Histological analysis of arteriovenous anastomosis-like vessels established in the corpus luteum of cows during luteolysis.

BACKGROUND: The mechanisms regulating the function and regression of the corpus luteum (CL) have not yet been elucidated in detail. The regressed CL of cows was previously reported to be filled with unusual vessels like arteriovenous anastomosis (AVA); however how these vessels are being established during luteolysis remains unknown. METHODS: The bovine CL at different luteal stages and regressing bovine CL induced by prostaglandin F2α (PGF) were histologically analyzed using light and electron microscopic levels. The changes in mRNA expression of genes encoding α-smooth muscle actin (SMA; Acta2) and transforming growth factor ß1 (Tgfb1) in luteal tissues were analyzed by quantitative RT-PCR. RESULTS: AVA-like vessels appeared in the regressed CL with a diameter less than 1.5 cm in which no functional luteal cells and macrophages were observed. Epithelioid cells in the AVA-like vessel wall were immunoreactive for SMA, and the lumen of the vessels were narrow. Immunoreaction for SMA was found in the tunica media of typical arteries and arterioles, and pericytes around capillary vessel. Cells with elongated cytoplasmic processes -resident fibroblasts expressing vimentin- distributed in the CL parenchyma without any association with blood vessels are also immunoreactive for SMA, and accumulated around arteries and arterioles during the late-luteal stage. In the regressed CL, walls of arteries and arterioles consisted of more than two layers of epithelioid cells positive for both SMA and desmin, suggesting that they are myofibroblasts transformed from fibroblasts. The percentage of the area positive for SMA and the mRNA expression of Acta2 were significantly increased in the regressed CL; however, they did not alter when a luteolytic dose of PGF was injected in vivo and collected within 24 h after the injection. On the other hand, Tgfb1, a known regulator for myofibroblast transformation, was significantly increased in PGF-induced regressing CL as well as in the CL during the late-luteal stage. CONCLUSIONS: SMA-positive myofibroblasts accumulates around the arteries and arterioles to form AVA-like vessels during luteolysis in cows. PGF indirectly regulates myofibroblast transformation through enhancing the expression of TGFß1. These peculiar AVA-like vessels may be involved in the regulation of blood flow in the bovine CL during luteolysis.

Mesoscopic and continuum modelling of angiogenesis.

Angiogenesis is the formation of new blood vessels from pre-existing ones in response to chemical signals secreted by, for example, a wound or a tumour. In this paper, we propose a mesoscopic lattice-based model of angiogenesis, in which processes that include proliferation and cell movement are considered as stochastic events. By studying the dependence of the model on the lattice spacing and the number of cells involved, we are able to derive the deterministic continuum limit of our equations and compare it to similar existing models of angiogenesis. We further identify conditions under which the use of continuum models is justified, and others for which stochastic or discrete effects dominate. We also compare different stochastic models for the movement of endothelial tip cells which have the same macroscopic, deterministic behaviour, but lead to markedly different behaviour in terms of production of new vessel cells.

'In parallel' interconnectivity of the dorsal longitudinal anastomotic vessels requires both VEGF signaling and circulatory flow.

Blood vessels deliver oxygen, nutrients, hormones and immunity factors throughout the body. To perform these vital functions, vascular cords branch, lumenize and interconnect. Yet, little is known about the cellular, molecular and physiological mechanisms that control how circulatory networks form and interconnect. Specifically, how circulatory networks merge by interconnecting 'in parallel' along their boundaries remains unexplored. To examine this process we studied the formation and functional maturation of the plexus that forms between the dorsal longitudinal anastomotic vessels (DLAVs) in the zebrafish. We find that the migration and proliferation of endothelial cells within the DLAVs and their segmental (Se) vessel precursors drives DLAV plexus formation. Remarkably, the presence of Se vessels containing only endothelial cells of the arterial lineage is sufficient for DLAV plexus morphogenesis, suggesting that endothelial cells from the venous lineage make a dispensable or null contribution to this process. The discovery of a circuit that integrates the inputs of circulatory flow and vascular endothelial growth factor (VEGF) signaling to modulate aortic arch angiogenesis, together with the expression of components of this circuit in the trunk vasculature, prompted us to investigate the role of these inputs and their relationship during DLAV plexus formation. We find that circulatory flow and VEGF signaling make additive contributions to DLAV plexus morphogenesis, rather than acting as essential inputs with equivalent contributions as they do during aortic arch angiogenesis. Our observations underscore the existence of context-dependent differences in the integration of physiological stimuli and signaling cascades during vascular development.

Occurrence and importance of glomus organs (Hoyer-Grosser's organs) in the skin of the equine and bovine mammary gland.

Glomus organs (Hoyer-Grosser's organs) were frequently found in the corium and the subcutis of the skin of the equine and bovine mammary gland. They were most frequently situated in the border zone between the stratum profundum and the stratum superficiale corii. These specialized vascular structures (arterio-venous anastomosis) were present in all investigated skin areas. Although the glomus organs varied in size and shape, they possessed common histologic structures: an arteriole entered the connective capsule of the glomus and divided into strongly convoluted arterio-venous channels; the arteriovenous channels united in the end to form a venule; the mentioned vascular elements were covered by a connective capsule and were thus united to an organ-like structure. Questions concerning their occurrence, their functional interpretation, their relevance, the size of the glomus organs as well as the possible involvement of the lymphatics were discussed.

Fine structure of the human coccygeal body: a light and electron microscopic study.

There is only limited data on the microscopic structure of the coccygeal body in classical textbooks. Although there are several articles describing the light microscopic structure of the organ, the data presented are rather conflicting especially when comparing glomus tumors and normal coccygeal bodies. We therefore examined the pericoccygeal soft tissues histologically with the aim of obtaining further evidence. At the light microscopic examination, coccygeal bodies were distinguished as sections of several blood vessels encapsulated by a connective tissue capsule. Small arteries within the coccygeal bodies had an unusual endothelial lining, resembling pseudostratified epithelium or neuro-epithelium. We observed increased numbers of glomus cells in a few samples. A highly tortuous course run by the small arteries was observed in the serial sections. At their electron microscopic examination, endothelial cells exhibited the features of columnar epithelial cells, though some appeared to be vacuole rich. The basal surface of the cells showed basal infoldings, but not as extensive as those of absorptive cells. External lamina, subplasmalemmal vesicles, bundles of microfilaments, groups of membrane bound organelles including mitochondria and endoplasmic reticulum were observed in glomus cells. In conclusion, it is rather difficult to use the term tumor for the coccygeal bodies when relying solely on the number of cells observed, because some segments of the coccygeal bodies were rich in glomus cells. It is more likely that the coccygeal bodies show variability in different individuals. The characteristics of the endothelium of the small arteries in coccygeal bodies needs to be examined in a wider range of specimens in order to be validated as a unique entity.

Effects of anastomoses on solute transcapillary exchange in countercurrent systems.

OBJECTIVE: To investigate effects of anastomoses between the descending vasa recta (DVR) and the ascending vasa recta (AVR) on the distribution of small solutes in the interstitial fluid of the renal medulla. METHODS: Countercurrent capillary loops, surrounded by a secretory epithelium, were used to model microvessels in the renal medulla. Anastomoses between the DVR and AVR were modeled as a decrease in cross-sectional area of the vessels and a decrease in flow velocity from the base to the tip of the capillary loop. When experimental data were used to evaluate parameters of the model, it was seen that diffusive transport of solute in the axial direction of the capillary was negligible, and the equations could be greatly simplified. RESULTS: General formulae of the solute concentration distribution were derived for different degrees of shunting between the two limbs of the capillary loop. Analytical solutions for the steady-state solute distribution were obtained when the sizes of capillaries and flows in them were assumed to decrease linearly with the distance from the base to the tip of the capillary loop. When the effects of reduction in the size of the limbs were compared with the effects of reduction of flow velocities on solute distribution, it was found that, in the presence of anastomoses, change in flow velocity increases the axial gradient of the solute concentration more than change in the cross-sectional area. The combined effects of a decrease in flow velocity and cross-sectional area can easily double the axial gradient of the solute concentration for a modest degree of anastomotic shunting. CONCLUSIONS: In this study, we separated effects of anastomotic flow between the DVR and AVR from other factors affecting the complicated countercurrent solute exchanges in the renal medulla. Results from the model show that anastomoses increase the solute concentration in the medullary interstitium and also the axial gradient of the solute concentration there.

Cytoskeletal characterization of arteriovenous epithelioid cells.

Data on the cytoskeleton of epithelioid cells in arteriovenous anastomosis (AVA) are sparse, but there is evidence that the (myo)-epithelioid cells of the AVAs represent a specialized smooth muscle cell type with less contractile properties. We demonstrated the expression of alpha-smooth muscle actin, smooth muscle myosin, calponin, caldesmon, and caveolin in epithelioid cells of rabbit ear and in human toes, finger tips, and glomus tumors by means of indirect immunofluorescence techniques and immunoelectron microscopy. Epithelioid cells in rabbit ear did not express vimentin, but it was present in human toes, finger tips, and glomus tumors. Epithelioid cells in human toes, finger tips, and glomus tumors did not express desmin, but it was present in rabbit ear. Epithelioid cells did not express cytokeratins. The epithelioid cells examined showed only a weak expression of the protein smoothelin, which occurs exclusively in contractile smooth muscle cells. Immunoelectron microscopical demonstration of (alpha-smooth muscle actin revealed a striking difference in the arrangement of actin filaments in the epithelioid cells as compared to that in the smooth muscle cells of blood vessels. The epithelioid cells contained a loose array of actin filaments, whereas the smooth muscle cells contained tightly packed parallel actin bundles. In the present study we observed a correlation between the lack of contractile marker protein expression in epithelioid cells and the presence of only a few filaments, although the epithelioid cells are alpha-smooth muscle actin positive. The reduced number of contractile elements in the epithelioid cells of rabbit and human anastomoses suggests a lower contractility of epithelioid cells compared to that of the surrounding smooth muscle cells in anastomoses. A second interesting difference between both cell types is the high number of caveolae in epithelioid cells. Immunoelectron microscopy showed a compact distribution of caveolae at the epithelioid cell border, but a more dispersed distribution of caveolae in the cytoplasm of the blood vessel endothelium. The benign glomus tumor was characterized by an expression pattern of cytoskeletal proteins similar to that of epithelioid cells, confirming its description as a benign tumor.

Nitrergic innervation and nitrergic cells in arteriovenous anastomoses.

Nitrergic innervation and nitrergic epithelioid cells were studied in arteriovenous anastomoses of the tongue, ear, eye, and glomus organ of the finger in different species (rat, rabbit, dog, and man), by means of immunohistochemistry for nitric oxide synthase and enzyme histochemistry utilizing the catalytic activity of this enzyme (the NADPH-diaphorase reaction). Nitrergic perivascular fibers of the tongue were concentrated along the arterial tree and were maximal at the arteriovenous anastomoses in all species. Generally, fewer fibers were located around comparable segments of the episcleral eye vasculature. Only a few nitrergic fibers were found in the canine and rabbit ear, and in the glomus organ of the human finger; however, epithelioid cells in the tunica media of arteriovenous anastomoses of these organs were NADPH-diaphorase-positive and were moderately immunoreactive for nitric oxide synthase. In the epithelioid cells, the reaction product of the NADPH-diaphorase could also be demonstrated by transmission electron microscopy. The epithelioid cells were negative for the panneural and neuroendocrine marker PGP 9.5 confirming the myocytotic nature of these nitrergic cells. Thus, nitric oxide might play a role in mediating the vessel tone of arteriovenous anastomoses via nitrergic nerves or epithelioid cells.

Three-dimensional cytoarchitecture of the media of arteriovenous anastomoses in the rabbit ear.

Arteriovenous anastomoses in the rabbit ear were examined with scanning electron microscopy to elucidate the structural differentiation of the media of the shunt. Arterial, intermediate, and venous segments in the shunt and two layers of the media in the intermediate segment were differentiated based on cell shape and cell organization. In the arterial segment, smooth muscle cells were spindle-shaped, either elongated or short, with a few branches, and were arranged circularly or diagonally with respect to the vessel's long axis. There were also stellate muscle cells with radiating processes. In the intermediate segment, the smooth muscle cells of the outer layer of the media were also arranged circularly and resembled the elongated cells in the arterial segments, but they were more irregular in shape and had more processes than those of the arterial segment. The epithelioid cells of the inner layer of the media were oval or polygonal and oriented irregularly with respect to the vessel's long axis, clustering to form longitudinal plicae. The smooth muscle cells of the venous segment were flat with many lateral processes and formed a thin, discontinuous layer. The smooth muscle cells in the arterial segment and those of the outer layer of the intermediate segment exhibited a highly rugged surface texture, indicating their strong contractility; the epithelioid cells and the smooth muscle cells in the venous segment exhibited a generally smooth surface, indicating less contractility. The intermediate segments were supplied with a dense nerve plexus. The intermediate segments, therefore, may be actively involved in the regulation of blood flow under neuronal influence.

"Intermediate zone" of mammalian spleens: light and electron microscopic study of three primitive mammalian species (platypus, shrew, and mole) with special reference to intrasplenic arteriovenous communication.

The intermediate zone (IZ) of nonperfused and perfused spleens in three species of primitive mammals (shrew, mole, platypus) was studied morphologically. The IZ is a tissue zone consisting of plexiform vessels, probably venous capillaries, and is located transitionally between the white and red pulp. The IZ is separated from the white pulp by the arterial net (AN), in which the white pulp arteries terminate. Development of the IZ differs between the three species examined being distinctive in the platypus and shrew. The IZ is thin in the mole spleen. A closed type of arteriovenous (A-V) anastomosis was demonstrated in or around the IZ in the two Insectivora species examined. In the shrew spleen, peripheral arterial branches running within the IZ anastomose with the AN around the follicle. The AN anastomoses eventually with venous plexiform vessels of the IZ around the nonfollicular area of the white pulp to form a closed system. In the mole spleen, A-V anastomoses were noted between white pulp arteries (follicular and AN) and veins of the red pulp, either by direct communication or through fenestrated IZ vessels compatible with the plexiform vessels of the shrew spleen. A-V anastomosis in the IZ is probable, but not confirmed, in the platypus spleen, as analysis was limited to a nonperfused specimen. Well-developed ellipsoids were noted around arterial terminals of the IZ in the shrew spleen. Ellipsoids were also noted around all arterial terminals of the mole spleen directed to the red pulp. Most ellipsoids of the mole spleen appeared located within the IZ. No ellipsoids were present around arterial terminals of the IZ in the platypus spleen. Closed circulation was noted in terminals of the pulp artery in spleens of all three species. All pulp arteries of the mole spleen are postellipsoid segments of white pulp (AN and follicle) arteries. No ellipsoids were found around terminals of the pulp artery (penicillar artery) in shrew and platypus spleens. The IZ is probably homologous to the perilymphatic sinusoid (vein) of the lungfish spleen and may be regarded as part of the red pulp. The IZ may be representative of primitive mammalian spleens that have closed circulation. The marginal zone (MZ) of common mammalian spleens is probably a modified IZ by differentiation (remodelling) of the intrasplenic vein. In this process, withdrawal of venous vessels from the IZ occurred, leaving a lymphoreticular zone with open circulation (MZ). The marginal sinus reported in some mammalian spleens is probably a modified AN formed during this process. Possible morphological alterations of the spleen in vertebrate phylogeny are discussed.

Innervation of arteriovenous anastomoses in the brood patch of the domestic fowl.

The innervation of blood vessels in the brood patch (thoracic skin) of the domestic fowl was studied by use of the catecholamine fluorescence technique, acetylcholinesterase staining, and the immunoperoxidase technique for demonstration of vasoactive intestinal polypeptide (VIP). Large arteries and veins were sparsely innervated, whereas arteriovenous anastomoses (AVAs) were densely innervated by adrenergic, acetylcholinesterase-positive, and VIP-immunoreactive nerve fibres. The rich supply of different vasomotor nerves to AVAs emphasizes the importance of these vascular shunts in regulating blood flow and, in turn, the transport of heat to the brood patch. Furthermore, the presence of VIP-immunoreactive nerve fibres in the vasculature of the brood patch suggests that VIP might be the mediator of the previously reported cold-induced vasodilatation.

Wall structure of arteriovenous anastomoses in the rabbit ear. Combined light-, scanning- and transmission electron-microscopic studies.

The wall structure of arteriovenous anastomoses in the rabbit ear was investigated. (1) Clusters of epithelioid smooth muscle cells form 3-4 longitudinally oriented plicae. The channel shows a single, irregularly outlined lumen, and its wall is very thin between adjacent plicae. (2) Endothelial cells covering the plicae protrude into the lumen, thus suggesting active contraction or shortening of the plicae. (3) The tunica adventitia is composed of 4-6 sheaths of flat fibroblasts, which may serve as a barrier to prevent loss of neurotransmitters. Processes of some of the fibroblasts also extend into the tunica media. (4) The tunica media is composed of an outer circular layer of typical smooth muscle cells, and an inner longitudinally running plica of ramified smooth muscle cells. Wide intercellular spaces between these ramified cells are filled with collagen fibrils, microfibrils, amorphous intercellular substances, and fibroblasts. Fibroblasts form close membrane contacts with each other, and with the smooth muscle cells. (5) Fibroblasts and other connective tissue components may function as an elastic support during active motility of the anastomotic channel.

Autonomic innervation of the arteriovenous anastomoses in the dog tongue. A histochemical and ultrastructural study.

The innervation of the arteriovenous anastomoses in the dog tongue has been investigated. At the light-microscopic level, the vessels were found to be densely supplied with adrenergic and AChE-positive nerve plexuses and less densely with the quinacrine-binding nerve plexus. At the electron-microscopic level, at least two apparently different types of axon profiles were identified: Small vesicle-containing axons, characterized by many small granular vesicles, variable numbers of small clear vesicles and large granular vesicles. Storage of endogenous amines and uptake of exogenous amines into most small granular vesicles and many large granular vesicles was demonstrated. These axons stained only lightly with reaction products for AChE activity and thus seemed to be adrenergic in nature. Some axons contained numerous large granular vesicles, whose cores occasionally stained with uranyl ions; this suggests a co-localization of ATP or peptides as neurotransmitters. Small granular vesicle-free axons, containing small clear vesicles and large granular vesicles in variable ratio. Most cores of these large granular vesicles were heavily stained with uranyl ions. No storage or uptake of amine into the synaptic vesicles was detected. Some axons appeared to be typically cholinergic, some, typically non-adrenergic, non-cholinergic, and the rest, intermediate between the two. All axons stained heavily with reaction products for AChE activity, suggesting their cholinergic nature.

Absence of "epitheloid cells" in AV-anastomoses (AVA) for electron microscopy.

AV-anastomoses were studied in rabbit auricles. After perfusion-fixation with glutaraldehyde and post-osmication AV-anastomoses are to be identified only when serial sections are analyzed. "Epitheloid cells" were not seen in the walls of the anastomosing segments.