The role of the thymus in development of lympho-hemopoietic tissues. The effect ofthymectomy on development of blood cells, bone-marrow, spleen, and lymph nodes

The origin and development of the immune system with a view to stem cell therapy

Careful study of the phylogeny and ontogeny of the three components of the immune system reveals that the macrophage, lymphatic, and hematopoietic systems originate independently of each other. Chronologically, the most ancient is the macrophage system, which arises in the coelomic cavity as mesenchymal ameboid cells having the properties to recognize self from non-self and to ingest foreign particles. The lymphatic system later develops from the endoderm of pharyngeal pouches, where the thymic anlage differentiates. The lymphocytes that originate here seed all lymphatic organs and retain the ability to divide and thereby form multiple colonies (lymphatic nodules) in the respiratory and digestive tract; further diversification of lymphocytes follows after confrontation with antigens. The last component of the immune system to appear is the hematopoietic system, which originates from the splanchnic mesoderm of the yolk sac as hematogenic tissue, containing hemangioblasts. The hematogenic tissue remains attached to the outer wall of the vitelline vessels, which provides an efficient mechanism for introducing the hematogenic tissue into the embryo. In an appropriate microenvironment, the hemangioblasts give rise to sinusoidal endothelium and to hemocytoblasts - the bone marrow stem cells for erythrocytes, myeloid cells, and megakaryocytes. The facts and opinions presented in this article are not in agreement with the currently accepted dogma that a common "hematolymphatic stem cell" localized in the marrow generates all of the cellular components of blood and the immune system.

Quantitative studies of lymphocytes and other cell populations in the bone marrow of neonatally thymectomized C3H mice

The effects of neonatal thymectomy on the development of the lymphoid, erythroid and granulocytic cell populations in mouse bone marrow have been assessed by quantitative techniques. The numbers per unit volume of bone marrow of 17 cell types were determined in neonatally thymectomized and sham thymectomized C3H mice at two, four and eight weeks of age, and compared with those of normal C3H mice. After neonatal thymectomy the numbers of small lymphocytes, large and medium-sized lymphoid cells, and erythroid cells reached normal levels at two weeks but fell progressively to 18%, 22% and 42% of normal, respectively, by eight weeks. In sham thymectomized mice these cell populations did not differ significantly from normal. Immature and mature granulocytes were elevated in numbers two weeks after either neonatal thymectomy or sham thymectomy, suggesting a transient non-specific stimulation of granulocytopoiesis. During continuous infusion of 3H-thymidine for ten days in neonatally thymectomized mice aged four weeks and eight weeks many bone marrow small lymphocytes remained unlabeled. The results demonstrate that early postnatal development of bone marrow lymphoid and erythroid cells proceeds normally in the absence of the thymus, in accord with the concept of the bone marrow as a primary site of lymphocyte production and differentiation. In addition, some slowly-renewing small lymphocytes in bone marrow appear to be thymus-independent cells.