In vitro methods in haemopoiesis and lymphopoiesis.

The haemopoietic system is conveniently represented as a series of cell populations which differ in their capacity for self-renewal and differentiation. In this population the earliest, functionally recognisable cell is the spleen colony-forming cell or CFU-S (Till and McCulloch, 1961) which is assayed by its ability for form colonies in the spleens of irradiated mice. This cell has extensive self-renewal ability and is pluripotent, i.e., it can give rise to all the myeloid and lymphoid elements (reviewed in Metcalf and Moore, 1971). Although generally recognised as the earliest haemopoietic stem cell, recent work indicates that there may be an 'age structure' in this cell population, particularly with regard to self-renewal ability (Micklem et al., 1975; Rosendaal et al., 1976; Schofield, 1979). Nonetheless, from the pluripotent stem cells are derived the variety of myeloid and lymphoid committed progenitor cells, which although limited in self-renewal capacity, undergo proliferation to give rise to the mature blood cells. In recent years, there has been a rapid expansion in the technology required for the growth of most, if not all, of these cell populations in vitro and concomitant advances in the nature of the regulatory molecules and cellular interactions required for their proliferation and development. In this article are briefly reviewed the various stem cell, myeloid and lymphoid in vitro systems. For a more extensive analysis of some of these systems, the reader is referred to the excellent review by Metcalf (1977).