Reticulo-fibroblastoid stromal cell progenitors (CFU-RF) in murine bone marrow.

The hemopoietic inductive microenvironment (HIM) of the bone marrow is responsible for secretion of growth factors that regulate hemopoiesis. It is composed of an extracellular matrix and a complex variety of cell types with a range of functions related to blood cell development. In order to understand how such a complex system operates, it will first be necessary to determine the role(s) of the integral parts. Several of the stromal cell types have been identified morphologically in various culture systems, and some of their functions have been elucidated. We have identified a new stromal cell type in mouse bone marrow that appears similar if not identical to its human counterpart. When bone marrow cells are placed in methylcellulose/plasma clot culture with phytohemagglutinin-stimulated human leukocyte-conditioned medium in the presence of bovine calf serum (BCS), mercaptoethanol, and hydrocortisone, extensive branching colonies develop within 14 days. These "reticulo-fibroblastoid" (RF) colonies arise from a putative reticulo-fibroblastoid colony-forming unit (CFU-RF) stem cell, and many become adipocytic by day 14; the addition of fresh medium, methylcellulose, and BCS on day 7 inhibits this change. The batch of human citrated plasma used in the culture system and the type and source of stimulating factor all influence the number of RF colonies that develop as well as the percent of colonies that become adipocytic. Whether this adipogenesis represents functional maturity or terminal differentiation is not yet known. Information gained on the role of these RF cells in normal and impaired hemopoiesis should contribute to the elucidation of the complicated interactive role of the microenvironment in the support and modulation of hemopoiesis.

Radioprotection conferred by dextran sulfate given before irradiation in mice.

Dextran sulfate (DS) has been observed to cause mobilization (fivefold) of hemopoietic stem cells (HSC) and leukocytes, primarily lymphocytes, into the peripheral blood of mice within 2-3 h after intraperitoneal (i.p.) injection. This effect was dose dependent and was prolonged for several hours when the high-molecular-weight version DS500 (500,000 daltons) was used. When DS500 was given 1-3 days before irradiation, hemopoietic recovery was markedly enhanced. Postirradiation injection was ineffective. By ten days after irradiation (7.0 Gy), the number of endogenous spleen colonies (CFUs) and the splenic mass were much larger if DS pretreatment had been given. This effect was dependent on the dose of DS500 and on the time administered, 60 mg/kg producing a maximal effect when given three days before irradiation. DS500 caused a transient anaphylactoid shock, however, in most mice--mild at low doses but potentially lethal at doses above 40 mg/kg (10% mortality within 1-3 days after 60 mg/kg). The following results were obtained with 50 mg/kg, a compromise dose causing minimal mortality (3%) given three days before irradiation. Reticulocyte reappearance was earlier in irradiated mice given DS500, indicating earlier erythropoietic recovery. Some of these reticulocytes were resistant to lysing agents, so their appearance could be detected using the Coulter electronic cell counter, as well as in stained blood smears. The 30-day mortality due to bone marrow failure after irradiation was significantly decreased in DS-treated mice below 9.5 Gy, and the LD50/30 was increased by 0.5 Gy. This study shows that dextran sulfate exerts a radioprotective influence on the hemopoietic system and hence survival when administered prophylactically.