Epithelial membranes are comprised of epithelial tissue and connective tissue. These membranes serve as linings and covering for various body structures, and they also form glands. Two primary types of epithelial membranes exist: serous membranes and mucous membranes. Epithelial tissue has several uses, including absorption of materials, protection from foreign sources, providing a sense of touch, secreting bodily fluids, and transporting materials between cells.
The structure of epithelial membranes is basic. A top layer of epithelial tissue is joined to underlying connective tissue by a basement membrane. This middle layer is created by proteins and carbohydrates secreted by both epithelial and connective tissue. The connective tissue layer houses nerves and blood vessels that supply the upper layers, and it also binds the membrane to the bodily structures. When epithelial cells fold into the connective tissue, glands are formed.
The outer portion of epithelial membranes — epithelial tissue — may be composed of a single layer of cells known as simple epithelium. Multiple layers of cells constitute stratified epithelium. The former type of epithelium is named by the shape of its nuclei, such as cuboidal or columnar. Simple epithelial cells are found lining structures such as blood vessels, the lungs, the reproductive organs, and various glands. Some simple cells contain sense receptors that aid in touch, while others, like pseudostratified epithelial cells, have short hairs called cilia that aid in moving body materials.
Stratified epithelium, on the other hand, lines structures that must withstand greater rigor, like the esophagus, the urinary tract, and the skin. Two further types of stratified epithelial classifications are keratinized and transitional. Keratinized epithelium is unique due to its tough, waterproof exterior comprised of dead cells. In contrast, stretching capabilities characterize transitional epithelium. All three tissue cell layers create epithelium: endoderm, mesoderm, and ectoderm.
Mucous membranes, also called mucosae, are one major type of epithelial membrane. They line portions of the body that have an outside opening. Their primary function is absorption and secretion. Therefore, the digestive tract, the excretory tract, the reproductive tract, and the respiratory tract all possess mucous membrane coverings.
The other main epithelial membrane type consists of serous membranes. These structures derive their name from the serous fluid they produce. The fluid is secreted by epithelial tissue and protects organs in close proximity from injuring each other. As such, serous membranes line and protect internal organs and the organ cavities that contain them. The pleura, which lines the thoracic cavity, is one example of a serous membrane.