Fundamentals of Anatomy and Physiology

94

Introduction

The basic units of tissue (TISH-you) are groups of cells.
These cells will have a similar function and a similar
structure. Tissues are classified based on how these cells
are arranged and what kind and how much material is
found between the cells. Cells are either tightly packed or
separated by interstitial material. The study of tissue is
called histology (hiss-TALL-oh-jee).
The four basic types of tissue are epithelial, connec-
tive, muscle, and nervous. Each type is further subdivid-ed
into specific examples. These tissues combine to form
organs. The various organs make up the systems of the
body that allow us to function and survive in our complex
world.

Epithelial Tissue.

Epithelial (ep-ih-THEE-lee-al) tissue functions in four
major ways.

1. It protects underlying tissues: Our skin is epithelial
   tissue and protects us from the harmful rays of the sun
   and certain chemicals. The lining of our diges-tive tract
   is made of epithelial tissue and protects underlying
   tissue from abrasion as food moves through the tract.


2. It absorbs: In the lining of the small intestine, nutri-
   ents from our digested food enter blood capillaries and

get carried to the cells of our body.^

3. It secretes: All glands are made of epithelial tissue;
   the endocrine glands secrete hormones, the mu-cous
   glands secrete mucus, and our intestinal tract contains
   cells that secrete digestive enzymes in ad-dition to
   the pancreas and the liver, which secrete the major

portions of digestive enzymes.^

4. Epithelial tissue excretes: Sweat glands excrete
   waste products such as urea.
   When epithelial tissue has a protective or absorbing
   function, it is found in sheets covering a surface, like the
   skin or intestinal lining. When it has a secreting function,
   the cells involute from the surface into the underlying
   tissues to form glandular structures. Only a minimal, if any,
   amount of intercellular material is found in epi-thelial
   tissue. The cells are very tightly packed together and thus
   this tissue is not as easily penetrated as other tissues.

Epithelial cells are anchored to each other and to
underlying tissues by a specialized membrane called the
basement membrane. This membrane acts like the adhe-
sive on a tile floor, the tiles being the epithelial cells. It

Chapter 5

is very important because it acts as an anchor for the at-

tached side of the epithelial cells and it provides protec-tion

for other underlying tissue like connective tissue.

Epithelial tissue can be named according to shape and

structures that might be on the free or outer edge of the

cells. This surface can be plain or it can have rows of cilia

(those that line the respiratory tract), a flagellum (the sperm

cell), microvilli (folds), and secretory vesicles (those that

line the small intestine). Epithelial tissue can be one layer

or several layers thick.

Classification Based on Function

Epithelial cells are classified as either squamous, cuboi-dal,

or columnar. Squamous (SKWAY-mus) cells are flat and

slightly irregular in shape (Figure 5-1). They serve as a

protective layer. They line our mouth, blood and lymph

vessels, parts of kidney tubules, our throat and esopha-gus,

the anus, and our skin. If exposed to repeated irri-tation like

the linings of ducts in glands, other epithelial cells can

become squamous i n appearance.

Cuboidal (kyoo-BOY-dal) cells look like small cubes

(Figure 5-2). They are found in glands and the lining tissue

of gland ducts (sweat and salivary), the germinal coverings

of the ovaries, and the pigmented layer of the retina of the

eye. Their function can be secretion and protection. In areas

of the kidney tubules, they function in absorption.

Columnar cells are tall and rectangular looking

(Figure 5-3). They are found lining the ducts of certain

glands (e.g., mammary glands) and the bile duct of the

liver. They are also found in mucus-secreting tissues such

as the mucosa of the stomach, the villi of the small

intestine, the uterine tubes, and the upper respiratory tract.

Many of these cells are ciliated.

Classification Based on Arrangement

The four most common arrangements of epithelial cells are

simple, stratified, pseudostratified, and transitional. As

epithelial cells are named, a combination of the clas-

sification of both shape and arrangement is used. The

simple arrangement is one cell layer thick. It is found in

the lining of blood capillaries, the alveoli of the lungs, and

in the loop of Henle in the kidney tubules. Refer to Figure

5 - 2 for simple cuboidal epithelium found in the lining of

glandular ducts. Refer to Figure 5-3 for simple columnar

epithelium found in the villi of the small intes-tine and the

lining of the uterus. The stratified arrange-ment is several

layers of cells thick. Refer to Figure 5-1B for stratified

squamous epithelium found lining our