Fig. 1.

Composition of adult and fetal thymocyte progenitor subsets. a Identification of subsequent stages of early intrathymic differentiation by flow cytometry. Representative histograms illustrating cell surface expression of CD44 and CD25 on lineage negative (double negative, DN) thymocytes in adult (top) and E15.5 fetal thymus (bottom). DN thymocytes were segregated into four populations DN1–4 by expression of CD44 and CD25, as defined by quadrant gating (left panels) or by strictly positioned gates to identify more homogeneous subsets (middle panels). DN1 was CD44⁺CD25^–, DN2 was CD44⁺CD25⁺, DN3 was CD44^lo/intCD25⁺, and DN4 was CD44^lo/intCD25^–. Early thymic progenitors (ETPs) were defined as CD117⁺ (receptor-type tyrosine kinase c-Kit) cells in DN1 (right panels). The ETP is a major constituent of adult and fetal DN1. b Representative histograms illustrating expression of CD24, CD90.2, CD127, CD27, CD49d, LPAM-1 on adult (black solid) and E15.5 fetal (gray filled) on DN1 ETP, DN2 and DN3 progenitor thymocytes. Isotype matched controls represent negative staining (adult: faint black, fetal: faint gray lines). c Cell cycle analysis of adult versus fetal thymocyte subsets. Representative histograms illustrating the frequency of cells in S/G2/M in adult (top) and fetal (bottom) DN1 ETP, DN2 and DN3 progenitor thymocytes. DN2 populations contained the highest frequency of cells in cycle, whereas DN3 cells were largely non-dividing