PREVALENCE OF CD 8 / CD 4 RATIO IN THE FETAL THYMIC PARENCHYME IN DOWN ’ S SYNDROME

In the thymic cortex, massive numbers of densely packed small thymocytes (thymic lymphocytes) predominate, occupying the interstices of the epithelial reticulum, which in histological sections they largely obscure, and forming about 90 % of the total weight of the thymus. A distinct subcapsular zone is present, housing the thymic stem cells, prothymocytes and lymphoblasts undergoing mitotic division. The first stem cells to enter the thymus in the embryo come from the yolk sac and liver during their haemopoietic phases, possibly, as in birds, being attracted by thymic chemotactic substances. During later periods it is probable that all thymic lymphocytes originate in the bone marrow, or at least have sojourned there, before passing in the bloodstream to the thymus.The cortex has two rather ill-defined zones: an outer cortex with a framework of Types 1–3 epitheliocytes and a deep cortex where Type 4 cells occur. Thymocytes undergo mitosis in all cortical zones as the clones of differentiating T cells mature, gradually moving deeper in the cortex. In rodents, cell cycling times of 8 hours have been recorded in the outer cortex, but no estimates exist for the human thymus. The appropriate conditions for the proliferation and differentiation of thymocytes appear to be produced by their close proximity to neighbouring epitheliocytes (9). Although the nature of these interactions is not clear, it may involve the release from the epitheliocytes of soluble mitogenic and differentiation factors as well as induction of changes through intercellular contact. During this process, thymocytes differentiate along the T-cell line, acquiring the CD3 + marker and T-cell receptors for antigen, and also switching into different subpopulation of T cells (3). The great range of different T-cell receptor types, running into many millions, is also established here by the expression of variable genes and related mechanisms (3). As time passes, the differentiating thymocytes enter the medulla, and migrate through the walls of venules and lym-