Summary. Quantitative histological examinations were carried out on large numbers of spleen nodules obtained in heavily irradiated mice by injection of bone‐marrow and foetal liver cell suspensions. Endogenous colonies and clones arising from irradiated bone‐marrow cells have also been analysed. It was found that the large majority of clones consisted of cells of either erythroid or myeloid or mega‐karyocytic type. A smaller percentage of clones were mixed with association of the various cell types in all possible combinations. Statistical analysis of the data revealed consistent differences in the distribution of clones derived from various sources. Attention is particularly focused on the mixed clones with a discussion of their significance in the light of the mono‐ or poly‐phyletic origin of blood cells.
Cells obtained from the blood-forming organs of donor animals and injected, under appropriate conditions, into heavily irradiated mice, can colonize, among other organs, the spleen of the hosts, giving rise to discrete and differentiated nodules of haemopoietic tissue (Till & McCulloch, 1961). It has been demonstrated that the number of nodules is directly proportional to the number of injected cells and that such nodules may be regarded as clones arising from single cells (Till & McCulloch, 1961; Becker, McCulloch & Till, 1963). The postulate of classical haematological theories (Maximov & Bloom, 1934; Ferrata, 1935) that a proportion of the cells in blood-forming organs is capable of indefinite division with renewal of the stem line and of differentiation into mature blood cells has thus been experimentally demonstrated.
Whether colony-forming cells should actually be regarded as the very primitive stem cells of the haemopoietic tissues (McCulloch, 1963; Lewis & Trobaugh, 1964) or as the earliest step of a differentiative haemopoietic line (Kay & Davies, 1964) has not yet been fully agreed upon.
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