Morphometric analysis of thymic medullary non-lymphoid cell changes during postnatal development

Thymic non-lymphoid cells have been shown to influence the differentiation of T-lymphocytes. Large numbers of non-lymphoid cells are concentrated in the thymic medullary zone. A morphometric analysis of these cells in the medulla identified little change in their absolute number and volume density during thymic development in the rat. Expansion of thymic medullary volume with age was documented by planimetry. Several subtypes of medullary non-lymphoid cell were identified by electron microscopy, including both squamous and cystic epithelial cells, interdigitating cells and macrophages. Changes in their relative proportion during development were determined with the percentage of squamous epithelial cells declining, interdigitating cells increasing and the other cell types exhibiting little change. Subcellular evidence of secretory activity was sought by analysis of changes in the volume density of organelles in each of the non-lymphoid classes. Cystic enlargement in epithelial cells and a decrease in the volume density of macrophage inclusion bodies were noted but no clear morphometric trend could be correlated with increased secretory activity expected in the developing endocrine thymus.