The distribution of red cells in the spleen

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.

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.

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.

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.

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.

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.