The ultrastructure of scid mouse thymus (a small encapsulated epithelial mass within the precardial fat pad) is described. The epithelium did not form cortex or medulla and hence remained relatively undifferentiated. Small unmyelinated nerves innervated the capsule, the major blood vessels and were distributed between the epithelial cells. Fenestrated blood vessels were common. Thymocytes were not identified but numerous granulocytes, mast cells and some fibroblasts, macrophages and interdigitating cells were present. All stages of granulopoiesis were observed in scid thymus. A very small number of immunoreactive ER-MP58 cells indicated bone marrow derived myeloid precursor cells, and low numbers of ER-MP12+ and ER-MP20+ mononuclear cells indicated stages of myeloid cells committed to the granulocyte/macrophage lineage. Cells containing proliferating nuclear cell antigen (cells in G1, S and G2-M stage) were present throughout the thymic mass. BALB/c thymuses contained cortical foci of p53+ cells whereas in scid mice, p53 positive cells were scattered singly throughout the thymus. This study indicates that the presence of moderately extensive myelopoiesis within the scid mouse thymus has potential for the study of extramedullary haematopoiesis, and also that it is important to bear this function in mind when using the scid mouse as an immunological model for thymus reconstitution and for creating 'organoid' cultures.
