The Lymphocyte Production Pathway in Bone Marrow: Possible Significance of the Size Spectrum of Lymphocytes and their Precursors

S ummary . It is now generally accepted that transitional (‘lymphoid’) cells are the precursors of small lymphocytes. Such cells have a heterogeneous size spectrum and show high proliferative capacity. To facilitate the study of the kinetics of lymphocyte production, a detailed investigation of cell sizes of the transitional cell‐lymphocyte compartment was carried out using a Coulter counter modified to permit a very rapid and accurate examination of cells in suspension. Enriched populations of ‘lymphoid’ cells, obtained after i o d rebound from hypoxia at half an atomosphere, were enriched further by bovine albumin and Ficoll gradients to give density fractions containing two types of cells. Differential counts of stained smears of these fractions enabled a comparison to be made between the size distribution and the specific cell types. Four distinct cell types were characterized in terms of volume and density: small and intermediate‐sized lymphocytes (volume 53–59 f1, albumin fractions 19–23%), small transitional cells (154–150 f1, 21–23%), medium transitional cells (206–218 f1, 17–19%) and large transitional cells (350–400 f1, 21–27%). These findings are consistent with the view that there are at least three mitoses in the course of the lymphocyte production pathway in the bone marrow.