No abstract

In humans, the greater omentum is a fatty peritoneal fold that extends from the greater curvature of the stomach to cover most abdominal organs. It performs many functions, which include acting as a reservoir of resident peritoneal inflammatory cells, a storage site for lipid, and a regulator of fluid exchange in and out of the peritoneal cavity. Most importantly, the omentum readily adheres to areas of inflammation and peritoneal damage, often leading to adhesion formation. Despite its clinical importance, the omentum remains an understudied organ, and discrepancies exist as to its exact morphology. This study uses a combination of phase contrast microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to elucidate the structure of the greater omentum of both human and mouse and determine whether it possesses a typical surface mesothelial cell lining similar to other serosa. Results indicated that both human and murine omenta were of similar structure and composed of two distinct types of tissue, one adipose-rich and the other translucent and membranous. The adipose-rich regions were well-vascularised and covered by a continuous mesothelial cell layer except at the sites of milky spots. In contrast, translucent areas were poorly vascularised and contained numerous fenestrations of varying size. The possible function and developmental origin of these gaps is unclear; however, their role in promoting omental adhesion formation and in the successful use of omental graft material is discussed.

All adhesions contained functional blood vessels and most showed evidence of cell proliferation. The presence of vascular endothelial growth factor A and its receptor in human adhesions suggests ongoing angiogenic activity. This study demonstrates that adhesions are vascular structures with evidence of tissue remodeling and suggests potential for new prevention strategies involving antiangiogenic therapies.

Although empyema affects more than 65,000 people each year in the United States and in the United Kingdom, there are limited data on the pathogenesis of pleural infection. We investigated the pathogenesis of empyema using animal and cell culture models of Streptococcus pneumoniae infection. The pathological processes during the development of empyema associated with murine pneumonia due to S. pneumoniae (strain D39) were investigated. Lungs were examined using histology, and pleural fluid and blood bacterial colony-forming units, cytokine levels, and cellular infiltrate were determined over time. Bacterial migration across mesothelial monolayers was investigated using cell culture techniques, flow cytometry, and confocal microscopy. After intranasal inoculation with 10 7 S. pneumoniae D39 strain, mice developed pneumonia associated with rapid bacterial invasion of the pleural space; raised intrapleural IL-8, VEGF, MCP-1, and TNF-a levels; and caused significant intrapleural neutrophilia followed by the development of fibrinous pleural adhesions. Bacterial clearance from the pleural space was poor, and in vitro assays demonstrated that S. pneumoniae crossed mesothelial layers by translocation through cells rather than by a paracellular route. This study describes key events during the development of S. pneumoniae empyema using a novel murine model of pneumonia-associated empyema that closely mimics human disease. The model allows for future assessment of molecular mechanisms involved in the development of empyema and evaluation of potential new therapies. The data suggest that transmigration of bacteria through mesothelial cells could be important in empyema development. Furthermore, upon entry the pleural cavity offers a protected compartment for the bacteria.Keywords: Streptoccocus pneumoniae; empyema; animal models; translocation; mesothelial cells Pleural infection is a global problem that affects over 65,000 patients each year in the United States and in the United Kingdom and is associated with significant morbidity and mortality (1-5). Empyema is defined by the presence of bacteria or pus in the pleural cavity and usually develops as a complication of pneumonia. Important questions regarding the disease pathogenesis remain unanswered, which may in part account for the lack of recent therapeutic advances. A key limiting factor in this area of research is that there is no suitable preclinical animal model of empyema. Studying the evolution of pleuropulmonary infection ideally involves repeated sampling of the lung and pleural cavity, which is not feasible in patients with pneumonia, and the animal models used to date have significant limitations (6). Published in vivo models rely on intrapleural delivery of bacteria to create localized pleural infection without concurrent pneumonia and many use species-specific microorganisms, such as Pasteurella multocida (7-9), with limited relevance to humans. These models require concomitant administration of systemic antibiotics to control infection and fatalities (8), ...

Omental adhesions are not static scar tissue as traditionally thought, but undergo active adipose tissue remodelling over-time.