Immunohistochemical and ultrastructural evidence for myelopoiesis in the scid/scid mouse thymus

The ultrastructure of scid mouse thymus (a small encapsulated epithelial ma ss within the precardial fat pad) is described. The epithelium did not form cortex or medulla and hence remained relatively undifferentiated. Small un myelinated nerves innervated the capsule, the major blood vessels and were distributed between the epithelial cells. Fenestrated blood vessels were co mmon. Thymocytes were not identified but numeous granulocytes, mast cells a nd some fibroblasts, macrophages and interdigitating cells were present. Al l stages of granulopoiesis were observed in scid thymus. A very small numbe r of immunoreactive ER-MP58 cells indicated bone marrow derived myeloid pre cursor cells, and low numbers of ER-MP12(+) and ER-MP20(+) mononuclear cell s 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 th at the presence of moderately extensive myelopoiesis within the scid mouse thymus has potential for the study of extramedullary hematopoiesis, and als o is important to bear this function in mind when using the scid mouse as a n immunological model for thymus reconstitution and for creating 'organoid' cultures.