Epithelial stem cells in vivo.
Cellular topography within the highly polarized surface epithelia can be used to identify the location of the stem cells. In some instances, this can be quite precise and allows the characteristics of stem cells to be studied. Our current knowledge of the stem cell population in murine epidermis and small intestinal crypts is reviewed. In the epidermis, the stem cells would appear to make up about 10% of the basal layer and are distributed towards the centre of the basal layer component of the epidermal proliferative unit. These cells have a long cell cycle and are probably the same cells that retain both tritiated thymidine and radioactively labelled carcinogens for long periods of time. This label retention permits the labelling of the putative stem cell compartment. Over recent years, there has been an accumulation of information indicating various types of heterogeneity within the basal layer, much of which can be interpreted in relation to cellular hierarchies. In the small intestine, cell positions can be fairly precisely identified and the stem cell zone identified. Complex modelling of a wide range of cell kinetic experiments suggests that each crypt contains between 4 and 16 steady state functional stem cells. Radiobiological experiments suggest that up to 32 cells may be capable of clonal regeneration. The repopulation of the clonogenic cell compartment has been determined and the doubling time measured to be 19.7 h. Such studies should throw further light on the behaviour of stem cells and identify the timing of periods of increased and decreased cell proliferation (activation and suppression of controls).