EPITHELIAL TISSUE

 TYPES OT ISSUES 

         

ANIMAL TISSUE

Tissue - In multicellular animals, a group of similar cells along with intracellular substances perform a specific function. Such an organization is called tissue.

Animal Tissues 

The structure of the cells vary according to their function. Therefore, the tissue are different and are broadly classified into four types :

• Epithelial 

• Connective 

• Muscular 

• Neural

EPITHELIAL 

An Epithelium is a tissue composed of one or more layers of cells covering the external surface of the body and viscera (Internal organs), or lining the cavities in the body and viscera.

Origin - Epithelia arises from all the three primary germ layers : Ectoderm, Mesoderm and Endoderm, of the embryo. For Example, the epidermis arises from the ectoderm, coelomic epithelium from the mesoderm,and epithelial lining of the alimentary canal from the endoderm.

Structure - Epithelial tissue consist of variously shaped cells closely arranged in one or more layers, forming continuous sheets. There is a little intercellular Material between the cells. The cells are held together by intercellular junction. The epithelia rest on a thin, noncellular basement membrane. The epithelial generally lack blood vessels, but still have a good power of repair after injury. They receive useful materials from the blood flowing in the blood vessels of the connective tissue across the basement Membrane and transfer waste matters to the blood in the same way. Nerve endings may penetrates the epithelium. 

Basement membrane - The basement membrane consists of two layers : outer thin basal lamina secreted by the epithelial cells, and inner thick fibrous or reticular lamina contributed by the underlying connective tissue. Both are composed of collagen fibrils associated with glycoprotein.  The basement Membrane provides elastic support, and acts as a selective diffusion barrier. 

Junction between Epithelial cells - The common intracellular junction include tight junction, gap junction, desmosome, interdigitation and intracellular bridge.

Cell surface specialization - The free surface of the epithelial cell may bears special structures, such as microvilli,stereocilia, cilia and flagella, for specific purposes. The difference between microvilli, Stereocilia and Cilia are tabulated. 

Functions.

Epithelia serve a variety of functions -

• Protection - Epithelia cover the surface and line the cavities. Therefore, they are sometimes called surface tissue. They protect the underlying or overlying tissue from -

• Mechanical injury

• Drying up 

• Infection 

• Harmful chemicals 

• Formation of exoskeleton - Surface epithelia produce many exoskeleton structures, such as scales, feather, hair, claws, nails, hoofs and horns, in tetrapod vertebrates. 

• Secretion- Epithelial lining the cavities give rise to glands that provide valuable secretions such as mucus,gastric juice.

• Absorption - Epithelial lining of the intestine absorbs digested food. 

• Excretion - Epithelial lining of uriniferous tubules in the kidneys eliminate nitrogenous waste material as urine. 

• Respiration- Epithelium of the lungs alveoli brings about exchange of oxygen and carbon dioxide between blood and air. 

• Sensation - Sensory epithelium in sense organs, e.g, nasal chambers, picks up stimuli from the environment and send nerve impulses to the central nervous system. 

• Conduction - Ciliated epithelium moves fluid, mucus and other materials in the organs it lines. 

• Reproduction - Germinal epithelium of the seminiferous tubules and ovaries produce spermatozoa and ova respectively. 

• Regeneration - The ability of epithelia to regenerate quickly helps in the healing of wounds. 

• Contractility - Myoepithelium, by its contraction,expels secretions ( milk, saliva, sweat)   from the respective gland. 

classification 

Epithelia are classified on the basis of cell layers and shape of cells.  There are two main types of animal epithelia : 

• Simple 

• Compound 

SIMPLE EPITHELIUM 

The simple epithelia consist of a single layer of cells resting on a basement membrane. They cover a moist surface, where there is little wear and tear by friction. They are further of five types according to the form and structure of their cells : Squamous, cuboidal, columnar, ciliated and pseudostratified. 

Simple Squamous Epithelium 

Structure - This epithelium consists of thin, flat, disc-like cells closely fitted like the tiles in a floor. Hence, it is also called pavement epithelium. The cell outlines are roughly polygonal in surface view. The nucleus is also flattened like the cell. It often lies at the center of the cell and cause a slight bulge. 

Location - The squamous epithelium lines the Bowman's capsule of the urinary tubules and the alveoli of the urinary tubules and the alveoli of the lungs.

Simple squamous epithelium also lines the blood vessels and the coelom. At these places, it is known as endothelium and mesothelium respectively. The cell outlines of endothelium and mesothelium are wavy. These epithelium are termed tessellated epithelia. 

Function - Protection, excretion, gas exchange, and coelomic fluid. 

SIMPLE CUBOIDAL EPITHELIUM 

STRUCTURE - This epithelium consists of cells that are about as tall as wide. The cell outlines are polygonal in surface view. The nucleus is rounded and lies at the center of the cell. 

Location - The cuboidal epithelium lines the small salivary and pancreatic duct.  Hence it is non secretory. It also lines the sweat, salivary and thyroid glands,where it is secretory. It also lines the proximal part of the urinary tubules. Here the cuboidal cell have numerous microvilli at their free ends. The latter give brush like appearance, and the epithelium is called brush bordered cuboidal epithelium. The villi increase the surface area of the free ends of the cells for proper reabsorption of materials. The cuboidal epithelium also covers the ovaries and lines the sperm - producing tubules of the testes. At these place it is known as germinal epithelium because it produces new cells for the formation of gametes. 

FUNCTION - Simple cuboidal epithelium performs the functions of protection, secretions, absorption and gamete formation. 

SIMPLE COLUMNAR EPITHELIUM 

STRUCTURE- The epithelium consists of cells that are much taller than wide. The cell outlines are polygonal in surface view. The nucleus is elongated along the long axis of the cell and may have a variable position. 

Location - The columnar epithelium lines the stomach, intestine and gallbladder. It also forms the gastric glands, intestinal glands and pancreatic lobules. Here it has a secretory role and is called glandular epithelium. Certain cells of the columnar epithelium lining the stomach and intestinal secrete a slimy substance, the mucus, that forms a fine coating over the epithelium. These cells are called globalt cells because they look like a goblet when with a vacuum full of mucus. The epithelium containing mucus - secreting cells, along with the underlying supporting connective tissue, is called mucous membrane, or mucosa. Other columnar cells of the intestinal lining are specialized for the absorption of water and digested food. They bear microvilli at the free ends to increase the surface area. The columnar epithelium of the small intestine is known as brush - border columnar epithelium. 

FUNCTION- simple columnar epithelium provides protection, secretion, and absorption. 

SIMPLE CILIATED EPITHELIUM 

STRUCTURE - The epithelium consists of cells that bear fine, vibratile cytoplasmic processes, the cilia, on the free surface.  A cilium arises from a minus basal granule lying below the cell membrane. Mucus - Secreting goblet cells almost invariably occur at intervals among the ciliated cells.  Mucus spreads over the epithelium as a thin layer. Cillia, by lashing action, move mucus and other substances over the epithelium. The ciliated epithelium is of 2 types :

• CUBOIDAL CILIATED EPITHELIUM - It consists of cuboidal cells that bear cilia on the free surface. It lines certain parts of urinary tubules of the kidney. It keeps the urine moving. 

• COLUMNAR CILIATED EPITHELIUM - It consists of columnar cells which bear cilia on the free surface. It lines the nasal passages, oviduct (Fallopian tubes), terminal bronchioles, Ventricles of the brain, and central canal of the spinal cord of the embryo. 

FUNCTIONS -  It provides protection as well as movement of mucus, urine, eggs and cerebrospinal fluid in a particular direction.

PSEUDOSTRATIFIED EPITHELIUM 

STRUCTURE - This epithelium is one -cell thick but appears 2-layered because all the cells do not reach the free surface, but all cells are attached to the basement membrane, hence the name pseudostratified. The tall cells have oval nuclei that lie at different levels. The short cells have rounded nuclei. Mucus - Secreting goblet cells occur here and there in the epithelium. The Pseudostraified is of two types : Columnar and ciliated. 

• PSEUDOSTRATIFIED COLUMNAR EPITHELIUM 

This epithelium has columnar cells without cilia. It lines the large ducts of certain glands such as parotid salivary glands and the urethra of the human male. It is also a component of olfactory Mucosa.

• PSEUDOSTRATIFIED CILIATED COLUMNAR EPITHELIUM -

This Epithelium has columnar cells. The tall cells bear cilia at the free surface, the short cells without cilia. The pseudostratified columnar ciliated epithelium lines the trachea and large bronchi.The movement of its cilia push the mucus laden with dust particles and bacteria toward the larynx. 

FUNCTIONS - It provides protection, movement of secretions from glands, urine and semen in urethra, and mucus loaded with dust particles and bacteria in trachea.