146
UNIT ONE
Nonciliated simple columnar epithelium lines the uterus
and portions of the digestive tract, including the stomach
and small and large intestines. Its cells are elongated, so this
tissue is thick, which enables it to protect underlying tissues.
Simple columnar epithelium also secretes digestive fl
uids
and absorbs nutrients from digested food.
Simple columnar cells, specialized for absorption, often
have many tiny, cylindrical processes extending from their
free surfaces. These processes, called
microvilli,
are from
0.5 to 1.0 µm long. They increase the surface area of the cell
membrane where it is exposed to substances being absorbed
(f
g. 5.5)
.
Typically, specialized, fl ask-shaped glandular cells are
scattered among the cells of simple columnar epithelium.
These cells, called
goblet cells,
secrete a protective fluid
called
mucus
onto the free surface of the tissue (see F
g. 5.4).
Pseudostratif
ed Columnar Epithelium
The cells of
pseudostratif
ed
(soo
do-strat
ı˘-fı
¯d)
columnar
epithelium
appear stratiF
ed or layered, but they are not. A
layered effect occurs because the nuclei are at two or more
levels in the row of aligned cells. However, the cells, which
vary in shape, all reach the basement membrane, even
though some of them may not contact the free surface.
Pseudostratified columnar epithelial cells commonly
have cilia, which extend from the free surfaces of the cells.
Goblet cells scattered throughout this tissue secrete mucus,
which the cilia sweep away
(f
g. 5.6)
.
PseudostratiF
ed columnar epithelium lines the passages
of the respiratory system. Here, the mucous-covered linings
are sticky and trap dust and microorganisms that enter with
the air. The cilia move the mucus and its captured particles
upward and out of the airways.
Simple Squamous Epithelium
Simple squamous
(skwa
mus)
epithelium
consists of
a single layer of thin, fl attened cells. These cells F t tightly
together, somewhat like fl oor tiles, and their nuclei are usu-
ally broad and thin
(f g. 5.2)
.
Substances pass rather easily through simple squamous
epithelium. This tissue is common at sites of diffusion and F l-
tration. Simple squamous epithelium lines the air sacs (alveoli)
of the lungs where oxygen and carbon dioxide are exchanged.
It also forms the walls of capillaries, lines the insides of blood
and lymph vessels, and covers the membranes that line body
cavities. However, because it is so thin and delicate, simple
squamous epithelium is easily damaged.
Simple Cuboidal Epithelium
Simple cuboidal epithelium
consists of a single layer of
cube-shaped cells. These cells usually have centrally located,
spherical nuclei
(f g. 5.3)
.
Simple cuboidal epithelium lines the follicles of the thyroid
gland, covers the ovaries, and lines the kidney tubules and
ducts of certain glands—such as the salivary glands, pancreas,
and liver. In the kidneys, it functions in tubular secretion and
tubular reabsorption; in glands, it secretes glandular products.
Simple Columnar Epithelium
Simple columnar epithelium
is composed of a single layer
of elongated cells whose nuclei are usually at about the same
level, near the basement membrane
(f g. 5.4)
. The cells of
this tissue can be ciliated or nonciliated.
Cilia,
7 to 10 µm in
length, extend from the free surfaces of the cells, and they
move constantly. In the female, cilia aid in moving the egg
cell through the uterine tube to the uterus.
TABLE
5.1
|
Types of Intercellular Junctions
Type
Characteristics
Example
Tight junctions
Close space between cells by fusing cell membranes
Cells that line the small intestine
Desmosomes
Bind cells by forming “spot welds” between cell membranes
Cells of the outer skin layer
Gap junctions
Form tubular channels between cells that allow exchange of substances
Muscle cells of the heart and digestive tract
TABLE
5.2
|
Tissues
Type
Function
Location
Distinguishing Characteristics
Epithelial
Protection, secretion, absorption, excretion
Cover body surface, cover and line
internal organs, compose glands
Lack blood vessels, cells readily divide, cells are
tightly packed
Connective
Bind, support, protect, ±
ll spaces, store fat,
produce blood cells
Widely distributed throughout the body
Mostly have good blood supply, cells are farther
apart than epithelial cells, with extracellular
matrix in between
Muscle
Movement
Attached to bones, in the walls of hollow
internal organs, heart
Able to contract in response to speci±
c stimuli
Nervous
Transmit impulses for coordination,
regulation, integration, and sensory reception
Brain, spinal cord, nerves
Cells communicate with each other and other
body parts
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