CELL CLASSIFICATION AND KINETIC ASPECTS OF NORMOBLASTIC AND MEGALOBLASTIC ERYTHROPOIESIS

Mitotic indices and 3 H‐thymidine flash labelling indices have been determined in a total of 6000 erythroid cells from patients with megaloblastic anaemia (vitamin B 12 deficiency) and 4000 erythroid cells from patients with increased, normoblastic erythropoiesis. In the anaemic states there is a lack of mitoses in the more mature erythroid compartments relative to the number of mitoses in the early erythroid precursors; this must reflect skipped division and/or cell death in the later precursors. In order to further locate the deficit of mitoses, erythropoietic cells were subdivided in a way which aimed at stratifying them according to cell generations. It appears that there are four consecutive generations of recognizable proliferating red cell precursors. Balance considerations of mitotic figures suggest that in stressed normopoiesis all cells which enter generation III divide, whereas only about one‐half of cells leaving generation III divide again in generation IV. In megaloblastic erythropoiesis it appears that only about one of three cells which leave generation III divide in generation IV. The data suggest that in megaloblastic anaemia, skipped division and/or cell death to a large extent take place in generation IV or at the transition from III to IV. In normoblastic erythropoiesis, the ratio labelled cells/mitotic cells is rather independent of cell maturation. In contrast, this ratio varies considerably in megaloblastic erythropoiesis, from 25:1 in early forms to 4‐5:1 in late forms. As an explanation of the lack of mitoses, relative to cells in DNA‐synthesis, in the early stages and the relative surplus of mitoses in the late stages it is proposed that cell cycle and cytological boundaries do not coincide in all cells. The present observations can be accounted for if a significant fraction of cells change their morphology (from basophilia to polychromasia) between their DNA‐synthesis phase and the subsequent mitosis. It cannot be decided whether in addition there is a death function between DNA‐synthesis and mitosis in the large basophilic megaloblasts, megaloblastic system could absorb a direct entry from the large basophilic cells amounting to perhaps about one‐half of the flux through the S‐pool of the large basophilic cells without being more dominated by very large cells than is actually the case; still, in large measure this will depend on the time from entry into the polychromatic pool until the subsequent mitosis or possible cell death. The alternative is a significant death function between S‐phase and mitosis at the level of the large basophilic E1‐E2 cells (generation I + II).