Cells of the epithelium are set very close to each other, separated by very thin films of extracellular material.

Neighbouring cells are held together by cell junctions.

The epithelial tissue rests on a noncellular basement membrane, which separates it from the underlying connecting tissue.

The basement membrane is a non-cellular membrane made of two layers:

(i) Upper thin layer called basal lamina, made up of glycoproteins and mucopolysaccharides and secreted by epithelial cells.

(ii) Lower thick fibrous layer called reticular lamina made of reticular fibres and collagen fibres which are a part of underlying connective tissue.

Blood vessels are absent in the epithelial tissue. Materials are exchanged between epithelial cells and vessels of the connective tissues by diffusion across the basement membrane.

The epithelial tissue is classified into simple and compound epithelia.

Specialized junctions between epithelial cells:

To provide mechanical support for the tissue plasma membrane of adjacent epithelial cells modified to form following structures called as Intercellular Junctions.

Tight junctions (Zonula occludens) : help to prevent substances from leaking across the tissue. Plasma membranes in the apical parts become tightly packed together or are even fused.

Interdigitations : These are interfitting, finger like processes of the cell membranes of the adjacent cells.

Intercellular Bridges : These are minute projections that arise from adjacent cell membrances.

They make contact with one anther.

Gap Junctions : Facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.

Intermediate Junctions (= Zonula adherens) : These usually occur just below tight junctions. The intercellular space at these places contains a clear, low electron density fluid. There is a dense plaque like structure on cytoplasmic side of each plasma membrane from which fine microfilaments of actin (protein) extend into the cytoplasm. There is no intercellular filaments between the adjacent cell membranes. There is an adhesive material at this point. They probably serve anchoring functions.

Desmosomes ( =Macula adherens) : Perform cementing to keep the neighbouring cells together. These are like zonula adherens but are thicker and stronger and are disc like junctions. They have intercellular protein. The plaque-like structures (= protein plate) are much thicker. The microfilaments which extend from protien plat are called tonofibrils. Desmosomes serve anchoring function. Hemidesmosomes (single sided desmosomes) are similar to desmosomes, but the thickening of cell membrane is seen only on one side. Hemidesmosomes join epithelial cells to basal lamina (outer layer of basement membrane).

Classification of Epithelial Tissues

1. Simple Epithelium

It is formed of a single layer of cells.

The adjacent cells are held together by means of desmosome, resting on the basement membrane.

Simple epithelium occurs mainly on secretory and absorptive surfaces.

It helps in nutrition, excretion and secretion but not for protecting the underlying tissue.

(i) Squamous Epithelium :

It consists of a layer of thin, flat, scale-like cells with prominent nuclei.

The cells have irregular boundaries that fit closely into those of neighbouring cells.

It forms the inner lining of lung alveoli and blood vessels (Endothelium).

It is also known as pavement or tesselated epithelium.

(ii) Cuboidal Epithelium

It has cells which are polygonal in outline, but appear cuboidal in vertical section.

It lines small salivary and pancreatic ducts and thyroid vesicles.

The cells participate in secretion, excretion and absorption.

The cells of cubical epithelium in absorptive surfaces often bear microvilli on their free ends. This gives a brush-like appearance to their free border.

They are, therefore, called brush-bordered cubical epithelial cells e.g., in proximal tubules of kidneys.

Microvilli greatly increase the area of the free surface of the cell and thereby enhance absorption.

(iii) Columnar Epithelium :

It is characterised by the presence of tall cells shaped like polygonal columns.

The nucleus is usually located at the base of the cell.

Columnar epithelium covers the inner surface of the intestine, stomach and gall bladder.

It also occurs lin gastric and intestinal glands.

Its function is secretion or absorption.

The intestinal mucosa is lined by Brush Bordered Columnar Epithelium which is highly absorptive.

(iv) Ciliated Epithelium :

It consists of columnar or cubical cells bearing cilia on their free surfaces.

The function of the cilia is to move particles, free cells or mucus in a specific direction over the epithelial surface.

Ciliated columnar epithelium lines the inner surfaces of some hollow organs such as fallopian tubes, bronchioles and small bronchi.

Ciliated columnar epithelium lining the ventricles of brain and spinal canal is called as ependyma.

Cilia is of two types

(a) Kinocilia are motile cilia with 9 + 2 organisation,

(b) Stereocilia - Basal granule absent, non-motile, Ciliated columnar epithelium 9 + 2 organisation is absent. Stereocilia are found in some parts of the male reproductive tracts such as the epididymis and vas deferens.

(v) Pseudostratified Epithelium :

It covers the inner linings of trachea and large bronchi.

Although made up of a single layer of columnar cells, it appears two-layered, because some cells are shorter than the others and have their nuclei at different levels.

The shorter cells lack cilia and secrete mucus which traps particles on the epithelial surface.

The longer cells are ciliated.

The ciliary movements propel the mucus and the particles towards the larynx.

Pseudostratified non ciliated columnar epithelium tissue is found in urethra of male and parotid salivary gland.

Squamous showing some keratinisation

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Special Types of Epithelium

(a) Neuro sensory epithelium :

In between piller shaped supporting cells modified sensory cells are present. On the free end sensory hair is present. Base of these cells is attached with sensory nerve.

e.g. – Gustatory Epithelium – Cover taste bud of tongue and receive taste sensation.

– Olfactory epithelium – Schneidarian membrane receive smell sensation.

– Stato – acoustic – Lining of internal ear.

– In retina of eye receive optic sensation.

(b) Myoepithelium : Around mammary and sweat gland

(c) Pigmented epithelium (Cuboidal) : In Retina of eye

2. Compound Epithelium

It consists of more than one layer of cells.

Only the cells of the deepest layer rest on the basement membrane.

Being multilayered, compound epithelia have little role in secretion or absorption, but they provide protection to underlying tissues against mechanical, chemical, thermal or osmotic stresses.

Compound epithelia may be stratified or transitional.

(i) Stratified Epithelium :

It has many layers of epithelial cells.

The deepest layer is formed by cuboidal cells.

But the morphology of the superficial layers varies in the different kinds of stratified epithelia.

In stratified cuboidal epithelium, the superficial cells are cuboidal.

It lines the inner surfaces of larger salivary and pancreatic ducts.

Stratified non-keratinised Squamous Epithelium covers moist surfaces such as those of buccal cavity, pharynx and oesophagus.

It has several superficial layers of living squamous cells and deeper layers of interlinlked polygonal cells.

Stratified Keratinised Squamous Epithelium covers the dry surface of skin.

It has many superficial layers of horny, scale-like remains of dead squamous cells and several deeper layers of living polygonal cells.

Heavy deposits of the insoluble protein keratin in the dead superficial cells make the epithelium impervious to water and highly resistant to mechanical abrasions.

In contrast, nonkeratinised stratified epithelia cannot prevent water loss and afford only moderate protection against abrasions.

(ii) Transitional Epithelium :

It is much thinner and more stretchable than the stratified epithelium.

It has a single layer of cuboidal cells at the base, 2-3 middle layers of large polygonal or pear-shaped cells and a superficial layer of large, broad, rectangular or oval cells.

It lines the inner surface of the urinary bladder and ureters.

It allows considerable expansion of these organs to accommodate urine, because stretching considerablly flattens and broadens the cells of superficial and middle layers.

3. Glandular Epithelia

The cells of glandular epithelia are generally columnar or cuboidal.

The glandular epithelium can be classified into two types : unicellular, consisting of isolated glandular cells (e.g., goblet cell of alimentary canal), and multicellular (e.g. salivary glands), consisting of cluster of cells.

A gland with a single unbranched duct is called a simple gland.

The secretory part of the gland consists of epithelial cells arranged in the form of tubes (tubules) or sacs (acini, alveoli) or a combination of both.

The duct is also made up of epithelial cells.

A gland with a branched system of ducts is called a compound gland.

In these glands, the secretory tubule or acinus may be coiled or branched, opens into the single duct of the gland.

Compound glands are present in the pancreas and sub-mandibular salivary glands.

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Types of simple gland

(a) Simple tubular: Simple tubular glands are present in the intestine (e.g., Crypts of Leiberkuhn).

(b) Simple alveolar: Terminal part forms alveolus e.g., Mucous glands in skin of frog, poison glands in toad.

(c) Simple coiled tubular: Terminal part is coiled e.g., sweat glands.

(d) Branched tubular: Gastric glands in stomach.

(e) Branched alveolar e.g., Sebaceous g.land .

Types of compound gland

(a) Compound tubular gland: e.g., mammary glands of prototherians.

(b) Compound saccular or alveolar gland: e.g., Salivary glands, (sub-maxillary and sub-lingual).

(c) Compound tubulo alveolar or tubulo saccular: They are tubular as well as alveolar and are found in mammary glands, pancreas, parotid salivary gland, Cowper's glands and Bartholin glands.

(ii) Exocrine glands have a secretory portion which contains the cells for secretion of milk, digestive enzymes, mucus, saliva, ear wax, oil and ducts which transport their secretions to the respective sites of action, for example, salivary gland, tear gland, gastric gland and intestinal glands. When a gland performs both exocrine and endocrine functions, it is called a mixed gland or Heterocrine gland (e.g., the pancreas, testis, ovaries).

(iii) On the basis of mode of secretion, glands can be :

(a) Holocrine glands: In holocrine glands (e.g., sebaceous gland), the product of secretion is shed with the whole cell leading to its destruction.

(b) Merocrine glands: When the secretory granules leave the cell by exocytosis (simple diffusion) with no loss of other cellular material , the glands are called merocrine glands (e .g. , the pancreas, salivary glands, intestinal glands and sweat glands).

(c) Apocrine glands: In apocrine glands (e.g., mammary gland and axillary sweat glands), only the apical portion of the cytoplasm is discharged along with the secretory product.