Contribution of the mouse spleen to post-hemorrhagic erythropoiesis

Serum soluble transferrin receptor concentration is an accurate estimate of the mass of tissue receptors

OBJECTIVE

Serum levels of the soluble transferrin receptor (sTfR) vary depending on the erythropoietic activity and iron status. In vitro, sTfR shed in the incubation medium correlates well with cellular TfR, but this relationship has never been established in vivo. To determine the value of serum sTfR as a quantitative marker of the body mass of tissue TfR, we designed experiments to examine the correlation between serum sTfR and tissue TfR in rats with various degrees of erythropoietic activity or iron status.

MATERIALS AND METHODS

We studied changes in erythropoietic activity in normal rats as well as in animals experiencing hemolysis, phlebotomy-induced iron deficiency, transfusion- or thiamphenicol-induced erythroid aplasia, or inflammation. At the end of follow-up, ferrokinetic studies were performed and animals were sacrificed. Organs were isolated and homogenized to determine the total mass of tissue TfR from the sum of tissue solubilized TfR in the bone marrow, spleen, liver, and blood cells (direct method). An indirect method was developed to derive the corporeal mass of tissue TfR from a representative marrow sample.

RESULTS

As expected, serum sTfR and total mass of tissue TfR varied as a function of iron status and erythropoiesis. Relative erythroid expansion in the spleen was greater than in the bone marrow. With the exception of phlebotomized animals, the indirect method correlated very well with direct measurements of the total mass of tissue TfR (r = 0.97, p < 0.0001). There was a close relationship between the total mass of tissue TfR and the total mass of serum sTfR (r = 0.79, p < 0.0001). Serum sTfR represented approximately 5-6% of the total mass of tissue TfR in most experimental situations, but this ratio was twice as high during iron-restricted erythropoiesis. In addition, the ratio could be higher or lower in nonsteady-state situations, because changes in tissue TfR occurred faster than those of serum sTfR.

CONCLUSIONS

Serum sTfR represents a constant proportion of the total mass of tissue TfR over a wide range of erythropoietic activity. However, iron deficiency results in a higher proportion of serum sTfR, and the pace of change in serum sTfR levels is slower than that of tissue TfR mass.

The genetic toxicity of 3,3',4,4'-tetrachloroazobenzene and 3,3',4, 4'-tetrachloroazoxybenzene: discordance between acute mouse bone marrow and subchronic mouse peripheral blood micronucleus test results

3,3',4,4'-Tetrachloroazobenzene (TCAB) and 3,3',4, 4'-tetrachloroazoxybenzene (TCAOB) are dioxin-like chemicals that were investigated for toxicity in 13-week gavage studies in male and female B6C3F(1) mice and F344N rats by the National Toxicology Program. As part of the comprehensive toxicological investigation of these chemicals, peripheral blood smears from mice treated 5 days per week for 13 weeks with 0.1-30mg/kg/day TCAB or TCAOB were analyzed for the frequency of micronucleated (MN) normochromatic erythrocytes (NCE). Both chemicals produced significant increases in MN-NCE in male and female mice. In contrast to these positive results in subchronic exposure studies, no significant increases were seen in acute bone marrow MN tests in male mice administered three daily injections of 50-200mg/kg/day TCAB and TCAOB. The results with TCAB and TCAOB suggest that the routine integration of MN tests with subchronic toxicity studies may allow detection of mutagenic activity for some chemicals that fail to elicit responses in short-term, high dose tests. In addition, the integration of mutagenicity tests into general toxicity tests reduces the use of laboratory animals and the cost of the testing.

Comparison of erythropoietic response to androgen in young and old senescence accelerated mice

In this study, to clarify whether the functional capacity of hemopoietic progenitor cells and the micro-environment of aged mice are identical with those of the young, we investigated the changes in the number of hemopoietic progenitor cells and the production of regulatory cytokines from splenic cells as well as changes in the serum levels of cytokine in senescence-accelerated mice (SAM) after administration of 19-nandrolone decanoate (19-ND), a synthetic androgenic anabolic steroid. 19-ND induced an increase in erythroid colony-forming units (CFU-E), erythroid burst-forming units (BFU-E), and granulocytic-macrophage committed progenitor cells (CFU-GM) in bone marrow and spleen; especially remarkable increases were observed in the splenic CFU-E in both young and old mice. Antigen expression analysis of hemopoietic organs revealed that total TER-119+ cells per spleen of young and old mice with androgen treatment rose 2.6- and 3.2-fold over their respective control values. The responsiveness of hemopoietic progenitor cells to androgen did not change with age. Injection of 19-ND into young and old mice markedly enhanced the erythropoietin levels but not IL3 and GM-CSF levels in the serum of both groups. Cytokine production assessed by pokeweed mitogen-stimulated spleen condition medium showed an age-related decline. Androgen treatment could not influence IL-3 and GM-CSF production of spleen. These findings suggest that the spleen of both old and young mice served as the major site of regenerative repopulation of hemopoietic progenitors, especially the late erythroid progenitors in 19-ND-treated mice. The proliferative reserve of erythropoiesis with androgen treatment in aged mice was not reduced more than that in treated-young mice.

Effects of colloidal resuscitation fluids on reticuloendothelial function and resistance to infection after hemorrhage

The effects of three resuscitation fluids, hydroxyethyl starch (HES), Haemaccel, and fresh autologous blood, on reticuloendothelial system phagocytic and catabolic functions and resistance to infection after 40% hemorrhages in BALB/c mice were studied. The mice, anesthetized with isoflurane, were bled over a 10-min period, left hypovolemic for 30 min, and then resuscitated with their shed blood or the same volume of asanguineous fluid. Normothermia was maintained throughout the experiments. The uptake and catabolism of intravenously injected double-labelled sheep erythrocytes (51Cr-125I-SRBC) in liver and spleen were determined at 1 and 48 h after hemorrhage. No significant changes in the uptake or catabolism of SRBC in liver or spleen were found at 1 h after hemorrhage and resuscitation with any of the fluids. However, at 48 h a significant increase in liver uptake of SRBC was seen in animals resuscitated with either Haemaccel or HES compared to that in animals resuscitated with shed blood or in animals subjected to a sham operation. The increase in liver uptake was accompanied by a small decrease in spleen uptake in animals resuscitated with Haemaccel but not with HES. No great changes in catabolic activity were seen at 48 h, although activity levels tended to be higher in animals resuscitated with Haemaccel. Separate groups of animals were challenged by an intraperitoneal injection with live Escherichia coli at 1 or 48 h after hemorrhage and resuscitation. Sixty-four percent of the animals resuscitated with shed blood survived the challenge with E. coli at 1 h after hemorrhage, whereas only 10 and 0% survival was seen for animals resuscitated with Haemaccel and HES, respectively. At 48 h survival was 80% for shed-blood-resuscitated animals and 60 and 70% for Haemaccel- and HES-resuscitated animals, respectively.

Steric effects on the haemolytic activity of aromatic disulphides in rats

Certain derivatives of diphenyl disulphide are known to cause haemolytic anaemia in rats, by a mechanism possibly involving intra-erythrocytic redox cycling with concomitant generation of 'active oxygen' species. In ring-substituted diphenyl disulphide derivatives, electronic effects of substituents have been shown markedly to affect the rate of 'active oxygen' production in vitro and toxicity in vivo. In the present study, the influence of steric effects of substituents on these parameters has been investigated. The severity of the haemolysis induced in groups of seven rats by oral dosing with 4,4'-dimethoxydiphenyl disulphide and 4,4'-dimethyldiphenyl disulphide at doses of 500 mumol/kg/day for 6 days was greater than that of the 2,2' isomers and the haemolytic activity of a series of 2,2'-dialkyl derivatives decreased with increasing size of the alkyl group. In vitro, the haematin-catalysed oxidation rates and the rates of redox cycling of the corresponding thiols in the presence of glutathione were similarly influenced by steric hindrance. The structure-activity relationships identified in the present investigation, together with knowledge of the electronic effects of substituents, should permit accurate prediction of the toxicity of new or untested aromatic thiols and disulphides.

A mathematical model of erythropoiesis in mice and rats. Part 4: Differences between bone marrow and spleen

In a preceding analysis we hypothesized that the most important parameter controlled by erythropoietic regulation in vivo is the degree of amplification (number of cell divisions) in the CFU-E and erythroblast cell stages. It was concluded that erythropoietic amplification in vivo is controlled according to a sigmoidal dose-response relationship with respect to the control parameter which is the haematocrit (or haemoglobin concentration). Here, this hypothesis is extended to include the differences in murine bone marrow and splenic erythropoiesis that are described and quantified by different dose-response relationships. Comparing several sets of experimental data with mathematical model simulations, this approach leads to the following conclusions: (i) in the unperturbed normal steady state at least one extra erythropoietic cell division takes place in the spleen compared with the bone marrow; (ii) a strong erythropoietic stimulus, such as severe bleeding or hypoxia, can induce five to six additional cell divisions in the spleen but only two to three additional divisions in the bone marrow; this results in a considerable increase in the spleen's contribution to erythropoiesis from about 10% in normal animals to over 40% during strong stimulation; (iii) under erythropoietic suppression, such as red cell transfusion, a similar number of cell divisions is skipped in both organs and the splenic contribution to erythropoiesis remains unchanged. In conclusion, the concept that bone marrow and spleen microenvironments differ in the dose-response relationship for erythropoietic regulation provides an explanation for the changing contribution of splenic murine erythropoiesis following a variety of experimental treatments.

A mathematical model of erythropoiesis in mice and rats. Part 2: Stimulated erythropoiesis

A mathematical model of erythropoietic cell production and its regulation process has been proposed in a preceding paper. It is primarily based on the assumption that the number of cell divisions taking place in the CFU-E and erythropoietic precursor stages is regulated depending on the oxygen supply of the tissue. Quantitative dose-response relationships for in vivo erythropoiesis are suggested. Here, we demonstrate that this model adequately reproduces data obtained in situations of stimulated erythropoiesis in mice and rats. In detail, this implies a quantitative description of the following processes: (1) Changes in tissue oxygen tension (Pto2) following removal of red cells (bleeding, haemolytic anaemia) or increase in plasma volume (dilution anaemia) or decrease in atmospheric oxygen pressure (hypoxia). (2) Pto2 dependent erythropoietin (EPO) production. (3) Dose-response of EPO on erythropoietic amplification (up to two to four additional mitoses). (4) The changes of the marrow transit time. Model simulations are compared with experimental data for changes of erythropoiesis during hypoxia, EPO-injection, and different forms of anaemia. A satisfactory agreement suggests that the model adequately describes and correlates different direct and indirect ways to stimulate erythropoiesis. It quantifies the role and relative contribution of the haematocrit, haemoglobin concentration, atmospheric oxygen pressure, tissue oxygen pressure, and plasma volume as triggers in erythropoietic stimulation under various conditions. Furthermore, the model may allow to optimize the scheme of EPO-administration and to find the maximum increase of erythropoiesis for a given amount of erythropoietin.

Response of the murine hematopoietic system to chronic infection with Mycobacterium lepraemurium

Mycobacterium lepraemurium infection of mice produces a chronic lethal disease that is characterized by massive accumulation of macrophages throughout the mononuclear-phagocyte system. We studied the influence of M. lepraemurium infection on the composition and function of the hematopoietic system. Medullary erythropoiesis was virtually abolished, as reflected by a small number of erythroid elements and a decrease in the number and frequency of erythroid progenitors in the bone marrow, together with reduced uptake of 59Fe into bone marrow hemin. On the other hand, erythropoiesis was observed in the spleen, as demonstrated by a large number of erythroid cells, a sixfold increase of 59Fe uptake, and a pronounced increase in the number of erythroid progenitors. A considerable increase of monocyte progenitors was observed in the spleen, and a more modest increase was observed in the bone marrow. This increase may be accounted for, at least in part, by greatly increased levels of macrophage-colony-stimulating factor in the serum of infected mice. Thus, M. lepraemurium infection produces important changes in the hematopoietic system, during the course of which the spleen becomes the major hematopoietic organ.

Haemopoietic regulation and the role of the macrophage in erythropoietic gene expression

The macrophage is considered as an "active" component of the haemopoietic cellular microenvironment with respect to erythropoietin (epo) production during embryonic, foetal and adult erythropoiesis. Emphasis is placed on steady-state rather than pathophysiological conditions. In addition, the signals capable of affecting the functional capacity of the macrophage with regard to colony stimulating factor and epo production are also taken into account. Evidence is given demonstrating that a subpopulation of resident macrophages in vitro and in the mouse bone marrow, under normal conditions, can express the epo gene. These results indicate that erythropoiesis can be regulated by short-range or cell-to-cell interactions within the bone marrow.

Comparative cytogenetic analysis of bone marrow damage induced in male B6C3F1 mice by multiple exposures to gaseous 1,3-butadiene

Groups of male B6C3F1 mice (N = 12) were exposed to ambient air or to gaseous 1,3-butadiene (BD) at 6.25, 62.5, and 625 ppm for 10 exposure days (6 hr + T90/day). Exposure to BD induced in bone marrow: 1) a significant increase in the frequency of chromosomal aberrations (CA); 2) a significant elevation in the frequency of sister chromatid exchanges (SCE); 3) a significant lengthening of the average generation time (AGT); 4) a significant depression in the mitotic index (MI); and, as measured in the peripheral blood, 5) a significant increase in the proportion of circulating polychromatic erythrocytes (%PCE), and 6) a significant increase in the level of micronucleated PCE (MN-PCE) and micronucleated normochromatic erythrocytes (MN-NCE). The most sensitive indicator of genotoxic damage was the frequency of SCE (significant at 6.25 ppm), followed by MN-PCE levels (significant at 62.5 ppm), and then by CA and MN-NCE frequencies (significant at 625 ppm). The most sensitive measure of cytotoxic damage was AGT (significant at 62.5 ppm), followed by %PCE (significant at 625 ppm), and then by MI (significant by trend test only). Because each cytogenetic endpoint was evaluated in every animal, a correlation analysis was conducted to evaluate the degree of concordance among the various indicators of genotoxic and cytotoxic damage. The extent of concordance ranged from a very good correlation between the induction of MN-PCE and the induction of SCE (correlation coefficient r = 0.9562) to the lack of a significant correlation between the depression in the MI and any other endpoint (r less than 0.37).

Hematopoiesis and aging. V. A decline in hematocrit occurs in all aging female B6D2F1 mice

Longitudinal studies of hematocrits were done in aging B6D2F1 female mice at 54, 64, 91, 105 and 115 weeks of age. A modest decline in hematocrit was observed in 41/42 mice; we have previously shown that the decreased hematocrit of aged as compared to young mice is due to an expansion of plasma volume. Mice which died spontaneously after 91 weeks had lower hematocrits at 91 weeks and 105 weeks than did those which survived to 115 weeks. At each time interval, a sub-group of mice was killed and uptake of 59Fe into blood, foreleg, spleen and liver was studied and total nucleated cells per humerus was determined. The results were generally compatible with the thesis that aging mice maintain normal rates of erythropoiesis under basal conditions. Thus, it would appear that a decrease in hematocrit can be considered an expected part of the aging process in this mouse.

Marrow mass and distribution in murine skeletons cleaned by beetles as compared to cut up carcasses and a further simplification of the latter technique

Distribution of 59Fe into various bone groups of the complete murine skeleton was studied using two methods of dividing up the bones: 1) our previously reported technique of simply cutting up a skinned, eviscerated carcass and 2) separating bones from skeletons cleaned of overlying tissue by beetles, Dermestes species. The total percentage of injected 59Fe recovered in the sum of all skeletal parts, the percentage of total skeletal 59Fe found in each bone group, and the overall accuracy of determining these values were quite similar for the two techniques. The only statistically significant difference shown was a modest decrease in the percentage of total skeletal iron found in ribs plus sternum plus cervical and thoracic spine in beetle-cleaned as compared to cut up groups and we would not consider this to be of biological significance. Cutting up carcasses is the simpler of the two techniques but there are circumstances in which beetle digestion would be advantageous. In addition, we collected data on the reproducibility and precision of determining the percentage of 59Fe injected which is found in a "pulled off" foreleg plus scapula and of the distribution of 59Fe within three cut up pieces from the leg and within the scapula. These data can be used as a measure of overall changes in marrow mass and/or distribution, or at least they can be used as a screening procedure to detect such. This simple procedure adds potentially useful values for fully interpreting hematopoietic changes in the mouse.

Toxicity of aromatic disulphides. III. In vivo haemolytic activity of aromatic disulphides

Diphenyl disulphide, 4,4'-diaminodiphenyl disulphide, 2,2'-diaminodiphenyl disulphide, 4,4'-dimethyldiphenyl disulphide and 4,4'-dinitrodiphenyl disulphide, when administered orally to rats, induced haematological and pathological changes indicative of erythrocyte destruction in vivo. No evidence of haemolysis was detected, however, in animals receiving diphenyl disulphide-2,2'-dicarboxylic acid or dibenzyl disulphide. The order of activity of the various aromatic disulphides in provoking in vivo haemolysis was similar to that previously recorded for 'active oxygen' generation and erythrocyte damage in vitro. The results of this investigation suggest that in vivo haemolysis may be anticipated from any disulphide or thiol which undergoes appreciable autoxidation at neutral pH. While aromatic or alpha beta-unsaturated thiols and disulphides would be expected to be the most active haemolytic agents, other thiols or disulphides may precipitate the destruction of erythrocytes whose defences against oxidative attack are deficient.

Hematopoiesis and aging III: Anemia and a blunted erythropoietic response to hemorrhage in aged mice

Whether the hematocrit normally declines in the aged or whether such a decline represents inapparent disease in addition to aging is a matter of dispute. Female B6D2f1 mice were studied at ages 3, 13, or 27-28 months, and there was no difference in hematocrit between the younger groups. The hematocrit of 45 aged mice was slightly lower than that of 66 younger mice; mean 43% vs 49% (p less than .001). However, rather unexpectedly, the total red cell mass was not decreased in the aged; rather, the plasma volume was expanded. Survival of mature red blood cells did not differ significantly between young and aged mice. Mice were bled 0.4 ml from the orbital sinus for 4 days, reducing the hematocrit of all groups to a nadir of 20-25%. Recovery of hematocrit began more slowly in aged than in young mice. That this reflected a difference in erythropoiesis rather than a difference in plasma volume equilibration was suggested by studies with 59Fe. 59Fe was given following the second bleed, and 1 day later RBC 59Fe was more than twice as high in young mice than in groups of aged mice. Aged mice that did not appear healthy had been excluded. Aged mice were divided into a group with significant amounts of gray hair and/or patches of hair loss and two groups with normal-appearing hair; the latter was subdivided into those weighing less (25-26 g) or more (30-34 g) than most aged mice. Neither hair condition nor weight influenced hematocrit or response to bleeding. These results suggest, but do not prove, that a mild "dilutional" anemia and a blunted erythropoietic response to hemorrhage may be an expected part of the murine aging process.

Heme metabolism in liver and spleen of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU)-treated rats

The effect of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), an anticancer alkylating agent of the nitrosourea group, on liver and spleen enzymes involved in the control of heme metabolism was studied. A single oral dose of 50 or 100 mg/kg CCNU caused a time-dependent loss in weight of both spleen and liver. Seven days after CCNU treatment (100 mg/kg) the weights were at 45 and 65% of controls respectively. The activity of delta-aminolevulinic acid synthetase (ALA-S), the rate-limiting enzyme in heme biosynthesis, declined in spleen and liver to 11 and 24% of control values, respectively, 7 days after CCNU treatment. Heme oxygenase activity, the rate-limiting enzyme of heme breakdown, was moderately increased in liver and spleen following CCNU administration. In liver, heme oxygenase activity was 142% of control values at 24 hr, and in spleen the activity was 180% of controls at 1 week. Pretreatment of the animals with phenobarbital (PB) (40 mg/kg/day, i.p.) for 4 days caused a reversal in the decline of liver weight with no effect on the decline in spleen weight following CCNU treatment. Similarly, PB pretreatment reversed the decline in hepatic ALA-S activity after CCNU administration but had no effect on the decline in splenic ALA-S activity. This study indicates that CCNU causes significant decreases in the activity of enzymes of heme biosynthesis in spleen and liver. The CCNU hemotoxicity in the liver was reversed by PB pretreatment whereas the splenic hemotoxicity was unchanged.

Repeated exposure of C57Bl mice to inhaled benzene at 10 ppm markedly depressed erythropoietic colony formation

Exposure of C57Bl mice to 10 ppm benzene (the current occupational exposure limit) for 6 h/day, 5 days/week causes a progressive depression in the in vitro colony forming ability of one of the erythroid progenitor cells, the colony-forming unit-erythroid (CFU-E). Colony growth of cells from exposed mice was only 5% of control colony growth after 178 days of exposure. Burst-forming-cell growth was depressed to 55% of control growth after 66 days but returned to control growth values at 178 days. In addition, benzene-exposed mice exhibited depressions in the numbers of splenic nucleated red cells and in the numbers of circulating red cells and lymphocytes. These results suggest that low-level exposure to benzene may be hematotoxic.

Distribution of 35S in mice after oral administration of alpha-dithioacetamidinium chloride. An autoradiographic investigation

The distribution of 35S in mice has been investigated by whole-body autoradiography after oral administration of the 35S labelled S-75 (2,2'-Dithiobis(N-[(1-adamantyl)-methyl]-acetamidine)dihydrochloride). The substance was rapidly absorbed, and the highest concentration occurred in the liver and kidneys. During the highest radiation protective activity (after about 45 min) a substantial concentration was found in the red pulp of the spleen. This supports previous findings that S-75 has its most marked protective effect on the splenic haemopoiesis. Previously, it was shown that cysteamine is more evenly distributed in the body than S-75, which is in agreement with its more generalized protective effect than S-75.

Comparison between two radiation protective substances in irradiated mice. Effect on splenic haemopoiesis

The radiation protective action of 2,2'-Dithiobis(N-[(1-adamantyl)-methyl]-acetamidine)-dihyrochloride (S-75) and cysteamine was compared in splenectomized and non-splenectomized mice. Cysteamine was found to have better and more general protection properties. Several indications of a specific effect of S-75 on the spleen were observed. It is suggested that the protection properties of S-75 should be tested in another laboratory animal not having such a marked splenic haemopoiesis as the mouse.

The granulocytopoiesis in the spleen of newborn rats

The granulocytopoiesis in the spleen of newborn, 1h-old rats was analysed. It was found that the transit times of mycleocytes and metamyelo cytes were remarkably shorter than in adult animals, and that the mitotic frequency of myelocytes was only nearly half as high. The average generation time of immature myelopoietic cells was estimated to be 8.4h, the S-phase duration 4.0h. The findings may be attributed to an elevated input from stem cells in contrast to the undisturbed granulocytopoiesis of adult rats, which is obviously maintained by a higher divisional rate.

Postirradiation erythropoietic recovery in splenectomized mice

The influence of splenectomy on erythropoietic recovery of lethally X-irradiated mice injected with different doses of syngeneic bone marrow was studied. Splenectomized animals showed less activation than unoperated controls in the lower dose range; however, by increasing the number of injected cells the response obtained was similar in both groups. Providing an adequate number of stem cells is administered to the splenectomized recipients, an enhanced erythropoietic activity of the graft may compensate for the absence of the spleen, which is an important organ in postirradiation recovery. Suggested explanations for this observation are lack of an haematopoietic inhibitory effect of the irradiated spleen, or changes in the environment provided by the spleenless host.

Inverse relationship between splenomegaly and stem cell compartment size in mice treated with nitrogen mustard

Following the administration of similar doses of nitrogen mustard (4 mg/kg) to different strains of mice, wide variations in the subsequent degree of splenomegaly were observed, implying strain differences in the role of the spleen in the compensatory erythropoietic response to haematopoietic stress. This investigation was undertaken to determine whether or not these differences were related to the size of the haematopoietic stem cell compartment size in the various strains of mice. Groups of 4 different strains of mice (Swiss Webster, A/J, C57BL/6J and CS1/ASH) were injected i.v. with nitrogen mustard (4 mg/kg body weight) and autopsied at regular intervals up to 20 d post-injection. At autopsy, the wet weight of the spleen was determined. Subsequently, groups of the same 4 strains of mice were exposed to single doses of wholebody gamma-irradiation in the range of 500-900 rads. 9 d after gamma-irradiation the mice were autopsied, their spleens removed, and the number of endogenous spleen colonies determined. The greatest degree of splenomegaly was observed in the C57BL/6J mice. The Swiss Webster mice showed no splenomegaly during the time period studied. There existed a linear inverse relationship between the maximum degree of splenomegaly observed and the dose of wholebody gamma-irradiation required to completely eliminate endogenous spleen colonies. This data is in accord with the hypothesis that there exists an inverse relationship between the extent of splenomegaly observed following haematopoietic stress and the haematopoietic stem cell compartment size.

On the in vivo action of erythropoietin: a quantitative analysis

The composite response of the erythron to exogenous erythropoietin has been studied in normal, splenectomized, and polycythemic mice. After stimulation the normal animal doubled its marrow nucleated red cells by the 3rd day with little further change by the 5th. Nucleated red cells within the spleen began to increase sharply on the 2nd day and, by the 5th, exceeded those in the marrow. The total nucleated erythroid response represented a fourfold increase. Reticulocytes lagged behind the expansion of the nucleated red cell mass, but by the 5th day the original ratio was re-established. Hemoglobin synthesis was increased, but the ratio of hemoglobin synthesized in nucleated red cells and reticulocytes was basically unchanged. Early displacement of marrow reticulocytes into circulation and the production of a larger red cell also occurred. No evidence of a change in the number of erythroid mitoses was found; only a slight decrease in the average cell cycle time was demonstrated. Thus, whereas erythropoietin stimulation induced several changes in erythropoiesis, the increased number of cells entering into the maturing pool appeared to be of greatest quantitative significance.Splenectomy reduced the proliferative response of the erythron over 5 days stimulation to three-fourths that found in the normal animal. This difference, also reflected in a proportionately lower reticulocyte response and increment in circulating red cell mass, suggests that erythropoiesis within the mouse marrow is spatially or otherwise restricted and that the spleen provided a supplemental area of erythroid expansion.