EPITHELIAL TISSUE

Origin and Characteristics

Epithelial tissue forms sheets that cover the body's outer surface, line the internal surfaces, and form the glandular tissue. Underneath all types of epithelial tissue is an extra­cellular matrix, called the basement membrane. A basement membrane consists of the basal lamina and an underlying reticular layer. The terms basal lamina and basement mem­brane are often used interchangeably. Epithelial cells have strong intracellular protein filaments (i.e., cytoskeleton) that are important in transmitting mechanical stresses from one cell to another. Cells of epithelial tissue are tightly bound together by specialized junctions. These specialized junctions enable these cells to form barriers to the move­ment of water, solutes, and cells from one body compart­ment to the next. Epithelial tissue is avascular (i.e., without blood vessels) and must therefore receive oxygen and nu­trients from the capillaries of the connective tissue on which the epithelial tissue rests (Fig. 4-21). Epithelial tis­sues contain neural receptors (i.e., pressure, thermal, and pain) that serve to sample the internal and external envi­ronments. To survive, epithelial tissue must be kept moist. Even the seemingly dry skin epithelium is kept moist by a nonvitalized, waterproof layer of superficial skin cells called keratin, which prevents evaporation of moisture from the deeper living cells.

Epithelia are derived from all three embryonic layers. Most epithelia of the skin, mouth, nose, and anus are de­rived from the ectoderm. Linings of the respiratory tract, the gastrointestinal tract, and the glands of the digestive tract are of endodermal origin. The endothelial lining of blood vessels originates from the mesoderm. Many types of epithelial tissue retain the ability to differentiate and undergo rapid proliferation for replacing injured tissue.

Types of Epithelial Cells

Epithelial tissues are classified according to the shape of the cells and the number of layers that are present: simple, stratified, and pseudostratified. The terms squamous (thin and flat), cuboidal (cube shaped), and columnar (resembling a column) refer to the cells' shape (Fig. 4-22).

Simple Epithelium. Simple epithelium contains a single layer of cells, all of which rest on the basement membrane.

 

TABLE 4-1 Classification of Tissue Types

Tissue Type

Location

Epithelial Tissue

Covering and lining of body surfaces Simple epithelium

Squamous

Cuboidal

Columnar Stratified epithelium

Squamous keratinized

Squamous nonkeratinized

Cuboidal

Columnar

Transitional

Pseudostratified Glandular

Endocrine

Exocrine Neuroepithelium Reproductive epithelium

Connective Tissue

Embryonic connective tissue

Mesenchymal

Mucous Adult connective tissue

Loose or areolar

Dense regular

Dense irregular

Adipose

Reticular Specialized connective tissue

Bone

Cartilage

Hematopoietic

Muscle Tissue

Skeletal Cardiac Smooth

Nervous Tissue

Neurons Supporting cells

Lining of blood vessels, body cavities, alveoli of lungs Collecting tubules of kidney; covering of ovaries Lining of intestine and gallbladder

Skin

Mucous membranes of mouth, esophagus, and vagina

Ducts of sweat glands

Large ducts of salivary and mammary glands; also found in conjunctiva

Bladder, ureters, renal pelvis

Tracheal and respiratory passages

Pituitary gland, thyroid gland, adrenal, and other glands Sweat glands and glands in gastrointestinal tract Olfactory mucosa, retina, tongue Seminiferous tubules of testis; cortical portion of ovary

Embryonic mesoderm Umbilical cord (Wharton's jelly)

Subcutaneous areas

Tendons and ligaments

Dermis of skin

Fat pads, subcutaneous layers

Framework of lymphoid organs, bone marrow, liver

Long bones, flat bones

Tracheal rings, external ear, articular surfaces

Blood cells, myeloid tissue (bone marrow)

Skeletal muscles

Heart muscles

Gastrointestinal tract, blood vessels, bronchi, bladder, and others

Central and peripheral neurons and nerve fibers Glial and ependymal cells in central nervous system; Schwann and satellite cells in peripheral nervous system

Simple squamous epithelium is adapted for filtration; it is found lining the blood vessels, lymph nodes, and alveoli of the lungs. The single layer of squamous epithelium lin­ing the heart and blood vessels is known as the endothe-lium. A similar type of layer, called the mesothelium, forms the serous membranes that line the pleural, pericardial, and peritoneal cavities and cover the organs of these cav­ities. A simple cuboidal epithelium is found on the surface of the ovary and in the thyroid. Simple columnar epithelium lines the intestine. One form of a simple columnar epi­thelium has hairlike projections called cilia, often with specialized mucus-secreting cells called goblet cells. This form of simple columnar epithelium lines the airways of the respiratory tract.

Stratified and Pseudostratified Epithelium. Stratified ep­ithelium contains more than one layer of cells, with only the deepest layer resting on the basement membrane. It is designed to protect the body surface. Stratified squamous keratinized epithelium makes up the epidermis of the skin. Keratin is a tough, fibrous protein existing as filaments in the outer cells of skin. A stratified squamous keratinized epithelium is made up of many layers. The layers closest to the underlying tissues are cuboidal or columnar. The cells become more irregular and thinner as they move closer to the surface. Surface cells become totally filled with keratin and die, are sloughed off, and then are re­placed by the deeper cells. A stratified squamous nonkera­tinized epithelium is found on moist surfaces such as the

 

 

ventral abdominal walls

FIGURE4-20 Cross section of a human embryo illustrating the de­velopment of the somatic and visceral structures.

mouth and tongue. Stratified cuboidal and columnar epi-thelia are found in the ducts of salivary glands and the larger ducts of the mammary glands. In smokers, the nor­mal columnar ciliated epithelial cells of the trachea and bronchi are often replaced with stratified squamous epi­thelium cells that are better able to withstand the irritat­ing effects of cigarette smoke.

Pseudostratified epithelium is a type of epithelium in which all of the cells are in contact with the underlying in­tercellular matrix, but some do not extend to the surface. A pseudostratified ciliated columnar epithelium with gob-

Microvilli

Apical surface

Cell junctions

Epithelial cells

Basement membrane

Connective

tissue

Nerve fiber

— Blood vessel

RE4-21 Typical arrangement of epithelial cells in relation to un­derlying tissues and blood supply. Epithelial tissue has no blood supply of its own but relies on the blood vessels in the underlying connective tissue for nutrition (N) and elimination of wastes (W).