YS-EMp can contribute to FL hematopoiesis. FL lobes from 10-dpc embryos (CD45.1⁺) were cocultured with either freshly isolated or 4-day in toto–cultured 8-dpc YS cell suspensions. After 10 days, FL lobes were scored for the presence of donor-derived B cells (B220, IgM) and myeloid cells (Gr-1, Mac-1). Plots are gated on donor-derived cells. YS cells can only give rise to a myeloid progeny in the FL environment, after an organ culture period.