Membranes of the body are thin sheets of tissue that line cavities, cover surfaces, or separate tissues or organs. Some are composed of both epithelial and connective tissues; others consist of connective tissue only.

Epithelial Membranes

Sheets of epithelial tissue overlying a thin supporting framework of areolar connective tissue form the epithelial membranes in the body. Blood vessels in the connective tissue serve both connective and epithelial tissues. There are three types of epithelial membranes: serous, mucous, and cutaneous membranes (figure 4.24).

Serous membranes, or serosae, line the ventral body cavity and cover most of the internal organs. They secrete serous fluid, a watery fluid, which reduces friction between the membranes. The pleurae, pericardium, and peritoneum are serous membranes. Recall that the epithelium of a serous membrane is a special tissue called mesothelium.

Mucous membranes, or mucosae, line tubes or cavities of organ systems, which have openings to the external environment. Their goblet cells secrete mucus, which coats the surface of the membranes to keep the cells moist and to lubricate their surfaces. The mucus also helps to trap foreign particles and pathogens, which limits their ability to enter the body. The digestive, respiratory, reproductive, and urinary tracts are lined with mucous membranes.

The cutaneous membrane is the skin that covers the body. Unlike other membranes, its free surface is dry, composed of nonliving cells, and exposed to the external environment.

Connective Tissue Membranes

Some specialized membranes are formed only of connective tissue, usually dense irregular connective tissue. Here are four examples:

  1. The meninges are three connective tissue membranes that envelop the brain and spinal cord.
  2. The perichondrium is a connective tissue membrane covering the surfaces of cartilage. It contains blood vessels, which supply cartilage through diffusion.
  3. The periosteum is a connective tissue membrane that covers the surfaces of bones. It contains blood vessels that enter and supply the bone.

    4. Synovial membranes line the cavities of freely movable joints, such as the knee joint. They secrete watery synovial fluid, which reduces friction in the joint.