Localization of membrane-associated sialomucin on the free surface of mesothelial cells of the pleura, pericardium, and peritoneum

Abstract

Strong anionic sites, as recognized by deposition of cationic colloidal iron even at pH 1.5, were distributed on the free surfaces of the mesothelia of the mouse pleura, pericardium, and peritoneum. Methylation inhibited colloidal iron staining on the surface, and successive saponification restored it. Digestion with neuraminidase or hydrolysis of sialic acid with H2SO4 erased the colloidal iron staining. Lectin Limax flavus agglutinin (LFA), which is specific for sialic acid, labeled the free surface of the mesothelium. All these findings strongly suggested that the surface substance contained sialic acid. Moreover, prior treatment with LFA inhibited the mesothelial surface stain with colloidal iron. In transmission electron microscopy, the colloidal iron (pH 7.3)-stained substance took the shape of fine strands of 50-300 nm in length. These characteristics of the substance on the mesothelial surface correspond well with biochemical properties of membrane-associated sialomucin, whose strong and abundant negative charges produce repulsive forces between facing serosal surfaces. This may contribute to prevent serosal adhesion and to reduce friction during movements of organs.

Pleural Lubrication

“…This hystochemical finding fits with the morphological findings of Wang [12,30] and Ohtsuka et al [32] showing that sialomucin occurs on the surface of the mesothelial glycocalyx. On the other hand, it seems to contrast with the lack of increase in µ of the pleural mesothelium after neuraminidase treatment, despite the fact that this enzyme cleaves most of the sialic acid of the sialomucin occurring in the glycocalyx of pleural mesothelium of diaphragm specimens.…”

“…Their presence has been later suggested also on pleura, peritoneal and pericardial mesothelium [12,[30][31][32][33]. Actually, by transmission electron microscopy, Wang [12] and Ohtsuka [32] found that colloidal iron stain of mesothelial glycocalyx is removed by neuraminidase treatment, which cleaves sialic acid from sialomucin.…”

“…This macromolecule might also occur in the coat of the mesothelial surface, because it has been found in small concentration both in the pericardial [39], and pleural [16] liquid of rabbits. However, Ohtsuka et al [32], like Wang [12], found that while neuraminidase removed colloidal iron staining of their mesothelium specimens, hyaluronidase did not affect this staining. Moreover, D'Angelo et al [8], showed that addition of hyaluronan (2.5 mg/mL) in Ringer solution after mesothelial blotting with filter paper lowers µ of rabbit pleural mesothelium more than the addition of Ringer solution; this effect is such that after the addition of this macromolecule µ value is brought essentially back to its pre-blotting one [26].…”

“…Actually, by transmission electron microscopy, Wang [12] and Ohtsuka [32] found that colloidal iron stain of mesothelial glycocalyx is removed by neuraminidase treatment, which cleaves sialic acid from sialomucin. In line with the earlier suggestion of Andrews and Porter [11], that the polyanionic nature of the surface of the mesothelial glycocalyx may protect from friction, Ohtsuka et al [32] proposed that the negative charges of sialomucin produce repulsive forces between facing serosal surfaces, and may, therefore reduce friction; nevertheless, little information seems available on the lubricant proprieties of sialomucins on mesothelial surfaces as well as on the spatial and functional relationships between mucopolysaccharides coat and phospholipids layers in these sites.…”

Electron Microscopic Cytochemical Studies of Anionic Sites in the Rat Spleen.

“…In view of these characteristic morphological features of the mammalian spleen, several electron microscopic studies have been conducted on the splenic blood vessels, especially with regard to their distribution patterns and fine structures [1,3,8,14]. Cell-surface glycoconjugates in various tissues are known to play essential roles in the maintenance of the peritoneal cavity through prevention of peritoneal adhesion [15,16], control of cell adhesion and migration [4,13] and cell surface adhesion receptors [26]. Glycoconjugates in the endothelium lining the blood vessels in the rat spleen have been studied by electron microscopic cytochemical analyses [22,24].…”