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PART 1 Organization of the Human Body

tissues, like the adhesive on Scotch TM tape. It plays an importantrole in supporting and guiding cell migration during tissue repair.The basement membrane is typically porous, which allows sub-stances to move to and from the epithelial tissue above it. A fewepithelial tissues, such as those in lymphatic capillaries and liversinusoids, do not have basement membranes, and some epithelialtissues, such as those in some endocrine glands, do not have afree surface or a basal surface with a basement membrane.4. Cell and matrix connections . Specialized cell contacts bindadjacent epithelial cells together and to the extracellularmatrix of the basement membrane.5. Nonvascular . Blood vessels in the underlying connective tissuedo not penetrate the basement membrane to reach the epithe-lium; thus, all gases and nutrients carried in the blood mustreach the epithelium by diffusing from blood vessels acrossthe basement membrane. In epithelial tissues with many layersof cells, diffusion must also occur across cells, and the mostmetabolically active cells are close to the basement membrane.6. Capable of regeneration. Epithelial tissue has the ability toreplace damaged cells with new epithelial cells. Undifferentiatedcells (stem cells) continuously divide and produce new cells. Insome types of epithelial tissues, such as those in the skin andthe digestive tract, new cells continuously replace cells that die.

Classification of Epithelial Tissues

Epithelial tissues are classified primarily according to the number ofcell layers and the shape of the superficial cells. There are threemajor types of epithelium based on the number of cell layers in each:

1. Simple epithelium consists of a single layer of cells, with eachcell extending from the basement membrane to the free surface.2. Stratified epithelium consists of more than one layer ofcells, but only the basal layer attaches the deepest layer to thebasement membrane.3. Pseudostratified columnar epithelium is a special type ofsimple epithelium. The prefix pseudo - means false, so this typeof epithelium appears to be stratified but is not. It consists ofone layer of cells, with all the cells attached to the basementmembrane. There appear to be two or more layers of cellsbecause some of the cells are tall and extend to the free surface,whereas others are shorter and do not extend to the free surface.

There are three types of epithelium based on idealized shapesof the epithelial cells:1. Squamous (skw ā ′m ŭ s) cells are flat or scalelike.2. Cuboidal (cubelike) cells are cube-shaped—about as wide asthey are tall.3. Columnar (tall and thin, similar to a column) cells tend to betaller than they are wide.

In most cases, an epithelium is given two names, such as sim-ple squamous, stratified squamous, simple columnar, or pseudostrati-fied columnar. The first name indicates the number of layers, andthe second indicates the shape of the cells at the free surface(table 4.1). Tables 4.2–4.4 provide an overview of the major typesof epithelial tissues and their distribution.Simple squamous epithelium consists of one layer of flat, orscalelike, cells that rest on a basement membrane (table 4.2 a ).Stratified squamous epithelium consists of several layers of cells.Near the basement membrane, the cells are more cube-shaped, but atthe free surface the cells are flat or scalelike (table 4.3 a ). Pseudostrati-fied columnar epithelial cells are columnar in shape (taller than theyare wide) and, although they appear to consist of more than onelayer, all the cells rest on the basement membrane (table 4.4 a ).

Functions of Epithelial Tissues

The major functions of epithelial tissue are

1. Protecting underlying structures. For example, the outer layerof the skin and the epithelium of the oral cavity protect theunderlying structures from abrasion.2. Acting as a barrier. Epithelium prevents many substances frommoving through it. For example, the skin acts as a barrier to waterand reduces water loss from the body. The skin also prevents manytoxic molecules and microorganisms from entering the body.3. Permitting the passage of substances. Though epithelium actsas a barrier for some substances, it also permits many othersubstances to move through it. For example, oxygen andcarbon dioxide are exchanged between the air and blood bydiffusion through the epithelium in the lungs. Epithelium actsas a filter in the kidney, allowing many substances to passfrom the blood into the urine but retaining other substances,such as blood cells and proteins, in the blood.4. Secreting substances. Mucous glands, sweat glands, and theenzyme-secreting portions of the pancreas are all composedof epithelial cells that secrete their products onto surfaces orinto ducts that carry them to other areas of the body.5. Absorbing substances. The plasma membranes of certain epi-thelial tissues contain carrier proteins (see chapter 3), whichregulate the absorption of materials.

ASSESS YOUR PROGRESS

7. List six characteristics common to most types of epithelialtissue.

8. What are the distinct cell surfaces found in epithelialtissue? Describe them.

9. List and describe the major functions of epithelial tissue.

TABLE 4.1

Classification of Epithelium

SquamousCuboidal Columnar

SquamousNonkeratinized (moist)KeratinizedCuboidal (very rare) Columnar (very rare)

Columnar

Roughly cuboidal to columnarwhen not stretched andsquamouslike when stretched

Simple (single layer of cells)

Stratified (more than one layer ofcells)

Pseudostratified (modification of simple epithelium)

Transitional (modification ofstratified epithelium)