Sequestration and possible maturation of reticulocytes in the normal spleen

How the spleen reshapes and retains young and old red blood cells: A computational investigation

The spleen, the largest secondary lymphoid organ in humans, not only fulfils a broad range of immune functions, but also plays an important role in red blood cell’s (RBC) life cycle. Although much progress has been made to elucidate the critical biological processes involved in the maturation of young RBCs (reticulocytes) as well as removal of senescent RBCs in the spleen, the underlying mechanisms driving these processes are still obscure. Herein, we perform a computational study to simulate the passage of RBCs through interendothelial slits (IES) in the spleen at different stages of their lifespan and investigate the role of the spleen in facilitating the maturation of reticulocytes and in clearing the senescent RBCs. Our simulations reveal that at the beginning of the RBC life cycle, intracellular non-deformable particles in reticulocytes can be biomechanically expelled from the cell upon passage through IES, an insightful explanation of why this peculiar “pitting” process is spleen-specific. Our results also show that immature RBCs shed surface area by releasing vesicles after crossing IES and progressively acquire the biconcave shape of mature RBCs. These findings likely explain why RBCs from splenectomized patients are significantly larger than those from nonsplenectomized subjects. Finally, we show that at the end of their life span, senescent RBCs are not only retained by IES due to reduced deformability but also become susceptible to mechanical lysis under shear stress. This finding supports the recent hypothesis that transformation into a hemolyzed ghost is a prerequisite for phagocytosis of senescent RBCs. Altogether, our computational investigation illustrates critical biological processes in the spleen that cannot be observed in vivo or in vitro and offer insights into the role of the spleen in the RBC physiology.

The spleen, the largest secondary lymphoid organ in humans, not only fulfils a broad range of immune functions, but also plays an important role in red blood cell (RBC) life cycle. In this study, we perform a computational study to simulate the passage of RBCs through interendothelial slits (IES) in the spleen at different stages of their lifespan, a critical biological process that cannot be observed in humans. Our simulation results illustrate a specific role of spleen in shaping young RBCs, which points to a probable missing step in current in vitro RBC culture protocols that fail to generate a majority of typical biconcave RBCs. Our results also reveal that intra-splenic mechanical constraints likely contribute to the final clearance and elimination of aged RBCs. Altogether, we demonstrate that our computational model can provide mechanistic rationales for experimental studies, offer insights into the role of the spleen in the RBC physiology and help the optimization of in vitro RBC culture techniques.

Erythrocyte flow through the interendothelial slits of the splenic venous sinus

The flow patterns of red blood cells through the spleen are intimately linked to clearance of senescent RBCs, with clearance principally occurring within the open flow through the red pulp and slits of the venous sinus system that exists in humans, rats, and dogs. Passage through interendothelial slits (IESs) of the sinus has been shown by MacDonald et al. (Microvasc Res 33:118-134, 1987) to be mediated by the caliber, i.e., slit opening width, of these slits. IES caliber within a given slit of a given sinus section has been shown to operate in an asynchronous manner. Here, we describe a model and simulation results that demonstrate how the supporting forces exerted on the sinus by the reticular meshwork of the red pulp, combined with asymmetrical contractility of stress fibers within the endothelial cells comprising the sinus, describe this vital and intriguing behavior. These results shed light on the function of the sinus slits in species such as humans, rats, and dogs that possess sinusoidal sinuses. Instead of assuming a passive mechanical filtering mechanism of the IESs, our proposed model provides a mechanically consistent explanation for the dynamically modulated IES opening/filtering mechanism observed in vivo. The overall perspective provided is also consistent with the view that IES passage serves as a self-protective mechanism in RBC vesiculation and inclusion removal.

Evaluation of splenic accumulation and colocalization of immature reticulocytes and Plasmodium vivax in asymptomatic malaria: A prospective human splenectomy study

BACKGROUND

A very large biomass of intact asexual-stage malaria parasites accumulates in the spleen of asymptomatic human individuals infected with Plasmodium vivax. The mechanisms underlying this intense tropism are not clear. We hypothesised that immature reticulocytes, in which P. vivax develops, may display high densities in the spleen, thereby providing a niche for parasite survival.

METHODS AND FINDINGS

We examined spleen tissue in 22 mostly untreated individuals naturally exposed to P. vivax and Plasmodium falciparum undergoing splenectomy for any clinical indication in malaria-endemic Papua, Indonesia (2015 to 2017). Infection, parasite and immature reticulocyte density, and splenic distribution were analysed by optical microscopy, flow cytometry, and molecular assays. Nine non-endemic control spleens from individuals undergoing spleno-pancreatectomy in France (2017 to 2020) were also examined for reticulocyte densities. There were no exclusion criteria or sample size considerations in both patient cohorts for this demanding approach. In Indonesia, 95.5% (21/22) of splenectomy patients had asymptomatic splenic Plasmodium infection (7 P. vivax, 13 P. falciparum, and 1 mixed infection). Significant splenic accumulation of immature CD71 intermediate- and high-expressing reticulocytes was seen, with concentrations 11 times greater than in peripheral blood. Accordingly, in France, reticulocyte concentrations in the splenic effluent were higher than in peripheral blood. Greater rigidity of reticulocytes in splenic than in peripheral blood, and their higher densities in splenic cords both suggest a mechanical retention process. Asexual-stage P. vivax-infected erythrocytes of all developmental stages accumulated in the spleen, with non-phagocytosed parasite densities 3,590 times (IQR: 2,600 to 4,130) higher than in circulating blood, and median total splenic parasite loads 81 (IQR: 14 to 205) times greater, accounting for 98.7% (IQR: 95.1% to 98.9%) of the estimated total-body P. vivax biomass. More reticulocytes were in contact with sinus lumen endothelial cells in P. vivax- than in P. falciparum-infected spleens. Histological analyses revealed 96% of P. vivax rings/trophozoites and 46% of schizonts colocalised with 92% of immature reticulocytes in the cords and sinus lumens of the red pulp. Larger splenic cohort studies and similar investigations in untreated symptomatic malaria are warranted.

CONCLUSIONS

Immature CD71+ reticulocytes and splenic P. vivax-infected erythrocytes of all asexual stages accumulate in the same splenic compartments, suggesting the existence of a cryptic endosplenic lifecycle in chronic P. vivax infection. Findings provide insight into P. vivax-specific adaptions that have evolved to maximise survival and replication in the spleen.

Reticulocytosis in non-anaemic cats and dogs

OBJECTIVE

To evaluate the proportion of blood samples diagnosed with reticulocytosis without anaemia in cats and dogs and report the aetiology and mortality rate of affected animals.

MATERIALS AND METHODS

Retrospective multicentre study including haematological examination of 3956 cats and 11,087 dogs admitted to seven German veterinary clinics (2012 to 2014). The proportion of blood samples with reticulocytosis without anaemia was calculated, and after exclusion of multiple measurements of the same animal, clinical data were evaluated. Animals with reticulocytosis without anaemia were classified as healthy or diseased, and diseased patients were assigned to 12 disease groups. Pretreatment (i.e. non-steroidal anti-inflammatory drugs, glucocorticoids, dipyrone) was recorded.

RESULTS

The proportion of blood samples with reticulocytosis without anaemia was 3·1% (124/3956) in cats and 4·4% (492/11,087) in dogs. Overall, 1·8% (2/111) of cats and 1·5% (7/458) of dogs with reticulocytosis without anaemia were healthy. Blood loss/anaemia, cardiac/respiratory disorders, gastrointestinal disorders and inflammatory disorders as well as cancer were the most frequent underlying diseases. Pretreatment was noted in 39·5% (43/111) of cats and 42·4% (194/458) of dogs. The mortality rate was 37·8% (42/111) in cats and 29·7% (136/458) in dogs with reticulocytosis without anaemia; the median survival time in non-survivors was 1 day (range: 0 to 376 days in cats, 0 to 444 days in dogs).

CLINICAL SIGNIFICANCE

In both species, reticulocytosis without anaemia was observed in a low proportion of blood samples (dogs>cat). Though a bias towards sick animals is possible in our sample, reticulocytosis without anaemia was mainly seen in diseased animals and associated with a mortality rate of approximately one-third of patients.

Residual pyruvate kinase activity in PKLR-deficient erythroid precursors of a patient suffering from severe haemolytic anaemia

OBJECTIVE

Here, we present a 7-year-old patient suffering from severe haemolytic anaemia. The most common cause of chronic hereditary non-spherocytic haemolytic anaemia is red blood cell pyruvate kinase (PK-R) deficiency. Because red blood cells rely solely on glycolysis to generate ATP, PK-R deficiency can severely impact energy supply and cause reduction in red blood cell lifespan. We determined the underlying cause of the anaemia and investigated how erythroid precursors in the patient survive.

METHODS

PK activity assays, Western blot and Sanger sequencing were employed to determine the underlying cause of the anaemia. Patient erythroblasts were cultured and reticulocytes were isolated to determine PK-R and PKM2 contribution to glycolytic activity during erythrocyte development.

RESULTS

We found a novel homozygous mutation (c.583G>A) in the PK-R coding gene (PKLR). Although this mutation did not influence PKLR mRNA production, no PK-R protein could be detected in the red blood cells nor in its precursors. In spite of the absence of PK-R, the reticulocytes of the patient exhibited 20% PK activity compared with control. Western blotting revealed that patient erythroid precursors, like controls, express residual PKM2.

CONCLUSIONS

We conclude that PKM2 rescues glycolysis in PK-R-deficient erythroid precursors.

Stress reticulocytes lose transferrin receptors by an extrinsic process involving spleen and macrophages

As they mature into erythrocytes during normal erythropoiesis, reticulocytes lose surface transferrin receptors before or concurrently with reticulin. Exosome release accounts for most of the loss of transferrin receptors from reticulocytes. During erythropoietic stress, reticulocytes are released early from hematopoietic tissues and have increased reticulin staining and transferrin receptors. Flow cytometry of dually stained erythrocytes of mice recovering from phlebotomy demonstrated delayed loss of reticulin and transferrin receptors during in vitro maturation compared to in vivo maturation, indicating that an in vivo process extrinsic to the reticulocytes facilitates their maturation. Splenectomy or macrophage depletion by liposomal clodronate inhibited in vivo maturation of reticulocytes and increased the numbers of reticulin-negative, transferrin receptor-positive cells during and after recovery from phlebotomy. This reticulin-negative, transferrin receptor-positive population was rarely found in normal mice. Transmission electron microscopy demonstrated that the reticulin-negative, transferrin receptor-positive cells were elongated and discoid erythrocytes, but they had intracellular and surface structures that appeared to be partially degraded organelles. The results indicate that maturation of circulating stress reticulocytes is enhanced by an extrinsic process that occurs in the spleen and involves macrophage activity. Complete loss of reticulin with incomplete loss of surface transferrin receptors in this process produces a reticulin-negative, transferrin receptor-positive erythrocyte population that has potential utility for detecting prior erythropoietic stresses including bleeding, hemolysis and erythropoietin administration, even after recovery has been completed. Am. J. Hematol. 91:875-882, 2016. © 2016 Wiley Periodicals, Inc.

Clinical pharmacokinetics and pharmacodynamics of erythropoiesis-stimulating agents

The cloning of the EPO gene in the early 1980s allowed for the development of recombinant erythropoietins and analogues [erythropoiesis-stimulating agents (ESAs)], offering an alternative to transfusion as a method of raising haemoglobin (Hb) levels, which have been used for more than 20 years to treat anaemia in millions of anaemic patients. There are now a number of ESAs available worldwide for the treatment of anaemia, approved for different routes of administration (intravenous and subcutaneous) and dosing intervals (three times weekly, weekly, biweekly and monthly). In this review, we discuss the pharmacokinetic characteristics, including absorption, distribution and elimination processes, across the different ESAs. Incomplete and slow lymphatic absorption, with limited extravascular distribution, and minor contributions of the target-mediated drug disposition to the overall elimination are the common characteristics across the marketed ESA. Additionally, we assess the similarities and differences of ESAs related to pharmacodynamics in the context of the different biomarkers used to monitor the magnitude and duration of the effect, and introduce the concept of the minimum effective concentration of the ESA. The relationship between the minimum effective concentration and the half-life suggests that the time during which drug concentrations are above the minimum effective concentration is the main determinant of ESA efficacy in increasing Hb levels. The tolerance phenomenon and its physiological mechanism and implications for ESA dosing are discussed. Finally, the areas of future clinical pharmacology research are envisioned.

The quantification of reticulocyte maturation and neocytolysis in normal and erythropoietin stimulated rats

A technique has recently been proposed for obtaining the reticulocyte (RET) age distribution from the flow cytometric reticulocyte count. It allows for a quantitative characterization of reticulocyte dynamics. In this work this technique was applied to characterize the blood, bone marrow and spleen reticulocytes in homeostatic and erythropoietically stimulated rats in order to determine the reticulocyte maturation times in the bone marrow and blood; and to confirm the presence of ineffective erythropoiesis (neocytolysis). The latter was done by comparing the reticulocyte removal rate from blood with bilirubin formation after erythropoiesis stimulation. A single subcutaneous dose (4050 IU/kg) of recombinant human erythropoietin (rHuEPO) was administered to rats, then their reticulocytes were stained with thiazole orange and the distribution of the fluorescent signal measured using flow cytometry. The obtained signal distribution of the reticulocytes was transformed to the age distribution and a set of basic parameters reflecting reticulocyte dynamics was determined. Bilirubin concentrations were measured to directly assess the presence of reticulocyte irreversible removal. The bilirubin formation was found to be considerably modulated by rHuEPO and corresponded well to the determined reticulocyte removal rate. The initial increase and subsequent decrease of the reticulocyte maturation time in blood was quantitated and directly linked with RET mobilization from the bone marrow. A substantial number (60%) of reticulocytes is sequestrated during homeostasis in rats. This number increases and then decreases after rHuEPO administration, as also reflected by bilirubin formation. Flow cytometry seems to be an excellent method for studying RET dynamics and the presence of young RBC neocytolysis.

Effects of racing on reticulocyte concentrations in Greyhounds

BACKGROUND

Greyhounds have several hematologic variables that are outside of the respective reference intervals of other dog breeds. In addition, increases in HCT, total protein and HGB concentration, and RBC and WBC counts occur immediately after exercise; these values return to resting values within a few hour after racing.

OBJECTIVE

This study evaluated the effects of exercise on the concentration of reticulocytes in circulating blood in racing Greyhounds. We hypothesized that reticulocyte numbers are significantly increased immediately after a race, and return to baseline within one to 2 h postrace.

METHODS

Fifty actively racing Greyhounds at the Wheeling Island Racetrack and Casino were included in the study. Samples were collected by jugular venipuncture one day prior to racing at the kennel (resting), immediately after racing, and one to 2 h after the race (recovery). Reticulocyte counts were determined with an IDEXX ProCyte Dx Hematology Analyzer (IDEXX Laboratories, Inc., Westbrook, ME, USA). Due to a nonparametric distribution, the results were statistically compared using the Friedman test.

RESULTS

Reticulocyte concentrations were significantly different among the 3 sample collection times (P < .0001). There was a significant increase in reticulocyte concentration immediately after racing (P < .001); one to 2 h after racing, the reticulocyte numbers decreased significantly (P < .001) to counts comparable to resting samples.

CONCLUSION

The increase in reticulocyte concentration is probably related to splenic contraction secondary to the release of catecholamines, although premature bone marrow release could also account for these changes. Thus, it is important to consider a Greyhound's activity and degree of excitement when interpreting selected hematologic data in a clinical setting.

Haemopoiesis in mammalian bone marrow

The bone marrow supports haemopoiesis of all blood cell types and delivers mature cells to the blood. Haemopoiesis is characterized not only by the differentiation and proliferation of haemopoietic stem cells but by a number of physically associated cell types. These include macrophages, lymphocytes and, when haemopoiesis is intense, a multinucleate branched stromal cell. The venous vasculature of the bone marrow is associated with both haemopoiesis and the delivery of blood cells to the circulation. The wall of the vascular sinus consists of an endothelium lying upon a basement membrane. On the outside surface of the basement membrane lie adventitial cells or pericytes which branch out into the haemopoietic space forming a scaffolding upon which haemopoietic clusters are arranged. These cells move away from the wall of the vascular sinus to permit maturing blood cells to penetrate the endothelium and enter the circulation. Under other circumstances, adventitial cells accumulate fat, becoming the adipocytes of marrow.

Evaluation of the reticulocyte micronucleus assay in patients treated with radioiodine for thyroid cancer

In the case of accidental radiation exposure, biological dosimetry has an important role. Previous studies have indicated that the flow cytometric micronucleus (MN) assay in human transferrin receptor positive reticulocytes (Tf-Ret) in blood could be a sensitive biomarker for chromosome damage. In the present investigation, the utility and sensitivity of this method was studied in 44 young patients from Belarus, who were treated with 131I for thyroid cancer. Red marrow (RM) is the critical organ in radioiodine therapy (RIT). In our patients, it was exposed to 100-700 mSv low-dose rate irradiation within 2-4 days. About 3 days after 131I administration, the frequency of micronucleated-Tf-Ret (f(MN-Tf-Ret)) increases within 1 day to a maximum and declines in the following 2-5 days to its value before treatment. A total dose of 100 mSv was easily detectable. The sensitivity of the assay after acute irradiation may be 50 mSv. The method should be useful for monitoring individuals after a radiation accident, provided blood samples can be obtained within a few days after exposure. The time-course of f(MN-Tf-Ret) is interpreted using a model, which considers the exponential exposure of red marrow in RIT as well as the kinetics of erythroblast maturation and reticulocyte migration into the peripheral circulation. Similar modelling was done on published data of MN in immature mouse erythrocytes. Striking similarities in the kinetic and in the yield of MN-induction were found between these two species. This lends support for the use of the mouse as a model for the MN-induction in humans.

Intermediary spleen microvasculature in canis familiaris- morphological evidences of a closed and open type

Numerous studies have been made regarding circulation via the red pulp of the spleen, and intense controversy surrounds the question as to whether or not endothelial continuity exists between arterial and venous vessels. Aware of this intense controversy, and in order to perform investigation over the spleen of dogs infected with a parasitic disease (future reports shall be done), the authors studied the vascularization of the normal dog spleen in order to define its normal pattern and evaluate the eventual changes of the circulation pattern under the parasitic condition. These studies led us to report, unequivocally, using complementary vascular replective techniques, that the normal dog's intermediary circulation is morphologically closed and of the open kind also. These findings are contrary to the thesis that defends the existence of a physiologically closed and morphologically open circulation in the dog spleen. Lymphatic vessels in the spleen of the dog are also demonstrated.

Temporal differences in membrane loss lead to distinct reticulocyte features in hereditary spherocytosis and in immune hemolytic anemia

Spherocytic red cells with reduced membrane surface area are a feature of hereditary spherocytosis (HS) and some forms of autoimmune hemolytic anemia (AIHA). It is generally assumed that membrane loss in spherocytic red cells occurs during their sojourn in circulation. The structural basis for membrane loss in HS is improper assembly of membrane proteins, whereas in AIHA it is due to partial phagocytosis of circulating red cells by macrophages. A hypothesis was formed that these different mechanisms should lead to temporal differences in surface area loss during red cell genesis and during sojourn in circulation in these 2 spherocytic syndromes. It was proposed that cell surface loss could begin at the reticulocyte stage in HS, whereas surface area loss in AIHA involves only circulating mature red cells. The validity of this hypothesis was established by documenting differences in cellular features of reticulocytes in HS and AIHA. Using a novel technique to quantitate cell surface area, the decreased membrane surface area of both reticulocytes and mature red cells in HS compared with normal cells was documented. In contrast, in AIHA only mature red cells but not reticulocytes exhibited decreased membrane surface area. These data imply that surface area loss in HS, but not in AIHA, is already present at the circulating reticulocyte stage. These findings imply that loss of cell surface area is an early event during genesis of HS red cells and challenge the existing concepts that surface area loss in HS occurs predominantly during the sojourn of mature red cells in circulation.

Reticulocyte cellular indices: a new approach in the diagnosis of anemias and monitoring of erythropoietic function

Reticulocyte analysis has been extended from the simple enumeration of reticulocytes to precise measurements of mRNA content and of cellular indices such as volume, hemoglobin (Hb) concentration, and content. Assessment of reticulocyte maturity is based on the fluorescence intensity of reticulocytes, which depends on RNA content. The appearance of high fluorescence reticulocytes has been shown to be associated with engraftment in the setting of bone marrow or peripheral stem cells transplantation, although it is still not clear how this parameter can improve quality or cost of care compared with the traditional use of absolute neutrophil counts. Reticulocyte indices have been studied especially in the setting of iron deficiency and functional iron deficiency during recombinant human erythropoietin (r-HuEPO) therapy. Reticulocyte hemoglobin content (CHr) may allow prompt identification of an imbalance between r-HuEPO therapy and iron availability by detecting the presence in reticulocytes of iron-restricted erythropoiesis. Diagnosis of simple iron deficiency can also be achieved in a more cost-effective fashion by using CHr in conjunction with the regular complete blood count (CBC), rather than relying on the traditional biochemical parameters of iron metabolism. Response to therapy of megaloblastic anemia can also be monitored with CHr. These new reticulocyte parameters provide a real-time assessment of the functional state of erythropoiesis.

Use of reticulocyte cellular indices in the diagnosis and treatment of hematological disorders

Automated counting of reticulocytes has markedly increased the precision and accuracy of this assay compared with the traditional manual counts. In addition, several new reticulocyte parameters are now available to clinicians and pathologists. This review examines the potential role of these parameters in the diagnosis and management of anemias. Reticulocyte maturity can now be assessed based on the staining intensity of reticulocytes, which is proportional to their RNA content. However, the clinical value of the numerical estimate of the immature reticulocyte fraction has not been yet demonstrated. In the bone marrow transplant setting, there is no clear evidence that the use of this index results in improved care of these patients, and many studies have failed to show its superiority compared with the traditional white cell count, especially for autologous transplants. Direct measurement of reticulocyte volume, hemoglobin concentration, and hemoglobin content are now available. Studies have shown that these parameters, and hemoglobin content in particular, allow a real-time assessment of the functional state of the erythroid marrow. In the setting of recombinant human erythropoietin therapy, studies of hemoglobin content have shown that this index allows an early detection of functional iron deficiency. Preliminary studies have also shown that this index may be helpful in the diagnosis of iron deficiency and in the monitoring of iron replacement therapy.

The high splenic hematocrit: a rheological consequence of red cell flow through the reticular meshwork

The spleen concentrates blood to twice arterial hematocrit, but how it does so is poorly understood. We hypothesize that hemoconcentration results from percolation of blood through the reticular meshwork (RM) between capillary endings and venous channels. The RM has a large flow cross section, producing low shear rates, and an enormous surface area for red blood cell (RBC) adhesion. If these conditions cause slowing of RBCs with respect to plasma, increased hematocrits will occur (inverse Fahraeus effect). Movements of individual RBCs through the RM were studied from intravital microscopic videorecordings of Ringer-perfused mouse spleens. Analysis of three 70-microns paths showed characteristic "stop and go" motion, RBCs spending 0.015 to 9.71 see in any 7-microns segment, despite steady (+/- 15%) perfusate velocity. At some locations RBCs adhered to reticular cells/fibers by point attachment and at others they became caught over fibers. In general, RBCs were detained in the RM by surface interactions rather than by narrow channel. Perfusate ("plasma") velocity was estimated to be at least that of the fastest RBCs in each segment. Ratios of overall mean RBC and plasma velocities for three paths were 0.19, 0.20, and 0.37. Applying these velocity ratios to a feed hematocrit of 40% leads to predicted splenic hematocrits up to 78%. We conclude that this slowing of RBCs within the reticular meshwork provides the primary mechanism for generating the high intrasplenic hematocrit.

Properties of the exchangeable splenic platelets released into the circulation during exercise-induced thrombocytosis

The human spleen normally retains about one-third of the body's platelets in an exchangeable pool which can be released into the circulation by alpha-adrenergic stimulation. Some previous investigators concluded that the splenic platelet population was enriched in a subpopulation of large, young, dense platelets (megathrombocytes) but more recent research suggests that platelet size, age, and density are largely independent variables. In this investigation the properties of the splenic platelets were studied after their release into the circulation by acute strenuous exercise in 11 normal subjects. The exercise caused a rise in mean platelet count from 245 +/- 49 to 328 +/- 71 x 10(9)/L--a net increase of 24 +/- 6% after correction for haemoconcentration. The mean platelet volume (MPV) of citrated platelets increased from 6.38 +/- 0.78 to 6.59 +/- 0.68 fL after exercise (P less than 0.01)--a rise of 3.7 +/- 4.1% suggesting that the MPV of the splenic platelet population was about 20% greater than that of the normal circulating population. The age distribution of the platelets was studied by measuring the platelet monoamine oxidase (MAO) activity several days after irreversible inhibition by tranylcypromine, when the young platelets had normal MAO activity but the older platelets had only 20% of normal activity. The mean platelet MAO activity did not change after exercise, indicating that the age distributions of the circulating and splenic populations were very similar. The platelet contents of several putative markers of platelet age (sialic acid, serotonin, beta-thromboglobulin, beta-N-acetylglucosaminidase) were also unchanged after exercise. Modal platelet density decreased slightly but not significantly after exercise. The splenic platelet population has a larger MPV but appears to have similar age and density distributions to the basal circulating population.

Analysis of density-fractionated rat red blood cells of different ages by partitioning in two-polymer aqueous phase systems

Rats were injected with radioactive iron and bled at different times thereafter. This gave rise to cell populations in which the bulk of labeled cells corresponded in age to the time elapsed between injection and bleeding. Such cell populations were centrifuged and the 7-10% least dense and the 7-10% most dense cells subjected to counter-current distribution in a charge-sensitive two-polymer aqueous phase system which fractionates cells on the basis of surface properties. While it is known that there is a tendency for cell density to increase with cell age and that rat red cells of different ages have characteristic partition ratios, we were able, by applying partitioning analysis to density-separated cells, to gather data not obtainable by use of either method alone. These include: (1) the demonstration of two very young cell populations having different surface properties in the most dense fraction; (2) the finding that in the least dense layer labeled mature erythrocytes (at least those labeled 18 days and beyond) and unlabeled red cells have overlapping distribution curves which indicate that the label in the top fraction can represent young or early middle-aged but never old cells; and (3) the presence, as judged by surface properties, of some middle-aged red cells, in addition to old cells, in the most dense cell layer.

Reticulocyte crisis after splenectomy: evidence for delayed red cell maturation?

In whole blood from splenectomized subjects (n = 8) in a steady state an increased number of reticulocytes was observed (14.0 +/- 7.8% versus 3.6 +/- 2.4% controls: p less than 0.05). Cell fractionation on density showed that reticulocytes, as in normals, are more or less confined to the least dense fraction; cell fractionation on volume revealed that reticulocytes form a heterogeneous cell population with diverging volume, but sharing a low density. Immediately after splenectomy (n = 4), a reticulocytosis is observed, mainly manifest in the top fractions (262.0 +/- 49.0% versus 40.0 +/- 32.4% preoperatively; p less than 0.01). This reticulocytosis is not due to pre- or postoperative complications, but is associated with the splenectomy itself and probably is the consequence of a delayed, but ultimately occurring red cell maturation. The increase of reticulocytes is not accompanied by significant alterations in cell morphology, as far as cell volume is concerned.

Alterations in size, shape and osmotic behaviour of red cells after splenectomy: a study of their age dependence

In whole blood from splenectomized subjects (n = 20), red cells showed a significant increase of mean surface area (MSA), mean cell volume (MCV), MSA/MCV-ratio and osmotic resistance, with the mean cell haemoglobin concentration (MCHC) being decreased. Studies on red cell populations of different cell age revealed that the increase of MSA affects younger and older cells, whereas the increase of MCV can mainly be ascribed to young cells with low density. The increased osmotic resistance is mainly determined by older cells due to a more favourable MSA/MCV-ratio. Shortly after splenectomy (n = 5) the MSA of younger and older cells increased, whereas the increase of MCV affected only young cells with a lowered density; moreover, the MSA/MCV-ratio increased in older cells in particular, resulting in a relatively greater increase of osmotic resistance. An impaired maturation of the reticulocyte may underlie the initial increase of MSA and MCV of young cells, but the present results contradict the current view that delayed maturation explains the changes in morphology and osmotic resistance of asplenic red cells.

Microcirculatory pathways in normal human spleen, demonstrated by scanning electron microscopy of corrosion casts

Confusion regarding microcirculatory pathways in normal human spleen has arisen due to extrapolation from pathological material and from other mammalian spleens, not to mention difficulties in tracing intricate three-dimensional routes from the study of thin sections or cut surfaces of tissue. We examined microcirculatory pathways in normal human spleens freshly obtained from organ transplant donors. A modified corrosion casting procedure was used to obtain an open view of vessels and their connections. Our results demonstrate: 1) "arteriolar-capillary bundles" within lymphatic nodules and extensive branching of arterioles in the marginal zone (MZ); 2) the marginal sinus around lymphatic nodules; 3) the peri-marginal cavernous sinus (PMCS) outside the MZ or immediately adjacent to the nodule itself; the PMCS receives flow via ellipsoid sheaths and MZ, or directly from arterial capillaries, and drains into venous sinuses; 4) fast pathways for flow into venous sinuses via ellipsoid sheaths; 5) arterial capillary terminations in the reticular meshwork of the red pulp or MZ ("open" circulation); direct connections to venous sinuses also occur ("closed" circulation), although rarely; and 6) numerous open-ended venous sinuses in the MZ, allowing a large proportion of the splenic inflow to bypass the red cell filtration sites in the reticular meshwork and at venous sinus walls.

Phagocytosis in the spleen of the sunfish Lepomis spp

A histological investigation of the filtering function of the spleen of the sunfish Lepomis spp. was conducted by light, scanning, and transmission electron microscopy. The parenchyma of the organ is predominantly red pulp, a system of splenic cords and sinuses. The white pulp consists of loose lymphoid tissue which forms a cuff around the pulp arteries. Filtering of particulate matter from the blood occurs in the red pulp by phagocytes of the pulp cords and ellipsoids (periarterial macrophage sheaths). The ellipsoids are pale-staining cuffs of macrophages and reticular cells in a framework of reticular fibres surrounding the arterial capillaries. Destruction of effete blood cells (especially erythrocytes) is confined to the pigment nodules; particulate matter is not taken up by the nodules. These yellow-brown bodies are dispersed throughout the red pulp and are bounded by a reticular capsule. They contain masses of phagocytes and have the appearance of a morula. They are associated with blood vessels and are surrounded by sinusoids. Prussian Blue stain shows the presence of haemosiderin within their phagocytes. The phagocytes of the pigment nodules are filled with inclusions such as residual bodies, siderosomes, and fragments of erythrocytes. The early filtering of particulate matter by the phagocytes of the pulp cords and ellipsoids may allow for a more efficient phagocytosis of erythrocytes by the pigment nodules, followed by storage and reutilization of iron-containing compounds uncontaminated by other phagocytosed material.

Vascular pathways in nonsinusal red pulp--an electron microscope study of the cat spleen

The red pulp of the cat spleen, including terminal segments of arterial capillaries, pulp venules, and the reticular meshwork, was studied by transmission electron microscopy. Splenic congestion and contraction were produced by barbiturate anesthetic and norepinephrine. Terminal segments of arterial capillaries were ampullary and flared. Blood escaped into surrounding pulp spaces through interendothelial gaps. Pulp venules originated as open-ended vessels in the reticular meshwork near trabeculae and drained into trabecular veins. Venule walls were thin and composed of squamous endothelial cells, a continuous basement membrane, and reticular cells. Venules in congested spleens had many mural apertures, but venules in contracted spleens had few. The interstices of the reticular meshwork in congested spleens contained large amounts of blood, which often was concentrated, many macrophages, lymphocytes, and plasma cells. Fewer blood cells and scant plasma were present in contracted spleens. The vascular arrangements are anatomically open. Blood takes pathways through the reticular meshwork from arterial terminations to pulp venules. Some pathways through the reticular meshwork probably function as closed vascular channels conveying rapidly flowing blood. Other pathways are functionally open and probably contain slowly moving blood that constitutes a reservoir of red cells. Macrophages formed associations with mature red cells and with reticulocytes. Mature red cells were attached to macrophages in a manner indicating erythrophagocytosis. Reticulocyte attachment had a different appearance and likely resulted in reticulocyte sequestration. Platelets bore pseudopodia which would impede their passage through irregular and cell-filled pulp spaces. The change in platelet shape probably is responsible for the formation of the splenic pool of platelets.

Electron microscopy of the red pulp of the dog spleen including vascular arrangements, periarterial macrophage sheaths (ellipsoids), and the contractile, innervated reticular meshwork

The vascular and stromal arrangements of the red pulp in congested and contracted dog spleens were studied by transmission electron microscopy. Each dog had been injected intravenously with Thorotrast to label actively endocytizing cells. Only macrophages ingested Thorotrast. The proximal portion of each arterial capillary was surrounded by a "periarterial macrophage sheath" (PAMS), a term we introduce to replace the term "ellipsoid". PAMS were composed of a fine meshwork of reticular cells and reticular fibers which held tightly-packed macrophages and interspersed blood cells. These macrophages, as well as those in the reticular meshwork of red pulp, contained Thorotrast, cell debris, and deposits of hemosiderin. The arterial capillary at the center of each PAMS was formed by parallel, rod-shaped endothelial cells and discontinuous layers of basement membrane and reticular-cell cytoplasm. PAMS were tapered at their distal ends; the terminal portion of the arterial capillary continued beyond the PAMS to end in the reticular meshwork of red pulp. Endothelial cells in the terminal arterial capillaries were separated by gaps through which blood cells passed into the spaces of the reticular meshwork of red pulp. The reticular meshwork was formed by reticular cells which appeared to be specialized for contraction. These cells were filled with thin filaments and possessed plasmalemmal dense bodies as found in smooth muscle cells. Furthermore, the reticular meshwork was innervated by unmyelinated adrenergic axons which probably were derived from nerves that followed arterioles. Axons were enclosed in surface invaginations of cells which were similar to reticular cells in shape and cytologic detail and which we called "axon-bearing reticular cells". Axon-bearing reticular cells were inserted between the branches of the reticular cells that formed the meshwork. Venous sinuses formed an anastomosing system of vessels draining into pulp veins which then joined trabecular veins. Sinuses were formed by parallel, rod-shaped endothelial cells encircled by strands of basement membrane and reticular-cell branches. Endothelial cells lay closely side by side except where interendothelial slits were opened by blood cells passing into the lumen or by pseudopodia of macrophages which lay outside the sinus. Cell traffic across the sinus wall was greatest in areas where blood cells were mixed with plasma. Congested spleens stored concentrated red cells in both sinuses and the reticular meshwork; contracted spleens were emptied of blood. The reticular meshwork may contract to assist trabecular and capsular smooth muscle in expelling stored red cells and effecting hemoconcentration.

Studies on splenic megakaryocytes. I. Recovery of megakaryocytes from the splenic venous outflow

Spleens from 14 healthy adult cats were isolated and perfused with cell-free Ringer's solution. Histological studies on the splenic megakaryocytes, as well as a search for splenic megakaryocytes in samples of the splenic venous outflow, were performed. The samples of the splenic venous outflow contained a lower cellular concentration when perfusion was prolonged, but the number of megakaryocytes appearing in the venous samples increased. At least five different forms of megakaryocytes were observed from smears of the venous outflow: (i) unipolar, (ii) multipolar, (iii) foamy, (iv) mobile and (v) nuclear types. These types were also found in splenic venous outflow of dogs, kittens and bled cats. It is suggested that not only platelets but also the larger megakaryocytes can pass out of the splenic circulation. This may imply that megakaryocytes can be culled, matured in the splenic red pulp, and ultimately released into peripheral blood.