Differentiation of Mouse Embryonic Stem Cells: V. Thymus-Like Environment Derived from Embryoid Bodies Implanted into Immuno-Incompetent Mice

Abstract

Mouse embryonic stem (ES) cells in culture can differentiate into late stages of many lineage-committed precursors. Under appropriate organ culture conditions, ES cells differentiate into lymphoid-like cells at an embryonic stage equivalent to day 10–14. These hematopoietic precursors are located in cup-shaped structures found in some embryoid bodies; we called such embryoid bodies "ES fetuses." We have followed the maturation of hematopoietic cells after implantation of ES fetuses into nude mice and SCID mice for 3 weeks. Nude mice seem to be better hosts than SCID mice for maturation of lymphoid cells. In chimeric nude mice, mature CD4⁺ and CD8⁺ T cells were found in the spleen. As nude mice lack a thymus, these T cells might have been educated by thymus-like epithelium generated from ES fetuses endogenously. Here we report finding organized epithelial structures resembling thymic cortex and medulla in the implanted ES fetuses. Using immunofluorescence and peroxidase staining, with flow cytometry, light and confocal microscopy, we describe the character of these structures. Antibodies against surface markers were used to characterize the stroma-like cells found in them. Moreover, the cup-shaped structure of embryoid bodies were determined to be endothelial in origin, and we report the finding of novel CD4^–, CD8αβ ⁺ dendritic-like cells.