Microvascular anatomy of olfactory and accessory olfactory (vomeronasal) organs in adult Xenopus laevis: scanning electron microscopy of vascular corrosion casts / Anatomía microvascular de órganos olfatorios y accesorios (vomeronasal) en adultos Xenopus laevis: microscopía electrónica de barrido de moldes de corrosión vascular

Microvascular anatomy and histomorphology of olfactory and vomeronasal organs in adult Xenopus laevis Daudin were studied by scanning electron microscopy of vascular corrosion casts and paraplast embedded stained serial tissue sections. Results show that the arterial supply is bilaterally by terminal arterioles of the medial branch of the nasal artery and by the palatal artery. Arterioles give rise to a capillary meshwork characteristic for respiratory surfaces in principal chambers and in dorsal and caudal areas of middle chambers. Anterior and inferior areas of the middle chambers own a distinctly different capillary network with conspicuous short capillary loops. Loops have a dilated tip and extend in acute angles towards the chamber lumen. The vomeronasal organ (VNO) locates beneath the olfactory organ. It has a medial to lateral extension and attaches with its caudal circumference to the medial nasal glands. Its capillary bed displays rectangular meshes which preferentially orientate along the long axis of the VNO. Locally, capillaries form short hairpin-like or strongly twisted loops with dilated tips which point towards the lumen of the VNO. These capillaries slow-down blood velocity and may lead to an increased exchange of oxygen, nutrients and water-borne odorants in the middle chambers and of pheromones in the VNO. In the latter vascular structures are present which might serve as a vascular pump.

Se estudiaron la anatomía microvascular e histomorfología de los órganos olfatorios y vomeronasales de Xenopus laevis Daudin adultos, mediante microscopía electrónica de barrido de moldes de corrosión vascular y secciones de tejido seriadas, teñidas e incluídas en paraplast. Los resultados muestran que el suministro arterial es bilateral por arteriolas terminales de la rama medial de la arteria nasal y por la arteria palatina. Las arteriolas dan lugar a un lecho capilar característico de las superficies respiratorias en las cámaras principales y en las áreas dorsal y caudal de las cámaras intermedias. Las áreas anterior e inferior de las cámaras centrales poseen una red capilar significativamente diferente con llamativos bucles capilares cortos. Los bucles tienen una punta dilatada y se extienden en ángulos agudos hacia la luz de la cámara. El órgano vomeronasal (VNO) se ubica debajo del órgano olfatorio. Se extiende de medial a lateral y se une con su circunferencia caudal a las glándulas nasales mediales. El lecho capilar muestra mallas rectangulares que se orientan preferentemente a lo largo del eje longitudinal del VNO. Localmente, los capilares forman bucles cortos en forma de horquilla o fuertemente retorcidos con puntas dilatadas que apuntan hacia la luz del VNO. Estos capilares ralentizan la velocidad de la sangre y pueden conducir a un mayor intercambio de oxígeno, nutrientes y odorizantes, a base de agua en las cámaras intermedias y de feromonas, en el VNO. En este último, están presentes estructuras vasculares que podrían servir como una bomba vascular.

Porcine liver vascular bed in Biodur E20 corrosion casts.

BACKGROUND: Pigs are frequently used as animal models in experimental medicine. To identify processes of vascular development or regression, vascular elements must be recognised and quantified in a three-dimensional (3D) arrangement. Vascular corrosion casts enable the creation of 3D replicas of vascular trees. The aim of our study was to identify suitable casting media and optimise the protocol for porcine liver vascular corrosion casting. MATERIALS AND METHODS: Mercox II® (Ladd Research, Williston, Vermont, USA) and Biodur E20® Plus (Biodur Products, Heidelberg, Germany) were tested in 4 porcine livers. The resins (volume approximately 700 mL) were injected via the portal vein. Corrosion casts were examined by macro-computed tomography, micro-computed tomography and scanning electron microscopy. RESULTS: For hepatectomies, the operating protocol was optimised to avoid gas or blood clot embolisation. We present a protocol for porcine liver vascular bed casting based on corrosion specimens prepared using Biodur E20® epoxy resin. CONCLUSIONS: Only Biodur E20®Plus appeared to be suitable for high-volume vascular corrosion casting due to its optimal permeability, sufficient processing time and minimum fragility. Biodur E20® Plus is slightly elastic, radio-opaque and alcohol-resistant. These properties make this acrylic resin suitable for not only vascular research but also teaching purposes.

The impact of VEGF and bFGF on vascular stereomorphology in the context of angiogenic neo-arborisation after vascular induction.

The aim of this in vivo study was to gather quantitative information on the three-dimensional morphology of a new vascular network under the influence of angioactive growth factors. For this purpose, the arteriovenous loop model was used in 10 Lewis rats to generate a bioartificial vascular assembly by means of vascular induction. In this model, an isolated organoid is created in the medial thigh of the animal by methods of tissue engineering. A fibrin gel containing vascular endothelial growth factor (VEGF(165)) and basic fibroblastic growth factor (bFGF) was used as a matrix in the effect group (GF+). Fibrin matrices devoid of growth factors were used as controls (GF-). A microvascular replica of the organoid was created by means of corrosion casting and the network was investigated on stereo-paired images obtained by scanning electron microscopy. Vectors of intercapillary and interbranching distances as well as the diameter of the pores in the intussusceptive events diameter and the ratio of sprouting versus intussusceptive angiogenic events were compared in the two groups. The results were highly significant. In the GF+ group there were more profound three-dimensional morphological traits of angiogenesis, whereas advanced neovascularisation in the phase of remodelling was demonstrated by a higher incidence of intussusception, compared to control. These results illustrate the importance of morphological studies with focus on the generation of three-dimensional vascular networks.

Microvascularization of the interhyoid muscle in larval Xenopus laevis (Daudin): Scanning electron microscopy of vascular corrosion casts and correlative light microscopy.

The interhyoid muscle in tadpoles of Xenopus laevis (Daudin) is an important part of the buccal pump, a functional unit that provides unidirectional flow of water through mouth and pharynx. In anuran tadpoles, this flow is crucial in both respiration (gas exchange) and food intake (ingestion). The microvascular anatomy of the interhyoid muscles of 43 tadpoles of X. laevis from developmental stages 49-60 was examined by scanning electron microscopy of vascular corrosion casts and correlative light microscopy of paraplast embedded Goldner stained serial tissue sections. Analysis of vascular corrosion casts of the interhyoid muscle showed that several descending branches of external carotid arteries supplied the interhyoid muscle. Arteries splitted into many arterioles at the dorsal surface of the interhyoid muscle and formed sheaths of longitudinally orientated capillaries around muscle fibers. Postcapillary vessels formed perpendicularly orientated arrays of collecting venules (mean diameter: 15.6 µm), which drained the interhyoid muscle from the ventral surface into external jugular veins. Cast analyses revealed sprouting angiogenesis at the capillary level and nonsprouting angiogenesis at distal domains of the venous system. Both means of angiogenesis that persisted throughout the developmental periods examined are thought to represent a superposition of concurrent developmental and physiological processes. The dense microvascular bed of the interhyoid muscle reflects its high demand for supply with oxygen and nutrients.

The dual role of zonula occludens (ZO) proteins.

ZO (zonula occludens) proteins are scaffolding proteins providing the structural basis for the assembly of multiprotein complexes at the cytoplasmic surface of intercellular junctions. In addition, they provide a link between the integral membrane proteins and the filamentous cytoskeleton. ZO proteins belong to the large family of membrane-associated guanylate kinase (MAGUK)-like proteins comprising a number of subfamilies based on domain content and sequence similarity. Besides their structural function at cell-cell contacts, ZO proteins appear to participate in the regulation of cell growth and proliferation. Detailed molecular studies have shown that ZO proteins exhibit conserved functional nuclear localization and nuclear export motifs within their amino acid sequence. Further, ZO proteins interact with dual residency proteins localizing to the plasma membrane and the nucleus. Although the nuclear targeting of ZO proteins has well been described, many questions concerning the biological significance of this process have remained open. This review focuses on the dual role of ZO proteins, being indispensable structural components at the junctional site and functioning in signal transduction pathways related to gene expression and cell behavior.

Microvascular anatomy of the large intestine in adult Xenopus laevis: scanning electron microscopy of vascular corrosion casts and correlative light microscopy.

The microvascular anatomy of the large intestine of the adult South African Clawed Toad, Xenopus laevis (Daudin), was studied by scanning electron microscopy (SEM) of vascular corrosion casts (VCCs) and correlative light microscopy. Observations showed the large intestine to be supplied by the haemorrhoidal artery and the posterior mesenteric artery and drain via the posterior haemorrhoidal vein into either the left or right posterior abdominal vein. Both arteries and veins showed a bipinnate supply/draining pattern with branches running circumferentially. Vessels embraced the gut wall while arteries and veins in most cases alternated along the gut length. Many short terminal arterioles arose from the circumferential arteries at almost acute angles and capillarized after a short distance. Capillary lengths were short and continued into numerous postcapillary venules which merged either in a leaf vein-like formation or in a rosette-like formation with up to four draining sites per supplying arteriole. The microvasculature was found to be well adapted 1) to sustain blood flow under different amounts of feces in the gut and 2) to provide optimal conditions for the resorption of water and salts from the gut lumen into the blood vascular system by the high number of venules and their conspiciouos rosette-like and leaf vein-like patterns.

Regression and persistence: remodelling in a tissue engineered axial vascular assembly.

In later stages of vasculoangiogenesis a vascular network is going through a metamorphosis for optimal perfusion and economy of energy. In this study we make a quantitative approach to phenomena of remodelling in a bioartificial neovascular network and suggest variance of calibre as a parameter of neovascular maturation. For this study, 18 male Lewis rats were subjected to the AV loop operation in combination with a hard porous biogenic matrix and an isolation chamber. The animals were allocated into three groups for different explantation intervals set to 2, 4 and 8 weeks, respectively. Collective attributes like vascular density, percent fractional area and variance of calibre were evaluated for a predefined region of interest (ROI). Late morphogenesis was evaluated by means of scanning electron microscopy. After the fourth week the absolute number of vessels within the ROI decreased (P < 0.03) whereas, on the contrary, the fractional area of all segments increased (P < 0.02). The variance in calibre was significantly increased in the 8-week group (P < 0.05). Lymphatic growth after week 4, early pericyte migration as well as intussusceptive angiogenesis were identified immunohistologically. Phenomena of remodelling were evaluated quantitatively in a neovascular network and variance could be proposed as a parameter of net vascular maturation.

Morphological features of vasa vasorum in pathologically changed human great saphenous vein and its tributaries.

BACKGROUND: The question whether the primary increase of vasa vasorum (VV) of venous wall (i) plays an initial role in varicogenesis or (ii) is an expression of impairment of the nutritional conditions in superficial veins of lower extremities is not unambiguously solved yet. The aim of the study was to describe the arrangement of the VV within the wall of the human great saphenous vein (GSV) qualitatively, and of its tributaries at different stages of varicosis and in other pathological states like thrombophlebitis or phlebosclerosis. MATERIAL AND METHODS: 22 patients deserving an aorto-coronary bypass surgery or GSV surgery were subdivided into three groups according to the staging of their varices and other pathology. The harvested GSV were prepared for light and scanning electron microscopy. One cadaverous specimen of GSV was injected with India ink. RESULTS: In specimens from reticular and primary large varices local intimal hyperplasia was regularly found, partially accompanied with a mild increase of VV. Tortuosities and irregular dilations of adventitial veins were also found. In patients with recurrent primary varices or thrombophlebitis severe intimal and medial hyperplasia, thrombosis and a striking increase of VV were found. The intima remained avascular in all cases. CONCLUSIONS: Remarkable increase of VV accompanies the most severe forms of varices as well as all cases of the extreme grades of phlebosclerosis, medial hyperplasia and thrombosis. We hypothesize that this increase in VV is rather a secondary vascular reaction to the impaired metabolic conditions within the venous wall than a primary varicogenic factor.

The venous graft as an effector of early angiogenesis in a fibrin matrix.

The arteriovenous loop (AV loop) model is gaining importance as a means of initiating and sustaining perfusion in tissue engineering constructs in vivo. This study represents an attempt to dissect the morphology of early arterialization and angiogenesis in the AV loop in a fibrin matrix with special focus on the interpositional venous graft (IVG) segment. An AV loop was constructed in 30 rats using the femoral vessels and an IVG. The AV loop was encased in an isolation chamber filled with a fibrin matrix. Evaluation methods included scanning electron microscopy (SEM) of corrosion casts, immune histology and micro magnetic resonance angiography (MRA). Direct luminal neovascular sprouting was evident between day 10 and day 14 from the vein and the IVG but not from the arterial segment. Arterialization of the IVG manifested itself on the corrosion casts as a gradual reduction in luminal caliber with onset after day 7. Microdissection of the microvascular replicas could demonstrate for the first time the presence of direct luminal sprouts from the IVG. MRA was used to display the shunt pattern of perfusion in the patent AV loop. From the three segments of the vascular axis in the AV loop the IVG is the most versatile for applications in the clinical as well as the experimental setting. Kinetics of angiogenesis warrant further investigation in the IVG.

Spatial growth and pattern formation in the small intestine microvascular bed from larval to adult Xenopus laevis: a scanning electron microscope study of microvascular corrosion casts.

The microvascular anatomy of the small intestine of metamorphosing tadpoles of the South African Clawed Toad, Xenopus laevis (Daudin) is studied from developmental stages 55 to 65 and in adults by scanning electron microscopy (SEM) of vascular corrosion casts (VCCs) and light microscopy. Up to stage 62, VCCs reveal a dense two-dimensional vascular network ensheating the intestinal tube, whose proximal portion forms a clockwise spiralling outer and its distal portion an anti-clockwise spiralling inner coil. Vessels of the intestinal network impose flat and run circularly to slightly obliquely. Locally, dense capillary plexus with small "holes" indicating ongoing intussusceptive microvascular growth (IMG) and vessel maturation, are present. The typhlosole, an invagination along the proximal portion of the small intestine, reveals a dense capillary bed with locally ongoing IMG. VCCs of stages 62/63 for the first time reveal a three-dimensional vascular bed with longitudinal intestinal folds of varying size and heights greatly enlarging the luminal exchange area of the intestinal tube. From stage 65 onwards, longitudinal intestinal folds undulate and, though smaller in size and less mature as indicated in VCCs by the presence of wider, sinus-like vessels with small "holes" interposed between, closely resemble the intestinal folds present in the small intestine of adult Xenopus. Our data suggest that maturation of the vascular pattern in the small intestine of X. laevis tadpoles takes place successively after stages 62-63, and growth during this period is preferentially by intussusception.

Valves in small veins and venules.

It is commonly believed that valves are absent in veins smaller than two millimetres in diameter. Consequently, current investigations on the pathophysiology of chronic venous disease (CVD) consider and evaluate only the valvular competence of large veins. The authors review literature from their own collections as well as from medical database searches to assess the functional relevance of these valves. Microscopic venous valves (MVVs) were first described in 1934 in the human digits and have subsequently been demonstrated in other parts of the human body as well as in many tissues and organs of animals. Their location and arrangement suggests that MVVs prevent blood reflux in small sized veins and restrict flow from postcapillary venules back into the capillary bed. This haemodynamic role of MVVs is strongly supported by the clinical finding that grafting skin rich in MVVs results in long-lasting healing leg ulcers attributable to CVD. The huge body of knowledge available concerning MVVs urges us to correct textbooks of anatomy. Studies on the pathophysiology of CVI should acknowledge that the valvular "chain" is not limited to large veins, but extends down to the venular level where MVVs play an important role in venous haemodynamics.

Analysis of microvascular trees by means of scanning electron microscopy of vascular casts and 3D-morphometry.

Arterial and capillary trees form by consecutive branching (mostly bifurcations) from a stem vessel, venous trees form by repeated merging of blood vessels. Diameters of stem (parent, mother) vessels and daughter vessels (branches), interbranching distances and branching angles between stem and daughter vessels lastly define the overall three-dimensional structure of the vascular network as well as the basic transport capacity of the system. Here we use scanning electron microscopy and 3D-morphometry to measure these variables from stereo paired images of vascular corrosion casts of the anterior cerebral artery and its main branches and from arteriolar bifurcations of the mesencephalic optic tectum in the actinopterygian fish, Acipenser ruthenus. We then calculate bifurcation indices, area ratios, asymmetry ratios and test for the optimality principles underlying the bifurcations studied. Our results show that arteriolar bifurcations in the optic tectum are in favor of the principles of minimum pumping power and minimum volume rather than the principles of minimum surface and minimum drag. We conclude that scanning electron microscopy of vascular corrosion casts in conjunction with 3D-morphometry is an excellent tool to thoroughly analyze vascular trees in healthy and diseased tissues and organs, as well as on an ontogenetic and phylogenetic scale.

Vasa vasorum of the human great saphenous vein.

The distribution of the vasa vasorum of the human great saphenous vein (GSV) was studied on veins taken both post-mortem and peroperatively. It was found that the stems of feeding vessels approach the venous wall at intervals of 1.5-2.5 cm; their smaller branches first passed the fascial compartments of the GSV and then entered the adventitia at intervals of 0.5-1.5 cm on both the stem and the largest tributaries of the GSV. In the stem regions vasa vasorum arteries and veins ran together but, between neighboring stems, isolated venae vasorum were regularly found which opened individually into terminal segments of the largest tributaries of the GSV. Neither by dissection nor by injection methods were venae vasorum found to open directly into the lumen of the GSV stem. The total thickness of the media ranged between 500 and 1300 micro m, according to the state of constriction of the venous wall before fixation. Two structurally different layers of GSV tunica media were present: an inner loose layer and an outer dense layer, both of similar thickness. The innermost capillaries of the vasa vasorum network were found in all cases on the border between the two layers of media. No lymphatic was found in any of the layers of GSV wall. From the findings the authors recommend extremely careful dissection of the GSV wall during in situ grafting surgery, to ensure the best viability of the venous wall.

Quantitative microvascular corrosion casting by 2D- and 3D-morphometry.

As a system of tubes (blood vessels) the cardiovascular system changes actively and passively diameters to adapt its transport capacities for respiratory gases, nutrients, heat, metabolites and waste products to and off the body's organs, tissues and cells. In most healthy organs blood vessels form a hierarchically arranged three-dimensional network with the geometry defined by vessel diameters, interbranching distances (defining branching frequencies and number of branching sites, i.e. nodes), intervascular distances, and branching angles. In the present study 2D- and 3D-morphometry is applied to quantify these parameters and their changes as they occur in resin casts during metamorphosis of the tadpole lung (2D-morphometry) and filter apparatus vasculature (3D-morphometry). It is shown that 2D-morphometry should be limited to the analysis of high powered images of flat two-dimensional vascular networks (example: tadpole lung alveolar vascular bed) to prevent underestimation of parameters. In contrast, 3D-morphometry can be applied over a wide range of magnifications whereby accuracy of measurements increases with the portion the structure to be measured occupies within the field of view. Together with a careful control of precasting conditions (application of vasoactive drugs, anaesthetics), casting conditions (pressure during rinsing and casting, amount of final shrinkage of casting media), and postcasting conditions (thermal burdening during maceration, sputtering, evaporation, and SEM inspection; thickness of conductive metal layers) 3D-morphometry enables to gain reliable data from resin casts of highly complex real vascular networks in healthy and diseased organs in the developing, juvenile, adult and aged state, as well as in different physiological states.

Eurosol versus fetal bovine serum-containing corneal storage medium.

PURPOSE: To evaluate the usability of Eurosol, a new medium-term corneal storage medium without components of bovine origin. METHODS: Ten pairs of human donor corneas were placed in tissue culture at 31 degrees C for 7, 14, 21, 28, or 35 days. One cornea of each pair was cultivated in conventional storage medium on Earls' minimum essential medium base containing 2% fetal bovine serum; the other one was stored in Eurosol. Corneas were examined with inverse light microscopy; corneal thickness was measured; and scanning electron microscopy was performed. RESULTS: No significant difference in corneal thickness and endothelial cell count was found at any time. Scanning electron microscopy showed a complete endothelial cell layer on all corneas. CONCLUSION. The findings indicate a potential clinical applicability of the tested serum-free medium-term storage medium, offering a safer alternative to conventional media containing fetal bovine serum.

Regression of blood vessels in the ventral velum of Xenopus laevis Daudin during metamorphosis: light microscopic and transmission electron microscopic study.

Structural changes of the ventral velum of Xenopus laevis tadpoles from late prometamorphosis (stage 58) to the height of metamorphic climax (stage 62) were examined by light and transmission electron microscopy. Special emphasis was given to the blood vessel regression. Early changes of velar capillaries were formation of luminal and abluminal endothelial cell processes, vacuolation, and cytoplasmic and nuclear chromatin condensation. At the height of metamorphic climax, transmission electron microscopy revealed apoptotic endothelial cells with nuclear condensation and fragmentation, intraluminal bulging of rounded endothelial cells which narrowed or even plugged the capillary, and different stages of endothelial cell detachment ('shedding') into the vessel lumen. These changes explain the 'miniaturisation' of the velar microvascular bed as well as the typical features found in resin-casts of regressing velar vessels which have been observed in a previous scanning electron microscopy study of the ventral velum.

Intussusceptive microvascular growth in the lung of larval Xenopus laevis Daudin: a light microscope, transmission electron microscope and SEM study of microvascular corrosion casts.

The remodeling of the uniform wide, plexus-like capillary bed of the lung of metamorphosing tadpoles of the South African clawed toad Xenopus laevis (Daudin) is studied from developmental stages 54 to 65 by scanning electron microscopy (SEM) of microvascular corrosion casts (VCCs), light microscopy (LM) and transmission electron microscopy (TEM). VCCs reveal that the remodeling of the existing uniform, plexus-like lung capillary bed into well-defined alveolar capillary meshworks starts in the caudal lung and then gradually proceeds cranially. Vascular remodeling is entirely by intussusceptive microvascular growth through insertion and enlargement of new and fusion of pre-existing capillary meshes. Analyses of lung tissue serial sections at the LM and TEM level confirm the presence of intracapillary cushions and tissue posts and correlate these structures in respect of size and location to the round to slit-like imprints and tiny "holes" found in VCCs. Additionally, SEM of VCCs give clear evidence that intussusceptive microvascular growth is also involved in the remodeling and maturation of alveolar arterioles and venules.

Lengths measurements in microvascular corrosion castings: two-dimensional versus three-dimensional morphometry.

In the present study we compared measurements of vessel lengths from (a) single-digital scanning electron microscope (SEM) images of microvascular corrosion casts (VCCs) of gill filters of tadpoles of Xenopus laevis Daudin by two-dimensional (2-D) morphometry (Optimas 6.5, Optimas Corp., Bothell, Wash., USA; planar measurements) and (b) digital stereopairs by three-dimensional (3-D) morphometry (3D-Morphometry, Minnich and Muska OEG, Salzburg). Depending on the spatial orientation of the vessels measured, we found a maximum difference of 58.84% (100 [3-D]-41.16 [2-D]) in vessel lengths by 3-D morphometry versus 2-D morphometry, which, in multiple (segmental) lengths measurements or when determining space angles, might be even higher. Based on results we consider 3-D morphometry of VCCs to be the method of choice for lengths measurements.

Corneal lathing using the excimer laser and a computer-controlled positioning system.

PURPOSE: To present the excimer laser corneal shaping system (ELCS-S), an add-on device to the Keratom, a commercially available 193-nm excimer laser built by Schwind. METHODS: The system is designed for the preparation of donor corneas under sterile conditions using the ultraviolet laser to offer greatest possible flexibility. Lenticules for planolamellar grafting and refractive epikeratoplasty, as well as donor buttons for penetrating keratoplasty can be computer-designed by the surgeon or technician and lathed with the system. RESULTS: Using the excimer laser corneal shaping system (ELCS-S) on human donor corneas, the central surface of the epikeratoplasty lenticule exhibited only narrow, flat concentric notches corresponding to the single lathing steps. Transmission electron microscopy revealed a damage zone of less than 0.3 microm in close approximation to the treated surface. The final thickness revealed a difference of less than +/-53 microm from the intended, initially programmed value. Ultrastructural studies showed the perpendicular stromal surface of the penetrating keratoplasty buttons to be smooth with minimal protrusion of Descemet's membrane. Endothelial injury was observed in a zone averaging between 40 and 100 microm adjacent to the cutting edge only. CONCLUSION: The excimer laser corneal shaping system (ELCS-S) allows a computer-controlled, surgeon-designed, sterile preparation of lamellar and penetrating corneal grafts with the use of the excimer laser. This could offer significant advantages in comparison to presently available systems for lamellar dissection and trephination.

Pulmonary lymphatic filling is increased in spontaneously hypertensive rats.

Extravascular lung liquid must rely on tissue-space pressure gradients to drive it into the lymphatics because the fluid is outside the lymphatic contractile pumping and valve control. Focal tissue pressure changes could result from muscular contraction in the blood vessel walls. Perivascular lymphatics usually lie within the adventitia of pulmonary blood vessels, and are generally more noticeable in veins than arteries. Spontaneously hypertensive rats have exaggerated focal pulmonary venous muscle (venous sphincters). These muscular tufts are often near initial lymphatics; if their contraction was important for lymph transport, spontaneously hypertensive rats could have more lymphatic filling in the areas of the pulmonary venous sphincters than normotensive rats. Because the focal muscularity is found in pulmonary veins more than arteries, veins may have more focal lymphatic filling than arteries. To test these hypotheses, lung histology and vascular and lymphatic casts of spontaneously hypertensive and normotensive rats were examined. Contracted venous sphincters were found on 108 of 127 veins with lymphatics in the spontaneously hypertensive rats and 5 of 41 in the normotensive rats P<0.01). The spontaneously hypertensive rats had deeper venous contractions and more lymphatic filling around both arteries and veins (P<0.01). In the hypertensive rats, the venous was greater than the arterial lymphatic filling (P<0.01). On the pleural surface, hypertensive rats also had greater lymphatic filling than controls (P<0.01). This anatomic evidence suggests that pulmonary venous sphincters are associated with focal lymphatic filling, and perivascular muscle action might be a component of the pulmonary lymphatic system.