Sequential Development of Thy-1⁺ Subpopulations in Regenerating Thymus after Bone Marrow Transplantation

Abstract

Mouse bone marrow contains multipotent stem cells which have been shown to function also as progenitor cells of the T cell lineage (1). The first cells that can be recognized as T cells during ontogeny and in newborns are found in the thymus and have Thy-1 surface markers (2,3). The density of Thy 1 on adult thymocytes is heterogenous. In general a low Thy 1⁺ and a high Thy 1⁺ subpopulation can be distinguished, which have been equated with medullary and cortical thymocytes, respectively. Shortman and Jackson (4) proposed a T cell differentiation model in which the two types of thymocytes belong to different subpopulations at least in adult mice. These subpopulations develop simultaneously and independently and each contain a proportion of cycling cells. In adult mice no evidence for a precursor product relationship between the two subpopulations has been established. In addition to this model that suggested a dualistic development, Mathieson et al. (5,6) presented evidence for a sequential and separated development of Lyt-1⁺ and Lyt-1,2,3⁺ thymocytes in fetal thymus.