The Caenorhabditis elegans distal tip cell (DTC) and the concept of a stem cell niche. (a) Left, the stem cell niche hypothesis from Schofield ; right, the C. elegans DTC (red) provides the stem cell niche for the germline stem cell (GSC) pool (yellow). (b) Images of the adult DTC and its processes. Left, cytoplasmic green fluorescent protein (green) highlights the DTC and its processes that embrace GSCs. Blue, germline nuclei; red, germline membranes. Modified from . Middle, electron microscopy (EM) image of DTC and its processes. Modified from . Right, scanning EM image of distal gonad; image courtesy of David Greenstein . An asterisk (*) marks one GSC in each image; arrowheads mark processes. (c) Molecular view of the niche and its control of GSC self-renewal or differentiation. Dark red, minimalist view of niche as the surface presenting Notch ligands; pink, broader view of niche including DTC as integral to providing the microenvironment for GSC control. (d) Expansion of niche concept based on investigations of DTC and Notch signaling in C. elegans. See text for explanation.