Exposure Of Bone Marrow Cells Research Articles

Acute lymphoblastic leukemia-secreted miRNAs induce a proinflammatory microenvironment and promote the activation of hematopoietic progenitors.

Leukemogenesis is proposed to result from the continuous interplay between inducive bone marrow (BM) microenvironments and malignant precursor cells. Recent findings point toward an abnormal production of proinflammatory mediators within the BM from acute lymphoblastic leukemia (ALL) patients, although the mechanism underlying this phenomenon is uncertain. Here, we have identified 3 miRNAs, miR-146a-5p, miR-181b-5p, and miR-199b-3p, as potential candidates for TLR8 ligation, which are overexpressed in ALL and show agonist functional binding. When purified from ALL exosomes, they demonstrated their capacity of inducing cytokine production by both, hematopoietic and stromal BM cells. Of note, the exposure of BM cells from ALL patients to the proinflammatory milieu resulting from these miRNAs agonist activity revealed the proliferation of normal progenitors, while poor effects were recorded in the leukemic counterpart. The unconventional roles of the tumor-secreted miRNAs as TLR8 agonist ligands may provide a novel mechanism contributing a tumor-microenvironment feedback loop by switching on proinflammatory pathways that further activate normal hematopoietic precursors and support ALL progression. Secreted B-ALL TLR8-agonist miRNAs are involved in the promotion of proinflammatory microenvironments that target normal hematopoietic cells. B-lineage ALL cells secrete exosomes containing miRNAs endowed with the ability of functionally binding TLR8 in hematopoietic and BM mesenchymal stromal cells. Upon TLR8 signaling, the activation of the NF-kB pathway induces secretion of proinflammatory cytokines that, in turn, promotes cell proliferation in early hematopoietic cell populations, driving a tumor-microenvironment-hematopoietic activation feedback loop that may reduce the normal hematopoietic stem and progenitor cell compartment and facilitate cancer progression.

IFN-γ-mediated hematopoietic cell destruction in murine models of immune-mediated bone marrow failure

AbstractInterferon gamma (IFN-γ) has been reported to have both negative and positive activity on hematopoietic cells, adding complexity to the interpretation of its pleiotropic functions. We examined the effects of IFN-γ on murine hematopoietic stem cells (HSCs) and progenitors in vitro and in vivo by using mouse models. IFN-γ treatment expanded bone marrow (BM) c-Kit+Sca1+Lin– (KSL) cell number but reduced BM KLCD150+ and KLCD150+CD48– cells. IFN-γ-expanded KSL cells engrafted poorly when tested by competitive repopulation in vivo. KSL, KLCD150+, and KLCD150+CD48– cells from IFN-γ-treated animals all showed significant upregulation in Fas expression. When cocultured with activated T cells in vitro, KSL and KLCD150+ cells from IFN-γ-treated donors showed increased apoptosis relative to those from untreated animals, and infusion of activated CD8 T cells into IFN-γ-injected animals in vivo led to partial elimination of KSL cells. Exposure of BM cells or KSL cells to IFN-γ increased expression of Fas, caspases, and related proapoptotic genes and decreased expression of Ets-1 and other hematopoietic genes. In mouse models of BM failure, mice genetically deficient in IFN-γ receptor expression showed attenuation of immune-mediated marrow destruction, whereas effector lymphocytes from IFN-γ-deficient donors were much less potent in initiating BM damage. We conclude that the activity of IFN-γ on murine hematopoiesis is context dependent. IFN-γ-augmented apoptotic gene expression facilitates destruction of HSCs and progenitors in the presence of activated cytotoxic T cells, as occurs in human BM failure.

Down-Regulation of c-Kit Receptor on Hematopoietic Cells By Stem Cell Factor (SCF; c-Kit-ligand): No Impact on Transplantation

Presence of the c-Kit tyrosine kinase receptor is a hallmark of the mouse hematopoietic stem cells (HSCs) and progenitors routinely used for their identification and separation. c-Kit is activated after binding of its ligand, the stem cell factor (SCF; c-Kit-ligand). c-Kit receptors with bound SCF form dimers that are rapidly internalized and degraded. This activates the c-Kit signaling pathways supporting cell survival, proliferation or quiescence and self-renewal. Although there is a consensus that c-Kit signaling is important for functioning of HSCs, published results are partly controversial.We have defined HSCs and progenitors as Lineage- Sca-1+c-Kit+ cells (LSK cells) and characterized them further by means of the CD150 and CD48 markers. We used anti-c-Kit antibody minus (FMO; Fluorescence Minus One) samples to distinguish between c-Kit+ and c-Kit- bone marrow cells and analyzed the distribution of c-Kit on the immature hematopoietic cells carrying different phenotypes. Further, we exposed bone marrow cells to a wide range of concentration of a recombinant mouse SCF in vitro and measured a change in c-Kit presence and distribution on these different cell types. Also, SCF was injected to mice in vivo and their bone marrow was similarly analyzed for a change in c-Kit expression. Bone marrow cells exposed to SCF concentrations that deeply down-regulated c-Kit receptors were transplanted to recipient mice, and their transplantation efficiency was compared to that of normal bone marrow.c-Kit was unevenly but characteristically distributed on different types of LSK CD150/CD48 cells, showing the highest and the most homogeneous density on cells with the LSK CD150+CD48- phenotype. Exposure of bone marrow cells to SCF in ranges of concentrations from 0.3-2000 ng/ml induced progressive down-regulation of c-Kit. However, the cells mostly remained c-Kit+(low). The response to SCF was the most prominent in a range of SCF concentrations between 1-100 ng/ml. Cells with the phenotype LSK CD150+CD48+were relative low-responders. In vivo administration of SCF to mice in doses exceeding 300 ng/mouse, either intraperitoneally or intravenously, had similar effect on c-Kit expression by bone marrow cells as their incubation with SCF in vitro.Next we investigated correlation of the intensity of c-Kit receptor expression on bone marrow cells with their repopulating capacity after transplantation. A significantly decreased c-Kit expression on transplanted cells, induced by exposure of the cells to SCF, did not decrease contribution of the cells to chimeric hematopoiesis in competitive transplantation assays. Formation of spleen colonies was also not affected in the CFU-S assay.Experiments which measured the effect of SCF administered to normal mice in vivo demonstrated an effect that lasted for less than 12 hours. c-Kit turnover on hematopoietic cells is thus rapid, and this fact may explain why down-regulation of c-Kit, on otherwise normal bone marrow cells, does not affect their capacity to be transplanted.In conclusion, c-Kit receptor density on hematopoietic cells does not appear to be a critical factor for the homing of transplanted hematopoietic stem and progenitor cells into the blood-forming tissues and their engraftment into specific niches. Also their performance in establishing productive hematopoiesis is not altered by the SCF-induced down-regulation of the c-Kit receptor density in time of their transplantation. DisclosuresNo relevant conflicts of interest to declare.

Tumor cells hijack macrophages via lactic acid.

Macrophages are among the most abundant cells in the tumor stroma and can contribute to neoplastic growth, invasion and metastatic diffusion by translating instructive signals delivered by transformed cells. These signals comprise soluble factors such as chemokines and cytokines.1 In many cancers, tumor-associated macrophages (TAMs) are constantly recruited to the tumor environment by the CCL2 chemokine that attracts CCR2+ monocytes circulating in the blood.2 It is generally accepted that the tumor environment polarizes TAMs to express a set of genes common to M2-type macrophages, a specialized subset intervening in inflammation resolution, tissue remodeling and control of parasitic infections.1 These genes include the neoangiogenesis-promoter vascular endothelial growth factor (VEGF) and the L-arginine-metabolizing enzyme arginase (ARG). A recent paper by Colegio et al.3 in Nature opens a new scenario, showing that TAMs can ‘sense’ metabolic changes typical of the malignant state.

A tamoxifen inducible knock-in allele for investigation of E2A function

BackgroundE-proteins are transcription factors important for the development of a variety of cell types, including neural, muscle and lymphocytes of the immune system. E2A, the best characterized E-protein family member in mammals, has been shown to have stage specific roles in cell differentiation, lineage commitment, proliferation, and survival. However, due to the complexity of E2A function, it is often difficult to separate these roles using conventional genetic approaches. Here, we have developed a new genetic model for reversible control of E2A protein activity at physiological levels. This system was created by inserting a tamoxifen-responsive region of the estrogen receptor (ER) at the carboxyl end of the tcfe2a gene to generate E2AER fusion proteins. We have characterized and analyzed the efficiency and kinetics of this inducible E2AER system in the context of B cell development.ResultsB cell development has been shown previously to be blocked at an early stage in E2A deficient animals. Our E2AER/ER mice demonstrated this predicted block in B cell development, and E2AER DNA binding activity was not detected in the absence of ligand. In vitro studies verified rapid induction of E2AER DNA binding activity upon tamoxifen treatment. While tamoxifen treatment of E2AER/ER mice showed inefficient rescue of B cell development in live animals, direct exposure of bone marrow cells to tamoxifen in an ex vivo culture was sufficient to rescue and support early B cell development from the pre-proB cell stage.ConclusionThe E2AER system provides inducible and reversible regulation of E2A function at the protein level. Many previous studies have utilized over-expression systems to induce E2A function, which are complicated by the toxicity often resulting from high levels of E2A. The E2AER model instead restores E2A activity at an endogenous level and in addition, allows for tight regulation of the timing of induction. These features make our E2AER ex vivo culture system attractive to study both immediate and gradual downstream E2A-mediated events.

Prevention of amputation caused by rheumatic diseases following a novel therapy of exposing bone marrow, occlusive dressing and subsequent epidermal grafting

Wounds with exposed bones caused by rheumatic diseases commonly result in amputation despite progress in our understanding of wound-healing mechanisms. To determine whether an experimental therapy of bone marrow exposure, an occlusive dressing and subsequent grafting of epidermal sheets accelerates healing and reduces the need for amputation in patients with rheumatic diseases. Fifteen patients, including those with rheumatoid arthritis or systemic sclerosis, who had wounds with exposed bones were treated either with the standard procedure, consisting of local wound care, debridement with a scalpel, bed rest and parenteral antibiotics (n = 8), or with a newly developed experimental procedure (n = 7). In that new procedure, the affected bone was initially exposed by debridement with a scalpel, followed by partial excision with a bone scraper until bleeding was observed from the exposed bone. The lesions were immediately covered with an occlusive dressing, and were eventually treated with epidermal grafts obtained from suction blisters. A comparison with standard therapy demonstrated that the time needed for wound healing was similar, but that the newly developed combination therapy reduced the risk of amputation (P = 0.020). No skin ulcers or erosions were observed for at least 1 year in five of seven patients (72%) due to the adoption of stable palmoplantar-type characteristics in grafts derived from the trunk epidermis. Our study indicates that exposure of bone marrow cells plus an occlusive dressing accelerates the healing of skin ulcers at least partly through the preparation of a healthy well-granulated wound bed and that subsequent epidermal grafting achieves site-specific differentiation through epithelial-mesenchymal interactions.

Rapid healing of intractable diabetic foot ulcers with exposed bones following a novel therapy of exposing bone marrow cells and then grafting epidermal sheets

Diabetic foot ulcers with exposed bones commonly result in amputation. To determine whether exposure of bone marrow cells and subsequent grafting of epidermal sheets accelerates healing and reduces the need for amputation. Thirty-eight patients with chronic wounds caused by diabetes mellitus were enrolled in this study. Epidermal sheets obtained from suction blisters of each patient were grafted on to diabetic foot ulcers without exposed bones (n = 10) and were compared with the standard treatment of local wound care, debridement with a scalpel when indicated, bed rest and parenteral antibiotics (n = 8). In another group of patients, diabetic wounds with exposed bones were treated either with the standard procedure (n = 9) or with a newly developed experimental procedure (n = 11). In that new procedure, the affected bone was initially exposed by debridement with a scalpel, followed by partial excision with a bone scraper until fresh bleeding was observed from the exposed bone. The lesions were then immediately covered with an occlusive dressing, and finally the wound was covered with an epidermal graft of skin harvested from suction blisters. Patients in each group were matched with their counterparts by age, sex, wound size, wound infection and wound duration, to compare the time needed for total skin repair and rates of amputation. Epidermal grafting significantly accelerated the healing of diabetic foot ulcers (P = 0.042) without exposed bones, with site-specific differentiation. The newly developed combination therapy resulted in the healing of all diabetic ulcers with exposed bones without the occurrence of osteomyelitis or the necessity for amputation (P < 0.0001). Our study indicates that early aggressive debridement of diabetic foot ulcers with exposed bones down to a bleeding vascularized base and then grafting epidermal sheets significantly improves healing and reduces the rate of amputation.

Lipopolysaccharides from Legionella and Rhizobium stimulate mouse bone marrow granulocytes via Toll-like receptor 2.

Lipopolysaccharide (LPS) derived from enterobacteria elicit in several cell types cellular responses that are restricted in the use of Toll-like receptor 4 (TLR4) as the principal signal-transducing molecule. A tendency to consider enterobacterial LPS as a prototypic LPS led some authors to present this mechanism as a paradigm accounting for all LPSs in all cell types. However, the structural diversity of LPS does not allow such a general statement. By using LPSs from bacteria that do not belong to the Enterobacteriaceae, we show that in bone marrow cells (BMCs) the LPS of Rhizobium species Sin-1 and of three strains of Legionella pneumophila require TLR2 rather than TLR4 to elicit the expression of CD14. In addition, exposure of BMCs from TLR4-deficient (C3H/HeJ) mice to the lipid A fragment of the Bordetella pertussis LPS inhibits their activation by the Legionella lipid A. The data show selective action of different LPSs via different TLRs, and suggest that TLR2 can interact with many lipid A structures, leading to either agonistic or specific antagonistic effects.

Effects of cyclic pressure on bone marrow cell cultures.

The present in-vitro study used bone marrow cell cultures and investigated the effects of cyclic pressure on osteoclastic bone resorption. Compared to control (cells maintained under static conditions), the number of tartrate resistant acid phosphatase (TRAP)-positive, osteoclastic cells was significantly (p<0.05) lower when, immediately upon harvesting, bone marrow cells were exposed to cyclic pressure (10-40 kPa at 1.0 Hz). In contrast, once precursors in bone marrow cells differentiated into osteoclastic cells under static culture conditions for 7 days, subsequent exposure to the cyclic pressure of interest to the present study did not affect the number of osteoclastic cells. Most important, exposure of bone marrow cells to cyclic pressure for 1 h daily for 7 consecutive days resulted in significantly (p<0.05) lower osteoclastic bone resorption and in lowered mRNA expression for interleukin-1 (IL-1) and tumor necrosisfactor-a (TNF-a), cytokines that are known activators of osteoclast function. In addition to unique contributions to osteoclast physiology, the present study provided new evidence of a correlation between mechanical loading and bone homeostasis as well as insight into the molecular mechanisms of bone adaptation to mechanical loading, namely cytokine-mediated control of osteoclast functions.

Comparative toxicity of known and putative metabolites of 1,3-butadiene in human CD34 + bone marrow cells

Species-specific susceptibility to the hematotoxic effects of 1,3-butadiene (BD) is well known. Previous studies have revealed that murine bone marrow is uniquely susceptible to toxicity following exposure to the parent compound in vivo or exposure of bone marrow cells to the monoepoxide metabolite, 3,4-epoxybutane, in vitro. Studies described herein compare the relative ability of putative and known BD metabolites to produce concentration dependent suppression of colony formation and cytotoxicity in human CD34 + bone marrow cells. Compounds evaluated included 3,4-epoxybutane, d, l-butane-bis-oxide, meso-butane-bis-oxide and (2S,3R)-3-epoxybutane-1,2-diol. In contrast to results previously observed in mice, only the bis-oxides produced significant suppression of colony formation at potentially relevant concentrations (10 −8 to 10 −3 M). No enantiospecific differences were observed between the meso- and d, l-bis-oxides and no significant lineage-specific differences in susceptibility to inhibition of clonogenic response were observed among early multi-potential myeloid and erythroid hematopoietic progenitor cells. The relative potencies of the bis-oxides were found to be comparable to that of the prototype hematotoxic compound, hydroquinone. These results confirm previous studies that reveal marked species-specific differences in the susceptibility of bone marrow cells to 3,4-epoxybutane. Moreover, these results suggest that the bis-oxides of BD are capable of suppressing the clonogenic function of human hematopoietic progenitor cells, if, in fact, they are produced in human bone marrow in significant concentration. Further interpretation of these findings requires a better understanding of the metabolism of BD in humans.

Chromosome Aberrations in Bone Marrow Cells from Japanese Patients with Thorotrastosis

Exposure of bone marrow cells to alpha-particle radiation causes various types of chromosome abnormalities and hematological malignancies. We performed chromosome analysis of hematopoietic stem cells from the bone marrow of 52 Japanese patients with thorotrastosis and 21 age-matched controls. The frequency of cells with stable chromosome abnormalities was significantly higher in the patients with thorotrastosis. Further studies found 14 clonal chromosome aberrations in cells from 11 patients (21.2%); clones observed in the cells from 2 of these patients had high frequencies of chromosome abnormalities. In one case, 68 to 100% of the cells analyzed had a large partial loss in the short arm of chromosome 1 and a translocation between the short arms of chromosomes 2 and 3 [46,XY,1p-,t(2p+;3p-)]. The cells from the other patient contained a clone with partial loss of both the short and long arms of chromosome 5 (46,XX,5p-,5q-). The frequency of this clone has been constant for the last 15 years (6-24%). We also analyzed bone marrow mononuclear cells from 17 of the patients for mutations of the TP53 tumor suppressor gene (formerly known as p53). However, no mutation was found in any of the cells, including those from the 2 patients with abnormal clones. Moreover, repeated medical examinations showed no evidence of leukemia or myelodysplasia in these patients. Our study suggests that exposure of bone marrow cells to alpha-particle radiation may induce clonal chromosomal aberrations at a high frequency.

Review: antidepressants reduce pain intensity in psychogenic pain or somatoform pain disorder

Environmental exposure to benzene results in both myelotoxicity and immunotoxicity. Although benzene-induced immunotoxicity has been well documented, no studies to date have addressed the possibility that benzene toxicity is due...

Aluminium accumulation in chronic renal failure affects erythropoiesis

Summary: Aluminium (Al) toxicity has been associated with anaemia in exposed patients with chronic renal failure (CRF). the present study was undertaken to determine whether the ingestion of Al citrate was able to affect erythropoiesis in rats with normal or impaired renal function. the renal insufficiency was induced by surgical procedures and control rats were sham operated. Twenty‐four rats were allocated to four groups of six rats each: (A) Sham; (B) Sham+Al; (C) CRF; and (D) CRF+Al. the groups B and D received daily doses of Al citrate (0.5 μmol/g bodyweight) and the groups A and C, deionized water, via the intragastric route. At the end of the experimental period (15 weeks) cultures of late erythroid progenitor cells (CFU‐E) stimulated with erythropoietin were performed and haematological parameters determined. the liver, kidney, brain, bone and serum Al amounts were quantified. the results are expressed as median and interquartile range. the CFU‐E growth was found inhibited in B and D groups (A: 100; B: 74/54‐83; C: 86/54‐98; D: 46/39‐53 %). the haematocrit values were significantly diminished in rats with renal insufficiency when compared to controls (A: 42/40‐43; B: 45/42‐46; C: 37/32‐40 and D: 37/24‐39 %). Serum Al accumulation was observed in B and D groups receiving Al (A: 8/5‐12; B: 36/36‐44; C: 5/5‐6; D: 45/26‐132 μg Al/l). No differences among groups were found in the liver and kidney Al contents, but uraemic state favoured Al accumulation in brain (A: 6/5.0‐9.0; B: 4/3.8‐4.3; C: 2/1.0‐3.0; D: 15/12.0‐21.0 μg Al/g tissue) and bone (A: 29/27‐31; B: 30/29‐39; C: 42/33‐48; D: 68/56‐79 μg Al/g tissue). We suggest that the heavy accumulation of Al in the bone compartment may result in a protracted endogenous exposure of bone marrow cells, affecting the erythropoiesis in vivo.

Effects of Pasteurella multocida toxin on porcine bone marrow cell differentiation into osteoclasts and osteoblasts.

The effect of Pasteurella multocida toxin (PMT) on porcine osteoclast and osteoblast differentiation was studied using in vitro cell culture systems. When grown in the presence of Vitamin D3, isolated porcine bone marrow cells formed multinucleated cells with features characteristic of osteoclasts. Exposure of bone marrow cells to Vitamin D3 and PMT during growth resulted in formation of increased numbers and earlier appearance of osteoclasts compared to controls. Ultrafiltered medium form PMT-treated cells likewise increased osteoclast numbers, suggesting that a soluble mediator may be involved in the action of PMT. When cell cultures were treated with fluorescein-labeled PMT, fluorescence was found within the cytoplasm of small, round cells that did not resemble either osteoclasts or osteoclastic precursor cells. Cultures of porcine bone marrow cells exposed to dexamethasone, ascorbic acid, and beta-glycerophosphate developed into osteoblastic cells that formed multilayered, mineralized nodules. Exposure of osteoblastic cultures to low concentration of PMT resulted in retarded cell growth, formation of decreased numbers of nodules and minimal to no mineralization in the nodules; higher concentration of PMT resulted in increased cellular debris and poor growth of cells, with no nodule formation. These findings suggest that PMT may induce turbinate atrophy in pigs by increasing osteoclast numbers and inhibiting osteoblastic bone formation. The effect of PMT on osteoclastic differentiation and growth may not be due to a direct effect on preosteoclastic cells, but rather due to alterations in the soluble mediator secretion by marrow stromal cells.

Functional lipopolysaccharide receptors of low affinity are constitutively expressed on mouse bone marrow cells.

Although lipopolysaccharide (LPS)-induced overproduction of cytokines, involved in the pathogenesis of septic shock, occupies the spotlight of endotoxin research, another LPS effect, the differentiation of various cell types including haematopoietic bone marrow cells (BMC), which is probably related to its radioprotective activity, deserves equal attention. We have previously established that nanomolar concentrations of LPS trigger in human BMC the expression of CD14 by an induction mechanism independent of CD14 or any other molecule anchored to the cell membrane by a glycosyl phosphatidylinositol glycolipid. We now show that this LPS-induced stimulation is triggered by the binding of a small number of LPS molecules (13,000 molecules/cell) to constitutive LPS receptors of low affinity (Kd = 480 nM). This interaction, which was inhibited by a synthetic LPS antagonist, appeared specific, reversible, saturable, time- and temperature-dependent, but was independent of divalent cations, and was inhibited by serum. Exposure of BMC to LPS did not induce a down-modulation of these receptors, but enhanced their sensitivity to trypsin degradation. Inhibition of LPS binding following different treatments correlated with inhibition of BMC stimulation, thus suggesting that the sparse constitutive receptors of low affinity are efficient signalling receptors for LPS.

Effect of Benzene on Fibroblastoid Colony-Forming Units in Mice

After exposure of C57BL6 x DBA/2 mice to benzene in air their number of bone marrow fibroblastoid precursor cells, CFU-F, was determined. The CFU-F exhibited an increasing plating efficiency, giving rise to a larger number of colonies and to colonies of greater size. This effect was dose dependent. When the mice were exposed for 16 weeks and were then allowed to rest, their CFU-F plating efficiency returned to normal within 6 weeks, but then increased again. Hematopoietic stem cells, such as CFU-S and CFU-C exhibited a dose-dependent depression. The in vitro exposure of bone marrow cells to benzene metabolites resulted in a dose-dependent depression of CFU-F numbers.

Interferon is a mediator of hematopoietic suppression in aplastic anemia in vitro and possibly in vivo.

We have investigated interferon as a mediator of hematopoietic suppression in bone marrow failure. Interferon production by stimulated peripheral blood mononuclear cells from patients with aplastic anemia was significantly higher than that observed in controls; spontaneous interferon production by these cells was also high for more than half of aplastic anemia patients. Circulating interferon, not detectable in normal individuals, was detected in 10 of 24 patients. Interferon is a potent inhibitor of hematopoietic cell proliferation and, therefore, may be the mediator of suppression in many in vitro models employing patients' cells and sera. The possible pathogenic importance of interferon in aplastic anemia was suggested by an increase in hematopoietic colony formation in vitro after exposure of bone marrow cells to antiinterferon antisera (277 +/- 71% increase for patients compared to 1.6 +/- 1.6% for normal individuals). Interferon levels in the bone marrow sera of aplastic anemia patients were high (mean = 203 international units (IU)/ml, n = 8), even in comparison to circulating levels in the same patients. Normal bone marrow sera also contained measurable interferon but at lower levels (41 IU/ml, n = 16), indicating that interferon may be a normal bone marrow product. High concentration of bone marrow interferon, possibly due to abnormal immunologic activity or a reaction to virus infection of the bone marrow, may mediate hematopoietic suppression in aplastic anemia patients.

Sedimentation velocity characterisation of the cell cycle of granulocytic progenitor cells in monkey hemopoietic tissue.

AbstractSedimentation velocity separation of Rhesus monkey bone marrow cells has demonstrated a reproducible but heterogeneous size distribution of cells capable of forming granulocytic colonies in agar culture (CFC's). This heterogeneity is shown to be due to the cell cycle status of the progenitor cell population. In vitro exposure of bone marrow cells to lethal doses of tritiated thymidine (H3TdR) either before or after separation restricts the size distribution of CFC's, greatly reducing the proportion of rapidly sedimenting cells. The calculation of the volume distribution of such cells before and after H3TdR exposure indicates that 55% of total CFC's in adult marrow are in G0 or G1 with a volume of 410 μ3, 42% are in S phase and of volume 450–950 μ3, and the remainder are in G2 and mitosis with a volume of between 600–950 μ3. CFC's in mid gestation fetal liver were larger than their adult counterparts and were of homogeneous volume indicative of a single non cycling population with no evidence of an S or G2 component. H3TdR exposure confirmed the non‐cycling status of these fetal progenitor cells.