Ultrastructural analysis of normal and immunized spleen of the snapping turtle,Chelydra serpentine

In the ultrastructural comparison of normal, unimmunized spleens with immunized spleens at key intervals after antigenic stimulation with keyhole limpet hemocyanin (KLH), we noted cellular and cytological features which reflect the cellular kinetics of the primary immune response, particularly with respect to plasma cell production. Although lymphoblasts and mature plasma cells are present in the white and red pulp, respectively, intermediate stages of the plasma cell line are rarely found in normal spleen. Following antigenic challenge, we found a marked increase in lymphoblasts in the white pulp, most of them containing short segments of rough endoplasmic reticulum suggesting initial differentiation toward plasma cells. Following an apparent migration of cells from the white to the red pulp, we found plasma cells in various stages of maturation in the red pulp cords and sinuses. The ultrastructural features of these cells reflect 'the differentiation of lympho blasts into mature plasma cells. Both immature and mature plasma cells usually possess dilated cisternae of rough endoplasmic reticulum, suggesting that they are capable of producing and storing a secretory product, presumably antibody. We also noted a large number of immature macrophages and monocytes in immunized spleens. These cellular events and their cytological characteristics are compared to those described in other vertebrate classes.     

Circulatory pathways in the sinusal spleen of the dog, studied by scanning electron microscopy of microcorrosion casts

Scanning electron microscopy of microcorrosion casts was used to visualize circulatory pathways in the sinusal spleen of dog. The examination of contracted versus dilated organs and variations of the volume of material injected gave an indication of flow dynamics. Minimal injections of material into contracted spleens produced filling of mainly the fastest routes for flow, whereas injections into dilated spleens primarily filled slower routes. This procedure yielded a more complete, three-dimensional picture of the arterial, intermediate, and venous pathways as a whole, and of the relative amounts of flow through different arterial routes. Evidence of flow from capillary lumina out into ellipsoid sheaths was plentiful in casts from dilated spleens, but rare in casts from contracted organs. The pattern of flow within and out of the marginal sinus has been elucidated: A circumferential filling occurs first, followed by a flow that radiates outward into the marginal zone and red pulp. Venous sinuses filled via two routes in addition to the generally accepted path from the reticular meshwork via fenestrations in sinus walls. First, many venous sinuses extending out from the marginal sinus and surrounding marginal zone originated as open-ended tubes continuous with the reticular spaces of the marginal sinus or marginal zone. Second, direct connections of arterial capillaries with venous sinuses in the red pulp were found. Evidence indicating that some mechanism is controlling the flow via these routes is discussed. The strikingly different arrangement of venous sinuses in the subcapsular region is demonstrated.     

Intracellular localization of enzymes in spleen. I. Reduced diphosphopyridine nucleotide cytochrome c reductase,cytochrome c oxidase,and succinic dehydrogenase in the rat and guinea pig

1. The intracellular distribution of nitrogen, DPNH cytochrome c reductase, succinic dehydrogenase, and cytochrome c oxidase has been studied in fractions derived by differential centrifugation from rat and guinea pig spleen homogenates. 2. In the spleens of each species, the nuclear fraction accounted for 40 to 50 per cent of the total nitrogen content of the homogenate, and the mitochondrial, microsome, and supernatant fractions contained about 8, 12, and 30 per cent of the total nitrogen, respectively. 3. Per mg. of nitrogen, DPNH cytochrome c reductase was concentrated in the mitochondria and microsomes of both rat and guinea pig spleens. Seventy per cent of the total DPNH cytochrome c reductase activity was recovered in these two fractions. The reductase activity associated with the nuclear fraction was lowered markedly by isolating nuclei from rat spleens with the sucrose-CaCl(2) layering technique. The lowered activity was accompanied by the recovery of about 90 per cent of the homogenate DNA in the isolated nuclei, indicating that little, if any, of the reductase is present in spleen cell nuclei. 4. Per mg. of nitrogen, succinic dehydrogenase was concentrated about 10-fold in the mitochondria of rat spleen, and 65 per cent of the total activity was recovered in this fraction. 5. Cytochrome c oxidase was concentrated, per mg. of nitrogen, in the mitochondria of both rat and guinea pig spleens. The activity associated with the nuclear fraction was greatly diminished when this fraction was isolated from rat spleens by the sucrose-CaCl(2) layering technique. Only 50 to 70 per cent of the total cytochrome c oxidase activity of the original homogenates was recovered among the four fractions from both rat and guinea pig spleens, while the specific activities of reconstructed homogenates were only 55 to 75 per cent of those of the original whole homogenates. This was in contrast to the results with DPNH cytochrome c reductase and succinic dehydrogenase where the recovery of total enzyme activity approached 100 per cent, and the specific activities of reconstructed homogenates equalled those of the original homogenates. The recovery of cytochrome c oxidase was greatly improved when only the nuclei were separated from rat spleen homogenates. 6. Data were presented comparing the concentrations (ratio of activity per mg. of nitrogen of the fraction to activity per mg. of nitrogen of the homogenate) of DPNH cytochrome c reductase in mitochondria and microsomes derived from different organs of different animals. 7. Data were presented comparing the activities per mg. of nitrogen of DPNH cytochrome c reductase in homogenates from several organs of various animals.     

Stereometry of the red pulp on the human spleen. I) Methodological remarks

The principal histometrical formulas to be applied to the analysis of human spleen's red pulp are illustrated. The discussion particularly concerns the formulas to evaluate the following measures per unit splenic volume:volume of the elements; surface area of sinus-splenic cord boundaries; mean breadth of cords; length of the sinus; mean sectional area of the sinus. Suggestions are given about the criteria to be followed in the proper assumption of the data which must be subjected to statistical estimation.     

Platelets, endothelial cells and macrophages in the spleen. An ultrastructural study on perfusion-fixed organs.

Rabbit spleens have been examined after perfusion fixation with and without prior washing with various fluids. The platelets were stored in the splenic sinuses and in the cord spaces as single platelets, or in loosely packed aggregates which appeared to be anchored to the endothelium by one or a few platelets. After washing prior to fixation most of the platelets disaggregated and regained their normal shape. Some platelets adhered to morphologically normal endothelium even after prolonged perfusion. Occasionally, platelets were observed inside splenic endothelial cells. Others were closely associated with macrophages, many of which also contained engulfed platelets. There was no morphological evidence of a particular platelet population being retained in the spleen after washing. In the sinuses special granule-rich cytoplasmic structures were observed. They were interposed between ordinary endothelial cells and contained a large number of small lysosome-like granules. Nuclei were never observed in these structures, probably because they consisted of pseudopod-like protrusions. Their origin and function are discussed. They may represent actively phagocytizing elements.     

Pathology of the spleen in hepatosplenic schistosomiasis. Morphometric evaluation and extracellular matrix changes

Histological, ultrastructural, morphometric and immunohistochemical data obtained from the study of spleens removed by splenectomy from 34 patients with advanced hepatosplenic schistosomiasis revealed that the main alterations were congestive dilatation of the venous sinuses and diffuse thickening of the splenic cords. Splenic cord thickening was due to an increase of its matrix components, especially type IV collagen and laminin, with the conspicuous absence of interstitial collagens, either of type I or type III. Deposition of interstitial collagens (types I and III) occurred in scattered, small focal areas of the red pulp, but in the outside of the walls of the venous sinuses, in lymph follicles, marginal zone, in the vicinity of fibrous trabeculae and in sidero-sclerotic nodules. However, fibrosis was not a prominent change in schistosomal splenomegaly and thus the designation "fibro-congestive splenomegaly" seems inadequate. Lymph follicles exhibited variable degrees of atrophy, hyperplasia and fibrous replacement, sometimes all of them seen in different follicles of the same spleen and even in the same examined section. Changes in white pulp did not seem to greatly contribute to increasing spleen size and weight, when compared to the much more significant red pulp enlargement.     

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.     

The architecture of the spleen of the yellow-bellied toad, Bombina variegata

The spleen of the yellow-bellied toad, Bombina variegata , consists of distinct white and red pulps. The well-developed white pulp is formed by a large central lymphocytic region around the numerous blood vessels and by its smaller peripheral ramifications, both surrounded by the more or less developed connective tissue boundary layer. Large peripheral sinuses of the white pulp, filled mostly with lymphocytes, are usually present at the inner side of this boundary. At the outer side of the boundary layer, the lymphocytic marginal zone is often observed. This zone merges into the erythrocyte-rich red pulp formed by cellular cords and small venous sinusoids.
The structure of the spleen of Bombina variegata differs considerably from the spleens of other anuran species studied so far. The highly developed white pulp and its distinct separation from the red pulp may be connected with the important role of the spleen as the main secondary lymphoid organ of B. variegata. The splenic compartmentalization makes the yellow-bellied toads a useful model for experimental immunobiological studies.     

Nature of human spleen red pulp cells with special reference to sinus lining cells

Summary A histological and cytological as well as enzyme histo- and cytochemical analysis (alpha-naphthyl acetate esterases, naphthol AS acetate esterases, naphthol AS-D chloroacetate esterases, acid and alkaline phosphatases) of human spleen cells in sections and imprints was carried out with special reference to sinus lining cells. These cells show strong naphthol AS esterase activity and no or only little alpha-naphthyl acetate esterase and acid phosphatase activity. Thus they can be distinguished from reticular cells in pulp cords and from other macrophages in cords and sinuses. From the morphological and enzyme histochemical aspect it can be deduced that the sinus lining cells are a distinct cell type of the human spleen. The comparison of these enzyme cytochemical findings with the results of biochemical and electron microscopical investigations suggests that reticular cells of pulp cords and littoral cells of sinuses also have different functions: reticular cells seem to have a high phagocytotic activity while littoral cells seem to be only facultatively phagocytic.This work was supported by the Deutsche Forschungsgemeinschaft.     

Ultrastructure of human spleen in transmission and scanning electron microscope

Despite new information concerning functional morphology of spleen, there are still some inaccuracies mostly regarding the spleen blood circulation. Billroth’s (splenic) cords are formed from three-dimensional network of fibroblastic reticular cells located among branched sinuses. Results from our study using scanning electron microscopy confirm an intimate contact between adjacent reticular cells and erythrocytes. Arterial terminals can be observed in the Billroth’s cords. The wall of sinuses reminds a sieve and it is lined with a special type of endothelium. In electron microscope, endothelial cells look like rods oriented parallel to the longitudinal axis of sinuses. Based on our observations fibroblastic reticular cells change to fixed phagocytes under no circumstances, hence they do not participate in phagocytosis. They may have a recognition function for cells circulating around them. According to our opinion, the open and the closed blood circulation are present in the human spleen simultaneously. Blood flowing in the closed circulation can help “absorption” of extra-vascular liquid and the blood elements into the vascular lumen. Due to sporadic occurrence of smooth muscle cells in the capsule and trabeculae, we assume that human spleen is not a blood reservoir, unlike the spleen in some other animals.     

A STUDY OF THE STRUCTURE OF SPLENIC SINUSES IN MAN AND IN THE ALBINO RAT WITH THE LIGHT MICROSCOPE AND THE ELECTRON MICROSCOPE

The splenic sinuses in the spleens of 5 human beings and 7 albino rats have been studied in the light microscope and electron microscope after fixation in Dalton's fluid and Palade's fluid and embedding in n-butyl methacrylate. Splenic sinuses are tortuous vascular channels of large but variable diameter which represent the first venous vessels in the spleen and make up almost the entire red pulp in man and in rats. These vessels are composed of reticulo-endothelial cells flattened to endothelial form and sheathed by a netted reticulum. The luminal surface of the endothelium is made highly irregular by delicate and variable cytoplasmic protrusions, slender corridors separating adjacent endothelial cells, anastomotic openings to other sinuses, bulgings of entire cells, and even thrusts of endothelium spanning the sinai lumen. The supporting reticulum presents a well developed latticed appearance in tangential sections of sinuses, but in most cuts is punctate or linear. The reticulum is composed of strands without limiting membranes, which, in substance, are amorphous and resemble basement membrane. Material identical in appearance to the substance of the reticulum may be present in the endothelium, suggesting that the reticulum is formed by endothelial cells. The endothelium also contains deposits of presumed ferritin and hemosiderin. The extreme luminal bulgings of endothelium suggest production of circulating monocytes or lymphocytes by detachment of endothelial cells. Sinuses are patent and collapsed to varying degrees. Patent sinuses are separated by collapsed sinuses and these collapsed sinuses appear to constitute splenic (Billroth) cords.     

Aerodynamic characteristics of the human right lung bronchial tree]

A new method of studying the aerodynamical characteristics of normal right bronchial tree in 17 natural preparations is described. The authors have found that 24 to 44 per cent of the total airflow entering the right main bronchus pass through the upper lobe bronchus, 9-26 per cent through the middle lobe bronchus and 38-56 per cent through the lower lobe bronchus. The data obtained show good agreement with the volume of the corresponding lobe expressed as percentage to the total right lung volume. Bronchial tree preparations differ greatly in their carrying capacities due to their corresponding aerodynamical resistances (+50% +25%). The relationship between resistance coefficient of the bronchial tree preparations and their geometrical parameters has been found.     

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.     

The red pulp of the human spleen. Structural basis of blood filtration

The paper summarizes the authors' findings indicative of an open blood circulation in the human spleen, particularly those obtained by immune and enzyme histo- and cyto-chemical methods, and by electron microscopy. In the red pulp the blood gets into the extravascular spaces of the pulp cords, where the individual blood components have to pass between numerous macrophages to reach the sinuses. The sinus wall is composed of elongated endothelial cells surrounded by waved annular or ring fibers of the basement membrane. In some areas annular fibers are joined by longitudinal fibers, giving rise to the filtration lattice, the fenestrated basement membrane. The sinus wall represents the last filter barrier which decises whether the blood elements get back into the blood or not. Extravasation of blood secures that all foreign as well as the altered own components, particularly cellular and particle ones naturally along with the normal constituents get from the circulating blood into extravascular spaces. In the next phase, however, the normal ones return into the circulation, whereas the abnormal components are removed from the extravascular tissue by means of the macrophagic and immune system of the spleen.     

INTRACELLULAR LOCALIZATION OF ENZYMES IN SPLEEN : I. REDUCED DIPHOSPHOPYRIDINE NUCLEOTIDE CYTOCHROMEc REDUCTASE, CYTOCHROMEc OXIDASE, AND SUCCINIC DEHYDROGENASE IN THE RAT AND GUINEA PIG

1. The intracellular distribution of nitrogen, DPNH cytochrome c reductase, succinic dehydrogenase, and cytochrome c oxidase has been studied in fractions derived by differential centrifugation from rat and guinea pig spleen homogenates. 2. In the spleens of each species, the nuclear fraction accounted for 40 to 50 per cent of the total nitrogen content of the homogenate, and the mitochondrial, microsome, and supernatant fractions contained about 8, 12, and 30 per cent of the total nitrogen, respectively. 3. Per mg. of nitrogen, DPNH cytochrome c reductase was concentrated in the mitochondria and microsomes of both rat and guinea pig spleens. Seventy per cent of the total DPNH cytochrome c reductase activity was recovered in these two fractions. The reductase activity associated with the nuclear fraction was lowered markedly by isolating nuclei from rat spleens with the sucrose-CaCl₂ layering technique. The lowered activity was accompanied by the recovery of about 90 per cent of the homogenate DNA in the isolated nuclei, indicating that little, if any, of the reductase is present in spleen cell nuclei. 4. Per mg. of nitrogen, succinic dehydrogenase was concentrated about 10-fold in the mitochondria of rat spleen, and 65 per cent of the total activity was recovered in this fraction. 5. Cytochrome c oxidase was concentrated, per mg. of nitrogen, in the mitochondria of both rat and guinea pig spleens. The activity associated with the nuclear fraction was greatly diminished when this fraction was isolated from rat spleens by the sucrose-CaCl₂ layering technique. Only 50 to 70 per cent of the total cytochrome c oxidase activity of the original homogenates was recovered among the four fractions from both rat and guinea pig spleens, while the specific activities of reconstructed homogenates were only 55 to 75 per cent of those of the original whole homogenates. This was in contrast to the results with DPNH cytochrome c reductase and succinic dehydrogenase where the recovery of total enzyme activity approached 100 per cent, and the specific activities of reconstructed homogenates equalled those of the original homogenates. The recovery of cytochrome c oxidase was greatly improved when only the nuclei were separated from rat spleen homogenates. 6. Data were presented comparing the concentrations (ratio of activity per mg. of nitrogen of the fraction to activity per mg. of nitrogen of the homogenate) of DPNH cytochrome c reductase in mitochondria and microsomes derived from different organs of different animals. 7. Data were presented comparing the activities per mg. of nitrogen of DPNH cytochrome c reductase in homogenates from several organs of various animals.     

The spleen of the African lungfish Protopterus annectens: freshwater and aestivation

We describe the structure of the spleen of the African lungfish Protopterus annectens in freshwater conditions, and after 6?months of aestivation. The spleen is formed by cortical tissue that surrounds the splenic parenchyma. The cortex is a reticulum that contains two types of granulocytes, developing and mature plasma cells, and melanomacrophage centres (MMCs). The parenchyma is divided into lobules that show a subcapsular sinus and areas of red pulp and white pulp. Red pulp contains vascular sinuses and atypical cords formed by delicate trabeculae. White pulp also contains vascular sinuses and cords. Structural data indicate that red pulp is involved in erythropoiesis, destruction of effete erythrocytes, and plasma cell differentiation. White pulp appears to be involved in the production of immune responses. Macrophages and sinus endothelial cells constitute the reticulo-endothelial system of the spleen. After aestivation, the number of MMCs increases, and spleen tissue is infiltrated by lymphocytes, granulocytes, and monocytes. Also, white pulp is reduced, and sinus endothelial cells undergo vacuolar degeneration. Lungfish spleen shares structural characteristics with secondary lymphoid organs of both ectothermic and endothermic vertebrates, but appears to have evolved in unique ways.     

Studies of a primitive mammalian spleen, the opossum (Didelphis virginiana)

Light microscopic sections of the adult opossum (Didelphis virginiana) spleen were observed to lack venous sinuses; this primitive mammalian spleen may be classified as non-sinusal in nature. In the spleen of the opossum, the capillary segments of the penicillar arteries lacked ellipsoid sheaths characteristic of certain mammalian spleens. Separating the lymphoid nodules from the surrounding red pulp was a distinct band of vascular tissue, the marginal zone. Arising from the central artery within the lymphoid nodule, vessels of capillary dimension were observed to terminate within the marginal zone and the area between lymphoid nodule and marginal zone. In addition to the vascular channels established by the terminal arterial vessels within the red pulp, the system of vessels within the marginal zone has been implicated as an important intermediate vascular channel within the spleen.     

Volume capacity and contraction control of the seal spleen

Cabanac, Arnaud, Lars P. Folkow, and Arnoldus Schytte Blix.Volume capacity and contraction control of the seal spleen. J. Appl. Physiol. 82(6):1989-1994, 1997.Volume changes in the spleens of hooded seals(Cystophora cristata) and harp seals(Phoca groenlandica) were measuredplethysmographically in vitro in response to epinephrine,norepinephrine, isoprenaline, phentolamine, and acetylcholine. Dilated spleens contracted forcefullywithin 1-3 min of -adrenoceptor activation with 1.0-5.0µg epinephrine/kg body mass, whereas stimulation of -adrenoceptorsand cholinergic receptors had little effect. The mass of dilated hoodedseal spleens corresponded to 2-4%(n = 7) of body mass, with volume (V;ml) relating to body mass (M; kg) as follows: V = 12.0M + 910 (r² = 0.96, n = 4). Thus the spleen of a 250-kghooded seal maximally expels 3.9 liters, or 13%, of its estimatedtotal blood volume. Average hematocrit in splenic venous outflow fromdilated spleens was 90 ± 3% (n = 3) in hooded seals and 85% (n = 2) inharp seals. From these data we have estimated that the aerobic divinglimit of a 250-kg hooded seal increases only 105 s, at the most, if complete emptying of the spleen occurs during diving, while the corresponding estimate for a 112-kg harp seal is 80 s.

     

Comparative histochemical and electron microscopic studies of the sinus and venous walls of the human spleen with special reference to the sinus-venous connections

Sinus and venous walls of normal human spleens were studied with enzyme histochemical and electron microscopic methods. Particular attention was paid to the connections between sinuses and veins. Histochemically the sinus lining cells revealed a distinct naphthol-AS-acetate-esterase activity but no reaction for alkaline phosphatase. Venous endothelial cells were positive for the latter but negative for the former enzyme. In the sinus-venous junctional area there were no endothelial cells with reactivity for both enzymes. Electron microscopically both the sinus lining cells and the venous endothelial cells could be clearly characterized and therefore easily distinguished from one another on morphological grounds. There were no clear ultrastrural indications of transitional forms between sinus lining cells and venous endothelial cells in the sinus-venous area. According to these findings, sinus lining cells represent a specialized endothelium, but one with practically no morphological similarities to the venous endothelium.     

Freeze-fracture investigation of the red pulp of human spleen

This study concerns the morphology of the human spleen in freeze-fracture replicas and compares this with the findings in ultrathin sections. The material investigated consisted of two spleens resected at gastrectomy and one resected because of splenomegaly in a case of hairy cell leukaemia. The current concepts concerning the ultrastructure of the spleen were generally confirmed with the freeze-fracture technique. The sinus walls were found, as expected, to consist of closely fitting endothelial cells, which were identifiable in freeze-fracture replicas by numerous caveolae of the cell membrane. Contrary to the opinion upheld in the literature, the sinus endothelial cells were occasionally found to be connected by desmosomes or maculae adhaerentes. Corresponding to the finding of desmosomes in ultrathin sections, focal collections of intramembranous particles were observed in freeze-fracture replicas and a positive immunohistochemical reaction for desmoplakin in the sinuses was found at the light microscopic level. The view generally held in the literature that sinus endothelial cells can exhibit tight junctions was not confirmed. However, such junctions were found between vascular endothelial cells. The ring fibres of the sinuses, which are closely connected to the sinus endothelial cells through contractile fibres, apparently have various functions. Firstly, they contribute towards maintaining mechanical stability. Secondly, they represent basement membranes through which exchange occurs between the sinus endothelial cells and their surroundings. This is indicated by the caveolae and vesicles that are often found here in large numbers and in focal collections. Hairy cells exhibit no features in freeze-fracture replicas to suggest a cytogenetic relationship to interdigitating reticulum cells.