Sidney S. Sobin

Morphometric Basis of the Sheet-Flow Concept of the Pulmonary Alveolar Microcirculation in the Cat

The purpose of this study was to describe quantitatively the geometric organization of the pulmonary alveolar capillary bed. Conventional representation of these vessels as a tight network of circular cylindrical tubes (tube-flow model) was contrasted with a suggested alternative of a continuous sheet bounded on two sides by endothelium held apart by connective tissue and cellular posts (sheet-flow model). A vascular space-tissue ratio, VSTR, is defined as the ratio of vascular lumen volume to the circumscribing tissue volume. The VSTR was obtained by planimetric measurements. Formulas were developed to relate VSTR to the geometric models. Cat lungs were perfused in situ with liquid silicone elastomer at 25 mm Hg, and the liquid elastomer was allowed to catalyze at 15 or 25 mm Hg static pressure while the airway pressure was maintained at 10 cm H2O. After the silicone became solid, the lungs were fixed with formalin-steam, and morphometric analysis was carried out on photographs of frozen sections. The mean values of VSTR of each cat (six lobes) were 0.91 to 0.92, irrespective of intracapillary pressure. These data are consistent with a sheet-flow model. Sheet thickness then becomes a significant variable.

The structure of the pulmonary intervalveolar microvascular sheet

Abstract To understand the morphologic basis of the compliance of the pulmonary interalveolar microvascular sheet, we investigated the histology and ultrastructure of the adult beagle interalveolar septum with special emphasis on the “posts” of the intercapillary interstitium. Special stains for collagen and elastin for light and electron microscopy were used; control preparations included tissues exposed to elastase and hyaluronidase prior to staining. In the interalveolar wall remote from vessels and ducts fine collagen fibers surround the posts, corss over the posts to adjacent capillaries, and pass through the posts from one side of the septum to the other in a curving arrangement. In cross-section the overall post configuration is biconcave and collagen is found at post borders in its course through the post. Elastin is more abundant and coarser than collagen, passes linearly through the posts, and is always intimately associated with collagen including the region of the alveolar-capillary membrane in the respiratory area. Following elastase hydrolysis, non-collageneous elements do not stain. Removal of post ground substance with hyaluronidase disrupts the fibroprotein pattern of connective tissue.

Age related alterations in the response of the pial arterioles to adenosine in the rat

To evaluate the effects of aging on vasoreactivity of pial arterioles to adenosine and barium chloride, an hydraulically intact cranial window preparation was developed in the rat. The microvasculature of anesthetized 3- and 24-month-old Fischer-344 rats was studied during superfusion with artificial cerebrospinal fluid with and without test agents and results determined by videomicroscopy techniques. In both cohorts, the response of pial arterioles to adenosine was both dose and vessel size dependent: arteriolar dilation increased with increasing concentrations of adenosine and at any given concentration the percent increase in diameter was greater in the smaller vessels. During adenosine superfusion the absolute changes and percent increase in vessel caliber were greater in the young rats. Arteriolar vasoconstriction due to barium chloride was vessel size dependent but there were no significant differences in response between young and aged rats. The results indicated an attenuated cerebrovascular response in aged rats to adenosine, but not to barium chloride. This may be due to a difference in the mode of action in these two compounds. Venules did not respond to adenosine at any concentration.

Elasticity of the smallest noncapillary pulmonary blood vessels in the cat.

Abstract We have previously described the compliance behavior of the pulmonary interalveolar capillary blood vessels in the cat and now characterize the compliance coefficient of the pulmonary arterioles and venules less than 50 μm in diameter in that species. These observations are unclassified with respect to arterioles and venules. Our method of silicone rubber microvascular casting under 0 to 23.75 cm H 2 O pressures was used and diameter measurements were made from histological slides. Since the exact position of the vessels in branching hierarchy was not known, we called these vessels juxta-alveolar. The diameters of the smallest noncapillary blood vessels were measured and were analyzed by extreme-value statistics. We obtain a compliance coefficient, α, of 0.274 μm/cm H 2 O over the pressure range measured, and a mean diameter of 14.5 μm for the smallest noncapillary blood vessels, 0 Δp . These vessels are compliant.

Histochemical characterization of the aging microvasculature in the human and other mammalian and non-mammalian vertebrates by the periodic acid-Schiff reaction.

Prior histochemical studies with the periodic acid-Schiff (PAS) reaction have shown altered biochemical composition in a limited part of the microvasculature (MV) in aging in two species of laboratory animals. We therefore studied, with the PAS reaction, all the components of the MV in multiple tissues from various immature, adult and aged mammals, including human, and immature and aged nonmammalian vertebrates. We now demonstrate that there is an altered biochemical composition of capillaries, arterioles and venules in various tissues with aging. These are first detectable somewhat beyond half the life-span in man (greater than 45 years), marmoset (5 years) and dog (8 years) and seen in old fish, reptiles and birds. The capillary wall is increasingly PAS+; in arterioles there are focal PAS+ areas in the media which increase in size and number with age and become hyalinized masses. The non-muscular venules are increasingly PAS+ apparently due to a polysaccharide staining of connective tissue elements. These histochemical changes in the MV with aging are in the extracellular matrix and appear to be a specific manifestation of aging in vertebrates. The consequences of such changes in MV aging may be important physiologically.

Determination of dimensions and geometry in fixed specimens

Abstract Determination of dimensions and geometry of the microvasculature from fixed tissue preparations imposes both technical and conceptual restrictions. The total microvascular bed is the complete bed available for flow; the dynamic bed is the functionally active open bed whose flow results from continuing modulation of the total bed by chemical and/or neural means. Preparation artifacts occur at any stage, especially non-physiologic in vivo preparations, loss of the physiological state during arrest of the microcirculation, and during tissue processing. Retention of the physiological state is accomplished by quick-freezing, chemical surface-fixation, or silicone microvascular casting. The shortcomings and artifacts of each are detailed. The wall of the entire vascular system is a tubular complex of collagen, elastin and reticulin on which smooth muscle is inserted; the fibroproteins and muscle are buried and anchored in a mucopolysaccharide matrix. Aqueous fixatives without quaternary ammonium salts dissolve mucopolysaccharides and result in collagen dispersal and apparently altered structure readily seen in electron micrographs. Dimension and geometry can be determined by current electronic-video techniques which offer increased rapidity of measurement, decrease or elimination of subjective factors in measurement and ease and rapidity in the reduction of analog measurement to numerical values without significant loss of resolution.

The pulmonary arteriole.

Abstract The terminal branches of the mammalian pulmonary arterial tree are called arterioles despite statements that these vessels in the adult have no smooth muscle. We examined these vessels in the cat (8 hr to 11 years) and man (24 hr to 76 years) by light microscopy and special staining to clarify the terminology problem. In both cat and man, the changes in arterioles from the newborn to the aged individual are similar and differ only in time course. At and shortly after birth, fetal pulmonary arterioles are found throughout the lung: Endothelial cells are polygonal and somewhat thickened, the smooth muscle of the media has plump nuclei with blunt ends and fine chromatin; the cytoplasm is pale and periodic acid-Schiff (PAS) negative; internal and external elastic laminae are present. Mixed in with these fetal vessels are a few arterioles which show occasional areas of PAS-positive material and nuclei which are elongating. Occasional arterioles are more strongly PAS positive and show further elongation of nuclei and increased condensation of chromatin. After weeks to months (species variation) almost all arterioles show wall thinning, flattened endothelial cells, smooth muscle cells which occupy most of the media with deeply stained elongated nuclei and scant but persistent cytoplasm which is deeply PAS positive, and persistence of an internal and external elastic layer. Such vessels persist in the aged cat and human. We have called these sequential changes in the pulmonary arteriole maturation changes. The functional correlates of these arteriolar alterations for the adult mammal are discussed.

Ultrastructural changes in the pulmonary arterioles in acute hypoxic pulmonary hypertension in the rat.

This study was planned to obtain ultrastructural details of the early changes in intra-acinar arterioles in acute hypoxic pulmonary hypertension that could lead to understanding the mechanisms in the development of chronic hypoxic hypertension. In the anesthetized rat, using 5-10% normobaric O2, within minutes after hypoxia, there are changes in endothelial cells characteristic of activation: prominence of cell body and protuberance of the nucleus, electron-dense membrane-bound bodies adluminally, increasing pseudopodia of the adluminal cell membrane, edema within (vacuoles) and beneath the endothelial cells with separation of the endothelial cells from the basal lamina. There is activation of platelets and leucocytes in the lumen and accumulation of platelets at the endothelium. Arteriolar wall edema rapidly increases, is excessive within 1 h, with dissection of the basal lamina and wall and cytolysis of wall components. At 24 h edema is reduced, the number of platelets is increased at the endothelium and fibroblasts are newly aligned within the arteriolar wall. At 48 h platelets further increase, a basal lamina develops in fibroblasts termed transitional cells and myofibrils occur subsequently to form smooth muscle. These findings suggest that activation of the endothelial cell is the initial event in a cellular cascade in the arteriolar hypoxic responses with fibroblast-to-smooth muscle transformation, which results in pulmonary arteriolar hyperplasia and vascular remodeling in hypoxic chronic pulmonary hypertension.

Changes in the microvasculature with age

Abstract The microvasculature of various organs of the rat and of the mesentery of the cat were examined for histochemical changes as a function of age, using the periodic acid-Schiff (PAS) reaction. Arterioles, minute arteries, and nonmuscular venules were histochemically unchanged to approximately 17 months of age in the rat and 8 years in the cat. Subsequently, focal areas of PAS-positive material developed in the media of arterioles and small arteries and increased in extent and severity with age. The adventitia of nonmuscular venules normally stains slightly positive due to the mucopolysaccharide coating of collagen fibers. With age this adventitial layer becomes more intensely PAS-positive. In the 26-month-old rat and 19-year-old cat, the media of arterioles and small arteries were extensively hyalinized. Lesions of arteriosclerosis were not present. These observations, in consort with prior observations of others in various mammals, indicate that there is a regular systematic alteration of various elements of the microcirculation with age. A possible relationship between these anatomical changes and tissue exchange is considered.

Electrical conductivity method for estimating right ventricular output and mathematical model

Abstract A constant-rate injection, dilution technique for estimating right ventricular output was employed in normal dogs. Five per cent NaCl solution was injected at the right atrial-inferior vena caval junction and changes in electrical conductivity were measured from pulmonary arterial blood. There was good agreement between right ventricular output estimates with this technique and mean pulmonary arterial flow measured electromagnetically. Evidence was presented that the added indicator cleared the right atrium within one cardiac cycle, under the conditions of the experiment. A mathematical model was constructed to express the relationship of indicator concentration in pulmonary arterial blood to the indicator injection rate, the residual fraction of right ventricular blood, and the heart rate.