The main evidence for stem cells comes from an in vivo assay of tissue formed following transplantation of stromal cells under the renal capsule or in diffusion chambers. The formation of fibroblastic colonies from a suspension of marrow cells

Marrow stromal tissue is a network of cells and extracellular matrix which physically supports the haemopoietic cells and influence their differentiation (Dexter, 1982). Macrophages and endothelial cells are often included in the stromal cell population (Allen & Dexter, 1982, 1984). However, in the present chapter, the term stromal will be restricted to cells of the stromal ‘fibroblastic’ system of marrow. By definition these include osteoblasts and preosteoblasts found near to bone surfaces, fibroblasts and reticular cells, terms commonly used for the soft connective tissue cells of marrow and blood vessel walls, and marrow adipocytes (Owen, 1985). Current evidence suggests that haemopoietic, stromal and endothelial cellular systems are histogenetically distinct in the post-natal animal under normal conditions (Le Douarin, 1979; Wilson, 1983; Simmons et al. 1987). As a working hypothesis for differentiation in the marrow stromal system we have proposed a scheme analogous to that in the haemopoietic system, where stromal stem cells give rise to committed progenitors for different cell lines (Fig. 1) (Owen, 1978, 1985). The main evidence for stem cells comes from an in vivo assay of tissue formed following transplantation of stromal cells under the renal capsule or in diffusion chambers. The formation of fibroblastic colonies from a suspension of marrow cells