title: Early events in lymphopoiesis: an update.
abstract: OBJECTIVE
  Cells of the immune system are replaced in large numbers throughout life, and the underlying mechanisms have been extensively studied. Whereas the pace of discovery in this area is unprecedented, many questions remain, particularly with respect to lymphocyte formation.
  
  
  RESULTS
  While transcription factors have long been a focus of investigation, microRNAs are also being implicated in lymphopoiesis. Lymphocytes are normally replaced in correct proportion to other blood cells, but ratios change dramatically during infections. Long-standing issues relating to T versus B lineage divergence remain but have been enriched with remarkable new findings about thymus seeding. There are indications that at least some age-related changes in lymphopoiesis may be reversible. Finally, knowledge obtained from studies of mice is slowly being extended to humans.
  
  
  CONCLUSIONS
  We can now appreciate that new lymphoid progenitors are drawn from a heterogeneous collection of hematopoietic stem cells through asynchronous patterns of gene expression. Complex interactions then occur between the gene products, preparing lymphoid progenitors to respond to environmental cues. Whereas unique markers describe the process of lymphocyte formation in humans, fundamental information now available should suggest ways to promote rebound from chemotherapy or transplantation and reverse declines associated with aging.
authors.affiliation: Immunobiology and Cancer Program, Oklahoma Medical Research Foundation, Oklahoma City, Oklahoma 73104, USA.
authors.forename: Qingzhao
authors.initials: Q
authors.lastname: Zhang
authors.forename: Ryuji
authors.initials: R
authors.lastname: Iida
authors.forename: Takafumi
authors.initials: T
authors.lastname: Yokota
authors.forename: Paul W
authors.initials: PW
authors.lastname: Kincade
mesh_terms: D000375:Aging / Q000502:physiology; D000818:Animals; D006801:Humans; D054503:Lymphoid Progenitor Cells / Q000502:physiology*; D038041:Lymphopoiesis / Q000502:physiology*; D051379:Mice; D035683:MicroRNAs / Q000502:physiology; D013601:T-Lymphocytes / Q000502:physiology; D015533:Transcriptional Activation / Q000502:physiology
chemical_list: D035683:MicroRNAs