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Tissues
Dr. Bruce Forciea
Welcome to the fascinating world of tissues. The
body is packed with different kinds of tissues.
Some are highly organized and some are not.
When studying tissues it helps to think about the
relationship between the structure of a tissue
and its function.
First of all, the study of tissues is known as
Histology. People who study histology spend a
lot of time looking in microscopes at the various
body tissues.
Let’s look at how tissues are categorized. There
are 4 main categories of tissues in the human
body:
Epithelium
Connective
Muscle
Nervous
Epithelium is a tissue that covers other structures.
Therefore one side is always exposed to the
outside (which could still be inside the body).
You will see epithelial tissue covering the inside of
body cavities and organs. The outer or
superficial portion of your skin is an epithelial
tissue.
Epithelial tissue does not have a blood supply.
Therefore nutrients must enter the tissue by
diffusion.
Remember diffusion?
Epithelial tissue is also anchored to other
structures via a basement membrane.
Epithelial tissue
Basement membrane
Here is an example of an epithelial tissue. Note that one
side of the tissue is exposed to the outside and the tissue
is connected by a basement membrane.
Epithelial tissue is categorized according to the
shape of the cells and number of layers.
With just a couple of exceptions…
Epithelial cells come in 3 basic shapes:
Squamous
Cuboidal
Columnar
Squamous cells are flat. From the side they look
something like a fried egg.
Cuboidal epithelium cells are shaped like their name implies.
Like little cubes.
Cuboidal epithelium
Columnar epithelium cells are rectangular in shape.
Columnar epithelium
Remember, epithelium is also categorized
according to number of layers.
A single layer of epithelium is called “simple.”
If there is more than one layer it is called “stratified.”
Here is an example of simple squamous epithelium. Look for a thin
layer of flat (squamous) cells.
Simple squamous epithelium is a common site for filtration. It is
commonly found in the lungs, walls of capillaries and inside of blood
and lymphatic vessels.
This is stratified squamous epithelium. One characteristic is as the
cells reach the surface they flatten out. This is a slide of the most
superficial layer of the skin known as the epidermis. Here we see the
cells becoming so flattened that they eventually come off.
Stratified squamous is also found in the oral cavity, anal canal and
vagina.
Here’s our picture of cuboidal epithelium. Since there is only one layer, we
can call this “simple cuboidal epithelium.”
Simple cuboidal epithelium is commonly found in ovaries, kidney
tubules, ducts.
Stratified cuboidal epithelium also lines ducts. It
can be found in the ducts of mammary glands,
sweat glands, salivary glands, pancreas
Here is a nice single layer of columnar cells. We can call this
“simple columnar epithelium.”
Simple columnar epithelium can be ciliated or non-ciliated. Ciliated
is found in the female reproductive tract. Non-ciliated is found in
the uterus and digestive tract.
Columnar epithelium can also be stratified. This
tissue is found in the vas deferens and pharynx.
It provides a thicker lining for some tubular
structures in the body.
A special type of cell known as a goblet cell is usually associated
with columnar epithelium. The goblet cell secretes mucous.
Cilia are also seen on columnar cells. The cilia and goblet cells
work together to move substances along the cells. An example of
this is in the respiratory system.
Cilia
There are some special cases of epithelium. We’ll
take a look at these now.
The first special case is called pseudostratified
columnar epithelium.
It is called pseudostratified because it looks like it’s
stratified (more than one layer) but it’s not.
It looks stratified because the nuclei of the cells are
at various levels. But in reality there is only one
layer.
This is an example of pseudostratified columnar epithelium. Note
that the nuclei are at different levels.
Nuclei don’t line up—
pseudostratified.
Nuclei all line up nicely in one row—
simple.
Here is pseudostratified next to simple columnar epithelium.
The other special case of epithelium is called
transitional.
Transitional epithelium looks somewhat like
stratified squamous, but there is a difference.
In transitional epithelium the cells
are rounded both at the base of the
tissue and the section exposed to
the outside.
Transitional epithelium is found in
the urinary bladder. The multiple
layers allow for the bladder to
distend and contract.
The epithelium also forms a barrier
to help protect the bladder from
infection.
To finish up our discussion about epithelium, we
will look at glandular epithelium.
Glandular epithelium can secrete substances into
the bloodstream (endocrine glands) or into ducts
(exocrine glands).
Exocrine Glands
• Can be classified by method of secretion.
– Merocrine—release substance via exocytosis
– Apocrine—lose small portion of cell body.
– Holocrine—release entire cell.
Next we will learn about another of the general
categories of tissues known as connective
tissue.
Connective tissue is the most abundant tissue in the body.
It consists of special cells called fibroblasts surrounded by
a matrix of intercellular material.
The matrix can contain fibers such as collagen, elastic,
and reticular fibers.
Other cells can exist in connective tissue such as
macrophages and mast cells (both are types of white
blood cells).
Connective Tissue Cells
• Fibroblasts
– Star-shaped cell
– Secretes protein into matrix producing fibers.
• Macrophages
– Act as scavengers (phagocytosis)
• Mast Cells
– Release heparin (an anticoagulant), histamine
(promotes inflammatory reactions).
Connective Tissue
• Fibers (produced by fibroblasts)
– Collagenous
•
•
•
•
•
•
•
Thick pieces of protein (collagen).
Not very elastic.
Hold structures together.
High tensile strength.
Ligaments, tendons.
Lots of collagenous fibers = dense CT (white fibers)
Few collagenous fibers = Loose CT.
Connective Tissue (elastic fibers)
– Elastic
•
•
•
•
Composed of bundles of elastin (elastic protein).
Not a lot of tensile strength but very flexible
Appear yellow in color.
Found in vocal cords, air passages of respiratory
system.
Connective Tissue (reticular fibers)
–Reticular
•Thin collagenous fibers that
form a supporting network.
There is also a special category of connective
tissue that contains:
Blood
Cartilage
Bone
The general categories of connective tissue are:
Loose
Dense
Reticular
Adipose
Elastic
Here is a picture of loose connective tissue. This is also in the skin
located deep to the epidermis.
Loose connective tissue (aka areolar tissue) is not very well
organized. It contains fibroblasts, matrix, and some fibers
scattered about.
Loose connective tissue is found in the dermis and subcutaneous
layers of the skin as well as surrounding muscles. Sometimes it is
called fascia.
Adipose connective tissue consists of cells
containing lipid (fat) called adipocytes. The lipid
is used to store energy to be used by the body if
needed.
Adipose tissue is also found around some organs
and joints. It forms a cushion for shock
absorption.
Adipose tissue also insulates the body.
Lipid
Nucleus of adipocyte
Here is a close up of adipose tissue. You can see the large lipid
containing adipocytes.
Adipocytes
Here’s another view of adipose tissue.
Reticular connective tissue consists of a thin
supportive network of collagen fibers. It is found
supporting the walls of the liver, spleen and
lymphatic system.
Reticular connective tissue is web-like and forms a supportive
network.
Dense connective tissue contains thick
collagenous fibers. It is found in ligaments and
tendons which have a high tensile strength.
Dense connective tissue has a poor blood supply
which is why tendons and ligaments do not heal
well.
There are also some elastic fibers and fibroblasts.
Elastic connective tissue contains more elastic
fibers than collagen fibers.
Elastic connective tissue is found in attachments
between vertebrae and in walls of some hollow
internal organs.
Next we will look at some of the tissues in the
category of specialized connective tissue.
Remember this contains blood, bone and cartilage.
We’ll start with cartilage. Cartilage is rigid and strong so it
can provide support and protection. It also forms a
structural model for developing bones.
Cartilage consists of cells called chondrocytes, a
chondromucoprotein matrix and collagenous and elastic
fibers.
Cartilage has no direct blood supply so nutrients must
enter by diffusion.
Since the nutrients for cartilage diffuse into the
tissue, the tissue needs water to help move
these substances in. As humans age cartilage
tends to “dry up” or become dehydrated which
lends to degeneration of the tissue.
The cartilage cells or chondrocytes also do not
divide very frequently which also contributes to
poor healing.
There are 3 types of cartilage:
Hyaline cartilage
Fibrocartilage
Elastic cartilage
Chondrocyte
Lacunae
Hyaline cartilage has the characteristic chondrocyte in lacunae
arrangement along with a “ground glass” appearance to the matrix. It is
found at the ends of bones, soft part of the nose, larynx and trachea.
Hyaline cartilage serves as a model for bone growth.
Another picture of hyaline cartilage.
Nice “ground glass” matrix!
Elastic fibers
Elastic cartilage also has the characteristic chondrocyte in lacunae along
with elastic fibers.
This cartilage is found in the larynx and the ear.
Fibrocartilage is characterized by rows of chondrocytes (in lacunae). It is a
very strong cartilage and is found in the intervertebral discs.
Bone is the most rigid of connective tissues. Its hardness comes from
mineral salts such as calcium phosphate and calcium carbonate. It is highly
organized into units called Haversian systems. The primary cell of bone is
the osteocyte.
Blood is also considered a connective tissue. It basically consists of
cells suspended in a matrix of fluid called plasma.
It transports gasses such as oxygen and carbon dioxide and
functions in clotting and immunity.
Next we will investigate muscle tissue. There are 3
types of muscles tissue:
Skeletal
Cardiac
Smooth
Skeletal muscle is striated. The striations are caused by the density of
overlapping protein filaments called actin and myosin.
Intercalated disk
Cardiac muscle is also striated but has a unique structure called an
intercalated disk. The disks are special intercellular junctions that
allow electrochemical impulses to be conveyed across the tissue.
Smooth muscle is not striated because the myofilaments are not as dense
as in cardiac and skeletal muscle. Smooth muscle is found in organs such
as in the gastrointestinal system and the arteries.
The last tissue we will investigate is nervous tissue.
Nervous tissue consists of nervous system cells
called neurons and supportive cells called glia.
Neuron
Glial cell
Nervous tissue contains neurons and glial cells.
Practice tissue identification at:
http://www.wisc-online.com/objects/index.asp?objID=AP1402
Review the following tables in your text:
5.4 p 156
5.6 p 162
5.7 p 175
This concludes the unit on tissues. Feel free to
review this presentation as often as needed.