Hematopoiesis and stem cells: plasticity versus developmental heterogeneity.

Hematopoietic stem cells (HSCs) provide for blood formation throughout the life of the individual. Studies of HSCs form a conceptual framework for the analysis of stem cells of other organ systems. We review here the origin of HSCs during embryological development, the relationship between hematopoiesis and vascular development and the potential plasticity of HSCs and other tissue stem cells.

Calpain-mediated cleavage of the cyclin-dependent kinase-5 activator p39 to p29.

The activity of cyclin-dependent kinase-5 (Cdk5) is tightly regulated by binding of its neuronal activators p35 and p39. Upon neurotoxic insults, p35 is cleaved to p25 by the Ca(2+)-dependent protease calpain. p25 is accumulated in ischemic brains and in brains of patients with Alzheimer's disease.

Intersections between blood cell development and leukemia genes.

Hematopoietic development is regulated in large part by transcription factors that control cell fate decisions and cellular differentiation. Several genes first discovered in the context of chromosomal translocations in leukemia also serve important functions in blood cell development. Gene-targeting experiments related to two of these factors, SCL/tal-1 and translocation-ets-leukemia (TEL), are reviewed here.