8.23: Tissue Membranes
A tissue membrane is a thin layer of cells that covers the outside of the body, the organs, internal passageways that lead to the exterior of the body, and the lining of the moveable joint cavities. There are two basic types of tissue membranes— connective tissue and epithelial membranes.
Connective Tissue Membranes
The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints. A synovial membrane is a type of connective tissue membrane that lines the cavity of freely movable joints like those of the shoulder, elbow, and knee. Fibroblast-like synoviocytes in the inner layer of the synovial membrane release hyaluronan, also called hyaluronic acid, into the joint cavity. The hyaluronan effectively traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body fluids.
Epithelial Tissue Membranes
The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, our skin. The mucous membrane is also a composite of connective and epithelial tissues. These epithelial membranes line the body cavities and hollow passageways that open to the external environment, and include the digestive, respiratory, excretory, and reproductive tracts. Mucus, produced by the epithelial exocrine glands, covers the epithelial layer. The underlying connective tissue, called the lamina propria, helps support the fragile epithelial layer. A serous membrane is an epithelial membrane composed of mesodermally-derived epithelium called the mesothelium that is supported by connective tissue. These membranes line the coelomic cavities of the body, which means that they cover the organs located within the cavities that do not open to the outside. Serous fluid secreted by the cells of the thin squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Three serous membranes line the thoracic cavity; the two pleura that cover the lungs, and the pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs. The skin, an epithelial membrane, is also called the cutaneous membrane. It is a stratified squamous epithelial membrane resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.1: Types of Tissues
