Immature and abnormal erythrocytes present in the normal, healthy 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.

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.

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.