The pleural curtain of the camel (Camelus dromedarius).

The visceral pleura of the camel (Camelus dromedarius) possesses a fibrous curtain of pleural threads or extensions along its basal margins, which extends into the pleural cavity of the costophrenic recesses. These threads are lined by mesothelium and have a core or stroma, which is largely collagenous. Small threads are avascular and nearly acellular. In larger proximal threads, blood vessels in the stroma are often arranged in a branching network, with irregular endothelia surrounded by several incomplete basal laminae. Lymphocytes and other inflammatory cell types aggregate in the stroma near blood vessels. The threads are lined by typical mesothelium except in patches close to the main pleural surface. These patches consist of layers of loosely applied cells with numerous cellular processes and features suggestive of phagocytosis. The position of the pleural curtain in the costophrenic recess and the presence of possibly phagocytotic cells suggest that the pleural curtain stirs, samples, and cleans the pleural fluid. The pleural curtain appears to be a feature of camelids and has also been seen in giraffes.