3.Intro to Epithelial Cells
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Transcript of 3.Intro to Epithelial Cells
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CYT 2113 Cytology I
Lesson 3: Introduction to Different
Types of Epithelial Cells I
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Epithelial Tissue
Consists of cells
Fitted tightly together to form a continuous
layer, or, sheet, of cells
Specialized for exchanging materials between Specialized for exchanging materials between
the cell and its environment
Epithelial tissue is organized into two general
types of structures: epithelial sheets and
secretory glands
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Epithelial sheets are layers of very tightly
joined cells that cover and line various parts of
the body
One surface of the sheet is typically exposed One surface of the sheet is typically exposed
because it covers the body (outer layer of the
skin) or lines a cavity, such as the lumen
(cavity) of the intestine
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The outer surface of an epithelial layer
attaches to the underlying tissue by a
noncellular basement membrane consisting of
tiny fibers and nonliving polysaccharide tiny fibers and nonliving polysaccharide
material that the epithelial cells produce
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In general, epithelial sheets serve as
boundaries that separate the body from its
surroundings and from the contents of cavities
that open to the outside, such as the digestive that open to the outside, such as the digestive
tract lumen
Only selective transfer of materials is possible
between regions separated by an epithelial
barrier
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The type and extent of controlled exchange
vary, depending on the location and function
of the epithelial tissue
For example, the skin can exchange very little
between the body and surrounding
environment, making it a protective barrierenvironment, making it a protective barrier
The epithelial cells lining the small intestine of
the digestive tract are specialized for
absorbing nutrients that have come from
outside the body
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Epithelial tissue perform many functions,
including:
Protection
The epithelial layer of the skin, the epidermis, The epithelial layer of the skin, the epidermis,
covers the entire body and protects it from
mechanical injury, chemicals, bacteria and
fluid loss
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Absorption
The epithelial tissue lining the digestive tract
absorbs nutrients and water into the body
SecretionSecretion
Some epithelial cells form glands that secrete
cell products such as hormones, enzymes or
sweat
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Sensation
Other epithelial cells are sensory receptors
that receive information from the
environmentenvironment
For example, epithelial cells in taste buds and
in the nose specialize as chemical receptors
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Epithelial Membrane
Sheet of epithelial tissue and a layer of
underlying connective tissue
Types of epithelial membrane: mucous
membrane and serous membrane
Mucous membrane / mucosa Mucous membrane / mucosa
Lines a body cavity that opens to the outside of
the body, such as the digestive or respiratory
tract
The epithelial layer secretes mucus that
lubricates the tissue and protects it from drying
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Serous membrane
Lines a body cavity that does not open to the outside of the body
Consists of simple squamous epithelium over a thin layer of connective tissue
This type of membrane secretes fluid into the This type of membrane secretes fluid into the cavity it lines
Examples of serous membranes are the pleural membranes lining the pleural cavities around the lungs and the pericardial membranes lining the pericardial cavity around the heart
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Classification of Epithelium
The traditional classification of epithelium is
based on two factors:
The number of cell layers and
The shape of the surface cellsThe shape of the surface cells
The terminology, therefore, reflects only
structure, not function
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Epithelium is described as
simple, when it is one cell layer thick
Stratified, when it has two or more cell layers
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The individual cells that compose an
epithelium are described as
squamous, when the width of the cell is
greater than its height
Cuboidal, when the width, depth and height Cuboidal, when the width, depth and height
are approximately the same
Columnar, when the height of the cell
appreciably exceeds the width (the term low
columnar is often used when a cells height
only slightly exceeds its other dimensions)
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In a stratified epithelium, the shape and height of the cells usually vary from layer to layer, but only the shape of the cells that form the surface layer is used in classifying the epitheliumepithelium
For example, stratified squamous epithelium consists of more than one layer of cells, and the surface layer consists of flat or squamouscells
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Two special categories of epithelium are
pseudostratified and transitional
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Pseudostratified Epithelium
Appears stratified, although some of the cells
do not reach the free surface; all rest on the
basement membrane
Thus, it is actually a simple epithelium Thus, it is actually a simple epithelium
The distribution of pseudostratified
epithelium is limited in the body
Also it is often difficult to discern whether all
of the cells contact the basement membrane
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For these reasons, identification of
pseudostratified epithelium usually depends
on knowing where it is normally found
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Pseudostratified Epithelium
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Pseudostratified Epithelium
Typical locations:
Trachea and bronchial tree
Ductus deferens
Efferent ductules of epididymisEfferent ductules of epididymis
Major functions:
Absorption
Secretion
Conduit (a channel through which water or other fluid is carried)
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Transitional Epithelium (Urothelium)
A term applied to the epithelium lining the
lower urinary tract, extending from the minor
calyces of the kidney down to the proximal
part of the urethrapart of the urethra
Urothelium is a stratified epithelium with
specific morphologic characteristics that allow
it to distend
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Transitional Epithelium
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Transitional (Urothelium)
Typical locations:
Renal calyces
Ureters
BladderBladder
Urethra
Major functions:
Barrier
Distensible property
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Simple Squamous
Composed of a single layer of cells, which are flat
and plate like
In histologic sections, the nuclei appear flattened
and the cytoplasm is indistinctand the cytoplasm is indistinct
Although squamous refers to any flat epithelium,
its use is restricted as many flat epithelia are
given more specific names, e.g. the flat
epithelium lining blood vessels being called
endothelium
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Simple Squamous
Typical locations:
vascular system (endothelium)
Body cavities (mesothelium)
Bowmans capsule (kidney)Bowmans capsule (kidney)
Respiratory spaces in lung
Major functions:
exchange, barrier in central nervous system
Exchange and lubrication
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Simple Squamous
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Simple Cuboidal
A simple cuboidal epithelium is composed of a
single layer of cells whose height, width and
depth are the same (they are not strictly
cuboidal)cuboidal)
In histologic section, such cells usually have a
centrally placed nucleus
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Simple Cuboidal
Typical locations:
Small ducts of exocrine glands
Surface of ovary (germinal epithelium)
Kidney tubules
Thyroid folliclesThyroid follicles
Major functions:
Absorption
Barrier
Secretion
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Simple Cuboidal
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Simple Cuboidal
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Simple Columnar
Composed of cells whose height is two or
three times greater than their width
The nuclei of columnar cells are basal and
arranged in an ordered layerarranged in an ordered layer
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Simple Columnar
Typical locations:
Small intestine and colon
Stomach lining and gastric glands
GallbladderGallbladder
Major functions:
Absorption and secretion
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Simple Columnar
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Stratified Squamous
Composed of several layers such that cells high
up in the epithelium are not in contact with the
underlying extracellular matrix
Stratified squamous epithelium derives its name Stratified squamous epithelium derives its name
from the flattened (squamous) appearance of
cells in the superficial part of the epithelium
Cells in the basal and middle layers of this type of
epithelium are in fact pyramidal or polygonal and
are not flattened
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Stratified Squamous
Typical locations:
Epidermis
Oral cavity and esophagus
VaginaVagina
Major functions:
Barrier
Protection
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Stratified Squamous
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Stratified Cuboidal
Typical locations:
Sweat gland ducts
Large ducts of exocrine glands
Anorectal junctionAnorectal junction
Major function:
Barrier
Conduit
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Stratified Cuboidal
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Glandular Epithelium
Glands are epithelial tissue derivative
specialized for secreting
Glands are formed during embryonic
development by pockets of epithelial tissue development by pockets of epithelial tissue
that invaginate and develop secretory
capabilities
There are two categories of glands: exocrine
and endocrine
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During development, if the connecting cells
between the epithelial surface cells and
secretory gland cells within the invaginated
pocket remain intact as a duct between gland pocket remain intact as a duct between gland
and the surface, an exocrine gland is formed
E.g. sweat glands and glands that secrete
digestive juices
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If the connecting cells disappear during
development and the secretory gland cells are
isolated from the surface, an endocrine gland
is formed
Endocrine glands lack ducts and release their
secretory products (hormones) internally into secretory products (hormones) internally into
the blood
E.g. the pancreas secretes insulin into the
blood, which transports this hormone to its
sites of action throughout the body
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Classification of Glands
Glands can be classified according to:
Their histologic organization
Possession of ducts
Type of material secretedType of material secreted
Manner in which material is secreted
Glands that consist of only a single cell are
called unicellular; aggregates of secreting cells
form multicellular glands
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Unicellular Glands
Unicellular, exocrine gland goblet cell
Found scattered among epithelial cell lining
the trachea, small intestine and colon
The cell has a narrow base and an expanded The cell has a narrow base and an expanded
apex filled with secretory granules
Goblet cells elaborate mucin, which, on
hydration, produces viscous lubricating fluid
called mucus
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The mucin is secreted onto an epithelial
surface
Hence, a goblet cell is classified as unicellular
exocrine glandexocrine gland
Unicellular endocrine glands also occur
They are numerous in the epithelium lining
the gastrointestinal tract and produce a
number of peptide hormones and/or amines
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They are also found in the epithelium lining
the respiratory system
These cells secrete into the extracellular space
the product of which may enter adjacent the product of which may enter adjacent
vasculature
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Multicellular Glands
The simplest form of multicellular exocrine
gland is the secretory sheet, exemplified by the
gastric lining epithelium, in which the secreting
cells form a continuous epithelial layer
Intraepithelial glands are small clusters of Intraepithelial glands are small clusters of
secretory cells that lie wholly within an
epithelial sheet, clustered about a small lumen
Cells of both types of gland secrete their
product onto the epithelial surface
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Complex Exocrine Glands
Exocrine glands secrete onto a luminal
epithelial surface either directly, as in
secretory sheets and intraepithelial glands, or
by a ductal systemby a ductal system
If the duct system of complex glands
branches, the gland is said to be compound
If the duct system does not branch, the gland
is classified as simple
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The secretory cells may be organized into
tubules, acini or alveoli
Simple and compound glands can be named
by the shape of the secretory portionby the shape of the secretory portion
Simple glands can be classified as simple
tubular, simple coiled tubular, simple
branched tubular or simple branched acinar
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Compound glands are subdivided into
compound tubular, compound accinar and
compound tubuloacinar
* The epithelial cells forming the closely packed * The epithelial cells forming the closely packed
tubular structures of complex glands remain
arranged as a single layer of cell despite the
complex appearance of the overall glandular
structure