Ultrastructure of in situ perfusion-fixed avian liver, with special reference to structure of the sinusoids

Broiler chicken and laying hen livers were fixed using a simple technique of in situ puncture perfusion of cacodylate-buffered fixative, which allowed characterisation of the fine structure of hepatic parenchyma, hepatocytes, bile ductules, and, in particular, the sinusoidal cells including endothelial, Kupffer, and Ito cells. Sinusoidal endothelial cells with their bulging perinuclear cytoplasm, evident in both transmission and scanning electron micrographs, were easily distinguishable from Kupffer cells, which possessed numerous pseudopodia. Bile ductular epithelium and hepatocytes of the laying hens contained large amounts of lipid. The ultrastructural characteristics of intercalated cells (putative extra-sinusoidal macrophages of chicken liver) are described and their possible role as precursors of Kupffer cells is discussed.     

Sinusoidal endothelial cells of the liver: Fine structure and function in relation to age

Liver endothelial cells form a continuous lining of the liver capillaries, or sinusoids, separating parenchymal cells and fat-storing cells from sinusoidal blood. Liver sinusoidal endothelial cells differ in fine structure from endothelial cells lining larger blood vessels and from other capillary endothelia in that they lack a distinct basement membrane and also contain open pores, or fenestrae, in the thin cytoplasmic projections which constitute the sinusoidal wall. This distinctive morphology supports the protective role played by liver endothelium, the cells forming a general barrier against pathogenic agents and serving as a selective sieve for substances passing from the blood to parenchymal and fat-storing cells, and vice versa. Sinusoidal endothelial cells, furthermore, significantly participate in the metabolic and clearance functions of the liver. They have been shown to be involved in the endocytosis and metabolism of a wide range of macromolecules, including glycoproteins, lipoproteins, extracellular matrix components, and inert colloids, establishing endothelial cells as a vital link in the complex network of cellular interactions and cooperation in the liver. Fine structural studies in combination with the development of cell isolation and culture techniques from both experimental animal and human liver have greatly contributed to the elucidation of these endothelial cell functions. Morphological and biochemical investigations have both revealed little changes with age except for an accumulation of iron ferritin and a decrease in the activities of glucose-6-phosphatase, Mg-ATPase, and in glucagon-stimulated adenylcyclase. Future studies are likely to disclose more fully the role of sinusoidal endothelial cells in the regulation of liver hemodynamics, in liver metabolism and blood clearance, in the maintenance of hepatic structure, in the pathogenesis of various liver diseases, and in the aging process in the liver.     

Amphibia Kupffer cells

Amphibia Kupffer cells (i.e., liver resident macrophages) show many common characteristics when compared with Mammalia Kupffer cells: filopodia, microvillous-like structures, lamellipodia, fuzzy coat, coated vesicles, bristled vacuoles, nonspecific esterase activity, and pinocytotic and phagocytic activity are present both in Amphibia and Mammalia Kupffer cells. On the other hand, some differences are present between Kupffer cells of both zoological classes: phagocytosed red cells and their derivatives, iron-protein complexes, and lipofuscin bodies are normally present in Amphibia Kupffer cells, but absent in the same cells of healthy mammals. Worm-like structures are not seen in Amphibia and endogenous peroxidase activity is very weak in these animals compared with Mammalia. The most important difference lies in the ability of Amphibia Kupffer cells to produce melanins: in fact the tyrosinase gene is expressed, "melanosome centers" are present, and dopa oxidase activity is demonstrable.     

Alanine minimises hepatocyte injury after ischemia-reperfusion in an <i>ex vivo</i> rat liver model

Ischemia-reperfusion injury is a determinant in liver injury occurring during surgery, ischemic states and multiple organ failure. The pre-existing nutritional status of the liver, i.e., fasting, might contribute to the extent of tissue injury. This study investigated whether alanine, an amino acid precursor of glucose, could protect ex vivo perfused livers of fasting rats from reperfusion injury. The portal vein was cannulated, the liver removed and perfused in a closed ex vivo system. Isolated livers were perfused either with glucose 1 g/L and 10 g/L, or with equal concentrations of alanine (n = 10 in each group). The experiment consisted of perfusion for 15 min, ischemia for 60 min, and reoxygenation during 60 min. Enzymes, glucose, lactate and bilirubin were analysed in perfusate samples. The proportion of glycogen as well as activation of caspase 3 was determined in biopsies. Alanine at a concentration of 10 g/L attenuated enzymes release in the perfusate during reoxygenation when compared to glucose-treated groups. Lactate level in the perfusate was lowest in alanine groups. Ischemia-reperfusion and mainly alanine activated apoptosis, specifically in Kupffer and endothelial cells. Alanine presents a protective effect on normothermic ischemia-reperfusion injury of the fasting rat liver when compared to glucose     

Formation of a functional endothelium on vascular grafts

The lack of a functional endothelial cell lining on artificial polymeric vascular grafts severely reduces their effectiveness in replacing small caliber (< 6 mm) blood vessels. Techniques have now been developed to transplant autologous endothelial cells from one site in the body onto the surface of grafts prior to implantation. Pre-clinical animal trials provide evidence that grafts sodded with autologous, fat-derived, microvessel endothelial cells exhibit a stable, antithrombogenic lining of endothelium. The new endothelial cell lining exhibits morphologies identical with endothelium on native blood vessels. The effectiveness of endothelial cell sodding techniques in pre-clinical animal trials provides support for expanded clinical trials.     

Lymphatic vessels in the oral cavity: different structures for the same function.

A study using a light and transmission electron microscope was performed on some structural characteristics of the lymphatic capillaries in different regions of the human oral cavity. The lymphatic capillaries of dental pulp, masticatory mucosa (gingiva and peri-implant mucosa) and lining mucosa (cheek) were examined. Our attention was focused on the morphologic characteristics of the endothelial wall in the lymphatic capillaries. In particular, the connections between endothelial cells were investigated. In the lymphatic capillaries of the dental pulp, the endothelial wall was always very complex. It frequently presented protrusions of the endothelial cells that overlapped and formed intercellular channels. These channels were thus contained by the vessel endothelial wall with their extremities opening out towards the surrounding interstitium and the vessel lumen. The endothelial wall of the lymphatic capillaries of the cheek was very smooth and thin without complex intercellular junctions. The endothelial cells were joined by end-to-end junctions and open junctions were frequently observed. Intercellular channels were also found in the endothelial wall of lymphatic capillaries of the gingiva and the peri-implant mucosa. The presence of numerous clefts represented by the open junctions in the lymphatics of the cheek and the existence of complex intercellular adhesions with the formation of intercellular channels in the endothelial wall of the lymphatic capillaries of the dental pulp and gingiva induce us to believe that these may play a role in the various mechanisms used by lymphatic capillaries to absorb interstitial fluids. These mechanisms are based on the different morpho-functional characteristics of the surrounding tissue.     

A histological and immunohistochemical study on the parabronchial epithelium of the domestic fowl's (Gallus gallus domesticus) lung with special reference to its scanning and transmission electron microscopic characteristics

The current study was designed to give complete histo-and immunohistochemical features of the parabronchial epithelium of domestic fowl's (Gallus gallus domesticus) lung with special reference to Scanning electron microscope (SEM) and mean transmission electron microscope (TEM) features. The lung exhibited variable-sized atrial openings encircled by exchange tissue zones. The parabronchial atrial chambers appeared as ovoid and polygonal-shaped that separated by the well-developed interatrial septum. The deep atrial lumens had blood vessels pierced by openings that represent the infundibula. The parabronchial blood capillaries meshwork was branched and exhibited ovoid-shaped air capillaries with numerous extravasated blood vessels. By TEM, there were several air capillaries and groups of squamous and endothelial respiratory cells and the squamous cells had oval nucleus with evenly distributed chromatin. The endothelial respiratory cells had few microvilli on their free surfaces. The parabronchial tubes opened into a group of widened atria that had smooth muscle bundles at the interatrial septa. The atrial chambers led to narrow infundibula. Moreover, the lining epithelium of parabronchi, atria, infundibula, and air capillaries was formed by simple squamous epithelium. Air capillary walls were lined by two types of respiratory cells (Types-I and II). Collagen fibers were concentrated within the tunica externa layers of the parabronchial blood vessels as well as, they were observed in CT interparabronchial septa. Immunohistochemically, the elastin immunoreactivity was detected around the parabronchial blood vessels, at the base of each parabronchial atria, and in the area encircling the alveolar-capillary walls. Our work concluded that there are a relation between the fowl's lifestyle and the surrounding environmental conditions.     

Fine structure of hepatic sinusoids and sinusoidal cells in disease

Liver sinusoids are special capillaries that are limited by fenestrated endothelial cells, without a genuine basement membrane, surrounded by perisinusoidal cells storing vitamin A, and harbouring Kupffer cells and pit cells, resident macrophages, and large granular lymphocytes, respectively. Each nonparenchymal cell and parenchymal cell of the liver interacts with all others and with the extracellular matrix. Therefore, the functional ability of each cell is constantly being modified by the metabolic activity of the others. Human liver biopsies (132), needle or surgical, perfusion-fixed with glutaraldehyde and processed for transmission electron microscopy (TEM), and occasionally for scanning electron microscopy (SEM), were examined. The study included liver diseases (such as alcoholic liver diseases, benign and malignant liver tumors, cholestasis of various origins, fulminant hepatitis, acute rejection after orthotopic liver transplantation, Budd-Chiari syndrome), as well as general or extrahepatic diseases (such as diabetes, hemochromatosis, hypervitaminosis A, various hematological disorders), and normal controls. Ultrastructural abnormalities are described and illustrated under two different headings: (1) elementary lesions of sinusoidal cells (endothelial, Kupffer, perisinusoidal and pit cells), nonsinusoidal cells (in the space of Disse and/or in the lumen), the extracellular matrix; and (2) the major pathological entities including perisinusoidal fibrosis, capillarization of sinusoids, sinusoidal dilatation, and peliosis. In the discussion, an overview of the major abnormalities reported in the literature is presented, and some specific questions regarding (1) perisinusoidal fibrosis in liver with normal histology, (2) the overload of perisinusoidal cells with lipids in non-hypervitaminosis A intoxication and (3) the etiological relationship of sinusoidal dilatation, peliosis, perisinusoidal fibrosis, or sinusoidal tumors with drugs and toxic compounds are discussed. In the event that lesions are not specific to any diagnosis, the knowledge of the ultrastructure of sinusoids is extremely useful from the perspective of the liver as an ecosystem.     

Diversity in unity: the biochemical composition of the endothelial cell surface varies between the vascular beds

The vascular endothelium represents a population of squamous epithelial cells characterized by a particular histological localization (intima of blood vessels) and by several physiological functions such as the transport of substances between blood and tissues, the modulation of the vascular tone, the control of blood coagulation and that of the leukocyte extravasation. In spite of all these elements in common and of an identical embryonic origin, endothelial cells show definite morphological and physiological variations that divide them into types and subtypes, each specifically associated to various categories of organs. Even within the vasculature of the same organ, there are clear segmental (arterial/capillary/venous) differentiations of the endothelial cells. While the morphological and physiological differences between endothelial cells are well documented, there are very few data on the biochemistry underlying this heterogeneity. This work presents several data suggesting that, at present, the domain is ripe for a comprehensive analysis of this biochemical diversity, at least in what concerns the luminal aspect of the endothelial plasmalemma, a compartment of crucial importance in the biology and pathology of the cardiovascular system.     

Phagocytosis of apoptotic cells by liver: a morphological study

The present review deals with the morphological features of the removal of apoptotic cells by liver. The engulfment of cells undergoing apoptosis can be considered a specialized form of phagocytosis, playing a major role in the general tissue homeostasis in physiological and pathological conditions. In fact, defects of phagocytosis of apoptotic cells might have deleterious consequences for neighboring healthy cells, i.e., pathogenesis of inflammatory disease or dysregulation of the immune system. Phagocytosis of apoptotic cells by liver is a complex phenomenon, involving multiple molecular mechanisms of recognition (i.e., lectin-like receptors and receptors for externalized phosphatydilserine) of both parenchymal (hepatocytes) and nonparenchymal (Kupffer and endothelial cells) liver cells, often operating in cooperation. The data discussed in the present review are drawn from studies of phagocytosis of apoptotic cells in the liver, carried out with in vivo and in situ adhesion experiments as well as in vitro assays. Our results indicate that the three main liver cell types (hepatocytes, Kupffer, and endothelial cells) are able to recognize and internalize apoptotic cells by means of specific receptors (galactose and mannose-specific receptor; receptor for phosphatydilserine) and by cytoskeletal reorganization that favors the engulfment of the apoptotic cells. The "flags" for the identification of apoptotic cells by the liver are modifications of the surface of dead cells, i.e., sugar residues and phosphatydilserine exposition. Vitronectin receptor is not involved in such a recognition. The adhesions between modified cell surfaces of apoptotic cells and phagocytes generate cytoplasmatic signaling pathways that drive apoptotic cells to their final fate within the phagocytes (i.e., lysosomal digestion).     

Healthy and pre-eclamptic placental basal plate lining cells: quantitative comparisons based on confocal laser scanning microscopy

Immunocytochemical confocal laser scanning microscope images of the monolayer of cells lining the intervillus space at the basal plate of term placentae were analysed using stereology. Immunoreactively-distinct regions of this mosaic layer were measured. In basal plate from healthy pregnancies, trophoblast epithelium occupied 18.91% of the surface area and endothelium 60.81%. In pre-eclampsia the equivalent areas were 15.57% and 67.63%. Acellular fibrinoid covers the remaining area and this component decreases in area in pre-eclampsia. The statistically significant increase in the cellular endothelial compartment may be relevant to the hypertensive pathology of pre-eclampsia as endothelial signalling plays a major role in regulation of blood pressure.     

Histochemical analysis of lymphatic endothelial cells in lymphostasis

The ultrastructure of endothelial cells of intestinal lymphatics and the thoracic duct (TD) and the relation to lymphostasis were examined in rats and monkeys. Localization of 5'-nucleotidase (5'-Nase) and endothelial nitric oxide synthase (eNOS) was studied. In normal lymphatic endothelial cells, 5'-Nase reaction product was evenly deposited on the cell surface in vivo and on cultured TD endothelial cells (TDECs), whereas eNOS was evenly distributed throughout the nucleus and cytoplasm. TDECs had a long filamentous process extending towards the subendothelial extracellular matrix but became flat and regular within 30-40 minutes after gastric perfusion with olive oil. According to their electron-density, two types of cells were found in the TD endothelial layer. The cells with low electron-density exhibited stronger 5'-Nase activity. Valves were bicuspid formations and the valvular endothelial surface of the convex side showed weaker 5'-Nase activity than the concave side. During TD blockage-induced lymphostasis in rats, the 5'-Nase product was almost not discernible in the TDECs within 2 weeks. Larger vesicles were found in the endothelial cytoplasm of the ligated TD. Their number decreased after 6-12 weeks. The small intestinal lymphatics in the mucosa and submucosa were dilated, with numerous open intercellular junctions. The endothelial lining appeared to have reduced activities for 5'-Nase and eNOS in 9 of 11 experimental animals. The results indicated that the inability of the open intercellular junctions, normally working as one-way endothelial flap valves, may be a key morphological feature after TD blockage. Reduced eNOS and 5'-Nase may functionally influence contractile activity and transport capability of the lymphatic vessels in the lymphostasis.     

Mechano-Sensing and shear stress-shielding by endothelial primary cilia: structure,composition, and function

Primary cilium is an antenna-like and non-motile structure protruding from the apical surface of most mammalian cells including endothelial cells lining the inner side of all the blood vessels in our body. Although it has been over a century since primary cilia were discovered, the investigation about their mechano-sensing and other roles in maintaining normal functions of cardiovascular system has just started in recent years. This focused review aims to give an update about the current literature for the role of endothelial primary cilia in blood flow mechano-sensing and shear stress-shielding. To do this, we first summarized the characteristic features of endothelial primary cilia in terms of structure, dimension, molecular composition, and mechanical properties (e.g., bending rigidity), which are the dominant factors for their functions in mechano-sensing and transduction, as well as vascular protection from the blood flow-induced wall shear stress. We also described the experimental techniques and mathematical models for determining the dimension and mechanical properties of the primary cilium. Then we reviewed the molecular mechanisms underlying mechano-sensing and transduction by endothelial primary cilia and the mathematical model prediction for their roles in redistribution and reduction of wall shear stresses. Finally, we briefly discussed the common cardiovascular diseases, e.g., atherosclerosis, hypertension, and aneurysm, due to defects and malfunction of endothelial primary cilia and suggested potential targets for therapeutic treatments.     

Development of the avian lymphatic system

Recently, highly specific markers of the lymphatic endothelium have been found enabling us to reinvestigate the embryonic origin of the lymphatics. Here we present a review of our studies on the development of the lymphatic system in chick and quail embryos. We show that the lymphatic endothelium is derived from two sources: the embryonic lymph sacs and mesenchymal lymphangioblasts. Proliferation studies reveal a BrdU-labeling index of 11.5% of lymph sac endothelial cells by day 6.25, which drops to 3.5% by day 7. Lymphangioblasts are able to integrate into the lining of lymph sacs. Lymphatic endothelial cells express the vascular endothelial growth factor (VEGF) receptors-2 and -3. Their ligand, VEGF-C, is expressed almost ubiquitously in embryonic and fetal tissues. Elevated expression levels are found in the tunica media of large blood vessels, which usually serve as major routes for growing lymphatics. The homeobox gene, Prox1, is expressed in lymphatic but not in blood vascular endothelial cells throughout all stages examined, namely, in developing lymph sacs of day 6 embryos and in lymphatics at day 16. Experimental studies show the existence of lymphangioblasts in the mesoderm, a considerable time before the development of the lymph sacs. Lymphangioblasts migrate from the somites into the somatopleure and contribute to the lymphatics of the limbs. Our studies indicate that these lymphangioblasts already express Prox1.     

Ultrastructural characterization of isolated rat Kupffer cells by transmission X-ray microscopy

The ultrastructure of primary cultured rat Kupffer cells was studied using transmission X-ray microscopy as well as transmission electron microscopy. X-ray microscopical images of intact, hydrated Kupffer cells demonstrated structures such as cell nucleus separated by a nuclear membrane and filaments concentrated in the perinuclear area. Within the cytoplasm, a number of vacuoles were visible; some of these were crescent-shaped vacuoles that were half X-ray lucent, half X-ray dense; others were uniformly dense. The number of crescent-shaped vacuoles was predominant. After phagocytosis of haematite particles, enlarged vacuoles containing the ingested material were visible within the cytoplasm of Kupffer cells while crescent-shaped vacuoles were no longer detectable. Densitometric analysis of the two types of vacuole revealed that the X-ray absorption of the uniform vacuole was approximately half that of the dense part of the crescent-shaped vacuoles. This observation led to speculation on the existence of only one type of vacuole in the cytoplasm of Kupffer cells. The different morphological aspects — crescent-shaped versus uniform vacuoles — might be due to different three-dimensional orientation with respect to the image plane. Using transmission electron microscopy, the morphology of vacuoles differed more widely in diameter, density and shape. Two main types of vacuole were identified: electron-lucent and electron-dense. Based on the observation of only one type of vacuole by transmission X-ray microscopy, the different morphological aspects of vacuoles obtained by transmission electron microscopy could be explained by imaging several different sections of a crescent-shaped vacuole. From the present data it can be concluded that transmission X-ray microscopy is a versatile technique that reveals the ultrastructure of intact, unsectioned biological specimens in their aqueous environment, thereby allowing a more comprehensive interpretation of data obtained by transmission electron microscopy.     

隧道软岩洞口段纤维混凝土衬砌抗震性能研究

为了进一步提高强震区隧道软岩洞口段的震时安全性,依托白云顶隧道进口段工程,对采用钢纤维混凝土二衬、玄武岩纤维混凝土二衬和钢筋混凝土二衬的隧道软岩洞口段进行了大型振动台动力模型试验。试验结果表明:采用纤维混凝土二衬提高了支护结构的强度及韧性,可有效提高隧道结构的震时安全性;硬岩段隧道结构安全性由运动相互作用控制,采用纤维混凝土二衬,隧道结构所承受的地震惯性力变化很小,安全系数最小值提高较大,最高增长了71.07%;软岩段隧道结构安全性由运动相互作用和地震惯性相互作用共同控制,采用纤维混凝土二衬对于抵抗隧道结构所承受的地震惯性力作用明显,安全系数最小值最高增长了30.40%。该研究成果可为强震区软岩洞口段的抗震设防设计提供参考。     

Inhibition of angiogenesis at endothelial cell level

Targeting of proliferating endothelial cells may allow a new therapeutic strategy against malignant tumors. Endothelial cells are easily accessible through the blood stream and genetically stable, reducing the possibility of acquiring drug resistance. There are numerous candidates for angiogenesis targets of drug development, e.g. a single endothelial cell has 2,000-20,000 different receptors on a cell membrane and inside a cell there are hundreds of second messengers. Inhibitors that are active at endothelial cells can be placed in three main groups inducing blockage of endothelial cell activator, direct inhibition of endothelial cells, inhibition of endothelial specific signaling. This review summarizes different approaches already in clinical trials. Specific effort is taken to give a view of and a new perspective over clinically relevant basic mechanisms, e.g. VEGF, ETS and radionanotargeting. It is clear that aggressive development work both at the preclinical and clinical levels is still needed; however, breakthroughs in applied therapies are expected at any time.     

Three-dimensional reconstruction of colon carcinoma metastases in liver

Resection of liver metastases in patients with colon cancer increases survival but success depends on removal of all tumour tissue. For this purpose, understanding of spatial relationships between metastases and liver architecture is essential. Because metastatic cancer growth is essentially a three-dimensional (3D) event, we decided to apply 3D reconstruction techniques to study these spatial relationships between metastases and liver structures such as blood vessels, stroma and the liver capsule (Glisson’s capsule). Colon carcinoma metastases were experimentally induced in rat liver by injection of colon cancer cells (CC531) into the portal vein. Three weeks later, livers from these animals and control livers were removed and immediately frozen in liquid nitrogen. Thirty-seven to 110 consecutive sections were used for each 3D reconstruction of 26 metastases in eight livers. Contours of different structures were stained by (immuno)histochemical means, traced in each section and stored in a database. From the contour model, a volume model was generated. Among the 26 metastases, seven were found to grow distantly from the liver capsule. They were small and consisted of well-differentiated cancer cells that were totally surrounded by a basement membrane and stroma which was always connected with adjacent blood vessels of a portal tract. The remaining 19 metastases showed a more advanced pattern of development. Infiltration of poorly differentiated colon cancer cells progressed through the stroma at various sites and areas of direct contact between cancer cells and hepatocytes were frequently found. This type of outgrowth of cancer cells was only found when metastases had made contact with the liver capsule. However, some areas in sections of these advanced stages still resembled small metastases. On the basis of these findings, we conclude that stroma affects the differentiation pattern of cancer cells and has at least a dual role in tumour growth. On the one hand it limits invasion of cancer cells in the surrounding host tissue. On the other hand, stroma formation at the capsule, which consists mainly of granulation tissue, facilitates outgrowth of the tumours. Furthermore, our 3D reconstructions demonstrate the spatial heterogeneity of larger metastases and the importance of a 3D approach to understand growth and development of metastases in general and colon cancer metastases in the liver in particular.     

Identification of fenestrated capillary segments in microvascular corrosion casts of the rat exocrine pancreas

The capillary bed of the rat exocrine pancreas was studied by scanning and transmission electron microscopy of corrosion casts and tissue sections. Two types of capillaries were distinguished in corrosion casts. First, there were straight capillaries of relatively constant width (mean diameter 4.79±0.87 μm), which were characterized by numerous circular constrictions on their surface. About 37% of the capillaries belonged to this type. Second, there were undulating capillaries which showed smooth surfaced eccentric dilatations defined by similar surface constrictions. The bulging areas measured 8.43 ± 1.33 μm, the constrictions next to the bulges figured for 6.45 ± 1.53 μm. About 63% of the capillaries belonged to the second type. Two types of capillaries were also identified in tissue sections. First, there were capillaries with continuous endothelial lining (26% of capillary profiles; mean diameter 5.48 ± 1.67 μm); 27% of their endothelial lining was provided with underlying pericytes. Second, there were capillaries with fenestrated endothelium (64% of capillary profiles; mean diameter 6.16 ± 1.75 μm); 12% of their endothelial lining was accompanied by pericytes. According to frequency and dimension of these two types of capillaries, we conclude that bulged and undulating capillary casts correspond to fenestrated capillaries and straight capillary casts of constant width correspond to nonfenestrated capillaries. The frequency of crests on the surface of capillary casts correlates with the different frequency of pericyte processes on fenestrated and nonfenestrated capillaries. It is concluded that pericyte processes beneath the endothelium hold resistance against luminal pressure. Bulging areas of capillary casts correlate with fenestrated areas of endothelial lining, that is, areas which are not reinforced by pericyte processes.     

The surface structure of mouse peritoneal cells--a study with the scanning electron microscope

Normal and stimulated peritoneal cells have been examined with the Stereo-scan scanning electron microscope. Normal lymphocytes were identified by their small size; some had numerous small stubby or pointed processes on their surfaces. Macrophages were larger and had ridge-like processes. Many of the stimulated cells showed no significant deviation from normal. Others, all macrophages, showed a marked deviation from the normal spherical shape. Ridge-like processes were more prominent on their surfaces, and large flange-like processes and a few finger-like processes were seen. This suggests that on stimulation peritoneal cells become more deformable and therefore irregular in shape, and that their surfaces become rougher.