White Plains High School
Anatomy and Physiology I
Lecture Notes: Tissues
Chapter 4-Living Fabric
Recall the levels of organization:
atoms→molecules→organelles→cells→tissues
Tissues: groups of cells which performs a specific function
In the body, tissues can be classified into the following functions:
o
o
o
o
Epithelial – cover 1
Connective – support 2
Movement – muscle 3
Control – nervous 4
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Remember, two or more tissues together form an organ.
Epithelial Tissues
Epithelial cells can be classified into 2 types, those that cover surfaces and cavities and
those that form a glandular function. Regardless of the type, epithelial tissues have the
following functions:
o Protection
o Absorption
o Filtration
o Excretion
o Secretion
o Sensory Reception
A. Characteristics of epithelial tissues:
1) Polarity
Epithelial cells have an upper or free surface. This is called the apical surface.
The apical surface can be modified by microvilli or cilia. Microvilli are typically seen
with epithelial cells involved with absorption or secretion, for example the cells lining the
intestine. The microvilli can be so thick that it can be described as a brush boarder.
Cilia are seen with epithelial cells which propel substances along in a specific direction,
for example, the cells lining the respiratory track.
The bottom side of the epithelial cell is called the basal surface. It is supported by a non
cellular, adhesive sheet called the basal lamina. The basal lamina is made of
glycoproteins that are secreted by the basal region of the epithelial cell. (Remember the
RER→Golgi→Vesicles→Secretion) The basal lamina helps acts as a selective filter for
large proteins and determines which ones diffuse through to the epithelium.
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2) Specialized Contacts
Epithelial cells have contact with each other. These lateral contacts are the desmosomes
and tight junctions. The tight junction forms an impermeable junction between each
cell and thus prevents diffusion between the cells. The desmosomes help hold the cell’s
shape with anchoring junctions scattered between each cell. Recall that the desmosomes
have intermediate fibers which act like an internal scaffolding.
These contacts are typically not seen with glandular epithelium.
3) Supported by Connective Tissue
All epithelial cells have their basilar surface attached to a connective tissue net work.
There are two parts. The first is the basal lamina secreted from the epithelial cell. The
second is the reticular lamina that is made of collagen which is produced by fibroblasts.
Together the basal lamina and reticular lamina form the basement membrane. This
forms the foundations which anchor the epithelial cells. Cancerous epithelial cells can
breech this basement membrane and spread to other areas of the body.
4) Avascular but Innervated
Epithelial cells do not have a blood supply and depend on diffusion for the delivery of
nutrients and removal of wastes. They do have a nerve supply, innervated, for sensory
function.
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5) Regeneration
Do to their protective function; epithelial cells need to be continually replaced. Mitotic
figures are not uncommon.
B. Classification of epithelium
Each epithelial type has two parts to its name:
1) Number of layers present
2) General shape of the cell
Number of Layers
Epithelium is classified as either having a single layer or having multiple layers. Those
having a single layer are called simple, while those having multiple layers are called
stratified.
General Shape
Epithelial cells have 3 general shapes:
o Squamous
o Cuboidal
o Columnar
Simple Squamous Epithelium
These are very thin cells which are typically are very permeable. Diffusion is a high
priority. These can be found lining the alveolar spaces in the lung and in the kidney
glomerulus. 2 epithelial surfaces have special names:
o Endothelium – cells that line the blood vessels, heart and lymph
vessels
o Mesothelium – cells that line the serous membranes and their
organs.
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Simple Cuboidal Epithelium
This type of epithelium is typically found lining the ducts of glands and kidney tubules.
Their role is secretory and absorption. They have a cube shape with a centrally placed
nucleus.
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Simple Columnar Epithelium
These are a single layer of oblong cells which are tightly packed. Usually seen with
organs involved with absorption, they are usually modified with microvilli or cilia. A
common example is found lining the small intestine.
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Pseudo stratified Columnar Epithelium
These are columnar epithelial cells that vary in height; the nuclei do not appear at the
same height. Their apical surface is covered by cilia. These are found in the respiratory
tract mixed with cells that secrete mucous.
Stratified Epithelium
These have multiple layers and are more durable. Their major function is protection.
Mitotic figures are not uncommon in the basilar levels.
Stratified Squamous Epithelium
This is the most common type of stratified epithelium. The major example is skin. The
upper most layers are flat and are made up of dead or dying cells. The lower levels may
have cuboidal cells which slowly morph into squamous cells as they move up toward the
apical surface. In the skin the outer most layers contains cells filled with keratin and thus
this epithelium is correctly called keratinized squamous epithelium.
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Stratified cuboidal and columnar epithelium
Neither of these is very common. Stratified cuboidal is found in some sweat ducts and in
mammary glands while stratified columnar is found in the pharynx.
Transitional Epithelium
This type has cells that are cuboidal in the basal layer while the shape of the apical cells
varies. These typically have a dome shape epithelial cells that can stretch flat and look
squamous, found in tissues which need to expand. The most notable example is the
bladder.
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Glandular Epithelium
Glandular epithelium is more complex and varied than the epithelial cells which cover
surfaces or line tubules or vessels.
Glandular epithelium form glands which can consist of one or more cells that make and
secrete a product. This product can be:
o Water soluble proteins
o Nonpolar lipids or steroids
Glands are classified first based on the presences or absence of a duct for the secretory
products.
Endocrine Glands are ductless glands and secrete their products directly into the
capillaries. Major examples of this type reside with the endocrine system. The products,
hormones, vary in amount and type.
Exocrine Glands secrete their products onto a body surface or a cavity. Exocrine glands
are classified first based on cell number:

Unicellular Exocrine Glands are made of up of single cells which secrete
their products directly through exocytosis onto a surface. The only major
examples are the goblet cells and mucous cells found mixed with the
columnar epithelium in the digestive and respiratory tracts. They secrete
products called mucins (mucous), glycoproteins that lubricate.

Multicellular Exocrine Glands are made up many cells and are
composed of two parts.
 A epithelial lined duct
 A secretory unit called the acinus
Supportive connective tissue surrounds the secretory unit and provides
blood vessels and nerve fibers.
.
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The shape of the duct and of the secretory unit provides for another level of classification.
Duct Structure:


Simple- unbranched, straight duct
Compound- branched duct
Secretory Unit:


Tubular- if the secretory unit has the same diameter throughout like a tube
Alveolar- if the secretory unit is in the shape of a flask
It is important to note that both of these terms refer to the acinus or secretory unit.
Notice that for the simple, the duct does not branch but the secretory unit (Acinar) can.
Glands regardless of type (unicellular or multicellular) can have one of three types of
secretory processes:



Merocrine- products are secreted by exocytosis. This is seen with most
glands.
Holocrine- the cell ruptures and releases the product. New cells replace
the spent cells. The only example in humans are the sebaceous (sweat)
glands
Apocrine-the top of the cell pinches off. Possibly seen in the lactating
mammary gland although this is generally considered merocrine in nature.
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