By Janet Rae-Dupree, Pat DuPree

Perhaps because of its unique job of both protecting the outer body and lining internal organs, epithelial tissue has many characteristics that distinguish it from other tissue types.

Epithelial tissues, which generally are arranged in sheets or tubes of tightly packed cells, always have a free, or apical, surface that can be exposed to the air or to fluid. That free surface also can be covered by additional layers of epithelial tissue.

[Credit: Illustration by Kathryn Born, MA]
Credit: Illustration by Kathryn Born, MA

But whether it’s layered or not, each epithelial cell has polarity (a top and a bottom), and all but one side of the surface cells are tucked snugly against neighboring cells. The apical side sometimes has cytoplasmic projections such as cilia, hairlike growths that can move material over the cell’s surface, or microvilli, fingerlike projections that increase the cell’s surface area for increased adsorption and more efficient diffusion of absorbents and secretions.

Opposite the apical side is the basal side (think basement), which typically attaches to some kind of connective tissue.

Adsorption involves the adhesion of something to a surface, while absorption involves a fluid permeating or dissolving into a liquid or solid.

Epithelial tissue serves several key functions, including the following:

  • Protection: Skin protects vulnerable structures or tissues deeper in the body.

  • Barrier: Epithelial tissues prevent foreign materials from entering the body and retain interstitial fluids.

  • Sensation: Sensory nerve endings embedded in epithelial tissue connect the body with outside stimuli.

  • Secretion: Epithelial tissue in glands can be specialized to secrete enzymes, hormones, and fluids.

  • Absorption: Linings of the internal organs are specialized to facilitate absorption of materials into the body.

  • Filtration: The epithelium of the respiratory tract protects the body by filtering out dirt and particles and cleaning the air that’s inhaled. The blood is filtered in epithelial tissue in the kidneys.

  • Diffusion: Simple squamous epithelial cells form a semipermeable membrane that allows selective diffusion of materials to pass through under osmotic pressure, which contributes to the filtration function. Diffusion is also involved in transport processes at the tissue interface in absorption and secretion functions.

Single-layer epithelial tissue is classified as simple. Tissue with more than one layer is called stratified. Epithelial tissues also can be classified according to shape:

  • Squamous: Squamous cells are thin and flat.

  • Cuboidal: As the name implies, cuboidal cells are equal in height and width and shaped like cubes.

  • Columnar: Columnar cells are taller than they are wide.

Following are the ten primary types of epithelial tissues:

  • Simple squamous epithelium: This flat layer of scalelike cells looks like a fish’s scales and is useful in diffusion, secretion, or absorption. Each cell nucleus is centrally located and is round or oval. Simple squamous epithelium lines the lungs’ air sacs where oxygen and carbon dioxide are exchanged.

  • Simple cuboidal epithelium: These cube-shaped cells, found in a single layer that looks like a microscopic mattress, have centrally located nuclei that usually are round. They are found in the ovaries, line the ascending limb of the loop of Henle and the convoluted tubules in the kidney, and line some glands such as the thyroid, sweat glands, and salivary glands, as well as the inner surface of the eardrum, known as the tympanic membrane.

    Functions of this type of epithelium are secretion, absorption, and tube formation. The kidney tubules have microvilli that increase the area of adsorption for more efficient diffusion and absorption.

  • Simple columnar epithelium: These densely packed cells are taller than they are wide, with nuclei located near the base of each cell. Found lining the digestive tract from the stomach to the anal canal, the functions of this type of epithelium are secretion and absorption.

  • Simple columnar ciliated epithelium: A close cousin to simple columnar epithelium, this type of tissue has hairlike cilia that can move mucus and other substances across the cell. It’s found lining the small respiratory tubes.

  • Pseudostratified columnar epithelium: Pay attention to the prefix pseudo here, which means “false.” It may look multilayered because the cells’ nuclei are scattered at different levels, but it’s not. This type of epithelium is found in the large ducts of the parotid glands (salivary glands) and some segments of the male reproductive system, including the urethra.

  • Pseudostratified ciliated columnar epithelium: Another variation on a theme, this tissue is nearly identical to pseudostratified columnar epithelium. The difference is that this tissue’s free surface has cilia, making it ideal for lining air passages because the cilia’s uniform waving action causes dust and dirt particles trapped in a thin layer of mucus produced by interspersed mucus cells to move in one direction — away from the lungs and toward the throat and mouth.

    The surface of the mucus-producing cells scattered among the ciliated epithelial cells is covered with microvilli. The microvilli occur on epithelial surfaces where absorption and secretion take place.

  • Stratified squamous epithelium: This tissue is the stuff you see every day — your outer skin, or epidermis. This multilayered tissue has squamous cells on the outside plus deeper layers of cuboidal or columnar cells. Found in areas where the outer cell layer is constantly worn away, this type of epithelium regenerates its surface layer with cells from lower layers.

  • Stratified cuboidal epithelium: This multilayered epithelium can be found in sweat glands, conjunctiva of the eye, and the male urethra. Its function is primarily protection.

  • Stratified columnar epithelium: Also multilayered, this epithelium is found lining parts of the male urethra, excretory ducts of glands, and some small areas of the anal mucous membrane.

  • Stratified transitional epithelium: This epithelium is referred to as transitional because its cells can shape-shift from cubes to squamouslike flat surfaces and back again. Found lining the urinary bladder, the cells stretch and flatten out to make room for urine. When urination occurs, the cells relax and assume their original form.