There are four basic types of tissues in your body:
Epithelial
(
ep'-i-THE-le-al
)
tissue
covers
body
surfaces, forms glands, and lines body cavities, hollow
organs, and ducts.
Connective tissue protects and supports the body
and its organs, binds organs together, stores energy
reserves as fat, and provides immunity.
Muscular tissue generates the physical force needed
to make body structures move.
Nervous tissue detects changes inside and outside
the body and generates transmits nerve impulses that
coordinate body activities to help maintain homeostasis.
Most epithelial cells and some muscle and nerve cells are
tightly joined into functional units by points of contact be-
tween their plasma membranes called cell junctions. Cell
junctions perform different functions in different tissues:
1.
Tight junctions fuse cells together tightly to prevent
substances from passing between the cells. In tissues
that line the stomach, intestines, and urinary bladder,
tight junctions prevent the contents of these organs
from leaking out.
2.
Some cell junctions hold cells together so that they
don’t separate while performing their functions:
• Adherens
(
ad-HER-ens
) junctions have a dense
layer of proteins just inside the plasma membrane
called a plaque that runs along microfilaments to
form a belt or strap-like structure called an adhesion
belt. Two adjacent cells are joined by transmembrane
glycoproteins that insert into the corresponding
adhesion belts. This arrangement resists separation
even when stretched.
• Desmosomes
(
DEZ-mo-soms
)
are
like
adherens
junctions, but the plaque binds to intermediate
filaments and does not form a belt. Instead of two
cells adhering along a belt, they adhere at specific
spots.
• Hemidesmosomes resemble half of a desmosome.
They do not adhere adjacent cells but rather attach
cells to membranes.
3.
Gap junctions form channels that allow ions and
molecules to pass between cells. This permits cells in
a tissue to communicate and enables nerve or muscle
impulses to spread rapidly among cells.
Let’s take a closer look at tissues, first examining epithe-
lial tissues.
Epithelial Tissue Covers Body Surfaces
There are two types of epithelial tissue or epithelium:
covering and lining epithelium and glandular epithelium.
Covering and lining epithelium forms the outer cover-
ings of skin and internal organs, lines body cavities, and
makes up the sense organs, along with nerve tissue. Glan-
dular epithelium forms secretory glands, such as sweat
glands. Both types of epithelial tissues have the following
five general features:
1.
Consist of continuous sheets (single or multiple layers)
of closely packed cells with little extracellular material
between them
2.
Have three types of surfaces:
Apical (free surface).
Exposed to body cavity, lining of
an internal organ, or exterior of the body
Lateral surface.
Faces adjacent surrounding cells
Basal surface.
Attached to a
basement membrane,
which
is a thin, extracellular structure made of protein
fibers and which lies between the epithelial cells and
the underlying connective tissue
In stratified epithelia, the uppermost layer is the apical
layer, while the deepest layer is the basal layer.
3.
Lack blood vessels and get their nutrients from the
connective tissue below by diffusion
4.
Have a nerve supply
5.
Divide rapidly and continuously to replace worn-out
and injured cells
Let’s look at how epithelial tissue is classified.
C overing and lining epithelium Covering and lin-
ing epithelium is classified according to cell shape and
number of layers. Epithelial cells have different shapes
that affect their functions (Table 3.2):
Squamous
(
SKWA-mus
)—Thin and flat cells that
allow substances to pass rapidly through them
Cuboidal—Cube-shaped
cells
that
may
have
microvilli at their apical surface for secretion or
absorption
72 CHAPTER 3
Cells and Tissues
previous page 107 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 109 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off