STRUCTURE

• Epithelial tissue is characterized by its distribution over the surface, both external and internal, and the structure, which have been derived from such surfaces. The epithelial cells are always closely packed in one or more layers and the intercellular material between them is very little. Intercellular substance i.e., tissue fluid mainly consists of Glycoprotein.


• Cells upon Basement membrane of MUCOPOLYSACCHARIDES + GLYCOPROTEINS + Collagen fibre layer; Selectively permeable.


• Being at the surface, the epithelial tissue is nearest to the environment, hence plays an important role in the life activities of the animal. It is concerned with protection, respiration, secretion, assimilation and elimination of waste products.


• Epithelium derived from the mesoderm is termed mesothelium which is found in the peritoneum that underlines the body wall and covers the viscera. Mesothelium on the inner surface of the blood vessels and heart is called endothelium.


• The true coelomic cavities are called peritoneal cavities. A true coelom is a body cavity which is lined by mesoderm.This mesodermal lining of embryo gives rise to peritoneum of adult. A body cavity is the cavity present between body wall and visceral (internal) organs of an individual. Thus a coelom will be seen as -



Fig. 1.5 showing Coelom and Peritoneum




• Epithelium is often associated with a thin connective tissue layer, the lamina propria or basement membrane. Such an association on the inner surface of alimentary canal and other tracts is known as Mucous membrane.


• A similar association of the Coelomic epithelium forms the Serous membrane, which lines the Coelomic cavities and also forms the mesenteries serving to suspend the viscera in the Coelomic cavities. Serous-membrane cells produces a watery lymph-like fluid that, besides other functions, serves to prevent damage due to friction amongst the parts of the viscera by lubricating their surfaces.


• Epithelial cells, in a single layer, form a simple or unilaminar epithelium, several layers one above the other constitute a Stratified or Multilaminar epithelium.

or

• With a single layer of cells.


• Present at surfaces where material transfer takes place by secretion, absorption or diffusion,


• Normal wear and tear - negligible. This can be -





• Simple plate like or flat or Scale like cells, usually polygonal;


• Serve for easy exchange of material across them. These cells looks like -





Fig. 1.6–Simple squamous epuithelium



• Examples - At pericardial, Perineural & Peritoneal cavities + Cavities of blood, lymph vessels, Heart, terminal bronchioles, airsacs, alveoli of lungs, Bowman's capsules, descending limb of henle's loop, rete testes & some small gland ducts.


• This epithelium can be called Tessellated when Plasma membrane wavy due to extensive interdigitations. (In particular Mesothelium).


• For Example Coelomic epithelia (ciliated) +(Endothelium) Epithelia of heart, lymph & Blood vessels.
  
  







• Of Cuboidal cells with equal height and width;


• suited for an active role in Secretion, absorption and excretion. These cells looks like -







Fig. 1.7 Simple cuboidal epithelium




• Examples include linings of convoluted parts of uriniferous tubules, ciliary body +Choroid, Thin bronchioles, sweat glands + Germinal epithelia.







• Tall & Prismatic cells, Usually with some mucus secreting goblet cells looks like -



–

Fig. 1.8 Simple columnar epithelium

• Examples include Gut epithelium from stomach to anus, Absorptive cells of intestine (with brush border), Bile duct, Gallbladder Linings etc. Together with lamina propria these called mucous membranes. Other examples are - lining of uterine tubes, tympanic cavity, small bronchi (lung)), Oviducts, Ureters, neurocoel of brain & spinal cord ciliated


• Pseudostartified Columnar epithelium contain single layer of cells which looks like multilayered


• Its cells are mostly ciliated & with Goblet cells. Ex. Pharynx, nasal passage, trachea, large bronchi, vasa diferentia, epididymis, male urethra + Larger gland ducts.


• Pseudostratified is more common in Non chordates.
  
  



Startified






• Bi or Multilaminar epithelia, Regular wear and tear seen, lowest layer germinating usually.


• This can be of following types -







Fig. 1.9 Keratinizing stratified epithelium





Specialised

1.Transitional eg., In the wall of urinary bladder, ureters and upper part of urethra. In relax conditions it looks like Multilaminar structure while during conditions of tension it usually looks like a trilaminar structure.



Fig. 1.10 Transitional epithelium

–

2.Neurosensory-eg.,Schneiderian membrane [olfactory mucosa],lining of internal ear, eye retina, epithelial covering of tongue.

3.Pigmented-eg., Epithelial cells of basal layer of retina.

GLANDULAR EPITHELIA

Glands are basically of two types

[a] Unicellular glands Ex. Goblet cell

[b] Multicellular Glands These are of three-types

(i) Exocrine Duct glands

(ii) Endocrine Ductless glands

(iii) Mixed.

Exocrine Glands :-- These may be



• Simple(Secretion through Single unbranched duct) Eg. Intestinal, Mammalian Sweat glands, Cutaneous glands of frog.


• Tubular Eg. SimpleIntestinal glands; CompoundGastric glands & Kidney.


• Acinous (rounded) Eg. SimpleCutaneous glands of frog; Compound Liver + Some Gastric glands.


• Compound With branched ducts. Eg. Liver, Pancrease, Salivary duct etc.


• Alveolar (Flask shaped) Eg. Sebaceous glands of mammals & Salivary glands


• Holocrine Eg. Sebaceous gland of mammals


• Merocrine or Epicrine Secretion by diffusion Eg. Goblet cells, most sweat glands, Salivary & intestinal glands.


• Apocrine Mammary & Some Sweat glands; Apical part + Secretary products both discharged.







Merocrineglands are of following types -



(i) Serous (Sweat, intestinal & Parotid glands) Secrete a clear watery fluid.

(ii) Mucous glands Secrete mucous, Ex. Goblet cells of gut epithelium, some gastric glands, Glands of colon etc.

(iii) Mixed Ex. Pancreas, most gastric glands, Submandibular Salivary glands.



· Connective Tissue






• In connective tissues the fibres are found as suspended in the gelatinous ground substance which is made up of proteoglycons and glycoproteins and relatively fixed due to the presence of Hyaluronic acidlikemolecules. The fibres are of following three type

[a] Collagen fibres of collagen protein;

[b] Reticular fibres of reticulin protein;

[c] Elastic fibres of yellow elastin protein; (the detailed description of these fibres is given structural details of loose areolar connective tissue)

The connective tissue proper is of following three types-

[a]

;

[b]

;

[c]

;

• The loose or areolar tissue is the most generalized and most common one which is present under skin, in between and around muscles, nerves and blood vessels, in the submucosa of gastrointestinal tract, between lobes and lobules of compound glands and in mesentries. This tissue binds parts together and facilitates their movements.

Following type of cells are present in the tissue-

[a]

-

• Most numerous and largest cells;


• synthesize proteins {Collagen, Reticulin, Elastin} + mucopolysaccharides.


• Their aggregations are seen in healing wounds, repairing tissues and in tissues where growth is taking place.


• Vit. C is essential for their best efficiency.


• In connective tissues some undifferentiated stem cells are found which are considered as Totipotent i.e., they are capable of giving birth to any other kind of cells.

[b]

-

1. The cells which work as scavengers i.e., have the capability of destroying the foreign particles, dead and injured cells.


2. -These are of two types

[i]Fixed macrophages with filopodia;-----------------

[ii] Wandering macrophages with simple lobopodia.

1. For engulfing a larger foreign body many {up to100} macrophages coalesce to form a multinucleate GIANT CELL also called EPITHELOID CELL.


2. Some specialised macrophages can be -

1. Dust cells- macrophages of lungs;


2. Kupffer cells- macrophages of liver;


3. Microglia-macrophages of brain + spinal cord.

[c]

-

• These are small, spherical or oval cells which are present along small blood vessels, lymphatics and nerves.


• These cells secrete Heparin {proteoglycon}, Histamine and Seratonin {both proteins}.


• Heparin is present as the anticoagulant in our body while the rest two are concerned with allergic reactions of our body. Among the rest two the histamine is VASODIALATOR and seratonin is the VASOCONSTRICTOR.

[d]

-

• These are the smallest cells, found less in nos., have the capability to move with pseudopodia. These are antibody producing cells.

[e]

-

• These are small cells and considered as most potential antibody forming cells of body.

[f]

-

• The cells which contain one or more fat globules.

[g]

-

• The cells which probably play role in inflammatory and allergic reactions of the body.

Besides, the tissue also contain following fibres-

[a]

-

• Long, unbranched fibres of Tropocollagen protein;


• soft and flexible but not elastic;


• with great tensile strength.


• If collagen is boiled it is converted in to gelatin.


• In man 1/3^rd of the proteins are collagen proteins.

[b]

-

1. These are delicate, freely branching inelastic fibres, made up of reticulin protein, found abundantly in the Embryos, newly born babies, and in healing +regenerating wounds.


2. Mostly these are replaced by collagen fibres in adults except from lymphoid and blood forming tissues.

[c]

-

• Of elastin protein, less numerous, branching, pale yellow in color.


• The elastin protein is probably the most resistant of all body proteins to any chemical change.


• In MUMMIES vessels are preserved due to well preserved elastin fibres.

• The dense fibrous connective tissue is of following types-

[a]

1. In the dermis of skin, in perichondrium, periosteum, muscles, nerves, in fibrous capsules of penis ,testes, blood vessels, glands and other organs.


2. The above written forms come under irregular dense connective tissue.


3. In ligaments and tendons due to the presence of regular fibres these are called regular dense fibrous connective tissue.


4. These tissues {those of skin} after chemical treatment {called tanning} are used in making leather.

[b]

-

• These tissue are present in highly elastic and mobile structures.


• eg., in certain ligaments-Ligamentumnuchae- In the neck of quadrupeds which support their head


• during grazing; Ligamentumflava- which connects adjacent vertebrae.


• These tissue are also present in vocal cords, capsules of spleen and lungs etc,.

• Specialised connective tissue : This will of following types-

-

1. This tissue is meant for fat storage and the cells are called ADIPOCYTES.


2. Eg., Panniculus adiposus (fat layer of humans); Blubber of Whales and Elephants; Hump of Camel; thick tail of marino sheep; yellow bone marrow. Because of this reason the tissue is also called ''fat depots''.


3. The human fat is of two types-

[a] White or yellow fat in which the adipocytes are with single large fat globule {

}- [b]Brown fat which is multilocular and is found in hibernating Mammals eg., Rodents + newly born babies.

-

1. Forms supporting network of bone marrow, thymus, spleen, lymph nodes, tonsils & other lymphoid organs. Together forms reticulo endothelial system which is the most potent immune system of our body.

-

• Present mostly in embryos with WHARTON'S JELLY as the ground substance.


• The Wharton's jelly contains mucopolysaccharides and found in umbilical cord, cock's comb, viterous body of eyeballs.

4.Pigmented connective tissue -

• Connective tissue of choroid and Iris +skin [dermis] of black people.




• The cells of this tissue contain large and branched pigment cells called Chromatophores or Melanophores containing Melanin pigment granules.