49.2 Epithelial tissue forms membranes and glands Characteristics of Epithelial Tissue capillaries, for example, where the thin, delicate nature of these membranes permits the rapid movement of molecules An epithelial membrane, or epithelium, covers every sur-(such as the diffusion of gases). A simple cuboidal epithelium face of the vertebrate body. Epithelial membranes are de- lines the small ducts of some glands, and a simple columna rived from all three germ layers. The epidermis, derived epithelium is found in the airways of the respiratory tract from ectoderm, constitutes the outer portion of the skin. and in the gastrointestinal The inner surface of the digestive tract is lined by an ep- terspersed among the columnar epithelial cells are numer ithelium derived from endoderm, and the inner surfaces of ous goblet cells, specialized to secrete mucus. The columnar the body cavities are lined with an epithelium derived from epithelial cells of the respiratory airways contain cilia on their apical surface(the surface facing the lumen, or cavity), Because all body surfaces are covered by epithelial mem- which move mucus toward the throat. In the small intes- branes, a substance must pass through an epithelium in tine, the apical surface of the columnar epithelial cells form order to enter or leave the body. Epithelial membranes fingerlike projections called microvilli, that increase the sur- thus provide a barrier that can impede the passage of some face area for the absorption of food. substances while facilitating the passage of others. For Stratified epithelial membranes are several cell layers land-dwelling vertebrates, the relative impermeability of thick and are named according to the features of their up- the surface epithelium(the epidermis) to water offers es- permost layers. For example, the epidermis is a stratified sential protection from dehydration and from airborne squamous epithelium. In terrestrial vertebrates it is further athogens(disease-causing organisms). On the other hand, characterized as a keratinized epithelium, because its upper the epithelial lining of the digestive tract must allow selec- layer consists of dead squamous cells and filled with a tive entry of the products of digestion while providing a water-resistant protein called keratin. The deposition of barrier to toxic substances, and the epithelium of the lungs keratin in the skin can be increased in response to abrasion, must allow for the rapid diffusion of gases producing calluses. The water-resistant property of keratin Some epithelia become modified in the course of em- is evident when the skin is compared with the red portion bryonic development into glands, which are specialized for of the lips, which can easily become dried and chapped be- secretion. A characteristic of all epithelia is that the cells cause it is covered by a nonkeratinized, stratified squamous are tightly bound together, with very little space between epithelium them. As a consequence, blood vessels cannot be interposed The glands of vertebrates are derived from invaginated between adjacent epithelial cells. Therefore, nutrients and epithelium. In exocrine glands, the connection between oxygen must diffuse to the epithelial cells from blood ves- the gland and the epithelial membrane is maintained as a sels in nearby tissues. This places a limit on the thickness of duct. The duct channels the product of the gland to the epithelial membranes; most are only one or a few cell layers surface of the epithelial membrane and thus to the external environment(or to an interior compartment that opens Epithelium possesses remarkable regenerative powers, the exterior, such as the digestive tract). Examples of constantly replacing its cells throughout the life of the ani- ocrine glands include sweat and sebaceous (oil) glands, mal. For example, the liver, a gland formed from epithelial which secrete to the external surface of the skin, and acces- tissue, can readily regenerate after substantial portions of it sory digestive glands such as the salivary glands, liver, and have been surgically removed. The epidermis is renewed pancreas, which secrete to the surface of the epithelium lin- every two weeks, and the epithelium inside the stomach is ing the digestive tract. replaced every two to three days. Endocrine glands are ductless glands; their connections There are two general classes of epithelial membranes: with the epithelium from which they were derived are lost simple and stratified. These classes are further subdivided during development. Therefore their secretions, called into squamous, cuboidal, and columnar, based upon the hormones, are not channeled onto an epithelial membrane shape of the cells(table 49.2). Squamous cells are flat, Instead, hormones enter blood capillaries and thus stay cuboidal cells are about as thick as they are tall, and colum- within the body endocrine glands are discussed in more nar cells are taller than they are wide detail in chapter 56 ypes of Epithelial Tissues Epithelial tissues include membranes that cover all Simple epithelial membranes are one cell layer thick. a body surfaces and glands. The epidermis of the skin is simple, squamous epithelium is composed of squamous ep an epithelial membrane specialized for protection, thelial cells that have an irregular, flattened shape with ta- whereas membranes that cover the surfaces of hollow pered edges. Such membranes line the lungs and ble organs are often specialized for transport. 988 Part XIlI Animal Form and FunctionCharacteristics of Epithelial Tissue An epithelial membrane, or epithelium, covers every sur￾face of the vertebrate body. Epithelial membranes are de￾rived from all three germ layers. The epidermis, derived from ectoderm, constitutes the outer portion of the skin. The inner surface of the digestive tract is lined by an ep￾ithelium derived from endoderm, and the inner surfaces of the body cavities are lined with an epithelium derived from mesoderm. Because all body surfaces are covered by epithelial mem￾branes, a substance must pass through an epithelium in order to enter or leave the body. Epithelial membranes thus provide a barrier that can impede the passage of some substances while facilitating the passage of others. For land-dwelling vertebrates, the relative impermeability of the surface epithelium (the epidermis) to water offers es￾sential protection from dehydration and from airborne pathogens (disease-causing organisms). On the other hand, the epithelial lining of the digestive tract must allow selec￾tive entry of the products of digestion while providing a barrier to toxic substances, and the epithelium of the lungs must allow for the rapid diffusion of gases. Some epithelia become modified in the course of em￾bryonic development into glands, which are specialized for secretion. A characteristic of all epithelia is that the cells are tightly bound together, with very little space between them. As a consequence, blood vessels cannot be interposed between adjacent epithelial cells. Therefore, nutrients and oxygen must diffuse to the epithelial cells from blood ves￾sels in nearby tissues. This places a limit on the thickness of epithelial membranes; most are only one or a few cell layers thick. Epithelium possesses remarkable regenerative powers, constantly replacing its cells throughout the life of the ani￾mal. For example, the liver, a gland formed from epithelial tissue, can readily regenerate after substantial portions of it have been surgically removed. The epidermis is renewed every two weeks, and the epithelium inside the stomach is replaced every two to three days. There are two general classes of epithelial membranes: simple and stratified. These classes are further subdivided into squamous, cuboidal, and columnar, based upon the shape of the cells (table 49.2). Squamous cells are flat, cuboidal cells are about as thick as they are tall, and colum￾nar cells are taller than they are wide. Types of Epithelial Tissues Simple epithelial membranes are one cell layer thick. A simple, squamous epithelium is composed of squamous ep￾ithelial cells that have an irregular, flattened shape with ta￾pered edges. Such membranes line the lungs and blood capillaries, for example, where the thin, delicate nature of these membranes permits the rapid movement of molecules (such as the diffusion of gases). A simple cuboidal epithelium lines the small ducts of some glands, and a simple columnar epithelium is found in the airways of the respiratory tract and in the gastrointestinal tract, among other locations. In￾terspersed among the columnar epithelial cells are numer￾ous goblet cells, specialized to secrete mucus. The columnar epithelial cells of the respiratory airways contain cilia on their apical surface (the surface facing the lumen, or cavity), which move mucus toward the throat. In the small intes￾tine, the apical surface of the columnar epithelial cells form fingerlike projections called microvilli, that increase the sur￾face area for the absorption of food. Stratified epithelial membranes are several cell layers thick and are named according to the features of their up￾permost layers. For example, the epidermis is a stratified squamous epithelium. In terrestrial vertebrates it is further characterized as a keratinized epithelium, because its upper layer consists of dead squamous cells and filled with a water-resistant protein called keratin. The deposition of keratin in the skin can be increased in response to abrasion, producing calluses. The water-resistant property of keratin is evident when the skin is compared with the red portion of the lips, which can easily become dried and chapped be￾cause it is covered by a nonkeratinized, stratified squamous epithelium. The glands of vertebrates are derived from invaginated epithelium. In exocrine glands, the connection between the gland and the epithelial membrane is maintained as a duct. The duct channels the product of the gland to the surface of the epithelial membrane and thus to the external environment (or to an interior compartment that opens to the exterior, such as the digestive tract). Examples of ex￾ocrine glands include sweat and sebaceous (oil) glands, which secrete to the external surface of the skin, and acces￾sory digestive glands such as the salivary glands, liver, and pancreas, which secrete to the surface of the epithelium lin￾ing the digestive tract. Endocrine glands are ductless glands; their connections with the epithelium from which they were derived are lost during development. Therefore, their secretions, called hormones, are not channeled onto an epithelial membrane. Instead, hormones enter blood capillaries and thus stay within the body. Endocrine glands are discussed in more detail in chapter 56. Epithelial tissues include membranes that cover all body surfaces and glands. The epidermis of the skin is an epithelial membrane specialized for protection, whereas membranes that cover the surfaces of hollow organs are often specialized for transport. 988 Part XIII Animal Form and Function 49.2 Epithelial tissue forms membranes and glands