Hemomicrocirculatory bed of the muscles of the shoulder and forearm

By means of classical anatomical techniques: injection of contrast masses into the vascular network, macro-microscopic preparation, translucency, roentgenography, and some histological techniques, peculiarities of the hemomicrocirculatory bed in muscles of the human arm and forearm have been revealed. Small arteries of the 3d-4th order run along the muscle fiber fasciculi. In the center of the 2d order muscle fasciculus, in its internal perimysium, arteriole and venule (or 2 venules) run; from them into the 1st order fasciculus, precapillary arterioles and postcapillary venules, connected by means of capillaries, run. The arteriole and the venule, accompanying it, together with the precapillary arterioles and postcapillary venules, branching off them, form a unit of the microcirculatory bed of the arm and forearm muscles (module). Well developed intramuscular arterial anastomoses, presence of isolated structural-functional units of the hemomicrocirculatory bed ensure functional prosperity of the human muscles.     

Mechanism of the development of functional hyperemia in the palatal salivary glands of the rat

At various regimens of the secretory activity in the palatile salivary glands, changes occurring in the transversal profiles of the postcapillary venules, but not of the blood capillaries, are most noticeable. Under food stimulation of secretion, the former dilate essentially, that can demonstrate certain functional hyperemia developing in the palatine salivary glands. Some previous experiments concerning interpretation of principles on the microcirculatory bed spatial organization give a good reason to suggest that dilatation of the postcapillary venules is connected with an increased blood perfusion in the canals of the preferrable blood stream. The postcapillary dilatation is possible because blood from the precapillaries gets into the capacitance blood microvessels and its volume at that moment is greater than the capacity of the venous microvessels. A suggestion is made that filtrative function of the palatile salivary glands depends on development of the functional hyperemia. It is possible, that this mechanism is universal, since owing to it, reflectory reactions of the salivary glands directed to the immediate secure of the oral cavity with a necessary amount of liquor become possible.     

Oxygen tension distribution in postcapillary venules in resting skeletal muscle

We tested the hypothesis that blood flow is distributed among capillary networks in resting skeletal muscle in such a manner as to maintain uniform end-capillary PO2. Oxygen tension in venules draining two to five capillaries was obtained by using the phosphorescence decay methodology in rat spinotrapezius muscle. For 64 postcapillary venules among 18 networks in 10 animals, the mean PO2 was 30.1 Torr (range, 9.7-43.5 Torr) with a coefficient of variation (CV; standard deviation/mean) of 0.26. Oxygen levels of postcapillary venules within a single network or single animal, however, displayed a much smaller CV (0.064 and 0.094, respectively). By comparison, the CV of blood flow in 57 postcapillary venules of 17 networks in 9 animals was 1.27 with a mean flow of 0.011 +/- 0.014 nl/s and a range of 3.7 x 10(-4) to 6.5 x 10(-2) nl/s. Blood flow of postcapillary venules within single networks displayed a lower CV (mean, 0.51), whereas that in individual animals was 0.78. Results indicate that among venular networks, heterogeneity of oxygen tension is less than that of blood flow and within venular networks the heterogeneity of oxygen tension is much less than that of blood flow. In addition, postcapillary PO2 was independent of flow among venules in which both were measured. Results of this study may be attributable to three factors: 1) O2 diffusion between adjacent capillaries and venules, 2) structural remodeling in regions of lower PO2, and 3) O2-dependent local control mechanisms.     

Evidence for endothelial cell-mediated regulation of macromolecular permeability by postcapillary venules

Local application of inflammatory mediators to the hamster cheek pouch produces an immediate increase in the number of leaking postcapillary venules as observed by intravital light microscopy. Leaks are illuminated by using fluorescein-labeled dextran given i.v. before mediator challenge. All mediators that have been tested produce a similar pattern of vascular leakage exclusively from postcapillary venules. Mediators can be characterized by their effects on vascular permeability and whether they produce dilation (bradykinin, prostaglandins [PGs]) or constriction (leukotrienes [LTs]) of arterioles. The rank order potency for vascular leakage is LTs greater than bradykinin greater than histamine greater than PGs. A linear regression for the relation between dose of mediator and number of leaky venules has been shown for several mediators, e.g., bradykinin, histamine, and LTs. Inhibition of mediator-induced vascular leakage is produced by a wide variety of substances subsequent to a direct effect on the venular endothelial cell. Morphological, physiological, and pharmacological findings are consistent, and provide evidence for the regulation of macromolecular permeability by the endothelial cells in the postcapillary venules.     

Analysis of the structural parameters of the blood flow pathways in the microcirculatory system]

The reconstruction of the mesenterium microcirculatory bed was performed intravitally in albino rats and cats after biomicrophotograms. The number, length and caliber of arterioles, pericapillary arteriolec, capillaries, postcapillary venules and venules of the mesenterium were measured. According to these data summary indices of the cross section, surface and volume of the vessels of various functional subdivisions of the microcirculatory bed were calculated. The blood volume entering the microcirculatory system of the albino rat's mesenterium is distributed in the vessels as follows: 8,4% -- arterioles, 10,2% -- pericapillary arterioles, 41,9% -- capillaries, 22,1% -- postcapillary venules and 17,4% -- venules. Similar correlations were found in the cat. The working surface of capillaries is 60--70% of the working surface of all the vessels of the mesenterial microcirculatory system. The evidence of the functional variability of the microcirculatory bed geometry depending on the tissue needs in blood supply is presented.     

Red blood cells initiate leukocyte rolling in postcapillary expansions: a lattice Boltzmann analysis

Leukocyte rolling on the vascular endothelium requires initial contact between leukocytes circulating in the blood and the vessel wall. Although specific adhesion mechanisms are involved in leukocyte-endothelium interactions, adhesion patterns in vivo suggest other rheological mechanisms also play a role. Previous studies have proposed that the abundance of leukocyte rolling in postcapillary venules is due to interactions between red blood cells (RBCs) and leukocytes as they enter postcapillary expansions, but the details of the fluid dynamics have not been elucidated. We have analyzed the interactions of red and white blood cells as they flow from a capillary into a postcapillary venule using a lattice Boltzmann approach. This technique provides the complete solution of the flow field and quantification of the particle-particle forces in a relevant geometry. Our results show that capillary-postcapillary venule diameter ratio, RBC configuration, and RBC shape are critical determinants of the initiation of cell rolling in postcapillary venules. The model predicts that an optimal configuration of the trailing red blood cells is required to drive the white blood cell to the wall.     

Functional morphology of the lymphatic system of the rat uterus during pregnancy

In the regional lymph nodes of the uterus the comparative volume of the paracortical zone significantly increases, especially within the period of the 13th-17th days of pregnancy. In the popliteal lymph node similar effect is not discovered. From the 7th up to the 11th day edema, vasodilatation, infiltration with special leucocytes are revealed. Endothelium of the postcapillary venules is hypertrophied, contains many migrating lymphocytes, which accumulate around the vessels mentioned. The volume of the microcirculatory bed is moderately increased. By the 17th day plasmoblasts, plasmocytes, Motta's cells, monocytes and especially macrophages appear in the paracortical zone. In B-zones and in medullary sinuses blasts, plasma cells, monocytes, macrophages, mitotically deviding cells increase in number. The part of the reticular cells decreases. The tensometric method demonstrates an increasing pressure of lymph in the iliac lymph node at pregnancy. Collateralies appear in the ovarian vein system, in the broad ligament of the uterus, in the lumbar area. The uterine vascular system is supposed to participate in adaptation to pregnancy. In genesis of the regional lymph node changes, discirculatory shifts, predominating during placental organogenesis, combine with phenomena of cell migration and proliferation (clearly revealed by the time when formation of the placenta is completed).     

Possible role for cell-surface carbohydrate-binding molecules in lymphocyte recirculation

We are investigating the hypothesis that carbohydrate-binding molecules on the cell surface are involved in the recirculation of lymphocytes from the bloodstream into lymphoid organs. This phenomenon requires the specific attachment of circulating lymphocytes to the endothelial cells of postcapillary venules. Using an in vitro assay to measure the adhesive interaction between lymphocytes and postcapillary venules, we have found that L-fucose, D mannose, and the L-fucose-rich, sulfated polysaccharide fucoidin specifically inhibit this binding interaction. L-fucose shows stereo-selective inhibitory activity at concentrations greater than 18 mM while fucoidin produces 50% inhibition at approximately 1-5 X 10(-8) M. Fucoidin appears to interact with the lymphocyte, and not the postcapillary venule, to inhibit binding. These data suggest that cell surface carbohydrates (fucoselike) and carbohydrate-binding molecules (cell surface lectins) may contribute to the specific attachment of lymphocytes to postcapillary venules.     

Ultrastructure of venules in the cat brain

Summary Intracerebral venules of the cat were examined to establish criteria for a distinct separation between the venous and arterial system, and to characterize, in greater detail, the mural construction of individual venules. The intracerebral venules were compared with those of other organs. Venules do not have a vascular wall composed clearly of endothelium, media, and adventitia, as is characteristic of arteries and arterioles. The venous endothelium has a similar structure to that of capillaries. The periendothelial cells of the venule differ in shape depending on the vascular diameter. The number of periendothelial cell processes in postcapillary venules increases progressively. Segments in which the basal lamina of the endothelium merges with that of the glia cover a smaller portion of the circumference than in venous capillary loops. In collecting venules, the endothelium is almost completely enveloped by periendothelial cells which have a larger number of filaments. There are no typical smooth muscle cells in the intracerebral venules. The perivascular space becomes wider in collecting venules, contains adventitial cells, phagocytes and a great number of collagen fibers.     

Pressure-induced leukocyte margination in lung postcapillary venules

Although pressure elevation in lung postcapillary venules increases endothelial P-selectin expression, the extent to which P-selectin causes lung leukocyte margination remains controversial. To address this issue, we optically viewed postcapillary venules of the isolated blood-perfused rat lung by real-time fluorescence imaging. To determine leukocyte margination in single postcapillary venules, we quantified the fluorescence of leukocytes labeled in situ with rhodamine 6G (R6G). Although baseline fluorescence was sparse, a 10-min pressure elevation by 10 cmH(2)O markedly increased R6G fluorescence. Both stopping blood flow during pressure elevation and eliminating leukocytes from the perfusion blocked the fluorescence increase, affirming that these fluorescence responses were attributable to pressure-induced leukocyte margination. A P-selectin-blocking MAb and the L- and P-selectin blocker fucoidin each inhibited the fluorescence increase, indicating that P-selectin was critical for inducing margination. Time-dependent imaging of blood-borne fluorescent beads revealed reduction of plasma velocity during pressure elevation. After pressure returned to baseline, a similar reduction of plasma velocity, established by manually decreasing the perfusion rate, prolonged margination. Our findings show that in lung postcapillary venules, the decrease in plasma velocity critically determines pressure-induced leukocyte margination.     

Irradiation increases the interactions of platelets with the endothelium in vivo: analysis by intravital microscopy

Adhesion of platelets to the endothelium is believed to be a major factor contributing to thrombosis and vascular occlusion after radiotherapy or endovascular irradiation. In the present study, platelet-endothelium interactions were analyzed in vivo by intravital microscopy in mesenteric venules of mice according to three parameters: (1) platelet rolling, (2) platelet adhesion, and (3) the presence of platelet clusters. A 10-Gy total-body irradiation of mice resulted in an increase in the frequency of appearance of these three types of platelet-endothelium interactions in postcapillary venules 6 and 24 h after exposure, whereas only minor alterations were seen in large venules. In addition, the duration of platelet adhesion was increased 24 h after irradiation in both postcapillary and large venules. However, P-selectin was not up-regulated on the platelet membrane and platelet-leukocytes were not seen rolling together, suggesting that changes in platelet-endothelial cell interaction result from endothelial cell activation rather than platelet activation. Our data suggest that irradiation transforms resting endothelial cells to a pro-adhesive surface for platelets, which could ultimately lead to thrombosis.     

Behavior of postcapillary venule pericytes during postnatal angiogenesis.

Autogeneic bone marrow was implanted into an artificially created cavity in a segment of rat sciatic nerve, after removal of nerve fascicles, without damaging the epineurium or surrounding microcirculation. Under these conditions, the bone marrow induces capillary growth and forms granulation tissue from surrounding tissues, the behavior of pericytes being studied in the preformed (preexisting) postcapillary venules of the latter. Beginning 20 h after bone marrow implantation, the pericytes of the preexisting postcapillary venules hypertrophy, with shortening of their processes, prominent nucleoli, dispersal of ribosomes into their free form, fragmentation of basal lamina, and increased DNA synthesis. The number of contact surfaces between pericytes and endothelium is noticeably lower than in controls. Many pericytes are in mitosis. Cells with a shape transitional between pericytes and interstitial fibroblast-like cells appear. In some cases, Monastral Blue (MB) was used as a marker of the cells in preexisting venule walls of the graft bed. In the earlier stages of the experiment, the MB labelling is restricted to the cytoplasm of pericytes and endothelial cells of postcapillary venules, and to the macrophages that occur in the space between pericytes and endothelium. Furthermore, the marker continues to be observed, at a later stage, in some of the following cells: pericytes and endothelial cells of the newly formed vessels, macrophages migrating into the interstitium, transitional cells between pericytes and fibroblasts, and typical fibroblasts of the granulation tissue. The present study provides greater evidence that preformed microvasculature pericytes are substantially activated during postnatal angiogenesis and granulation tissue formation, suggesting that they may contribute to the origin of new pericytes and fibroblasts.     

Ultrastructure and permeability of lymph node microvasculature in the mouse

Summary The microvasculature of lymph nodes and Peyer's patches consists of arterioles, capillaries and venules. The postcapillary segment comprises high-endothelial venules (HE venules) as well as ordinary venules. In order to study the ultrastructure of the microvasculature, particularly with respect to the nature of intercellular junctions, lanthanum and ruthenium red were used as tracers. Furthermore, to evaluate the permeability properties of the different segments of the microvasculature, intravenously injected horseradish peroxidase (HRP; MW: 40,000) was used.All segments of the microvasculature are permeable to HRP. However, the mechanism of transport across the vascular wall varies in the different segments, apparently correlated with a gradual decrease in number of transport vesicles and a gradual attenuation in the sealing of the endothelial cells. Tight junctions are present in arterioles, and it is assumed that HRP reach the basal lamina exclusively by vesicular transport. Incomplete or focal tight junctions are present in the capillaries, and both intercellular and vesicular pathways are observed. In the venules the intercellular pathway seems to be the dominant one, while vesicular transfer is negligible. However, some micropinocytic vesicles in the HE venule endothelial cells probably represent the initial stage of an intracellular digestion.     

Leukocyte-endothelial cell recognition: evidence of a common molecular mechanism shared by neutrophils, lymphocytes, and other leukocytes

The interaction of leukocytes with endothelial cells is intrinsic to the process of leukocyte extravasation, whether during the entry of blood polymorphonuclear leukocytes and monocytes into sites of acute and chronic inflammation, or during the homing of lymphocytes to lymphoid organs. A lymphocyte surface glycoprotein, defined by monoclonal antibody MEL-14, has been described that appears to mediate lymphocyte recognition of postcapillary venules in peripheral lymph nodes, and to control the migration of lymphocytes from the blood into these lymphoid organs. We now report that the antigenic determinant recognized by MEL-14 is present at high levels on other leukocytes as well, including neutrophils, monocytes, and eosinophils; and we demonstrate involvement of the MEL-14 antigen in neutrophil-endothelial cell interactions. MEL-14 immunoprecipitates a neutrophil surface protein of Mr approximately 100,000, similar in m.w. to the 80,000 to 90,000 dalton lymphocyte surface MEL-14 antigen, and it blocks the interaction of neutrophils with endothelial cells in an in vitro model of adhesion to postcapillary venules in lymph node frozen sections. Neutrophil binding to lymph node venules is also inhibited by PPME, a mannose-6-phosphate-rich yeast polysaccharide that is thought to mimic the endothelial cell ligand for the MEL-14-defined lymphocyte receptor. Interestingly, neither MEL-14 nor PPME exhibit a major effect on neutrophil binding to postcapillary venules in Peyer's patches, suggesting that as for lymphocytes, the neutrophil MEL-14 antigen is involved in recognition of tissue-specific endothelial determinants. Finally, we show that MEL-14 inhibits the capacity of neutrophils to migrate from the blood into sites of acute inflammation in the skin. These observations lead us to propose that receptors for tissue-specific endothelial determinants are utilized by neutrophils and lymphocytes and probably other leukocytes during the physiologic process of leukocyte extravasation in vivo.     

Vascular anastomoses of the blood microcirculatory bed of the human heart

Using a complex of morphological techniques both injective and non-injective, scanning electron microscopy including, the hemomicrocirculatory bed and vascular anastomoses have been studied in various parts of the human heart. In most cases anastomoses between the microcirculatory links are realized at the level of capillaries, precapillary arterioles and postcapillary venules. Venulo-venular anastomoses are demonstrated in the myocardium. Existence of terminal arterioles is discussed.     

Role of shear forces and adhesion molecule distribution on P-selectin-mediated leukocyte rolling in postcapillary venules

Rolling on the venular endothelium is a critical step in the recruitment of leukocytes during the inflammatory response. P-selectin is a key mediator of leukocyte rolling, which is an early event in the inflammatory cascade; this rolling is likely to be directly regulated by both local fluid shear forces and P-selectin site densities in the microvasculature. However, neither the spatial pattern of P-selectin expression in postcapillary venules nor the effect of local expression patterns on rolling behavior in intact functional venules is known. We investigated the influence of local shear forces and the spatial distribution of endothelial P-selectin in intact blood perfused post capillary venules in anesthetized mice using intravital confocal microscopy, high temporal resolution particle tracking, and immunofluorescent labeling. We demonstrated a shear-dependent increase in average leukocyte rolling velocity that was attributable to a shear-dependent increase in the occurrence of transient leukocyte detachments from the endothelial surface: translational velocity during leukocyte contact with the vessel wall remained constant. P-selectin expression was not different in venules with characteristically different shear rates or diameters but varied significantly within individual venules. In postcapillary venules, regions of high P-selectin expression correlated with regions of slow leukocyte rolling. Thus the characteristically variable leukocyte rolling in vivo is a function of the spatial heterogeneity in P-selectin expression. The study shows how the local hydrodynamic forces and the nonuniform pattern of P-selectin expression affect the behavior of interacting leukocytes, providing direct evidence for the local variation of adhesion molecule expression as a mechanism for the regulation of leukocyte recruitment.     

Scanning electron microscopic study of the capillary loops in the dermal papillae. Skin of the hand of the Japanese monkey (Macaca fuscata)

The microvasculature of the skin of the hand of Japanese monkeys was examined by means of scanning electron microscopy of corrosion casts. The vasculature of all areas of the skin of the hand was examined and divided into three structures excluding the nail bed: (1) In the ball of the finger, the typical structure of the capillary loops was studied. Capillary loops were formed out of not just one capillary vessel, but two or three vessels. Each capillary vessel arose and divided into several branches at the papilla, and they became descending limbs. After the loop passed a hairpin turn, the descending limbs were 1.5 times larger than the ascending limbs in the intrapapillary portion, and they became extrapapillary venules. The descending limbs connected with the postcapillary venules in the postpapillary portion and with the horizontal network. The postcapillary venules fused with each other (1-5 loops) to form the primary and secondary venous arcades. (2) In the thenar eminence, the capillary loops were a little lower, and their grooves were wider than in the ball of the finger. The characteristic structure in this area was the interpapillar capillary network. (3) In the lateral side of the finger, the number of capillary loops formed by the arterial capillary network of the subepidermal layer was smaller. The capillary loops here had the lowest height and a simple structure.     

Study of the lympho- and hemomicrocirculatory bed of lymph nodes by scanning electron microscopy

The spatial interrelation of the lymphatic and blood beds has been studied in the lymph node together with its stromal and lymphoid elements, using scanning electron microscopy of corrosive casts and native preparations. There is a great variety of pathways for lymph transport and blood vessels in dependence of their localization in the lymph node. Of a special interest are the pathways of lymphocytes migration across the walls in the cortical and medullary sinuses and in the postcapillary venules, as well as participation of the reticular tissue in the lymph node immune reactions.     

Experimental Cerebral Malaria Pathogenesis—Hemodynamics at the Blood Brain Barrier

Cerebral malaria claims the lives of over 600,000 African children every year. To better understand the pathogenesis of this devastating disease, we compared the cellular dynamics in the cortical microvasculature between two infection models, Plasmodium berghei ANKA (PbA) infected CBA/CaJ mice, which develop experimental cerebral malaria (ECM), and P. yoelii 17XL (PyXL) infected mice, which succumb to malarial hyperparasitemia without neurological impairment. Using a combination of intravital imaging and flow cytometry, we show that significantly more CD8+ T cells, neutrophils, and macrophages are recruited to postcapillary venules during ECM compared to hyperparasitemia. ECM correlated with ICAM-1 upregulation on macrophages, while vascular endothelia upregulated ICAM-1 during ECM and hyperparasitemia. The arrest of large numbers of leukocytes in postcapillary and larger venules caused microrheological alterations that significantly restricted the venous blood flow. Treatment with FTY720, which inhibits vascular leakage, neurological signs, and death from ECM, prevented the recruitment of a subpopulation of CD45^hi CD8+ T cells, ICAM-1+ macrophages, and neutrophils to postcapillary venules. FTY720 had no effect on the ECM-associated expression of the pattern recognition receptor CD14 in postcapillary venules suggesting that endothelial activation is insufficient to cause vascular pathology. Expression of the endothelial tight junction proteins claudin-5, occludin, and ZO-1 in the cerebral cortex and cerebellum of PbA-infected mice with ECM was unaltered compared to FTY720-treated PbA-infected mice or PyXL-infected mice with hyperparasitemia. Thus, blood brain barrier opening does not involve endothelial injury and is likely reversible, consistent with the rapid recovery of many patients with CM. We conclude that the ECM-associated recruitment of large numbers of activated leukocytes, in particular CD8+ T cells and ICAM+ macrophages, causes a severe restriction in the venous blood efflux from the brain, which exacerbates the vasogenic edema and increases the intracranial pressure. Thus, death from ECM could potentially occur as a consequence of intracranial hypertension.     

The hemomicrocirculatory bed in the wall of hollow organs of the gastrointestinal tract of dogs with portal hypertension

At portal hypertension, produced by means of experimental stenosis of the portal vein in the hemomicrocirculatory bed of hollow organs of the gastrointestinal tract, congestive phenomena and edema of walls in the organs are observed. Manifested dilatation is noted in the lumen of arterioles, venules, postcapillary venules and capillaries. At early stages after the operation average diameters of these vessels in the submucosal base of the small intestine become increased 3-7 times and they do not return to the initial size even at late stages. The precapillary sphincters are in the state of spasm. Overdistention of walls in microvessels of the venular part of the functional module results in their increased permeability, that is demonstrated as diapedesic hemorrhages. During formation of intraorganic and extraorganic peripheral pathways of the circulation, the congestive phenomena in the hemomicrocirculatory bed disappear gradually.