Relationships between hematopoietic stem cells and lymphocyte progenitors.

Remarkable advances have been made in characterizing highly purified hematopoietic stem cells and their lymphoid progeny. The majority of the cytokines and cell interaction molecules that comprise their environment are probably known, as are most cell cycle regulatory proteins, signaling molecules and transcription factors that mediate the lineage fate decisions and control population dynamics. However, it is becoming obvious that traditional differentiation diagrams are not adequate for depicting early changes and give overly rigid impressions about how cells of the immune system arise.