Immuno electron microscopic studies on cells synthesizing elastin

Localizing factors in atherosclerosis

Atherosclerotic vascular disease is the leading cause of death worldwide. Although the entire vascular bed is constantly exposed to the same risk factors, atheromatous lesions present a distinct intra-individual pattern of localization and progression, being consistently more frequent in specific segments of the arterial vascular bed. This peculiar distribution may be related to selective sensitivity of such locations to the influence of risk factors or to histopathological and flow differences, and has relevant clinical implications, as the prognosis of the disease varies according to localization. We here review the theories that have been formulated to explain such preferential locations, as its understanding can be useful to pursue diagnostic screening strategies and focused preventive measures.

Comparison of the endothelial surface and subjacent elastic lamina of anterior descending coronary arteries at the location of atheromatous lesions with internal thoracic arteries of the same subjects: a scanning electron microscopic study

AIM

Current theories fail to explain the localisation of atheromatous lesions or their variable incidence in different arteries of the same subject. The objective of this study was to compare by scanning electron microscopy (SEM) the endothelial surface and the subjacent elastic lamina of human coronary arteries at the location of areas showing infiltration by lipid and cells, with the same components of internal thoracic arteries of the same subjects.

METHODS

The endothelial surface and the subjacent elastic lamina of localised atheromatous areas of 146 anterior descending coronary arteries were compared with the same structural components of the internal thoracic arteries of the same subjects, using SEM, transverse paraffin sections and freeze-fracture. Some arteries were digested with formic acid to destroy the endothelium and interstitial tissue, and reveal the elastin fibre structure of the elastic laminae.

RESULTS

Coronary arteries showed localised defects of the endothelial surface and of the elastin fibre structure of the subjacent elastic membrane, with the presence of lipid and cells in transverse sections of the intima. Internal thoracic arteries showed such changes only rarely, more particularly in older age groups.

CONCLUSIONS

In localised areas of the coronary arteries showing infiltration of the wall by lipid and cells, there were imperfections of the endothelial surface and of the elastin fibre structure of the subjacent elastic lamina. These imperfections were not in general present in the endothelial surface, or subendothelial elastic lamina of the internal thoracic arteries in age groups below 50, and only rarely in older subjects.

Diffusion of gamma globulin into the arterial wall identifies localized entry of lipid and cells in atherosclerosis

BACKGROUND

The localization of atheromatous lesions in vulnerable arteries and their relatively rare occurrence in other arteries of the same subject cannot be explained by current theories of the aetiology of atherosclerosis.

OBJECTIVE

To determine whether abnormal diffusion of gamma globulin into the arterial wall from the lumen will identify defects of barrier function allowing localized entry of lipid and cells in atherosclerosis.

METHODS

Paraffin sections of left anterior descending coronary arteries and corresponding internal thoracic arteries from 80 human subjects aged 1-65 years were stained for gamma globulin by the immunoperoxidase technique. Duplicate sections were stained with orcein to demonstrate the elastin structure.

RESULTS

The barrier function of the luminal surface of the thickened intima was associated with the presence of an elastin lamina beneath the endothelial cells. With advancing age, the coronary arteries exhibited breakdown of this barrier function in localized areas with entry into the arterial wall of gamma globulin, lipid and cells. This was rare in the internal thoracic artery.

CONCLUSION

Lack of continuity or incomplete formation of this sub-endothelial lamina, which was seen particularly in the coronary artery, was associated with localized entry into the arterial wall of gamma globulin, lipid and cells from the circulating blood and with the development of atheromatous lesions.

The importance of a substantial elastic lamina subjacent to the endothelium in limiting the progression of atherosclerotic changes

This study examines the hypothesis that progressive intimal thickening and atherosclerosis in the larger pulsatile arteries arise from failure to maintain, subjacent to the endothelial cells, a substantial elastin membrane, a component which has been shown to be of special structural significance. The internal thoracic arteries of 293 subjects of all ages up to 60 years were compared histologically with the anterior descending coronary arteries of the same individuals by light- and electronmicroscopy and immunoperoxidase staining for macromolecules. The internal thoracic arteries usually developed a new robust reduplicated internal elastic lamina at an early age, no further intimal thickening, and no significant entry of lipid or cells to the intima. The coronary arteries showed areas of rapid intimal thickening with poor and incomplete reduplicated internal elastic laminae, entry of lipid, macrophages, and other cells to the intima. The reduplicated internal elastic laminae appeared to be formed primarily by the endothelial cells themselves. An elastin membrane subjacent to the endothelial cells appears to be essential. It provides a secure attachment for the cells and a barrier to the entry of macromolecules and cells to the intima. Its absence is associated with progressive intimal thickening and atherosclerosis.

Stellate cells storing retinol in the liver of adult lamprey, Lampetra japonica

Distribution, localization and fine structure of the stellate cells in the liver of lamprey, Lampetra japonica, were studied during the spawning migration by use of Kupffer's gold-chloride method, fluorescence microscopy for vitamin A (retinol) and electron microscopy. The stellate cells in the lamprey liver differ in some of their properties from those in mammalian livers. Stellate cells which store abundant retinol in lipid droplets, occur not only in the hepatic parenchyma, but also in the dense perivascular and capsular connective tissue of the liver and in the interstitium of pancreatic tissue. In the hepatic parenchyma these cells are located perisinusoidally or along thick bundles of collagen fibrils. The stellate cells display a number of large retinol-containing lipid droplets, granular endoplasmic reticulum, tubular structures, dense bodies. Golgi complex, microtubules, and microfilaments. In the space of Disse, the stellate cells and extracellular fibrilar components such as collagen fibrils and microfibrils (11-12 nm in diameter) are intervened between the two layers of basal laminae. Differentiation and possible functions of the stellate cells in the lamprey liver are discussed.

The secretion of tropoelastin by chick-embryo artery cells

Chymotryptic fingerprint analyses of tropoelastin a and tropoelastin b demonstrated a very close relationship between these two polypeptides synthesized in a cell-free system under the direction of chick-embryo polyribosomal mRNA. A similar study on tropoelastin polypeptides extracted in their hydroxylated and under-hydroxylated forms from artery cells incubated with [3H]valine in the absence and presence of alpha alpha'-bipyridine or 3,4-dehydroproline confirmed this close relationship and suggested that tropoelastins a and b are likely to be the products of a single gene. Pulse-chase experiments in which the synthesis and secretion of tropoelastin by artery cells were monitored demonstrated that, after a pulse with [3H]proline, the polypeptides rapidly appeared in the medium and the half-time of tropoelastin secretion was approx. 30 min. Further pulse-chase studies, in which [3H]tropoelastin contents of subcellular fractions were determined, showed that rough and smooth microsomal fractions contained maximal amounts of tropoelastin at different times. The quantity of tropoelastin in the smooth-microsomal fraction was always only a small proportion of that in the rough-microsomal fraction, suggesting rapid translocation of the polypeptides to the plasma membrane. Incubation of the cells with 0.1 mM-colchicine did not markedly alter the rate of secretion or the distribution of tropoelastin between the subcellular fractions, whereas when 1 microM-monensin was included in the incubations the polypeptides were retained in the rough microsomal fraction. The results are consistent with the proposal that tropoelastin may follow a pathway of secretion from rough endoplasmic reticulum to the plasma membrane via secretory vesicles.

Elastin biosynthesis in chick-embryo arteries. Studies on the intracellular site of synthesis of tropoelastin

Electrophoretic analyses of the products of cell-free translation of elastin mRNA isolated from 17-day chick-embryo thoracic arteries have demonstrated that the elastin mRNA codes for polypeptides that are slightly larger than the cellular tropoelastin polypeptides synthesized and secreted by matrix-free artery cells. Pulse-chase experiments with cells labelled with [3H]proline established that newly synthesized tropoelastin polypeptides were associated solely with membrane-bound particulate fractions. Cell-free translation of membrane-bound and free polyribosomes isolated from artery cells revealed that the tropoelastin mRNA was associated predominantly with the membrane-bound fraction. When rough-microsomal fractions, isolated from cells labelled with [3H]proline for 10 min, were treated with proteinases in the presence and in the absence of detergent, the nascent tropoelastin polypeptides were shown to be susceptible to proteolysis only when the integrity of the membranes was destroyed by detergent treatment. In similar experiments tropoelastin polypeptides synthesized by membrane-bound polyribosomes in the nuclease-treated reticulocyte lysate were also resistant to the proteolytic-enzyme treatment. The results suggest that tropoelastin polypeptides are synthesized on membrane-bound polyribosomes and discharged into the lumen of the endoplasmic reticulum with co-translational removal of a signal peptide.

Endogenous mammalian lectin localized extracellularly in lung elastic fibers

An affinity-purified antibody preparation raised against a beta-galactoside-binding lectin from bovine lung was used to localize a similar lectin in rat lung by immunofluorescence and by electron microscopy after on-grid staining visualized with colloidal gold conjugated second antibody. The endogenous mammalian lectin was found in smooth muscle cells and squamous alveolar epithelial (type I) cells and was concentrated extracellularly in elastic fibers of pulmonary parenchyma and blood vessels. The extracellular localization of this lectin suggests that it, like others, functions by interaction with extracellular glycoconjugates.

Secretion of elastin in the embryonic chick aorta as visualized by immunoelectron microscopy

Recently, significant advances have been made in characterizing the pathway of elastin biosynthesis from the biochemical point of view and a 70,000 dalton protein, designated tropoelastin, appears to be the primary translation product and soluble intermediate of the insoluble elastin. However, relatively little is known concerning the intracellular secretory pathway of tropoelastin. We previously developed an electron microscopic technique using elastin-specific antibody and ferritin-conjugated secondary antibody to identify intracellular elastin and to identify, provisionally, intracellular vesicles containing elastin ( Damiano et al., Conn. Tiss . Res. 8: 185-188, 1981). However, the method did not permit localization of elastin in other intracellular organelles. We now describe an improved post-embedding technique using the peroxidase-antiperoxidase method to detect the primary elastin antibody and have localized elastin in both the endothelial and medial cells of the embryonic chick aorta. Specific staining was visualized in the cisternae of the endoplasmic reticulum, in the Golgi apparatus, and in vesicles forming on the trans side of the Golgi. Some of these smaller vesicles appeared to fuse, forming larger vesicles which may have a storage function. Both types of vesicles were seen fusing with the cell plasma membrane, suggesting that elastin is secreted by an exocytotic process. These results suggest that tropoelastin follows the classical pathway for protein secretion.

Ultrastructure of mucocartilage in the larval anadromous sea lamprey, Petromyzon marinus L

The fine structure of mucocartilage, a tissue unique to larval lampreys, was examined in Petromyzon marinus L. This tissue is surrounded by a perichondrium of vascularized, dense connective tissue composed of fibroblasts, collagen fibrils, and elastic-like microfibrils, but it is avascular itself and consists of elastic-like microfibrils, ground substance, and a few diffusely scattered fibroblasts. Fibroblasts possess rough endoplasmic reticulum, may free ribosomes, a well-developed Golgi apparatus, a tubulo-vesicular network, and a number of secondary lysosomes containing crystalline material. The appearance of the organelles suggests the involvement of the cell in the synthesis and secretion of the ground substance and microfibrils. Tubular microfibrils, 11-13 nm in diameter, comprise the major portion of the matrix, and they are similar to those described in developing mammalian elastic tissue (Ross and Bornstein, 1969). The retention of the microfibrils may represent either a primitive form of elastic fiber in this "primitive" vertebrate or reflect the larval condition of the lampreys under examination. Scattered spherical to polyhedral-shaped mitrix granules and intergranular filaments make up the remainder of the matrix. It was concluded that mucocartilage in larval lampreys is not a conventional type of vertebrate connective tissue.

Elastin formation in heterotopic transplants of isolated arterial smooth muscle cells

Smooth muscle cells were isolated from the aorta of 5 day old rats by collagenase digestion and injected intramuscularly into animals of the same strain, where the cells reconstituted an elastic tissue with many similarities to that found in the media of the intact aortic wall. The transplants consisted of partly aligned smooth muscle cells surrounded by an extracellular matrix of microfibrils, elastic fibers, bundles of collagen fibrils, and small granules believed to represent proteoglycans. The production of extracellular matrix was much more efficient than in cultures of arterial smooth muscle cells. This cell transplantation system may be valuable in elucidating the mechanisms of normal growth and development of the arterial wall as well as the pathogenesis of various pathological processes.