A membrane is a thin sheet or layer of tissue that covers a structure or lines a cavity. Most membranes are formed from epithelium and the connective tissue on which it rests.

Types of Membranes
Serous membranes consist of three components: a layer of simple squamous epithelium called mesothelium, the basement membrane, and a layer of loose connective tissue. Serous membranes line cavities that do not open to the exterior, such as the pericardial, pleural, and peritoneal cavities.
Mucous membranes consist of epithelial cells, their basement membrane, and a thick layer of loose connective tissue called the lamina propria. Mucous membranes line cavities and canals that open to the outside of the body, such as the digestive and respiratory passages.
Synovial membranes consist of modified connective tissue and line joint cavities, such as those found in the knee. Synovial membranes do not contain epithelial tissue.