Page 157 - Fundamentals of anatomy physiology

144 Unit 1 Levels of Organization

4 of fine protein filaments (Figure 4–2e). It is secreted by the adja- protected location. ATLAS: Embryology Summary 2: The Development
cent layer of epithelial cells. The clear layer acts as a barrier that of Epithelia
restricts the movement of proteins and other large molecules
from the underlying connective tissue into the epithelium. Checkpoint
3. List five important characteristics of epithelial tissue.
The deeper portion of the basement membrane, the dense 4. Identify four essential functions of epithelial tissue.
layer, contains bundles of coarse protein fibers produced by 5. What is the probable function of an epithelial surface
connective tissue cells. The dense layer gives the basement
membrane its strength. Attachments between the fibers of the whose cells bear many microvilli?
clear layer and those of the dense layer hold the two layers to- 6. Identify the various types of epithelial cell junctions.
gether, and hemidesmosomes attach the epithelial cells to the 7. What is the functional significance of gap junctions?
composite basement membrane. The dense layer also acts as a See the blue Answers tab at the back of the book.
filter that determines what substances can diffuse between the
adjacent tissues and the epithelium. 4-3 Cell shape and number of layers

Epithelial Maintenance and Repair determine the classification of epithelia

Epithelial cells lead hard lives. They are exposed to disrup- Learning Outcome Describe the relationship between structure and
tive enzymes, toxic chemicals, pathogenic bacteria, and me- function for each type of epithelium.
chanical abrasion. Consider the lining of your small intestine,
where epithelial cells are exposed to a variety of enzymes and There are many different specialized types of epithelia. You can
abraded by partially digested food. In this extreme environ- easily sort these into categories based on (1) the cell shape,
ment, an epithelial cell may last just a day or two before it is and (2) the number of cell layers between the basement mem-
shed or destroyed. The only way the epithelium can maintain brane and the exposed surface of the epithelium. Using these
its structure over time is by the continual division of stem cells. two criteria, we can describe almost every epithelium in the
body (Table 4–1).
p. 129 Most epithelial stem cells, also called germinative
cells, are located near the basement membrane, in a relatively

Table 4–1 Classifying Epithelia
SQUAMOUS
CUBOIDAL COLUMNAR

Simple
Simple squamous epithelium Simple cuboidal epithelium Simple columnar epithelium

Stratified
Stratified squamous epithelium Stratified cuboidal epithelium Stratified columnar epithelium