Two Morphologically and Kinetically Distinct Populations of Lymphoid Cells in the Bone Marrow

Abstract

LYMPHOID cells account for 20–30 per cent of all nucleated cells in the bone marrow of common laboratory animals and in the human foetus. Evidence from a number of sources suggests that bone marrow lymphocytes may be functionally distinct from lymphocytes of lymph and lymphoid tissue in spite of morphological similarities¹. Lymphoid cells of the bone marrow comprise a heterogeneous population consisting of pachychromatic small lymphocytes which do not synthesize DNA and of leptochromatic transitional cells which synthesize DNA and divide². On the basis of differences in cytoplasmic basophilia, transitional cells themselves have been further subdivided³. It was shown that pale and basophilic transitional cells incorporated ³H-thymidine at markedly different rates after pulse label⁴. This observation suggested that transitional cells with basophilic or pale cytoplasm may represent cell populations with different kinetic behaviour. I report here preliminary experiments to characterize the cell cycle and estimate the rate of renewal for these cells.