The Thymic Microenvironment

All the mature cellular elements of the blood (erythrocytes, granulocytes, platelets, monocytes, and lymphocytes) require the continuous replacement of their respective populations throughout the life-span of the animal. Contemporary concepts point to the pluripotential hemopoietic stem cell as the ancestral cell for all the various differentiating populations. Recently, a model of the process of hemopoiesis from the stem cell to mature progeny has been described as consisting of three overlapping pyramidal-shaped hierarchies of cell compartments (24). Using this model of hemopoietic differentiation, which has been extensively investigated and described for myeloid elements, we wish to propose a parallel pyramidal system of differentiation for the T-lymphocyte series (Figure 19.1).