EPITHELIUM

TISSUE

Groups of cells which are similar in structure and which perform common or related functions.

There are (4) types of tissue:

1. Epithelial

2. Connective

3. Muscle

4. Nervous

TERMS TO KNOWTerms referring to the cell shapes

• Squamous = flat

• Cuboidal = cube

• Columnar = rectangular (column)

• Transitional = ability to change shape

Terms referring to the layers

• Simple = one layer

• Stratified = more than one layer

• Pseudostratified = false layered (appears to be more than one layer, but has only one layer)

EPITHELIUM TISSUE

A collection of closely apposed cells that lines the hollow organs and glands is called epithelium.

• Epithelium is avascular and is nourished by diffusion.• All epithelia are supported by loose connective tissue.• Between epithelium and connective tissue there is always

a basement membrane.

• Epithelial cells cover the internal and external surfaces of the body and also form parenchyma of various glands on the basis of which they are divided into two main groups

1: glandular epithelia

2: covering epithelia (surface epithelia)

EPITHELIAL CLASSIFICATIONS

Classified on the basis of shape of cell and number of cell layers Shape of cells

• squamous = thin, flat cells• cuboidal = cube-shaped cells• columnar = tall, elongated cells

Number of cell layers• simple = single layer• stratified = 2 or more layers

Apical surface

Basal surface Simple

Apical surfac

e

Squamous

Cuboidal

ColumnarClassification based on cell shape.

Glandular epithelium

Glandular epithelium is the membranous tissue made up of cells that covers all the glands in the body and secret fluids into ducts or fluids of the body.

• A gland consists of one cell or a group of cells• Endocrine Glands – secretions enter the interstitial fluid

• They secret Hormones• Exocrine Glands – secrete their products into ducts (tubes)

that empty at the surface of the covering/lining epithelium• They secrete Mucus, oil, earwax, digestive enzymes

COVERING EPITHELIA• The cells of the covering epithelia are arranged into one or

more layers which coat the body surface that is why it is also called surface epithelia.

Example: epidermis of the skin that covers the outer surface of the body is the best example

• Spaces and passages that open to outside are also lined by covering epithelia(respiratory, digestive, and urogenital passages)

• It also cover the internal(luminal surfaces) of heart, blood vessels and lymph vessels

Body cavities except joint cavities are also lined by surface epithelia

CLASSIFICATION OF COVERING EPITHELIA

Depending on the number of cell layers present covering epithelia are divided into following main types

1. Simple epithelia(unilaminar epithelia)

It has only single layer of cells.

2. Stratified epithelia(multilaminar epithelia)

Consist of two or more layers of epithelial cells.

SIMPLE EPITHELIA• It has only one layer of cells• Each cell has a free surface and a basal surface resting on basement

membrane.• According to the shape of the component cells simple epithelia are further

classified as under

1. simple squamous epithelium

This type has flat cells

2. simple cuboidal epithelium

It consists of cube like cuboidal cells

3. simple columnar epithelium

It consists of column shaped cells

4. simple pseudo stratified epithelium

Which really have one layer of cells but give the appearance of being stratified(having many layer of cells)

Simple squamous epithelium

Description: • Single layer of flattened cells• the simplest of the epithelia.• roughly polygonal• Cells are having serrated or wavy margins and from a

sheet• Has ovoid or flattened nuclei in located in center

Function: • Allows passage of materials by diffusion and filtration • secretes lubricating substances in serosae.Location: Kidney glomeruli; air sacs of lungs; lining of heart, blood vessels, and lymphatic vessels; lining body cavity (serosae).

b) Simple cuboidal epithelium

Description: • single layer of cube like cells with large, spherical central

nuclei.• the height and width of each cell is equal• the cells form a mosaic

Function: Secretion and absorption.

Location: Kidney tubules; ducts and secretory portions of small

glands; ovary surface.

(c) Simple columnar epithelium

Description: • Single layer of tall cells with round to oval nuclei located in the base

of the cell• some cells bear cilia• layer may contain mucus- secreting unicellular glands (goblet cells).• On surface view appear like a mosaic as like simple cuboidal cells

Function: Absorption and secretion of mucus, enzymes, and other substances;

ciliated type propels mucus (or reproductive cells) by ciliary action.

Location:• Nonciliated type lines most of the digestive tract (stomach to anal

canal), gallbladder, and excretory ducts of some glands; • ciliated variety lines small bronchi, uterine tubes, and some regions

of the uterus.

SIMPLE EPITHELIUM

(d) Pseudo stratified (columnar )epithelium

Description: • this is a modification of simple columnar epithelium in which

the nuclei are at different levels giving the false appearance of being stratified

• Single layer of cells of differing heights, some not reaching the free surface

• may contain mucus secreting cells and bear cilia.• All cells rest on the basement membrane but all do not

extend to the free surface• Those that reach the free surface are tall columnar cells and

are widest near the free surface but basal part of each cell narrows to form a slender process

(d) Pseudo stratified (columnar )epithelium

• A second kind of cells called basal cells are present between the columnar cells, they are conical in shape with a broad base resting on the basal lamina

• The basal cells taper early and do not reach to the surface

Function: Secretion, particularly of mucus; propulsion of mucus by

ciliary action.

Location: • Nonciliated type in male’s sperm-carrying ducts and

ducts of large glands• ciliated variety lines the trachea, most of the upper

respiratory tract.

(d) Pseudo stratified (columnar )epithelium

STRATIFIES EPITHELIUM

Epithelia that consists of two or more layers is called stratified epithelium.

Further classified on the basis of shape of cells in the superficial layer as under

1. Stratified squamous

2. Stratified cuboidal

3. Stratified columnar

(e) Stratified squamous epithelium

Description: • Thick membrane composed of several cell layers; basal

cells are cuboidal or columnar and metabolically active;

• surface cells are flattened (squamous); • basal cells are active in mitosis and produce the cells of the

more superficial layers.

• In places where the surafce cells are placed to more attrition and drying like skin epidermis the cells are non-nucleated and the cells cytoplasm has protein keratin and are called keratinized cells.

(e) Stratified squamous epithelium

• In wet areas the cells are nucleated and contain little or no keratin.

• In both sub-types of stratified squanous epithelium the most superficial layer constantly flake off from the surface.

• The basal layer produce new cells which move toward the surface and flake off.

Function: Protects underlying tissues in areas subjected to abrasion.

Location: • Nonkeratinized type forms the moist linings of the

esophagus, mouth, and vagina• keratinized variety forms the epidermis of the skin.

STRATIFIED CUBOIDAL EPITHEILIUM

• Consists of two layers of cuboidal cells

• Location: ducts of sweat glands

STRATIFIED COLUMNAR EPITHELIUM

Consists of columnar cells resting on one or more layers of roughly cuboidal cells.

Location:• lines ducts of main salivary glands and pancreas• Lines ductus deferens and male urethra

(f) Transitional epithelium

Description: • Resembles both stratified squamous and stratified cuboidal

epithelium.• basal cells are cuboidal or columnar• surface cells are dome shaped or squamous like, depending

on degree of organ stretch.

Function: Stretches readily and permits distension of urinary organ by contained urine.

Location: Lines the ureters, urinary bladder, and part of the urethra.

TWO SPECIAL VARIETIES OF EPITHELIAL CELLS

1: NEURO-EPITHELIAL CELLS:• These are columnar cells and have cilia or sterocellia on

their free surface.• Found in special sense organs like taste buds and

vestibular receptor system.• These cells are specialized as sensory receptors for the

reception of external stimuli.• Afferent nerve fibers terminate at neuro-epithelial cells

and form synapse with them.

TWO SPECIAL VARIETIES OF EPITHELIAL CELLS

2: MYO-EPITHELIAL CELLS:• Also called basket cells.• Their cytoplasm contain contractile filament(actin,myosin)• These cells contract when stimulated by nervous or neuro-

hormonal signals• Found around secretory acini of mammary ,salivary, lacrimal

and sweat glands.

Function:• Their contraction assist in the flow of secretion into the larger

channels.

Microvilli• Small slender, finger like projections found on surface of epithelial

cells• E/M reveals that they are cytoplasmic processes that extend from

cell surfaces• E/M also shows that each microvillus contains a central bundle of 20

to 30 actin filaments.• they Insert into terminal web

LOCATION:• Columnar cells lining the luminal surface of small intestine has

numerous microvilli• Also located in proximal convoluted tubules of kidney where they

are longer and appear as a brush border on L/M

FUNCTION:• Increase area for absorption as in small intestine• Facilitate the absorptive process

CILIUM• Hair like structure found on the free surface of those

epithelial cells which are specialized for transport of fluid or mucus .

• They are Motile processes of microtubules that move synchronously

• Visible under L/M measuring 5-10 micro-meter in length and 0.2 micrometer in diameter.

• They can move to and fro and are also called kinocillia.

• Distinguished from sterocillia which are nonmotile.

CILLIUM………….

• 9+2 microtubule arrangement, two central microtubules surrounded by 9 circularly arranged doublet peripheral microtubules

• The microtubules extend from the tip of the cilium to the base

FUNCTION:

Helps in • propulsion or

movement of fluidmucusparticulate

matter ovum

STEREOCILIA• Under L/M appear as thin, hair like structure which are in contact with

each other and form small tufts.• Under light microscope they resemble cilia but they are non-motile and

that is why they are called sterocillia (stero=solid or non-motile)• Long microvilli (NOT CILIA)• Non-motile• Average length of a sterocillium is 30 micrometer

LOCATION:• Located in the cells lining the ducts of epididymis• In neuroepithelium• In taste buds

FUNCTION:

Increase the surface area in epididymis to facilitate reabsorption.

In taste buds they respond to electrical signals

FLAGELLUM

• A long whip like motile projection from a cell is called flagellum.

• It has two central microtubules and 9 peripheral doublets like cilia but a flagellum has a greater length.

• Its movement is different from that of cilia

FUNCTION:• Help in cell movements

LOCATION:

Spermatozoon(sperm cells)