Ultrastructural studies of mouse thymic reticulum. II. Non-epithelial component.

In this study we describe two types of non-epithelial cells forming a part of the thymic reticulum: macrophages with high phagocytic function, present in the cortex and medulla of the organ, and interdigitated cells present at the corticomedullary junction and in the medulla. These cells, in relation with epithelial cells, form a meshwork, a thymic microenvironment which influences the differentiation and maturation of T lymphocytes. These non-epithelial cells were probably mobile and their precursors exist in bone marrow. It has not yet been determined whether they are both of the same lineage and whether there is or is not common lineage between macrophages and interdigitated cells. Their role as accessory cells in the immune response seems evident. We will compare our observations with those of other authors. We will also discuss several issues concerning these two cell types; their nomenclature, their interrelationship in the thymic reticulum, their function, and their relationship to other similar cells in situ and to cells isolated in vitro, which perhaps are similar.