Correlation Between Cytokinetically Resting Lymphocytes and Bone Marrow Restoration: Experiments Using a Discontinuous Albumin Gradient

Abstract

Rats were labeled with 3H-thymidine (3H-TdR) in utero and for 6 wk after birth in order to obtain 100% labeling of all bone marrow cells. Six weeks after the last 3H-TdR injection only cytokinetically resting cells were still labeled. At this time, the regenerative capacity of fractions obtained after centrifugation on a discontinuous albumin gradient was tested in 1200-R x-irradiated recipients, and the results were compared with the effect observed after transplantation of the same number of unfractionated bone marrow cells. One of the fractions obtained had a regeneratory capacity tenfold that of unfractionated cells. In contrast, when the response to a PHA stimulation test was evaluated, this fraction showed a decreased incorporation of 14C-TdR as compared to other fractions. In a second test system, fractions of bone marrow from 3H-TdR-labeled donors made hypoplastic by repeated injections of hydroxyurea were transfused into 1200-R x-irradiated recipients. Regenerative capacity was similar to that seen in the first experiment. The findings indicate a correlation between cytokinetically resting, small mononuclear cells and the regeneratory process in lethally x-irradiated recipients.

The Role of Blood Stem Cells in Hematopoietic Cell Renewal

“…The bulk of labeled cells disappeared from the circulating blood within two to three h but could not be traced to bone marrow sites. Today we know that the bulk of pluripotent stem cells is not in DNA synthesis but at rest [45], and it was only in studies using the reverse approach—activation of thymidine‐labeled resting mononuclear cells by partial‐body irradiation and/or by their transfusion into lethally irradiated recipient animals—that we came to the appropriate assumption that the pluripotent stem cell is a resting cell capable of migration through the blood [46], but not recircling through the lymph [47], that can home in hematopoietic stroma prepared to accept this seeding process in medullary but also in appropriate extramedullary sites [48].…”