Epithelium I

Khalid Said Mohammad, MB.,BCh, MSc, Ph.D.

Professor of Anatomy
Histology Course Director
Department of Anatomy, Research Director
Master Program, Molecular and Cell Biology Track Director
Kmohammad@alfaisal.edu
 Learning Objectives
 Describe the primary characteristics and functions of
epithelial tissues.
 Classify epithelium according to its structure and
organization.
 Describe how epithelial organization relates to its function
in different organs or locations
 Describe the ultrastructure of the basement membrane
 Describe the ultrastructure of the cell junctions
 The Basic type of Tissues
 Each tissue is an assemblage of similarly specialized cells united in
performing a specific function
 Each contains an extracellular matrix (ECM) and cells, associated
with one another in the variable proportions and morphologies
characteristic of each organ
 The human body is composed of only four basic tissue types:
1. Epithelial
2. Connective
3. Muscular
4. Nervous
Main characteristics of the four basic types of tissues
 Characteristic Features of Epithelial Cells
 Line and cover all body surfaces except the articular
cartilage, the enamel of the tooth and the anterior
surface of the iris
 Lack a direct blood and lymphatic supply. Nutrients
are delivered by diffusion
 Have very little extracellular matrix (ECM)
 The cohesive nature of an epithelium is maintained by

cell adhesion molecules and junctional complexes

 Anchored to a basement membrane
 Have structural and functional polarity
 Derive from the ectoderm, mesoderm and endoderm
 Most epithelial cells renew continuously by mitosis
 Functions of the Epithelium
The principal functions of epithelial tissues
include the following:
 Covering, lining, and protecting surfaces
(e.g., epidermis)
 Absorption (e.g., the intestinal lining)
 Secretion (e.g., parenchymal cells of glands)
 Excretion (kidney tubules)
 Gas exchange (lungs)
 Classifications of Epithelial Tissue
 The classification and nomenclature are based on two
parameters:

1. The shapes of individual cells

2. The arrangement of the cells in one or more layers
3. Other special features (will discuss later)
 Classifications of Epithelial Tissue
Squamous
 The shapes of individual cells
1. Flattened (squamous cells)
2. Have equal dimensions (cuboidal cells)
3. Taller than wider (columnar cells)
Columnar

Cuboidal
 Classifications of Epithelial Tissue
 The number of cell layers:
1. Single-cell layer (simple) Cuboidal
2. More than one layer (stratified)
 The stratified epithelium is classified according to
the cell shape of the superficial outer layer;
squamous, cuboidal, or columnar

Stratified
 Simple Squamous Epithelium
 Consists of a single layer of flattened cells Squamous

 When viewed from the side, the cells appear

flattened; in a top-down view they appear
round
 Minimizes the distance between the apical and
basal sides of the cell and thus maximizes
diffusion efficiency, both through and between
the cells
 Found in tissues and organs that utilize
diffusion for molecular movement like
capillaries
 Simple Cuboidal and Columnar Epithelium
 Cells in simple cuboidal or simple columnar
epithelia contain more cytoplasm Than
squamous epithelium
 Are involved in more active transport of
molecules from one side of the epithelium to
the other using pump and channel molecules
 Movement of these molecules requires ATP,
so these cells typically have large numbers
of mitochondria
 Classifications of Epithelial Tissue
Epithelium that does not “exactly” fit with the previous categories
Pseudostratified columnar epithelium:
 Appears stratified, all rest on the basement membrane but some do not
reach the free surface
 The nuclei of these two types of cells form two distinct rows, giving the
appearance of a stratified
epithelium
Typically have surface
specializations such as
cilia that aid in movement
of mucus or absorption of
other material
 Classifications of Epithelial Tissue
Epithelium that does not “exactly” fit with the previous categories
Transitional epithelium (Urothelium):
 specific morphologic characteristics that allow it to distend
 Special stratified epithelium that is not classified by the shape of cells at
surface as other epithelia (umbrella cells)
Lines part of the urinary
tract, e.g., the urinary
bladder
 Epithelial Tissue Orientation
 It is important to note that an epithelium is a
flat sheet of cells, and a section of tissue may
be cut through an epithelium at different Cuboidal
angles
 Think of the epithelium as a sheet of flat
object with some thickness If viewed from
the side, it will appear like the object labeled
“side view”. If viewed from above, the Top view
epithelium will look like the object labeled
“top view”
Side view
 Specialization of the Apical surface
 Cilia:
some simple & pseudostratified
columnar epithelia are ciliated and
hence are classified as simple
columnar or pseudostratified
columnar ciliated when the apical
surface domain possesses cilia

 Keratin: Layer of anucleate cells

at the surface of the stratified
squamous epithelium. Keratinized
stratified squamous
How to Classify Epithelium?

Transitional

Ciliated/non

Keratinized/non
 Basement Membrane
 The basal surface of all epithelia rests on a thin
extracellular, felt-like sheet of macromolecules
referred to as the basement membrane
 It is semipermeable filter for substances reaching
epithelial cells from below
 Glycoproteins and other components in this
structure can often be stained and visualized
with the light microscope
 Ultrastructure of the Basement Membrane
 With TEM, two parts of the basement membrane may be resolved.
Nearest the epithelial cells is the basal lamina, a thin, electron-dense,
sheet-like layer of fine fibrils, and beneath this layer is a more diffuse and
fibrous reticular lamina
 “basement membrane” and “basal
lamina” are sometimes used
interchangeably, but “basal lamina”
usually denotes the fine extracellular
layer seen ultrastructurally and
“basement membrane” the entire
structure beneath the epithelial cells
visible with the light microscope
 Molecular Structure of the Basement Membrane
Basal Lamina
The macromolecules of the basal lamina are
secreted from the basal sides of the
epithelial cell
Type IV collagen
Laminin: These are large glycoproteins that
attach to transmembrane integrin proteins
in the basal cell membrane
Nidogen and perlecan: Respectively a short,
rod-like protein and a proteoglycan, both of
these cross-link laminins to the type IV
collagen network
 Molecular Structure of the Basement Membrane
Reticular Lamina
 Reticular lamina contains type III
collagen and is bound to the basal
lamina by anchoring fibrils of type
VII collagen, both of which are
produced by cells of the
connective tissue
 Polarity
 Epithelial cells generally show polarity,
with organelles and membrane proteins
distributed unevenly within the cell
 The apical (uppermost) domain is
exposed to the lumen or external
environment
 The lateral domain faces neighboring
epithelial cells linked to each other by
cell adhesion molecules and junctional
complexes
 The basal domain is associated with a basement membrane that separates
the epithelium from underlying connective tissue. With the presence of
Cell-to-extracellular matrix (ECM) junctions
 Characteristic Features of Epithelial Cells
 Line and cover all body surfaces except the articular
cartilage, the enamel of the tooth and the anterior
surface of the iris
 Derive from the ectoderm, mesoderm and endoderm
 Most epithelial cells renew continuously by mitosis
 Lack a direct blood and lymphatic supply. Nutrients
are delivered by diffusion
 Have almost no free intercellular substances
 The cohesive nature of an epithelium is maintained by

cell adhesion molecules and junctional complexes

 Anchored to a basal lamina
 Have structural and functional polarity
 Epithelial Cell Junctions
 Epithelial cells adhere strongly to
neighboring cells and basal laminae,
particularly in epithelia subject to friction
or other mechanical forces
 Several membrane-associated structures
provide adhesion and communication
between cells
 Some are present in other tissues, but all
are particularly numerous and prominent
in epithelia
 Epithelial Cell Junctions
Tight or occluding junctions form a seal between adjacent cell
 Anchoring junctions are sites of strong cell
adhesion. Three major types Tight or occluding
junctions
 2 cell to cell,
 Desmosomes (Spot-like)
 Adherents junctions (Belt-like)
 2 cell to extracellular environment
 Focal Adhesions
 Hemidesmosome (Spot-like)
 Gap junctions are channels for
communication between adjacent cells
Focal Adhesions Hemidesmosomes
 Tight Junctions (Occluding Junctions)
 It is the most apical component in the junctional complex
between epithelial cells
 Not a continuous seal but a series of focal fusions
between the cells created by transmembrane molecules
 Separates the luminal space from the intercellular space
and connective tissue compartment
proteins of adjoining cells
 Three main transmembrane proteins
are found:
1. Occludin
2. Claudins
3. Junctional adhesion molecule
(JAM)
 Molecular Structures of the Tight Junctions
 Occludin and claudin have four
transmembrane domains with two
extracellular loops, but JAM has only
a single transmembrane domain,
and its extracellular portion
 Associated proteins of the occluding
junction (ZO-1, ZO-2, ZO-3) and
Interact with each other
 The associated proteins, ZO-1,
interacts with the cell cytoskeleton
binding actin filaments
 Anchoring Junctions: Adherent Junctions
 Encircles the epithelial cell,
usually immediately below the
tight junction
 Firmly anchoring a cell to its
neighbors Adherent Junctions
 Cell adhesion is mediated by
cadherins, transmembrane
glycoproteins of each cell that
bind each other in the presence
of Ca2+
 Anchoring Junctions: Adherent Junctions

 Cadherins bind catenins that link to

actin filaments with actin-binding
proteins. The actin filaments linked to
the adherens junctions form part of
the “terminal web,” a cytoskeletal
feature at the apical pole in many
epithelial cells
 E-cadherin, member of this family, It
also acts as an important suppressor
of epithelial tumor cells
 Anchoring Junctions : Desmosomes
 Is a major anchoring cell-to-cell junction
that provides a particularly strong
attachment
 Desmosomal attachment plaque anchors
intermediate filaments
 The intercellular space occupied by the
extracellular portion of desmogleins and
desmocollins, which are members of the
cadherin family of Ca2+ -dependent cell
adhesion molecules
In the presence of Ca2+ desmogleins and
desmocollins bind adjacent identical
molecules of neighboring cells
 Anchoring Junctions : Focal Adhesions
 They form a structural link between the actin
cytoskeleton and extracellular matrix proteins,
attaching actin filaments into the basal lamina

 Consists of:
 A cytoplasmic face bound to actin filaments through
acting binding proteins (α-actin, Paxillin, vinculin and
talin)
 A transmembrane connecting region (Integrin are the
main transmembrane family of proteins)
 An extracellular face that binds to the extracellular
matrix glycoproteins (fibronectin)
 Hemidesmosomes
 Found on the basal cell surface
 Provide increased adhesion to the basal lamina in certain epithelia subject to
abrasion and mechanical shearing forces that would tend to separate the
epithelium from the underlying connective tissue
such as cornea, the skin, and the
mucosa of the oral cavity
 It exhibits an intracellular attachment
plaque on the cytoplasmic side of the
basal plasma membrane
 Three major proteins have been
identified in the plaque: Plectin, BP 230,
Erbin
 Summary

 What is the epithelium

 Different types of epithelium and how to identify them
(simple, stratified, etc.,)
 Structure of the basement membrane/basal lamina
and the difference between the 2 names
 Epithelial polarity and its importance in epithelial
functions
 Different types of cell junctions
Thank you!

Questions?