Body membranes

Mucous membranes (mucosae) line tracts that open to the exterior — digestive, respiratory, urinary and reproductive. They consist of an epithelium (varying by site: stratified squamous in the mouth and oesophagus, simple columnar in the stomach and intestine, pseudostratified ciliated in the airways, urothelium in the bladder) over a loose connective-tissue lamina propria, and in the gut a thin muscularis mucosae. They are kept moist by mucus from goblet cells and glands, and contain abundant lymphoid tissue (MALT).

Serous membranes (serosae) line closed body cavities and cover the organs within them: the pleura (lungs), pericardium (heart) and peritoneum (abdominal and pelvic viscera). Each has a parietal layer lining the cavity wall and a visceral layer on the organ, continuous with each other, enclosing a potential space with a few millilitres of lubricating serous fluid. Histologically they are a simple squamous epithelium — mesothelium — on a thin areolar connective-tissue layer.

Synovial membranes line the cavities of synovial joints, bursae and tendon sheaths. They have no true epithelium: an intimal layer of macrophage-like (type A) and fibroblast-like (type B) synoviocytes — the latter secreting hyaluronan and lubricin into synovial fluid — over a vascular subintima. The cutaneous membrane is the skin — keratinised stratified squamous epidermis over dense irregular dermis — the only dry membrane.