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The effects of hyperosmolarity on the blood-brain barrier morphology and permeability were studied using light and electron microscopy in the adult rat cerebral cortex. Sucrose was used as the hyperosmolar agent. An extracorporeal rat-head perfusion technique was employed. The general effect was shrinkage of all layers of the arteriolar wall, endothelium, pericyte, and smooth muscle with a concomitant increase in the adventitial spaces. The0.28 M sucrose perfusate caused the entry of a small amount of peroxidase into adventitial cell cytoplasm with none in the spaces. In the0.42 M sucrose perfusates there were large accumulations of horseradish peroxidase (HRP) in the adventitial spaces limited by the glial basement membrane. None was seen in the neuropil space. Endothelial tight junctions appeared to be closed to peroxidase in the0.42 M group. Although a small number of pinocytotic vesicles were seen containing peroxidase, they did not seem adequate to account for the apparent transport of sucrose and the large quantity of HRP noted in the0.42 M sucrose-treated animals. We support the suggestion that shrinkage of endothelial cells, caused by the hyperosmolarity of the sucrose perfusate, resulted in a transient opening and closing of the tight junctional complex causing the rapid movement of sucrose and peroxidase into the adventitial spaces.

The existence of pericytes on the capillaries of lung alveoli has long been questioned. A systematic electron microscopic study has shown them to exist in human lungs, as well as in lungs of dogs, guinea pigs, and rats, whereas they could not be found in the lungs of the smallest mammal, the Etruscan shrew. The study of systemic capillaries (myocardium, pancreas) shows the pericyte to be characterised by a close association with the capillary basement membrane within which it is ensheathed, by the formation of branched cytoplasmic processes which approach the endothelial cell more and more the finer they become, by the presence of a web of cytoplasmic filaments along the membrane close to the endothelium, and by the exclusive or predominant occurrence of pinocytosis along the outer cell membrane; lysosomal elements are a rare occurrence. These features are comparable to those characterising smooth muscle cell processes in venules. Several cells and numerous cell processes showing all these characteristics have been found associated with lung capillaries. It is concluded that pulmonary alveolar capillaries are also provided with pericytes, but that these appear to be rarer than on systemic capillaries and that they are less densely branched. Their frequency furthermore appears to be proportional to body and lung size.

An electron microscopic study was made of the ultrastructure and permeability of the capillaries in the cerebral hemispheres of the lizard, Anolis carolinensis. The brain of Anolis is vascularized by a loop-type pattern consisting exclusively of arteriovenous capillary loops. The ultrastructure of the endothelium and the arrangement of the various layers from the capillary lumen to the central nervous tissue is similar to that of mammals. The endothelial cells form a continuous layer around the lumen and are joined by tight interendothelial junctions. The basal lamina of the endothelium is also continuous and encloses pericyte processes. The cells of the nervous tissue rest directly on the basal lamina of the capillary and are separated from each other by a 200 A space. Intravenously injected horseradish peroxidase (MW 40,000) and ferritin (MW 500,000) were used to study the permeability of the capillaries. The entry of horseradish peroxidase and ferritin into the intercellular spaces of the brain is restricted by the tightness of the interendothelial junctions. No vesicular transport of either tracer occurs; however, ferritin does enter the endothelial cells in vacuoles. No tracer molecules are present in the basal lamina, pericytes, or nervous tissue. The different responses of the endothelial cell to the tracers used in this study suggest that endocytotic activities of endothelial cells involve different processes. Vacuoles formed by marginal folds, vacuoles formed by endothelial surface projections or deep invaginations of the plasma membrane, 600–800 A vesicles, and coated vesicles all seem to differ in the nature of the substances which they endocytose.

In continuation of a previous paper on the middle-ear epithelium in chronic Secretory and suppurative otitis, the authors describe the conditions of the submucosa in the same diseases. First, the stroma and the cellular population in the submucosa of the normal middle ear are described, in particular the pericyte and its presumed potential properties as the precursor of histiocytes as well as fibroblasts. Moreover, the authors mention the evidence indicating that the formation of collagen fibrils takes place in an intracellular site, in relation to the endoplasmatic reticulum. The most important cellular changes in the submucosa in chronic Secretory otitis are a material increase in the number of fibrocytes, fibroblasts, and histiocytes as well as some increase in the number of neutrophilic leukocytes and mast cells. On the other hand, eosinophils were not present. This militates against allergy being the cause of Secretory otitis. Finally, a proliferation of the capillary network was found, with an increased number of pericytes and histiocyte-like cells in relation to the pericytes. In the stroma there was an increased number of collagen fibrils, without signs of degenerative changes. The studies showed no essential morphological differences between otitis with serous and otitis with mucous secretion. The quantitative and qualitative differences in the secretion must be explained on the basis of the previously demonstrated epithelial changes in these disease conditions.

Introduction: A hemangiopericytoma is a rare, soft-tissue tumor of vascular origin derived from a pericyte of Zimmerman, which is a modified smooth muscle cell that surrounds the small blood vessels. Hemangiopericytomas can occur wherever there are vascular capillaries. However, there were no previous reports of a hemangiopericytoma in the retroperitoneal space. Case presentation: We describe the first reported case of a hemangiopericytoma found in the retroperitoneal space. A 25-year-old man presented with a palpable tumor on the left abdomen. Preoperative imaging indicated that the tumor was in the retroperitoneal space without invasion of other organs. A complete resection was performed via a midline incision. The histological and immunohistochemical staining patterns supported the diagnosis of a hemangiopericytoma. Conclusion: A complete resection without piecemeal excision is the best way to treat a hemangiopericytoma. Recognizing the presence of a hemangiopericytoma in the retroperitoneal space requires appropriate surgery. DOI: http://dx.doi.org/10.3126/hren.v9i3.5593 HR 2011; 9(3): 210-213

Of recent years, owing to the work of Krogh and his collaborators, the idea that the contractility of blood capillaries resides in certain adventitial cells has been brought into prominence and has gained a wide acceptance. Vimtrup (5, 6, 7) succeeded in displaying, by histological methods, cells applied to the walls of capillaries. He stated that these cells were contractile, and that, by the shortening of their elongated processes, the capillary lumen was narrowed. These results were obtained, for the most part, in the frog, although analogous cells were described as existing on the capillaries of the mammalian skin.

Summary 1. A survey of accounts of the histology of the blood system in the various families of the Oligochaeta and Polychaeta has revealed a surprising lack of agreement among descriptions not only of the same family, but often of the same genus or species. There are several reasons for this. Many annelid vessels are very small and are difficult objects for histological investigation. They have rarely been examined in detail whilst alive. Few people have employed special staining methods for distinguishing connective tissue fibres from muscle fibres; nearly always both are present, and they have often been confused with each other. The lack of any comprehensive and analytic review of the scattered literature on the subject seems to be one of the reasons for many of the discrepancies. 2. Not only is there disagreement in factual descriptions of histological structure, but, since 1904, there has also been much discussion about the interpretation of observations, occasioned by two important rival theories of the origin of the annelid blood system (Lang, 1904; Vejdovsky, 1905). 3. Lang postulated that the haemocoel occupies the site of the blastocoel. The gut sinus is bounded internally by the basement membrane of the gut epithelium. The outer wall of the sinus and the walls of the vessels are derived from splanchnopleuric mesoderm, and consist of the peritoneum and its basement membrane (Leydig's ‘Intima’). An endothelium, where present, is a secondary component of the walls; its origin is obscure. 4. Vejdovsky considered that the gut sinus is intra-endodermal. The blood bathes the gut epithelium, the basement membrane of which is situated in the outer wall of the sinus. This outer wall, and the walls of the blood vessels, have both endodermal and mesodermal components. The latter is the peritoneum. The endodermal component consists of a skeletal coat (vasothelial membrane, equivalent to Leydig's intima) and vasothelial cells of endodermal origin. These cells, corresponding to the endothelium of other authors, are primary components of the walls. 5. It has been shown that Vejdovsky's observations disagree with those of previous workers, and that later research has not confirmed them. They are open to criticism because he used inadequate staining methods. 6. The theories of Lang and Vejdovsky led to controversy in the interpretation of observations. They also stimulated a large amount of research, much of which was uncritical. For example, two adherents of Vejdovsky's theory noticed a basement membrane on the gut epithelium, but reiterated the statement that the sinus is intra-endodermal. Similarly, many supporters of Lang's theory paid little attention to cells lying on the inner surface of the skeletal coat, because Lang considered that they are secondary components of the wall; they have frequently been dismissed as sessile blood corpuscles. 7. An analysis of all the literature on the histology of the blood system in the Oligochaeta and Polychaeta has led to the following conclusions, which should be regarded as tentative until more careful and detailed observations have been made. 8. The walls of the blood system contain a homogeneous connective tissue membrane. This skeletal coat is covered by a peritoneum which is differentiated in various ways in different vessels. On the inner surface of the skeletal coat lies a discontinuous endothelium. 9. The endothelium probably often takes the form of branched cells joined together by their processes. Endothelial cells are flat, and their flat oval nuclei are elongated in the direction of the length of the vessel. Fibrils have sometimes been found in the cells, but have never been shown to be contractile. However, there is evidence that the endothelium may in some cases be contractile. Iron and chloragosome-like bodies have occasionally been found in it. 10. The skeletal coat is a continuous homogeneous membrane of a collagenous substance. It is linked by fibres with connective tissue outside the vessel. It becomes longitudinally folded when the vessel contracts. 11. The peritoneum is differentiated in various ways, probably correlated with the size and contractility of the vessels. It may be a simple flat epithelium without fibrils; an epithelium with intracellular muscle fibrils; a muscle-epithelium; a coat of muscles with a cellular covering consisting both of peritoneal epithelial cells and of the cell bodies of the muscle fibres; a coat of muscles covered by a complete peritoneal epithelium. 12. The muscle fibres are of the nematode type, i.e. they consist of a cell body containing the nucleus, and processes containing the muscle fibrils. The nucleus is large and nearly spherical and has a conspicuous nucleolus. On small vessels the muscle cells are sometimes stellate, resembling the Rouget cells of vertebrate capillaries. Like Rouget cells, they can be vitally stained with methylene blue. 13. Various descriptions of the internal structure of the larger muscle fibres have been given. Some are apparently unstriped. Others have a double oblique striation. Others are said to show a true cross-striation. In some cases the fibre apparently contains both striped and unstriped fibrils. 14. Several theories of the origin of the various coats in annelid vessel walls have been advanced (notably by Lang and Vejdovsky). Evidence based on comparative anatomy, embryology and studies on regeneration suggests that Lang's theory is to be preferred. The haemocoel occupies the site of the blastocoel. The walls of the blood system are entirely mesodermal. The walls of the larger vessels, and the outer wall of the gut sinus are formed by the splanchnopleuric mesoderm, to which belong the peritoneum and the skeletal coat. Nothing is known about the origin of the endothelium. I wish to acknowledge the help I have gained by many discussions of this subject with Dr A. Stock. I am grateful to him and to Dr B. M. Walshe for reading and criticizing the manuscript.

Elektronenmikroskopisch konnen an den Capillaren der Stria vascularis Pericyten (Rouget-Zellen) nachgewiesen werden, deren contractile Funktion wahrscheinlich erscheint, da zarte Filamente in ihnen nachzuweisen sind. Morphologische Hinweise fur eine humorale Wirkung auf die Permeation der Capillarwand (Granula von Serotonin, Noradrenalin etc.) fanden wir nicht. Damit werden neue Indizien geliefert, das auch die Capillaren der lateralen Cochleawand eine vegetativ-humorale, contractile Regulation aufweisen. Die subtilen Untersuchungen von Perlman, Kimura u. a. mussen wohl so gedeutet werden, das durch die Eroffnung des Innenohres unphysiologische Bedingungen geschaffen werden, so das die freigelegten Gefasbahnen in eine Starre, ahnlich wie in einen Schock geraten.

Alterations in the structure of small cerebral vessels were studied with the electron microscope at various intervals following ionizing particle irradiation. The disruption of small vessels required heavier doses of irradiation than is required for destruction of neural or glial elements. The appearance of large phagocytes which might be identified with the compound granular corpuscle is dependent upon and correlated with perivascular reaction. These cells appear to be derived from the pericytes of small vessels and not from any glial element within the brain parenchyma. The appearance of dense bodies or lamellar inclusions in cytoplasm cannot constitute evidence of phagocytosis. The only cerebral element which appears to display macrophage activity seems to be derived from the vascular pericyte. It is suggested that mesodermal elements only extend beyond the basal lamina of vessels under conditions of tissue disruption. The implications of these findings with respect to the designation “microglia” is discussed.

The cephalic aorta of Octopus vulgaris has a fairly complete endothelium lining the lumen, a thick complete basement membrane, a layer of circularly orientated and a layer of longitudinally orientated muscle fibres. Presumptive synaptic endings are of two types. In the circular muscle, axons containing vesicles, contact club-shaped projections of the muscle. The gap between the pre- and postsynaptic membranes is less than 100 A and in some places apparently forms a “tight junction”. The second type of ending has been found in the longitudinal muscle; here axons full of vesicles end on the muscle. The ending is enclosed by a “mesaxon” of muscle and the synaptic gap is approximately 100 A. In the smaller blood vessels, axons end on myofilament-containing pericytes of blood vessels (equivalent to small arterioles). The endings contain vesicles and have a synaptic gap of 100 A. Only some of the pericytes seem to be innervated and transmission between one pericyte to another may be mediated by specialized junctions between the cells. The smaller non-myofilament containing vessels (equivalent to capillaries) are not thought to be innervated.

The region of the posterior horn of the spinal cord in the lumbosacral region of the rat was studied with the electron microscope. This zone contained neurons, astrocytes, oligodendrocytes and blood vessels. The neurons possessed irregular nuclei which occupied a large portion of the cell. The cytoplasm contained elaborate smooth surfaced membrane systems. The rough surfaced endoplasmic reticulum was small in amount and did not possess an orderly arrangement. Electron dense bodies were invariably found within the neurons. The morphological features, such as an extremely irregular nucleus and elaborate cisterns of smooth surfaced reticulum, indicate a well developed nucleocytoplasmic transport mechanism in the neuron. The vascularity of the area was evidenced by the presence of numerous blood vessels which generally had a perineuronal position. Pericytes were invariably found in close relationship to the endothelial cells of the blood vessels. The nucleus of the pericyte was remarkably dense. The cytoplasm of these cells was extremely attenuated and almost encircled the blood vessels.

(1) From this series of experiments carried out under good optical conditions, in one case as good as present lenses allow, we conclude that capillary contraction occurs in the rabbit's ear-chamber, that this contraction is under the control of the sympathetic nervous system, and that endothelial swelling causes the obliteration of the vessels. No positive evidence was obtained of the participation of Rouget cells in this contraction. (2) “Spikes” on the endothelium of slightly injured veins have been noted. Red corpuscles hang on these, often for several seconds. The chambers have been made in the laboratory by Mr. S. W. Bush, to whose skill we are greatly indebted. Mr. J. Kent has had charge of the animals. The drawing is by Mr. H. Axtell.

Previous work has shown that endogenous chemical mediators, of which histamine is the prototype, increase the permeability of blood vessels by causing gaps to appear between endothelial cells. In the present paper, morphologic and statistical evidence is presented, to suggest that endothelial cells contract under the influence of mediators, and that this contraction causes the formation of intercellular gaps. Histamine, serotonin, and bradykinin were injected subcutaneously into the scrotum of the rat, and the vessels of the underlying cremaster muscle were examined by electron microscopy. To eliminate the vascular collapse induced by routine fixation, in one series of animals (including controls) the root of the cremaster was constricted for 2–4 min prior to sacrifice, and the tissues were fixed under conditions of mild venous congestion. Electron micrographs were taken of 599 nuclei from the endothelium of small blood vessels representing the various experimental situations. Nuclear deformations were classified into four types of increasing tightness (notches, foldsl closing folds, and pinches. In the latter the apposed surfaces of the nuclear membrane are in contact). It was found that: (1) venous congestion tends to straighten the nuclei in al groups; (2) mediators cause a highly significant increase in the number of pinches (P < 0.001), also if the vessels are distended by venous congestion; (3) fixation without venous congestion causes vascular collapse. The degree of endothelial recoil, as measured by nuclear pinches, is very different from that caused by mediators (P < 0.001). (4) Pinched nuclei are more frequent in leaking vessels, and in cells adjacent to gaps (P < 0.001); (5) mediators also induce, in the endothelium, cytoplasmic changes suggestive of contraction, and similar to those of contracted smooth muscle; (6) there is no evidence of pericyte contraction under the conditions tested. Occasional pericytes appeared to receive fine nerve endings. Various hypotheses to explain nuclear pinching are discussed; the only satisfactory explanation is that which requires endothelial contraction.

The pathway by which intravenously injected ferritin molecules move from the blood plasma across the capillary wall has been investigated in the muscle of the rat diaphragm. At 2 min after administration, the ferritin molecules are evenly distributed in high concentration in the blood plasma of capillaries and occur within vesicles along the blood front of the endothelium. At the 10-min time point, a small number of molecules appear in the adventitia, and by 60 min they are relatively numerous in the adventitia and in phagocytic vesicles and vacuoles of adventitial macrophages. Thereafter, the amount of ferritin in the adventitia and pericapillary regions gradually increases so that at 1 day the concentration in the extracellular spaces approaches that in the blood plasma. Macrophages and, to a lesser extent, fibroblasts contain large amounts of ferritin. 4 days after administration, ferritin appears to be cleared from the blood and from the capillary walls, but it still persists in the adventitial macrophages and fibroblasts. At all time points examined, ferritin molecules within the endothelial tunic were restricted to vesicles or to occasional multivesicular or dense bodies; they were not found in intercellular junctions or within the cytoplasmic matrix. Ferritin molecules did not accumulate within or against the basement membranes. Over the time period studied, the concentration of ferritin in the blood decreased, first rapidly, then slowly, in two apparently exponential phases. Liver and spleen removed large amounts of ferritin from the blood. Diaphragms fixed at time points from 10 min to 1 day, stained for iron by the Prussian Blue method, and prepared as cleared whole mounts, showed a progressive and even accumulation of ferritin in adventitial macrophages along the entire capillary network. These findings indicate: (1) that endothelial cell vesicles are the structural equivalent of the large pore system postulated in the pore theory of capillary permeability; (2) that the basement membrane is not a structural restraint in the movement of ferritin molecules across the capillary wall; (3) that transport of ferritin occurs uniformly along the entire length of the capillary; and (4) that the adventitial macrophages monitor the capillary filtrate and partially clear it of the tracer.

AN important feature of the peripheral circulation is the close correspondence between the local needs of a given tissue and the amount of blood supplying it. Controlling mechanisms, to accomplish this, must involve more than the vasoconstrictor and dilator reactions of the terminal arterioles. Krogh attempted to ascribe the relative autonomy of the capillary circulation to the widespread occurrence of contractile, pericapillary elements, the Rouget cells. This has been considerably debated, and has never been adequately demonstrated. Moreover, there is still the question concerning the manner in which an adequate venous return can be maintained in spite of the supposed interposition, between arterioles and venules, of a network of extremely pervious capillaries1. The techniques hitherto used for observing the capillary vessels have been either too fragmentary or have lacked the required delicacy of experimental approach for ascertaining their highly reactive and finely balanced behaviour.

Retinal pericyte degeneration, a usual finding in diabetic retinopathy, has been shown to be a change probably peculiar to the retina rather than part of a more widespread angiopathy. Thus in a patient with diabetic retinopathy the characteristic eosinophilic degeneration of the pericyte nuclei ceased at the optic disc and could not be found in vessels of the optic nerves and brain.

The presence of blood group isoantigens in vascular endothelium cells was used in order to identify cells of endothelial origin in CNS tumors. 70 cerebrospinal tumors coded as “angioblastic meningiomas” and 30 cerebellar hemangioblastomas were examined for the detection of BG isoantigens by the specific red cell adherence (SRCA) test by Davidson (1972). In cerebellar and supratentorial hemangioblastomas a positive SRCA reaction in endothelial cells forming the prominent component of this neoplasm was opposed by a negative reaction in the large majority of interstitial (stroma) cells that argues against their endothelial origin. Identical behavior of SCRA test supports the histogenetic identity of hemangioblastomas and angioblastic meningiomas. A consistently negative SRCA reaction in cerebrospinal hemangiopericytomas except for the endothelial cells lining the capillaries supports the concept of a non-endothelial (pericytic) histogenesis of this neoplasm. Richly vascularized meningiomas with positive SRCA reaction restricted to vascular endothelial cells can be separated from angioblastic meningiomas, although transition forms are likely to occur. Angioblastic meningiomas suggested to originate from vasoformative elements (endothelium and pericytes) were found to represent 3.2 percent of a biopsy sample of 660 cases with cerebrospinal meningiomas. Their relationship to primary meningeal neoplasms needs further elucidation.

To evaluate the variations of capillary calibers and those of basal lamina (BL), pericytes, endothelium, and lumen within a sample of skeletal muscle and among samples from different anatomic sites, areas occupied by total capillary cross sections, by the lumen, endothelium, pericytes, and BL were measured on electron micrographs. Twenty capillary cross sections from each of the 41 specimens of skeletal muscle from 23 nondiabetic males were used. For all parameters, a marked variation was noted within each specimen, between specimens from identical sites of different individuals, and between different anatomic sites of the same individuals. In general, the capillaries are largest in the lower leg and foot muscles; the lumen is smallest at the levels of mid-thigh and lower leg; the capillaries of the lower extremity have a greater investment of BL, endothelium, and pericytes than the capillaries of the trunk and hand muscles. The areas occupied by endothelial cytoplasm, pericyte cytoplasm, and by BL are larger in bigger capillaries apparently owing to greater circumference. In addition, the capillaries of the leg, particularly those of the lower leg, have an absolutely greater investment of BL than capillaries from proximal sites. Reasons for these regional differences in the overall size of capillaries and in the investment of BL are not known.

Introduction Two types of cells have been identified on the retinal capillaries of humans 1-4 and animals. 1,5,6 One type is the endothelial cell which is the main component of the capillaries. A second type of cell has been called a mural cell by Kuwabara et al. 7 It also has been called an intramural pericyte, a pericyte, or an undifferentiated cell. The function and characteristics of these cells are not well established. Kuwabara and Cogan have assumed that the mural cells play a significant role in the retinal capillary function. 9 They suggest that inhibition of neovascularization may be one function of the mural cell, also that these cells may participate in the control of capillary tone. 8 The reaction of the endothelial and mural cells of the retinal capillaries and the other vascular structures to an experimentally produced allergic condition in the animal eye was investigated using the

A “blind” study on the ultrastructure of the dermal capillaries of Africans in Johannesburg, South Africa, is reported. None of the patients had a family history of diabetes mellitus, and all were normotensive. A group of African, nondiabetic, normotensive ward patients with normal glucose tolerance curves acted as controls. As a corollary, a small group of Indian (Asiatic) and white patients were included in the sample to determine if any capillary changes could be related to ethnic group, diet, or other circumstances. The results indicate that thickening of the basal lamina (basement membrane) in the dermal capillaries of Africans is not specific for diabetes mellitus, and that no significant ultrastructural differences exist among the diabetics of the three major ethnic groups. However, it appears that the capillary lesion is commoner in diabetics than in nondiabetics, and the role of the pericyte in the possible pathogenesis of the lesion is discussed.

The ultrastructure of bovine spinal ganglia and intercostal nerves is described. The capsule of the ganglion and the perineurium of the nerve were similar and consisted of an inner zone of four to six layers of perineural epithelial cells and an outer zone of collagen. The ganglionic neuron was entirely surrounded by a layer of satellite cells but in some neurons processes from the neuronal perikaryon penetrated the satellite cell sheath and terminated adjacent to the basement membrane of the satellite cell layer. Similar axonal projections were occasionally observed in nerve fibres at the nodes of Ranvier. Irrespective of size all neurons had a similar morphology. Bifurcation of the nerve fibre was occasionally seen in the ganglion. The nerve was composed of both myelinated and unmylinated fibres and nodes of Ranvier and Schmidt-Lantermann clefts were frequently encountered. Capillaries were surrounded by pericyte cells and in the interstitial tissues mast cells were frequently seen.

Electron microscopic examination of several thalamic nuclei undergoing neural degeneration following ablation of their cortical projection fields revealed reactive changes in vessels unrelated to direct mechanical trauma. A series of 30 rabbits were studied at post-operative survivals of one to 238 days. Many neurons display signs of retrograde atrophy within the first week accompanied by a distinct increase in the number of perivascular cells within the basal lamina surrounding venules and large capillaries. This reaction was optimally observed between 5 and 98 days post-operatively and was limited to the circumscribed zones of degeneration. In early stages, some vessels displayed margination of hematogenous elements, predominantly agranular leukocytes, of which some examples were seen in phases suggestive of diapedesis. Cells within the perivascular space exhibited diverse morphology. In addition to the indigenous “pericyte” population, there were numerous agranular leukocytes, and a few plasma cells and eosinophils. Within the second to third week postoperatively, some perivascular elements become enlarged and display electron-lucent lipid vacuoles and PTA-positive dense bodies of complex morphology, many of which are clearly lysosomal in nature. This cellular reaction persists for three to four months, after which time the cytoplasm displays evidence of cholesterol crystals and lysosomal transformations suggestive of terminal or residual bodies. Morphological criteria for identifying the possible origin of vacuolated, lysosomeladen perivascular cells are presented for a consideration of their possible derivation from pericytes and/or hematogenous elements. Of the several hypotheses considered to account for these findings, it appears likely that lipid and protein end-products of neural degeneration provide the stimulus for perivascular aggregation and subsequent transformation.

Unmyelinated axons in a Schwann cell sheath have been found in the rat lung, in alveolar wall deep within the acinus: they are rare. These fibres may be sensory, their position suggesting an association with the J receptor. Processes from the alveolar pericyte are commonly seen; these resemble an axon in that they contain numerous tubules similar in diameter to neurotubules and are found within a basement membrane.

The ultrastructure of capillaries and arterioles of the hamster dental pulp has been studied. Pieces of the incisor pulp were fixed in 1% OsO4 buffered with 0.14 M veronal acetate, embedded in epoxy resin and studied in an electron microscope. The basic cytoplasmic organization of endothelial cells confirmed observations on blood vessels of other organs made by previous investigators. A large number of intracytoplasmic fibrils were found in many endothelial cells, and their presence was discussed in relation to earlier reports on the contractility of endothelial cells. Previous observations on the structural details of the intercellular region of the endothelium were confirmed and elaborated upon. The manner in which the cytoplasmic flaps of adjoining cells were oriented indicated that, if the integrity of the capillary wall was maintained, only cells with capacity for ameboid movement could creep through the region. Thus they were thought to play a role as an effective barrier against the passage of red blood cells. Numerous micropinocytosis and pinocytosis vesicles were found in association with plasma membranes. The significance of their structure in the transport of materials across the capillary wall is discussed. The fine structure of the pericyte resembled that of the fibroblast. Portions of plasma membranes of adjacent smooth muscle cells in the media of arterioles were brought together closely and resembled attachment zones.

This rare tumor which contains a combination of mesenchymal elements is believed to be the only one of its kind appearing in the orbit. The mesenchymoma has been described in detail by Stout. 1,2 These tumors which are derived from mesenchyme appear microscopically to arise from supportive and reticuloendothelial tissues. Stout 1 believes that most of the mixed mesenchymal tumors are probably initiated as the result of a developmental fault and so are regarded as dysontogenic growths. He believes that metaplasia plays some part in their development. There has been considerable discussion regarding this tumor, for it demonstrates two tissues arising from basic mesenchyme. The principal one represents a hemangiopericytoma, in which the tumor cells are believed to arise from the pericyte of Zimmerman and lie outside the reticulum sheath of the capillary, differentiating it from the hemangioendothelioma. 3 In addition, there appears in this tumor a growth of chondroblastic

1. An electron microscopical analysis of the development of new capillary structures from vascularized (omentum) or avascularized (cardiac valves) rat tissue expiants in diffusion chambers placed in rat peritoneal cavity, has shown that the solid precapillary structures previously described (Aloisiet al., 1970) are always multicellular in nature, even when they appear as syncytial to the light microscopy. 2. The nature of the cells composing the solid pre-capillary structures appears to be different from that of common fibroblasts, an evolution of the latter to endothelial-like cells being nevertheless not excluded. 3. In these pre-capillary structures cellular differentiations take place precociously and lead to pericyte formation on the outer side and to degeneration of the inner cells. 4. The formation of a capillary lumen is therefore associated to such degenerative changes, and to the shedding and dissolution of the inner cells, a process which, combined with the apposition of new endothelial cells from outside may also account for the widening of the newly formed elementary vessel. 5. The morphogenetic value of these regressive phenomena in the formation of capillary lumina is discussed, as analogous to other phenomena in the embryo and particularly reminiscent of the formation of embryo vasculature from angioblast and from other mesenchymal anlagen.

The hyaloid vascular system is an extensive, transitory network of intraocular blood vessels. The hyaloid artery runs from the optic disc to the posterior pole of the lens, giving off branches which fill the vitreous—the vasa hyaloidea propria. When the hyaloid artery reaches the lens, it divides into branches which ramify along the posterior lens capsule—the tunica vasculosa lentis. The electron microscope reveals that the capillaries of the vasa hyaloidea propria and tunica vasculosa lentis are of the A-1-alpha structure, with a continuous nonfenestrated endothelial lining, a continuous basement membrane, and an interrupted pericyte lining. The hyaloid artery has the fine structure of a typical arteriole, with a prominent tunica media of smooth muscle. After their function is complete, the hyaloid capillaries involute, become occluded by macrophages, and eventually disappear.

Haemangiopericytoma, first described by Stout and Murray in 1942, is a rare vascular tumour, related to the glomus tumour, in which the proliferating cell is the pericyte of Zimmerman, situated in the capillary wall outside the reliculin sheath. This position distinguishes the pericyte from endothelial cells in the capillary wall. Haemangiopericytoma lacks the organoid formation of the glomus tumour. In accordance with its tissue of origin, the tumour may occur in widespread parts of the body. However, it is most commonly found in the musculo-skeletal system and skin. Fewer than 10 cases have been reported in the nasal cavity, and the following case is reported because of its rarity in this site.

The retinal vasculature of the pig was examined in histological sections, in whole mounts, and by electron microscopy. The pig retina usually has four major retinal arteries and veins and a capillary network in multiple layers similar to those of the human. There is no macula, fovea or tapetum in the pig. The arteries and veins are composed of endothelial cells separated from several layers of smooth muscle fibers by basement membranes. The capillaries consist of endothelial cells which surround a small lumen and are, themselves, surrounded by a complete basement membrane which occasionally splits to surround a pericyte. The structure of the retinal vasculature of the pig is compared with that of the human.

Cytological features and certain histological relationships in the avian pecten oculi in the domestic chicken are described in detail at both the light and electron microscopic levels. Comparisons and contrasts are made to relate the authors' electron microscopic observations on this specialized vascular organ with those of Tanaka (1960). The cleared, doubly injected in toto preparations of the adult pecten oculi shows that the basic plan of organization of the blood vessels in each pectineal fold consists of a supplying artery and a draining vein, and between these two main vessels are numerous anastomosing capillaries of varying calibre. The main artery (longitudinal artery), together with its ventrally associated longitudinal vein, runs along the base of the pecten and gives off a supply and draining branch to each of the pectineal folds (16–18 in number). Histologically, the avian pecten oculi consists of two tissue components: (1) a plexus of modified blood vessels and, interdigitated between the blood vessels, (2) numerous pigmented intervascular cells. No glial elements have been observed. The pectineal capillaries which form the main portion of the pecten are lined by a continuous layer of endothelial cells which typically possess some interesting ultrafine structural modifications and specializations. There are two membrane systems in a pectineal endothelial cell: (1) apically, the plasma membrane is highly plicated and (2) basally, there is also a folded membrane system which appears at intervals confluent with the bounding thick, laminar and fibrillar basement membrane. Groups of small vesicles are associated with the infoldings of the membrane systems. It is suggested that the vesicular profiles, as well as the infoldings of the membrane systems, may represent and function as an agranular endoplasmic reticulum in the pectineal endothelial cells. Preliminary studies have shown that intravenous injection of Diamox, which reduces intraocular pressure within 60 min after administration, in the avian eye causes marked changes in the ultrafine morphology of the membrane system of the pectineal endothelial cell. Both membrane systems are labile and are transformed into more complex highly interdigitated membrane systems within a relatively short time. Exactly how this striking morphological transformation of the membrane is accomplished is unknown, and is being investigated at the present time. The larger calibre pectineal blood vessels contain well defined intramural cells (pericytes, pericapillary cells, Rouget cells) which are completely surrounded by a laminar and fibrillar basement membrane. The intravascular pigmented tissue contains numerous dense osmiophilic pigment granules. Associated with each pigment granule are two or three mitochondria. No intercellular spaces are present between the pigment cells and the neighbouring pectineal capillaries in this material. The ultrafine morphology of elaborate membrane systems of the endothelial cells of the pectineal capillaries suggests (a) that it is a device for increasing cell surface, and (b) that this epithelium is probably concerned in active fluid transport. Experimental studies are in progress to elucidate, if possible, the dynamics of fluid transport in this specialized vascular organ.

In 1923 Zimmermann 1 a Swiss histologist, first described the pericyte, a pericapillary modified smooth muscle cell with dendritic processes which encircle capillaries and is believed to exert a contractile effect in regulating the capillary lumen. Subsequently Murray and Stout 2,3 demonstrated that 2 known vascular tumors, hemangiopericytomas and glomus tumors, could be classified as pericytomas since they were both characterized by abnormal proliferation of Zimmermann's pericyte. Inasmuch as the pericyte is common to both tumors, differentiation is often possible only by differences in morphologic architecture. The glomus tumor is characterized by, and derives its name from, its structural similarity to the neuromyoarterial glomus, a normal, subungual anatomical vascular complex which circumvents capillaries by allowing direct arteriovenous shunts. In hemangiopericytoma the pericyte proliferates in no such recognizable "organoid" pattern. These tumors are well documented for all age groups, in case reports and collected series in the adult literature. However, since neither glomus