterine development, and in all phases the initiation of hemopoiesis is the same. It consists of a differentiation of mesenchymal cells into free cells whose cytoplasm acquires a definite basophil character. These cells, often termed, hemocytoblasts, proliferate actively. Such a process occurs initially within the yolk sac during the third week of development. The hemocytoblasts become transformed into primitive erythroblasts by the elaboration and accumulation of hemoglobin within their cytoplasm. They differ from bone marrow-derived erythrocytes in that they are larger and retain their nuclei.

During the second phase, hemopoiesis occurs within the liver and spleen. It commences in the liver at about six weeks and continues actively until the middle of fetal life. Hemocytoblasts proliferate and differentiate into nucleated and non-nucleated red blood cells, leukocytes, and megakaryocytes. The formation of nucleated red blood cells gradually diminishes prior to the middle of fetal life and hemopoietic activity in the liver normally disappears at the time of birth. Hemopoiesis occurs within the spleen between the second and eight months. Erythropoiesis ceases at or just after birth, although lymphocytes continue to be produced within the spleen postnatally. Additionally, during the second phase, the thymus commences to produce lymphocytes from the second month on.

The third phase involves the bone marrow and the lymph nodes. Myeloid tissue of bone marrow appears in the third fetal month when the cartilaginous primordia of the bones become invaded by mesenchyme during the process of ossification. Again mesenchymal cells round off and become hemocytoblasts, which give rise to erythroblasts, myelocytes, monocytes, lymphocytes, and megakaryocytes. With the appearance of bone marrow, production of nucleated red blood cells ceases. Lymph nodes develop relatively late during fetal life, and production of myeloid elements is never a marked feature within them. However, they continue to function throughout life, like the spleen, in the production of lymphocytes.

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