ABSTRACT
In many contexts, self-renewal and differentiation of stem cells are influenced by signals from their environment, constituting a niche. It is postulated that stem cells compete for local growth factors in the niche, thereby maintaining a balance between the numbers of self-renewing and differentiated cells. A critical aspect of the niche model for stem cell regulation is that the availability of self-renewing factors is limited and that stem cells compete for these factors (Fig. 1). Consequently, the range and concentrations of the niche factors are of critical importance. Now that some of the few self-renewing factors have become identified, aspects of the niche models can be tested experimentally. In this chapter, we address mechanisms of signal regulation that take place at the level of signal-producing cells, constituting a niche for stem cells. We emphasize the biochemical properties and posttranslational modifications of the signals, all in the context of Wnt signaling. We propose that these modifications control the range of Wnt signaling and have critical roles in establishing niches for stem cells in various tissues.
Subject(s)
Drosophila/physiology , Stem Cells/cytology , Stem Cells/physiology , Wnt Proteins/physiology , Animals , Cell Differentiation , Cell Proliferation , Drosophila/cytology , Drosophila/genetics , Drosophila Proteins/chemistry , Drosophila Proteins/genetics , Drosophila Proteins/physiology , Embryonic Stem Cells , Humans , Models, Biological , Protein Processing, Post-Translational , Signal Transduction , Wnt Proteins/chemistry , Wnt Proteins/geneticsABSTRACT
Numerous investigators have found that serum prolactin levels increase after tonic-clonic and partial complex seizures, but the effect of syncope on prolactin levels has been studied little. Serum prolactin levels were measured following unexpected syncopal attacks in patients seeking emergency treatment in a community hospital. Levels sampled 18 to 60 minutes after syncopal episodes were increased in 8 of 11 cases. Follow-up prolactin levels, measured 17 to 222 days later, were normal in all eight cases in which they were initially increased. Most subjects had concurrent illness. Although the current study does not clarify whether it was the syncope, the concurrent illness, or both that caused the prolactin elevations, it implies that measurement of this hormone will not help the clinician in distinguishing between seizures and syncopal attacks.
