CYT 2113 Cytology I

Lesson 3: Introduction to Different

Types of Epithelial Cells I

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

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

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

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

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

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

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

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

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

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

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

Epithelium is described as

simple, when it is one cell layer thick

Stratified, when it has two or more cell layers

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)

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

Two special categories of epithelium are

pseudostratified and transitional

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

For these reasons, identification of

pseudostratified epithelium usually depends

on knowing where it is normally found

Pseudostratified Epithelium

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)

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

Transitional Epithelium

Transitional (Urothelium)

Typical locations:

Renal calyces

Ureters

BladderBladder

Urethra

Major functions:

Barrier

Distensible property

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

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

Simple Squamous

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

Simple Cuboidal

Typical locations:

Small ducts of exocrine glands

Surface of ovary (germinal epithelium)

Kidney tubules

Thyroid folliclesThyroid follicles

Major functions:

Absorption

Barrier

Secretion

Simple Cuboidal

Simple Cuboidal

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

Simple Columnar

Typical locations:

Small intestine and colon

Stomach lining and gastric glands

GallbladderGallbladder

Major functions:

Absorption and secretion

Simple Columnar

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

Stratified Squamous

Typical locations:

Epidermis

Oral cavity and esophagus

VaginaVagina

Major functions:

Barrier

Protection

Stratified Squamous

Stratified Cuboidal

Typical locations:

Sweat gland ducts

Large ducts of exocrine glands

Anorectal junctionAnorectal junction

Major function:

Barrier

Conduit

Stratified Cuboidal

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

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

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

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

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

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

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

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

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

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

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