Serosal membranes (pleura, pericardium, peritoneum). Normal structure, development and experimental pathology.

This monograph offers a comprehensive review of the present knowledge of the structure of the serosal coverings of the pleural, pericardial, and peritoneal cavities in humans and laboratory animals. The authors provide data from their own research--with transmission and scanning electron microscopy--on the structure of the main components of the serosal membranes: mesothelial cells, underlying basal lamina, and submesothelial connective tissue layer. Two main types of mesothelial cells (flat and cubic) are distinguished and their distribution on the parietal serosal sheets and on the visceral coverings of various organs is described. The openings between mesothelial cells (stomata) and their relations with lymphatic lacunae are described thoroughly. Special reference is made to the serosal accumulations of lymphoid tissue (milky spots). The transcellular and intercellular transport to and from serosal cavities is studied by means of horseradish peroxidase tracing experiments. The prenatal and postnatal developmental studies are focused on human and rat pleura. The alterations of serosal membranes after experimental hemothorax, pneumonectomy, and peritonitis caused by Pseudomonas aeruginosa application suggest the existence of early, reversible, and late, definite periods.

Mesothelial lamellar bodies in norm and experimental conditions. Transmission and scanning electron microscopic observations on the peritoneum, pleura and pericardium.

The ultrastructural characteristics of the mesothelial intracellular and extracellular lamellar bodies (LB) in norm, and especially in pathological conditions are still unknown. After routine fixation procedure, material from organs in the three serous cavities of Wistar rats, as control group were compared with animals following experimental hemothorax (EH) and experimental peritonitis (EP), using transmission and scanning electron microscopy (TEM, SEM). Different membrane-bound profiles, short strip-like structures and single LB characterize the control group. Five days after EH small groups of LB were observed. Single balloon-like profiles, numerous particles and larger groups of LB with wide varieties in size, form and membrane structure characterize the 8th day after EH. Thin and concentric membranes build single LB the 5th day of EP. Eight days after the same treatment, groups of LB, complex structures with several multilamellar centers, strip-like structures with rough granulo-filamentous material and uninterrupted covering over wide areas were observed. We conclude that constant components in the untreated rats are different membrane profiles, as initial lamellar formations and single typical LB with predominant intracellular position. The preferable sites for exocytosis of LB are the intercellular spaces. The present data demonstrate simultaneous findings of the two main groups of formations by using routine fixation in the control group and in the treated animals. The first represent LB as well as other round profiles, observed by TEM and particles, observed by SEM, which are widespread and probably have a basic role. The second ones are strip-like structures and its SEM-equivalents, i.e. uninterrupted covering. TEM-profiles and their SEM-images show significant organ differences and in some cases lack of correspondence between both electron microscopic techniques in the same treatment. Significant enlargement of the number of LB and the length of the strip-like structures characterize the mesothelium after pathologic conditions. LB with considerable differences in the structure and larger particles over the microvillous border predominate after EH. Strip-like profiles, continuous covering and smaller groups of LB with similar morphology and preferable submesothelial position characterize the mesothelium after EP.

Postinflammatory changes of the diaphragmatic stomata.

Numerous investigations concerning the fine morphology of diaphragmatic stomata have been performed, but its ultrastructural changes in experimental conditions remain unclear. The present study demonstrates the peritoneal side of the diaphragm in adult Wistar rats by transmission electron microscopy. Ten experimental animals were observed 5 and 8 days after Pseudomonas aeuriginosa instillation (PI) into the peritoneal cavity. A control group of 6 rats showed flat mesothelial covering on basal lamina (BL) and connective tissue layer, as well as cubic mesothelial cells, single stomata over underlying lymphatic lacunae (LL). Five days after PI the mesothelial cells had more numerous microvilli, microvesicles, vacuoles, lysosomes and a lesser number of specialized contacts. The multiplication of the extravasal cells and larger intercellular spaces lead to thickenings of the connective tissue around LL. LL were larger and located in close proximity of the mesothelium. Intercellular spaces in the mesothelial layer and different types of contacts between mesothelial cells and endothelial protrusions of LL (with common BL or without BL) were encountered. Eight days after PI the mesothelium, endothelium of LL, their BL and surrounding connective tissue were interrupted and structurally modified to form typical new channels--stomata. The larger portion of the channels were formed of mesothelial cells, while the endothelial cells participated in the submesothelial part. LL were more numerous than in the previous period, and were arranged in groups. LL increased their vertical (50.59 microm) and horizontal (155.57 microm) diameter, as compared with control animals (respectively 12.37 microm and 74.08 microm). Neighbouring LL were separated by thin or thick septae. Peristomatal mesothelial cells or more rarely endothelium formed valve- or bridge-like structures. Valves on the opposite side of LL were observed. Groups of electron-dense bodies characterized some tall endothelial cells of LL. Cubic mesothelium, endothelium of the LL, both BL, the cell connections that formed new stomata, LL and surrounding connective tissue underwent rapid and parallel changes after PI. Among these elements of the lymphatic regions mentioned above, the mesothelium and endothelium of LL had a main role in experimental conditions.

Electronmicroscopic observations on the visceral and parietal rat's pleura after contralateral pneumonectomy.

The mechanism of compensatory growth and healing of the pleura remains unresolved. Contralateral visceral and parietal (diaphragmatic and costal) pleura were investigated by transmission electron microscopy, following an experimental pneumonectomy (EP). Fifteen young-adult Wistar rats were divided into three groups and with survival times of 1, 5 and 8 days respectively after EP. Three sham-operated (thoracic cavity opened and closed) and three unoperated rats served as controls. One day following EP the superficial mesothelial cells have more microvilli and microvesicles, but a lower number of specialized contacts. Multiplication of extravasal cells leads to an increase of the thickness of the layer over the basal lamina and of the submesothelial layer. Five days after EP the superficial cells show a stratified arrangement in longer sectors of both pleural sheets. Along with typical mesothelial cells there are three new populations of cells: (1) with an abundant granular endoplasmic reticulum and secretory granules, (2) with fibroblast-like characteristics and (3) with a more extensive lysosomal system. The submesothelial layer is thickened due to newly formed blood vessels and collagen bundles. Eight days after EP the mesothelial cells build multi-row arrangement sectors and surround intercellular dilatations covered with microvilli. 'Activated' high mesothelial cells characterize the monolayer sectors. The submesothelial layer remains thicker due to larger collagen bundles and elastic fibers. The changes in the mesothelium and in the connective tissue layer suggest the existence of two periods. The first one is characterized by different mesothelial cell populations, new vasculogenesis and starting of fibrillogenesis. In the second period there are 'activated' mesothelial cells, pleural villi, groups of lymphatic lacunae and significant fibrillogenesis.

Ultrastructural observations on the human visceral pleura.

Visceral pleura with underlying lung parenchyma was obtained following thoracotomy carried out in 21 patients of both sexes, aged 27-56 years. The samples were taken from unaltered lung regions, and were studied using transmission electron microscopy (TEM). The main components of the visceral pleural tissue complex, mesothelium, basal lamina (BL), elastic membrane (EM), and submesothelial connective tissue layer, are described, with special reference to their relations with the corresponding structures of the lung interstitium. These elements have significant quantitative regional diversities but can be recognized in all investigated cases, and show constant structural uniformity. Two basic mesothelial cell types, squamous (flat) and cuboidal (high) are discerned, based on the differences in size, shape, cell organelles and membrane specialization. The cuboidal mesothelial cells are especially characteristic for the visceral pleura. In addition, an intermediate mesothelial cell that apparently represents a stage in the development of a squamous to a cubiodal mesothelial cell is fairly common. For more rarely, degenerative mesothelial cells are found.

The serous membranes in the cat. Electron microscopic observations.

Visceral and parietal pleura, peritoneum and pericardium of 26 adult cats were studied by means of transmission electron microscopy. The main components of the three serous membranes follow a general plan: mesothelium, basal lamina (BL) and submesothelial connective tissue layer. They show significant diversities in both sheets of the three serous membranes in the different organs and regions. The elastic membrane under the BL is an obligatory component of the visceral pleura. Two basic cell types - high and flat, as well as intermediate and degenerative cell forms are described in the mesothelial layer. The high cells are especially characteristic of the visceral sheets, while the flat cells predominate in the parietal sheets. The involvement of the mesothelium in the homeostasis in the cavities is discussed. A detailed characteristic of the BL of both sheets and its variation in individual organs is presented. Varieties of cells, collagen and elastic fibers, blood and lymph capillaries of the connective tissue layers of the visceral and parietal sheets are described with special reference to their relation to different underlying tissues. An attempt to find a structure-functional correlation of these observations is made. The transport capability of the pleura and peritoneum is investigated by the intrapleural and intraperitoneal application of horseradish peroxidase (HRP).

Development of the human fetal visceral pleura. An ultrastructural study.

The visceral pleura of human fetuses aged from 9 to 36 weeks of gestation was studied by means of transmission electron microscopy. The main components of the visceral pleura (mesothelium, basal lamina and submesothelial connective tissue layer) are formed in the fetal period. They develop asynchronously in different pleural areas, and do not reach maturity. Fetal differentiation of the lung pleura can be divided in two stages--early (until 17 gestation week) and late stage--up to birth. The high mesothelial cells appear later than the flat cells, but the first type predominates in the final covering layer during the period investigated. The most significant developmental phenomena of the mesothelium involve membranous differentiation (the microvillous covering, vesicular system and intercellular contacts). The different transport and secretory potentials of the mesothelial cells during the various prenatal periods are discussed. The mode of development of the basal lamina suggests its mesothelial origin. The elastic membrane appears during the late stage of fetal life. The components of the submesothelial connective tissue layer (fibroblasts, collagen and elastic fibres, blood and lymph vessels) undergo several phases of differentiation.

A combined electron microscopic investigation of the peritoneal mesothelium in the rat.

Visceral and parietal peritoneum of adult Wistar rats was studied by means of transmission and scanning electron microscopy. The structure of the peritoneum follows a general plan: mesothelium, basal lamina, and submesothelial connective tissue layer. Two basic types of mesothelial cells are described: flat and high (cubic), as well as transitional forms. A regularity of distribution of these cells in the visceral and parietal peritoneal sheets, and in the cover of individual organs and regions is described. A functional characterization of the different types of mesothelial cells is attempted, based on the differences of their cytoplasmic organization. The involvement of the mesothelium in the homeostasis in the peritoneal cavity is discussed.