Ontogeny of the rat thymus micro-environment: Development of the interdigitating cell and macrophage populations

The ontogeny of the rat thymus micro-environment and in particular the development of the interdigitating cell (IDC) and macrophage (Mø) populations has been studied. At day 15 of fetal life the thymus consisted of an epithelial primordium in which some Thy-1 positive thymocytes were present around local capillaries in a central area. At day 16 some Ia positive cells, which could not be further identified, and some monocyte-like Mø were observed in the central area. From day 17 the thymus became lobulated by ingrowth of small blood vessels with perivascular connective tissue from the surrounding capsule. An Ia positive epithelial reticulum developed which became populated by increasing numbers of thymocytes. Some strongly acid phosphatase positive Mø were present from this stage of development. From day 19 cortical and medullary areas could be distinguished in the thymus. The cortex consisted of an Ia positive epithelial reticulum in which closely packed thymocytes and scattered Mø were present. The medulla demonstrated a confluent Ia staining and consisted of an epithelial reticulum in which thymocytes, strongly non-specific esterase positive IDC and an occasional Mø were present. Also highly phagocytic IDC-like cells were observed in the medulla, most likely they comprise the population of differentiating IDC. Thymocyte proliferation areas, which were strongly pyroninophilic, were observed from day 21 in the cortex, just beneath the surrounding connective tissue capsule. A distinct cortico-medullary region with many Mø was present one week after birth. From this stage the IDC and Mø distribution was comparable with older thymi.