Biology Today - February 2018

II. Compound Epithelia (Multilayered Epithelia)

(a) Stratified epithelium

Stratified squamous epithelium

Consists of several cell layers. In the deepest

layers, the cells are mainly columnar and as

they grow towards the surface they become

flattened. It is of following two types:

(i) Keratinised stratified squamous

epithelium : In the outer few layers, the cells

replace their cytoplasm with a hard, water

proof protein, the keratin. These layers of

dead cells are called stratum corneum or

horny layer.

(ii) Non-keratinised stratified squamous

epithelium : It lacks keratin and is unable to

check water loss and provides only moderate

protection against abrasion.

Occurs in the epidermis of the skin of

land vertebrates.

Occurs in the oral cavity (buccal cavity),

tongue, pharynx, oesophagus, anal canal,

lower parts of urethra, vocal cords, vagina,

cervix (lower part of uterus), conjunctiva,

inner surface of eyelids and cornea of eye.

Protection of underlying

structure against drying

out, abrasions and

infection.

Stratified cuboidalepithelium

Consists of outer layer of cuboidal cells and

basal layer of columnar cells.

Forms the epidermis of fishes and many

urodeles (tailed amphibians such as

salamanders). It also lines the sweat

gland ducts and larger salivary and

pancreatic ducts.

Protection

Stratified columnarepithelium

Columnar cells present in both superficial and

basal layers.

Covers the epiglottis and lines mammary

gland ducts and parts of urethra.

Secretion and

protection

Stratified ciliated columnarepithelium

Outer layer consists of ciliated columnar cells

and basal layer of columnar cells.

Lines the larynx and upper part of the

soft palate.

Movement of

secretions

(b) Transitional epithelium

Also known as

Urothelium

Consists of 4 to 6 layers of cells. The cells

of deepest (= basal) layer are columnar or

cuboidal while the cells of middle layer are

polyhedral or pear-shaped. The cells of the

surface layer are large and globular or umbrella

shaped. When the epithelium is stretched, all

the cells become flattened.

Found in the renal calyces, renal pelvis,

ureter urinary bladder and part of the

urethra. Because of its distribution, it

is also called urothelium (epithelium

present in the urinary system). The

transitional epithelium of the urinary

bladder can be stretched considerably

without being damaged.

Permits distention. It

is also protective in

function

1. Neha Joshy - Alappuzha, Kerala


2. D.B. Rakshit Narayan

• Annanagar, Chennai

December-2017

Winners

All : Asteroidea

Have : Holothuroidea

Calcareous : Crinoidea

Ossicle : Ophiuroidea

Endoskeleton : Echinoidea

To easily remember the five classes of Echinoderms, the mnemonic

“All Have Calcareous Ossicle Endoskeleton” can be used as follows: