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The four main tissue membranes are mucous, serous, cutaneous, and synovial. Mucous membranes line body cavities that open to the exterior, such as the respiratory tract. Serous membranes cover closed body cavities and organs within them. The cutaneous membrane, or skin, is a dry, waterproof covering. Synovial membranes line free-moving joints and secrete lubricating fluid.

Mucous membranes consist of epithelial cells supported by areolar connective tissue called the lamina propria. Mucus and other fluids moisten these membranes, which line passageways including the digestive, respiratory, excretory, and reproductive tracts. This moist covering protects delicate tissues and facilitates the movement of materials through body cavities.

Synovial fluid is a natural lubricant secreted by synoviocytes in synovial membranes. It contains hyaluronan, which traps water to form a viscous fluid that enables bones in freely movable joints to move smoothly against one another with minimal friction. This fluid also exchanges water and nutrients with blood to maintain joint health.

Serous membranes line closed body cavities and organs that do not open to the exterior, such as the thoracic and abdominal cavities. They are composed of mesothelium supported by connective tissue and secrete serous fluid for lubrication. Mucous membranes, by contrast, line cavities and passageways that open to the external environment and are moistened by mucus.

The cutaneous membrane, or skin, consists of a keratinized stratified squamous epithelium called the epidermis attached to thick underlying connective tissue called the dermis. The apical surface is covered with dead, keratinized cells that protect the body from desiccation and pathogens, creating a waterproof barrier against the external environment.

Tissue membranes are classified into connective tissue membranes and epithelial membranes. Connective tissue membranes, such as synovial membranes, are formed solely from connective tissue and line movable joints and encapsulate organs. Epithelial membranes combine epithelium with supporting connective tissue and include mucous, serous, and cutaneous membranes.

Three serous membranes line the thoracic cavity: the two pleura that cover the lungs and the pericardium that covers the heart. These membranes secrete serous fluid that lubricates surfaces and reduces friction between organs and the cavity walls, allowing smooth movement during breathing and heartbeats.