Scanning and transmission electron microscopy and high resolution intravital video-microscopy of capillaries in the mouse exocrine pancreas, with special emphasis on endothelial cells

Identification of markers for quiescent pancreatic stellate cells in the normal human pancreas

Pancreatic stellate cells (PSCs) play a central role as source of fibrogenic cells in pancreatic cancer and chronic pancreatitis. In contrast to quiescent hepatic stellate cells (qHSCs), a specific marker for quiescent PSCs (qPSCs) that can be used in formalin-fixed and paraffin embedded (FFPE) normal human pancreatic tissue has not been identified. The aim of this study was to identify a marker enabling the identification of qPSCs in normal human FFPE pancreatic tissue. Immunohistochemical (IHC), double-IHC, immunofluorescence (IF) and double-IF analyses were carried out using a tissue microarray consisting of cores with normal human pancreatic tissue. Cores with normal human liver served as control. Antibodies directed against adipophilin, α-SMA, CD146, CRBP-1, cytoglobin, desmin, GFAP, nestin, S100A4 and vinculin were examined, with special emphasis on their expression in periacinar cells in the normal human pancreas and perisinusoidal cells in the normal human liver. The immunolabelling capacity was evaluated according to a semiquantitative scoring system. Double-IF of the markers of interest together with markers for other periacinar cells was performed. Moreover, the utility of histochemical stains for the identification of human qPSCs was examined, and their ultrastructure was revisited by electron microscopy. Adipophilin, CRBP-1, cytoglobin and vinculin were expressed in qHSCs in the liver, whereas cytoglobin and adipophilin were expressed in qPSCs in the pancreas. Adipophilin immunohistochemistry was highly dependent on the preanalytical time interval (PATI) from removal of the tissue to formalin fixation. Cytoglobin, S100A4 and vinculin were expressed in periacinar fibroblasts (FBs). The other examined markers were negative in human qPSCs. Our data indicate that cytoglobin and adipophilin are markers of qPSCs in the normal human pancreas. However, the use of adipophilin as a qPSC marker may be limited due to its high dependence on optimal PATI. Cytoglobin, on the other hand, is a sensitive marker for qPSCs but is expressed in FBs as well.

The Vascular Microarchitecture of the Human Fetal Pancreas: A Corrosion Casting and Scanning Electron Microscopy Study

OBJECTIVES

Detailed knowledge on the development of the pancreas is required to understand the variability in its blood supply. The aim of our study was to use the corrosion casting method combined with scanning electron microscopy to study the organization of the pancreatic microcirculation in human fetuses.

METHODS

The study was conducted on 28 human fetuses aged 18 to 25 gestational weeks. The fetal vasculature was appropriately prepared and then perfused with a low-viscosity Mercox CL-2R resin. The prepared vascular casts of the surface of the fetal pancreas were then examined in scanning electron microscopy and digitally analyzed.

RESULTS

The lobular structure of the pancreas has a strong impact on the organization of the microvasculature. The lobular networks were supplied by the interlobular arteries and drained by the interlobular veins. The vascular system of fetal human pancreas has many portal connections, including islet-lobule and islet-duct portal circulations, which likely play a key role in the coordination of both endocrine and exocrine pancreatic functions.

CONCLUSIONS

The organization of the microvascular network of the human pancreas in fetuses aged 18 to 25 gestational weeks is very similar to that of an adult but with more prominent features suggesting active processes of angiogenesis and vascular remodeling.

Novel anti(lymph)angiogenic treatment strategies for corneal and ocular surface diseases

The cornea is one of the few tissues which actively maintain an avascular state, i.e. the absence of blood and lymphatic vessels (corneal [lymph]angiogenic privilege). Nonetheless do several diseases interfere with this privilege and cause pathologic corneal hem- and lymphangiogenesis. The ingrowths of pathologic blood and lymphatic vessels into the cornea not only reduce transparency and thereby visual acuity up to blindness, but also significantly increases the rate of graft rejections after subsequent corneal transplantation. Therefore great interest exists in new strategies to target pathologic corneal (lymph)angiogenesis to promote graft survival. This review gives an overview on the vascular anatomy of the normal ocular surface, on the molecular mechanisms contributing to the corneal (lymph)angiogenic privilege and on the cellular and molecular mechanisms occurring during pathological neovascularization of the cornea. In addition we summarize the current preclinical and clinical evidence for three novel treatment strategies against ocular surface diseases based on targeting pathologic (lymph)angiogenesis: (a) modulation of the immune responses after (corneal) transplantation by targeting pathologic (lymph)angiogenesis prior to and after transplantation, (b) novel concepts against metastasis and recurrence of ocular surface tumors such as malignant melanoma of the conjunctiva by anti(lymph)angiogenic therapy and (c) new ideas on how to target ocular surface inflammatory diseases such as dry eye by targeting conjunctival and corneal lymphatic vessels. Based on compelling preclinical evidence and early data from clinical trials the novel therapeutic concepts of promoting graft survival, inhibiting tumor metastasis and dampening ocular surface inflammation and dry eye disease by targeting (lymph)angiogenesis are on their way to translation into the clinic.

Islet architecture: A comparative study

Emerging reports on the organization of the different hormone-secreting cell types (alpha, glucagon; beta, insulin; and delta, somatostatin) in human islets have emphasized the distinct differences between human and mouse islets, raising questions about the relevance of studies of mouse islets to human islet physiology. Here, we examine the differences and similarities between the architecture of human and mouse islets. We studied islets from various mouse models including ob/ob and db/db and pregnant mice. We also examined the islets of monkeys, pigs, rabbits and birds for further comparisons. Despite differences in overall body and pancreas size as well as total beta-cell mass among these species, the distribution of their islet sizes closely overlaps, except in the bird pancreas in which the delta-cell population predominates (both in singlets and clusters) along with a small number of islets. Markedly large islets (>10,000 mum(2)) were observed in human and monkey islets as well as in islets from ob/ob and pregnant mice. The fraction of alpha-, beta- and delta-cells within an islet varied between islets in all the species examined. Furthermore, there was variability in the distribution of alpha- and delta-cells within the same species. In summary, human and mouse islets share common architectural features that may reflect demand for insulin. Comparative studies of islet architecture may lead to a better understanding of islet development and function.

Real-time, multidimensional in vivo imaging used to investigate blood flow in mouse pancreatic islets

The pancreatic islets of Langerhans are highly vascularized micro-organs that play a key role in the regulation of blood glucose homeostasis. The specific arrangement of endocrine cell types in islets suggests a coupling between morphology and function within the islet. Here, we established a line-scanning confocal microscopy approach to examine the relationship between blood flow and islet cell type arrangement by real-time in vivo imaging of intra-islet blood flow in mice. These data were used to reconstruct the in vivo 3D architecture of the islet and time-resolved blood flow patterns throughout the islet vascular bed. The results revealed 2 predominant blood flow patterns in mouse islets: inner-to-outer, in which blood perfuses the core of beta cells before the islet perimeter of non-beta cells, and top-to-bottom, in which blood perfuses the islet from one side to the other regardless of cell type. Our approach included both millisecond temporal resolution and submicron spatial resolution, allowing for real-time imaging of islet blood flow within the living mouse, which has not to our knowledge been attainable by other methods.

Switching-on survival and repair response programs in islet transplants by bone marrow-derived vasculogenic cells

OBJECTIVE

Vascular progenitors of bone marrow origin participate to neovascularization at sites of wound healing and transplantation. We hypothesized that the biological purpose of this bone marrow-derived vascular component is to contribute angiogenic and survival functions distinct from those provided by the local tissue-derived vasculature.

RESEARCH DESIGN AND METHODS AND RESULTS

To address this hypothesis, we investigated the functional impact of bone marrow-derived vascular cells on pancreatic islets engraftment using bone marrow-reconstituted Id1(+/-)Id3(-/-) mice, a model of bone marrow-derived vasculogenesis. We show that, in this model, bone marrow-derived vasculogenic cells primarily contribute to the formation of new blood vessels within islet transplants. In contrast, graft revascularization in a wild-type background occurs by tissue-derived blood vessels only. Using these distinct transplant models in which bone marrow-and tissue-derived vasculature are virtually mutually exclusive, we demonstrate that bone marrow-derived vasculogenic cells exhibit enhanced angiogenic functions and support prompt activation of islets survival pathways, which significantly impact on islets engraftment and function. Moreover, gene profiling of vascular and inflammatory cells of the grafts demonstrate that neovascularization by bone marrow-derived cells is accompanied by the activation of a genetic program uniquely tuned to downregulate harmful inflammatory responses and to promote tissue repair.

CONCLUSIONS

These studies uncover the biological significance of bone marrow-derived vasculogenic cells in the response to injury during transplantation. Enhancing the contribution of bone marrow-derived vasculogenic cells to transplantation sites may help to overcome both limited angiogenic responses of the adult tissue-derived vasculature and untoward effects of inflammation on transplant engraftment.

Islet Vasculature as a Regulator of Endocrine Pancreas Function

The islets of Langerhans consist of endocrine cells embedded in a network of specialized capillaries that regulate islet blood flow. Despite evidence for a critical role of islet perfusion in endocrine pancreas function, there is information to support no fewer than three models of endocrine cell perfusion, emphasizing the lack of a universally accepted physiological theory. Islet blood flow is regulated by signals, such as hormones and nutrients that reach the islet vasculature from distant tissues via the bloodstream. In addition, islet perfusion determines communication between endocrine and exocrine cells and between different types of endocrine cells within islets. Interest in islet microcirculation has increased after improvements in islet transplantation, a therapy for diabetes mellitus that requires revascularization of grafted islets in a new host organ. Abnormal revascularization is thought to be partly responsible for differences in graft and native islet function. Similarly, angiogenesis has been shown to be a critical step in the transformation of islet hyperplasia to neoplasia.

The limbal vasculature

The cornea is an avascular structure whose closest point of approach to the systemic blood stream is provided by the limbal vessels. Activity within these structures provides the clinician with a sensitive indicator of contact lens performance and associated problems. In this paper, the anatomy and physiology of the limbal vessels is reviewed with particular attention to the control of capillary perfusion. Mechanisms whereby soft contact lenses can interact with this network of vessels are considered.

New polyurethane-based material for vascular corrosion casting with improved physical and imaging characteristics

Vascular corrosion casting has been established as a method to reconstruct the three-dimensional (3D) structure of blood vessels of organs and tissues. After replacing the blood volume with a low viscosity resin, the surrounding tissue is removed to replicate the vascular architecture, typically using scanning electron microscopy (SEM). To date available casting resins have had significant limitations such as lack of viscosity, leading to insufficient perfusion of smallest capillaries in organs and tissues of smaller species, interaction with surrounding tissue or fragility of resulting casts. We have reported here about a new polyurethane-based casting resin (PU4ii) with superior physical and imaging characteristics. Low viscosity, timely polymerization, and minimal shrinking of PU4ii produces high quality casts, including the finest capillaries. These casts are highly elastic while retaining their original structure to facilitate postcasting tissue dissection and pruning of casts. SEM images illustrate the high reproduction quality, including endothelial cell imprints, features that allow one to discern arterial and veinal vessels. For quantitative analysis, casts from PU4ii can be imaged using micro-computed tomography to produce digital 3D reconstructions. The inherent fluorescence of PU4ii is sufficient to reproduce casts with or without tissue using confocal microscopy (CM). Because of the simplified casting procedure, the high reproducibility and the superior reproduction quality, a combination of vascular corrosion casting using PU4ii with advanced imaging technologies has great potential to support the description of vascular defects and drug effects in disease models using mutant mice.

Ultrastructural and morphometric comparison of retinal and myocardial capillaries following acute ischaemia

The recovery of any tissue following a period of ischaemia is dependent on a patent microvasculature to restore blood flow. In the ischaemic myocardium, a reduction in capillary cross-sectional dimensions occurs, which is likely to contribute to "no-reflow" injury. Clinical and experimental evidence indicates that the retina is able to tolerate moderate periods of ischaemia without significant loss of function. The aim of the present study is to test the hypothesis that, as an end-arterial system, the retina possesses compensatory processes to maintain a functional microcirculation following acute ischaemia. Thirty minutes of no-flow global ischaemia was induced in isolated hearts of Wistar rats without reperfusion. The retina was also made ischaemic for 30 min using two experimental models: microsphere embolization and anoxic superfusion. Changes in capillary dimensions were assessed by ultrastructural morphometry. Following 30 min of myocardial ischaemia capillaries appeared swollen with a significant reduction in total capillary and luminal cross-sectional area. By contrast, ischaemic retinal capillaries showed minimal morphological changes and no significant alteration in dimensions. We have demonstrated notable differences in the response of retinal and myocardial microvessels to acute ischaemia. It is likely that the maintenance of capillary patency following short periods ischaemia in the retina is part of an adaptive mechanism to protect visual function.

Effects of free and liposome-encapsulated hemoglobin on choroidal vascular plexus blood flow, using the rabbit eye as a model system

PURPOSE

This study investigated the vasoconstrictive effects of both stroma-free and liposome-encapsulated cross-linked hemoglobin (Hb) on vascular plexus hemodynamics, using the choroid of the rabbit eye as a model system.

METHODS

Sequential subtraction of high-speed ICG fluorescence angiogram images facilitated visualization of the time-varying patterns of blood flow distribution in the choriocapillaris during the cardiac cycle. Differences between baseline and post-hemoglobin injection blood flow distributions were analyzed. Likewise, differences in the time-varying patterns of flow distribution between the particulate and liquid phases of blood during a cardiac cycle were investigated, since this may bear on differences in vasoactivity induced by circulating stroma-free vs. encapsulated Hb.

RESULTS

Cross-linked Hb induced a transient, but marked, localized reduction in choriocapillaris blood flow. This effect was significantly attenuated when liposome encapsulated cross-linked hemoglobin was administered. Plexus blood flow distribution was different for particulate and liquid ICG.

CONCLUSIONS

Differences in particulate and liquid ICG flow patterns suggest that one contribution to the different plexus blood flow patterns observed in the encapsulated and free Hb experiments may be due to differences in liquid and particle-bound Hb distribution within the plexus. The observed choriocapillaris blood flow reductions may be attributable to an aggregate endothelial cell contractility induced by presence of extra-cellular Hb in the choriocapillaris plexus.

Perivascular structures in corrosion casts of the human central nervous system: a confocal laser and scanning electron microscope study

Scanning electron microscopy (SEM) of microvascular corrosion casts revealed perivascular structures that resembled smooth muscle and pericyte cells. Although these structures have been studied in widely different experimental contexts, their origin, function, and distribution pattern in different tissues are not understood. Microvascular corrosion casts from 15 fresh human brains and 20 lumbar spinal cords were studied by SEM. In five cerebral hemispheres a fluorescent resin was injected in order to study the vascular bed by confocal laser scanning microscopy (CLSM). Microvascular casts showed two perivascular structures on their surfaces: plastic strips, which formed a muff around arteriolar vessels, and pericyte-like structures that were present around the capillary network. Their morphological characteristics and distribution were similar to those of smooth muscle cells and pericytes, respectively. The SEM study showed that these structures were not tightly joined to the cast surface, but were connected to the vascular cast by narrow plastic connections. The CLSM showed that the resin invaded the subendothelial space, thus giving rise to these structures. Perivascular structures associated with arteriolar and capillary vessels appear to represent smooth muscle cells and pericytes. They are formed by the passage of the resin to the subendothelial space, probably through weak endothelial cell junctions. The effusion of resin into the subendothelial space may represent evidence for the structural basis of myocyte and pericyte cell control. Chemical communication by substances released locally or transported to these cells through these junctions may regulate their functions, allowing them to regulate blood flow.

Morphometry of human coronary arterial trees

BACKGROUND

Some groups, including ours, have been generating arterial tree models using constrained constructive optimization (CCO). Arterial trees have been grown to arbitrary resolution without input of anatomical data. We performed this study to learn about the shortcomings that might have resulted from neglecting the anatomical data in CCO models.

METHODS

In a total of 450 segments obtained from 4 human cast hearts, the ratio ofbifurcating daughter segment radii (O < Sbif = r(2)/r(1) < 1) was examined, which corresponds to the split of the total flow of the mother segment. For any complete bifurcation, where the radii of the parent segments and the radii of daughters were known, the area expansion ratio was computed (Aexp = [r(1)2 + r(2)2]/r(parent)2).

RESULTS

The bifurcating ratio was found to be distributed in a nonnormal fashion, with a median of 0.76. The average area expansion ratio Aexp, characterizing the change of cross-sectional area of the vasculature from proximal to distal, was 0.93+/-0.26. The 'rate of branching' (d(i)/(d(0)) was defined by the segment diameter relative to the diameter of the root segment. Averaging the rate of branching over segments within each bifurcation level resulted in a decreasing function of bifurcation level.

CONCLUSIONS

This article provides new experimental data on branching geometry of coronary arteries (i.e., the trees evaluated in this study are purely delivering rather than conveying). Based on these facts, we suggest that the analytical bifurcation law in CCO might be replaced by the bifurcation rule obeyed on a stochastic basis only.

In vivo microscopy of the exocrine pancreas

Light microscopic studies of the living acinar pancreas, although limited in number, have revealed valuable information concerning dynamic aspects of microvascular and parenchymal structure and function. For example, it has been found that: 1) the living organ in anesthetized animals can be imaged with a resolution approaching the limit of the light microscope; 2) blood flow through individual capillaries in the exocrine pancreas is intermittent; 3) blood flow through these capillaries is regulated locally by smooth muscle precapillary sphincters and within individual capillaries by endothelial cells which are spontaneously contractile as well as responsive to vasoactive substances; and 4) the formation and release of zymogen granules occurs within 45-90 minutes in acinar cells stimulated with pancreozymin. This paper reviews these studies and some of the methods used to obtain them.

Microcirculation in pancreatic function

The pancreas is involved in two major bodily functions: production of hormones involved in the control of carbohydrate metabolism and the production of enzymes essential to digestion. Pancreatic function is mediated by both neurological and humoral control. The major pathway for humoral control is through the circulatory system, the level of action being in the microcirculation. This introductory paper explores the need for a deeper understanding of the dynamic morphology, i.e. the actual flow patterns in the microcirculation, as a function of the physiological state and demand to complement the careful ultrastructural mapping of the microvasculature. The current state of knowledge in this field is reviewed as a basis for identifying important areas of knowledge and ignorance, and some suggestions are made as to possible procedures for further experimental studies, particularly in the microscopic observation of the dynamics of the microcirculation with special emphasis on the need for transport studies in both directions across the microvascular wall.

Morphologic sites for regulating blood flow in the exocrine pancreas

The exocrine pancreas has a lobular structure and an intricate capillary network supplies the lobules. Casts of these capillaries are either straight and of constant width, provided with many shallow crests, or undulating and of varying diameter, provided with bulges and deeper constrictions. The mean capillary cast diameter is 6.32 microns (SD 0.53) and 3.91 microns (SD 0.84) at constriction sites. The first type corresponds to non-fenestrated capillaries, makes 24% of capillaries and is more frequently provided with pericytes (2.7 +/- 0.9 pericytes per capillary profile). The second type corresponds to fenestrated capillaries, comprises 76% of the capillaries and is less frequently provided with pericytes (1.5 +/- 0.6 pericytes per capillary profile). The endothelial cells of capillaries regularly form intermediate junctions and microvilli and contain microtubuli and cytoplasmic filaments. Intravital observations show that capillaries are capable of contracting and narrowing the capillary lumen. This contractility is accomplished by endothelial cells both at and apart from their nuclear regions while pericytes never contracted spontaneously during our in vivo observations. The capillary diameters estimated by intravital measurements, 3.53 microns (SD 1.05), are similar to cast measurements but differ at constricted segments from cast measurements. Flow reduction shows more variability in smaller capillaries and the flow is more reduced in capillaries of 5 microns diameter to about 40% of open capillaries vs. 68% in capillaries with 7.5 microns diameter. Veins are either provided with smooth muscle sphincters or with valves. These results indicate that corrosion casting accurately shows the geometry of capillaries. However, where the capillaries are drastically constricted, they might not be filled and therefore may be underestimated during measurements. Since none of the intravital luminal constrictions are small enough to reduce flow (smaller than 1 micron luminal diameter) and because many constrictions are effective to reduce flow, we conclude that capillaries of the exocrine pancreas are always capable of maintaining continuous blood flow yet can influence blood perfusion. The presence of venous valves in association with venous sphincters constitutes a new situation concerning blood drainage regulation in the exocrine pancreas.

Morphology and tyrosine-hydroxylase immunohistochemistry of the systemic secondary vessel system of the blue catfish, Arius graeffei

Fish have a secondary vessel system which emerges from the primary vasculature via large numbers of coiled origins. The precise role of this vessel system is unknown. Vascular casting techniques and scanning electron microscopy reveal that the secondary vessels of the blue catfish, Arius graeffei, originate from dorsal, lateral, and ventral segmental primary arteries and from the caudal dorsal aorta. These vessels anastomose with each other to form larger secondary arteries which parallel the primary vessels for their entire length. Secondary vessels do not appear to form a capillary bed in the skin in A. graeffei as they do in some fish species. Coiled secondary vessel origins are abundant within the tunica media and adventitia of the primary vessels from which they emerge. The origins of the secondary vessels are surrounded by the extensive cytoplasmic processes of specialized endothelial cells. These processes extend for up to 6 μm into the lumen of the primary vessel. Ultrastructurally the coiled secondary capillaries consist of an endothelial cell tube which is surrounded by a single layer of pericytes. These endothelial cells extend large numbers of microvilli into the lumen of the coiled secondary capillary. Nerve terminals are commonly associated with the coiled secondary capillaries. Immunohistochemistry has revealed the presence of tyrosine-hydroxylase, an enzyme involved in catecholamine synthesis in nerve varicosities close to secondary vessels in A. graeffei. This vessel system could therefore be regulated by adrenergic nerves. © 1996 Wiley-Liss, Inc.

Pulmonary lymphatics and their spatial relationship to venous sphincters

BACKGROUND

Pulmonary lymphatics are critical to clearing lung fluid. Although their structure can be shown with light and transmission electron microscopy, scanning electron microscopy of their casts can better show their number, size, shape, distribution, and degree of filling. This technique has identified four forms of lung lymphatics, but these forms have not been fully evaluated by tissue microscopy. A most important site of pulmonary edema formation, the pulmonary capillary, is just upstream from small veins which have focal, smooth muscle tufts termed venous sphincters. Because of their constricting potential, these sphincters may control lung perfusion and cause edema.

METHODS

With light and transmission electron microscopy of tissue and scanning electron microscopy of casts, the lymphatic forms were explored in relation to the tissue anatomy in rats without pulmonary edema and with mild-to-moderate edema caused by extended vascular rinsing.

RESULTS

The edematous lungs had increased sacculo-tubular lymphatics adjacent to the venous sphincters. These lymphatics were in the adventitial connective tissue and were partially endothelialized. As lymphatics became more tubular their endothelium became more complete. Collagen fibers traversed the lumen of these lymphatics even where endothelial cells were present and caused the lines on the surface of the lymphatic casts. Overlapping endothelial cells caused clefts on the casts.

CONCLUSIONS

Scanning electron microscopy of lymphatic casts better defines their ultrastructure and shows the spatial relationship of veins and their sphincters to venous lymphatics. Sphincter contraction may influence pulmonary lymph production which could affect other aspects of regional lung perfusion.

New aspects of microvascular corrosion casting: a scanning, transmission electron, and high-resolution intravital video microscopic study

We used intravital microscopy of small intestine and pancreas in order to show dynamic interactions between vascular wall and undiluted Mercox, because previous studies of ours have shown that Mercox diluted with monomeric methylmethacrylate penetrates cells in the vascular wall. Scanning and transmission electron microscopy were used to show three-dimensional pathways and correlating tissue structures, which cannot be identified in vivo. The microvascular diameters were not altered when the vasculature was flushed with saline/dextran solution using perfusion pressures between 70 and 140 mm Hg, but, in circumscribed areas, contraction of vascular wall was observed immediately after Mercox injection. This phenomenon was carried out by endothelial cells; pericytes were never present at the site of constrictions. Extravasation, i.e., leakage of the resin into the surrounding tissue, occurred in circumscribed areas regardless of the applied perfusion pressure. The resin also filled routes, which were not perfused with blood before casting. Scanning microscopy of corresponding specimens showed flattened cast channels, with impressions of valves and endothelial cell nuclear imprints characteristic of lymphatics. These results show that undiluted Mercox is a stimulus for vascular cellular components and that it changes the vascular wall permeability, resulting in extravasation and filling of lymphatics. Transmission electron microscopy showed that large vessels were homogeneously filled with resin and that cellular structures were not infiltrated with Mercox. Cut sections of the gold-coated surface of casts showed grooves up to 20 nm wide, suggestive of minimal deformation, while the abluminal surface of the metal film was almost smooth. Another proof of minimal deformation of undiluted Mercox casts is that the diameter of vessels was not altered during and after polymerization. Obtained casts are not fragile, as are casts of diluted Mercox, and phase separation does not occur, which would result in penetration of the cells in the vascular wall. For these reasons, the use of undiluted Mercox is recommended. Mixing 10 ml Mercox with 1 g catalyst resulted in complete polymerization within 5.5-7 min. This mixture can be used for casting biological specimens.