Sequestration and possible maturation of reticulocytes in the normal spleen

The presence, in the feline spleen, of a slowly exchanging red cell 'compartment' ([Formula: see text] 54 min) has been demonstrated previously. These red cells adhere to reticulum cells and sinus walls in the red pulp and have been shown to be larger in cellular volume and lighter in cellular density than the rest. This suggested that they might be younger cells and we have reported briefly that they contain a high proportion of reticulocytes. Using supravital stains we have measured the percentage of reticulocytes in the outflow from isolated spleens of cats and dogs, perfused with oxygenated Ringer solution. Reticulocyte counts increased from 0.4% to 99% as the perfusion progressed. The results show that the slow compartment consists entirely of reticulocytes. The ratio of reticulocytes to rubricytes in the spleen was found to be 75:1. Therefore the reticulocytes were not produced in the spleen but were accumulated from the circulating blood. The total number of reticulocytes so stored is 1.2 × 1010 cells, equivalent to 1.5 times the daily production in the whole animal. From these data we conclude that reticulocytes released from the bone marrow under physiological conditions are sequestered and matured in the spleen.

Physical characteristics of red cells collected from the spleen

Kinetics of cell washout, when isolated cat spleens are perfused with Ringer solution, have shown previously that the red cells stored in the spleen correspond to a system of three compartments (fast, intermediate, and slow); morphological studies, at different stages of the washout, have already identified these compartments as free cells in vascular channels, free cells within sinuses, and cells adhering to sinus walls, respectively. By collecting the venous outflow at three particular stages of the washout fairly pure samples (>85%) of the cells from each compartment have now been obtained. We have measured the density (phthalate method), volume, and osmotic changes (Celloscope) of these cells. Cells from the fast and intermediate compartments were not significantly different from those of arterial blood, but cells from the slow compartment were lighter (specific gravity difference was 0.0064; p < 0.01), larger in volume (5.0%; p < 0.01), and swelled 14% less in 200 mOsmol/l (p < 0.01). These differences indicated that cells from the slow compartment might be predominantly younger cells and this has been confirmed by finding reticulocyte counts of 58.0 ± 3.8 (S.E.) %. It is suggested that immature red cells, released from the bone marrow, may be sequestered in the spleen and matured.

The distribution of red cells in the spleen

Kinetics of red cell washout, when isolated cat spleens are perfused with Ringer solution, show that the spleen corresponds to a three-compartment system. To determine whether or not there exist morphological counterparts to these compartments we examined microscopic sections from 16 spleens perfused by different volumes of Ringer solution. Red cells could be divided into three groups: (1) free cells in vascular channels and sinuses, (2) cells adhering to reticulum cells or sinus endothelium, and (3) cells in the cytoplasm of macrophages. When 50 ml were perfused no free cells were seen in vascular channels. After 600 ml few free cells remained in the sinuses. Thereafter the number of cells adhering to the sinus wall decreased gradually and cell counts agreed with predictions from the washout curve. We conclude that the compartments of our model (fast, intermediate, and slow) represent, respectively, free cells in vascular channels, free cells within sinuses, and cells adhering to sinus walls. The only cells trapped irreversibly are the very few found in the cytoplasm of macrophages. It is suggested that the slow compartment represents red cells in a pre-phagocytosed stage, e.g. aged cells, abnormal cells containing inclusion bodies, and possibly, reticulocytes in the process of maturation.