Epithelial Tissue:
1. Definition of Epithelial Tissue:
An epithelium is a tissue composed of one or more layers of cells covering the external and internal surfaces of various body parts. Epithelial tissue also forms glands. The term “epithelium” (sing, of epithelia) was given by a Dutch anatomist Ruysch (1638-1731) to refer to the fact that epithelial (Gr. epi- upon, thelio- grows) tissues grow upon other tissues.
2. Location of Epithelial Tissue:
The epithelial tissues occur on external and internal exposed surfaces of the body parts where they form protective covering.
3. Origin of Epithelial Tissue:
Epithelial tissues evolved first and are also formed first in the embryo. The epithelial tissues arise from all the three primary germ layers: ectoderm, mesoderm and endoderm, of the embryo. For example the epidermis of the skin from the ectoderm, coelomic epithelium from the mesoderm and epithelial lining of alimentary canal (= gut) from the endoderm.
4. Features of Epithelial Tissues:
Epithelial tissues consist of variously shaped cells closely arranged in one or more layers. There is little intercellular material between the cells. The cells are held together by intercellular junctions. The epithelial tissues usually rest on a thin non-cellular basement membrane.
Usually blood vessels are absent in epithelial tissues. However the underlying connective tissues are generally well supplied with blood vessels. Nutrients enter epithelial tissues from the underlying connective tissues by diffusing through the basement membrane.
Nerve endings may penetrate the epithelial tissues. The epithelial tissues have a good power of repair (regeneration) after injury. A specialized epithelium, the stria vascularis of the cochlea of internal ear has blood capillaries within the thickness of the epithelium.
Basement Membrane:
The epithelial tissues usually lie on the non-cellular basement membrane which consists of two layers:
(i) Basal Lamina:
It is outer thin layer (near the epithelial cells), composed of mucopolysaccharides and glycoproteins, both secreted by epithelial cells. It is visible only with the electron microscope.
(ii) Fibrous or Reticular Lamina:
It is inner thick layer, composed of collagen or reticular fibres of the underlying connective tissue. It is visible with light microscope. The basement membrane provides elastic support. It also allows selective chemical exchange between epithelial tissues and surrounding blood vessels.
Specializations of the Plasma Membrane (= Cell membrane):
These specializations of plasma membrane are as follows:
(i) Microvilli:
These are minute finger like processes (= projections) which arise from free surface of the epithelial cells. They increase the absorptive surface area of the cells. Microvilli are found in the intestinal epithelium.
(ii) Stereo Cilia:
These are long, non-motile processes of the epithelial cells. They are found in the epithelium of epididymis and vas deferens (both are parts of male reproductive tract). They are also present in the hair cells of the internal ear.
(iii) Cilia and Flagelia:
They arise from the basal granules. Outer covering of the cilia and flagelia is continuous with the cell membrane. Cilia are present in the epithelium of respiratory tract, oviducts, etc. The cilia beat back and forward to propel the materials which come in contact with them. In the human body, only the spermatozoon (= sperm) has flagellum which helps in its locomotion.
(iv) Endocytic, Pinocytotic and Exocytic Vesicles:
These vesicles are associated with endocytosis (cell eating), pinocytosis (cell drinking) and exocytosis (cell vomiting) respectively.
5. Specialized Junctions between Epithelial Cells:
These are as follows:
(i) Tight Junctions (= Zonula occludens):
Plasma membranes in the apical parts of the adjacent epithelial cells become tightly packed together or are even fused to form the tight junctions. They check the flow of materials between the cells.
(ii) Gap Junctions:
These are meant for chemical exchange between adjacent cells.
(iii) Adhering Junctions:
They perform cementing function to keep neighbouring cells together. Adhering junctions are of three types: zonula adherens, desmosomes (macula adherens) and hemi-desmosomes.
(a) Zonula adherens:
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. They probably serve anchoring function.
(b) Desmosomes (= Macula adherens):
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 plaque-like structure into the cytoplasm are not of actin, but of a keratin like protein and these microfilaments are called tonofibrils. Desmosomes serve anchoring function.
(c) Hemi desmosomes:
(Single sided desmosomes) are similar to desmosomes, but the thickening of cell membrane is seen only on one side. Hemi desmosomes join epithelial cells to basal lamina (outer layer of basement membrane).
(iv) Inter-digitations:
These are finger-like processes of the cell membranes of the adjacent cells. They increase the area of surface contact between the adjacent cells and, therefore, their adhesion.
(v) Intercellular Bridges:
These are minute projections that arise from adjacent cell membranes. They make contact with one another.
6. Types of Epithelia Tissue:
On the basis of arrangement and shape of the cells, the epithelia are classified into two groups: simple epithelia and compound epithelia.
A. Simple Epithelia (Uni-layered Epithelia):
The cells are arranged in a single layer, forming one cell thick epithelium.
Simple epithelia are further divisible as follows:
1. Simple Squamous Epithelium (Pavement Epithelium):
Structure:
It is composed of large flat cells whose edges fit closely together like the tiles in a floor, hence it is also called pavement epithelium. The cells rest on a thin basement membrane. If the cells are seen from the surface, they seem to be polygonal in shape. The nuclei of the cells are flattened and often lie at the centre of the cells and cause bulging’s of cell surface.
Location:
This epithelium is present in the terminal bronchioles and alveoli of the lungs, wall of the Bowman’s capsules and descending limbs of loops of Henle of the nephrons of the kidneys, membranous labyrinth (internal ear), blood vessels, lymph vessels, heart, coelomic cavities, and rete testis of the testis.
In the blood vessels, lymph vessels and heart it is called endothelium. In the coelom, it is called mesothelium. The cells of endothelium and mesothelium may become wavy, hence these epithelia are called tessellated. The skin castings of frog are cast off in single layers, hence are treated as squamous epithelium.
Function:
Protection, excretion, gas exchange and secretion of coelomic fluid.
2. Simple Cuboidal Epithelium:
Structure:
As the name indicates the cells are cubical and more or less squarish in shape. In surface view, the cells look like polygonal. The nuclei are rounded and lie in the centre of the cells. The cells of cuboidal epithelium often form microvilli on their free surface.
This gives a brush-like appearance to their free border called brush bordered cuboidal epithelium. Microvilli increase absorptive surface area. In ovaries and seminiferous tubules of the testes it is called germinal epithelium because it produces gametes (ova and sperms respectively).
Location:
The cuboidal epithelium is present in the small salivary and pancreatic ducts, thyroid follicles, parts of membranous labyrinth, proximal and distal convoluted tubules of the nephrons of kidneys, ovaries, seminiferous tubules of testes, and ciliary bodies, choroid and iris of eyes, other sites of cuboidal epithelium are the inner surface of the lens, the pigment cell layer of the retina of the eye and sweat glands of mammalian skin.
Function:
Protection, secretion, absorption, excretion, gamete formation.
3. Simple Columnar Epithelium:
Structure:
The cells are elongated and placed side by side like column. The outer free surface of each cell is slightly broader. The nuclei are somewhat elongated along the long axis of the cells. Nuclei lie near the bases of the cells. Certain cells of this epithelium contain mucus (a slimy substance) and are called goblet (or mucous) cells as they look like goblet.
The epithelium containing mucus secreting cells, along with the under lying supporting connective tissue is called mucosa or mucous membrane. The latter is present in the stomach and intestine. The intestinal mucosa (= mucous membrane) has microvilli to increase the absorptive surface area and is called brush-bordered columnar epithelium which is highly absorptive.
Location:
It lines the stomach, intestine, gall bladder and bile duct. It also forms the gastric glands, intestinal glands and pancreatic lobules (present in the pancreas) where it has secretory role and is called glandular epithelium.
Function:
Protection, secretion and absorption.
4. Simple Ciliated Epithelium:
Structure:
The cells bear numerous delicate hair like outgrowths, the cilia, arising from basal granules. Mucus secreting goblet cells also occur in the ciliated epithelium. The cilia remain in rhythmic motion and create a current to transport the materials which come in contact with them.
The ciliated epithelium is of two types:
(i) Ciliated Columnar Epithelium:
It comprises columnar cells which have cilia on the free surface. This epithelium lines most of the respiratory tract and Fallopian tubes (oviducts).
It also lines the ventricles of the brain and the central canal of the spinal cord. It is also present in tympanic cavity of middle ear and auditory tube (Eustachian tube).
(ii) Ciliated Cuboidal Epithelium:
It consists of cubical cells which have cilia on the free surface. It occurs in certain parts of nephrons of the kidneys.
Functions of Ciliated Epithelium:
The main function of ciliary movement is to maintain a flow of mucus or liquid or suspended particles or bodies constantly in one direction. In the respiratory tract the cilia help to push mucus towards the pharynx (throat). In the oviducts the cilia help to move an egg towards the uterus.
The cilia of the ventricles (cavities) of the brain and central canal of the spinal cord help to maintain the circulation of cerebrospinal fluid present there. In the nephrons of the kidneys, cilia keep the urine moving.
5. Pseudo-stratified Epithelium:
Structure:
The cells are columnar, but unequal in size. The long cells extend up to free surface. The short cells do not reach the outer free surface. The long cells have oval nuclei, however, short cells have rounded nuclei. Mucus secreting goblet cells also occur in this epithelium.
Although epithelium is one cell thick, yet it appears to be multi-layered which is due to the fact that the nuclei lie at different levels in different cells. Hence, it is called pseudo stratified epithelium.
It is of two types:
(i) Pseudo stratified Columnar Epithelium:
It consists of columnar cells. It occurs in the large ducts of certain glands such as parotid salivary glands and the urethra of the human male. It is also present in the olfactory mucosa.
(ii) Pseudo stratified Columnar Ciliated Epithelium:
It consists of columnar cells. The long cells have cilia at their free surface, however, the short cells are without cilia. This epithelium occurs in the trachea and large bronchi. The movements of the cilia propel the mucus and foreign particles towards the larynx.
Functions of Pseudo stratified Epithelium:
Protection, secretion, movement of secretions from glands, urine and semen in the male urethra and mucus loaded with dust particles and bacteria from the trachea towards the larynx.
B. Compound Epithelia (Multi-layered Epithelia):
These are made up of more than one layer of cells. The compound epithelia may be stratified and transitional.
1. Stratified Epithelium:
It has many layers of epithelial cells in which the deepest layer is made up of columnar or cuboidal cells. This epithelium is classified on the basis of the shape of the cells present in the superficial layers.
It is of four types:
(i) Stratified Squamous Epithelium:
The cells in the deepest (= basal) layer are columnar or cuboidal with oval nuclei. It is called germinative layer (= stratum germinativum or stratum Malpighi). The cells of this layer divide by mitosis to form new cells. The new cells gradually shift outward.
In the middle layers the cells become polyhedral with rounded nuclei. These are called intermediate layers. The superficial layers are flat with transversely elongated nuclei. These layers are called squamous layers.
It is of two types:
(a) Keratinized Stratified Squamous Epithelium:
In the outer few layers, the cells replace their cytoplasm with a hard, water proof protein, the keratin. The process is called keratinization. These layers of dead cells are called stratum corneum or horny layer. The deeper layers have living polygonal cells.
The keratin is impermeable to water and is also resistant to mechanical abrasion (scraping). The horny layer is shed at intervals due to friction. This epithelium occurs in the epidermis of the skin of land vertebrates.
(b) Non-keratinized Stratified Squamous Epithelium:
As the name indicates, it does not have keratin. It is unable to check water loss and provides only moderate protection against abrasion (scraping). This epithelium occurs in the oral cavity (buccal cavity), tongue, pharynx, oesophagus, middle part of anal canal, lower parts of urethra, vocal cords, vagina, cervix (lower part of uterus), conjunctiva, inner surface of eye lids and cornea of eye.
(ii) Stratified Cuboidal Epithelium:
It has outer layer of cuboidal cells and basal layer of columnar cells. It forms the epidermis of fishes and many urodels (tailed amphibians such as salamanders). It also lines the sweat gland ducts and larger salivary and pancreatic ducts.
(iii) Stratified Columnar Epithelium:
It has columnar cells in both superficial and basal layers. It covers the epiglottis and lines mammary gland ducts and parts of urethra.
(iv) Stratified Ciliated Columnar Epithelium:
Its outer layer consists of ciliated columnar cells and basal layer of columnar cells. It lines the larynx and upper part of the soft palate.
2. Transitional Epithelium (= Urothelium; Fig. 7.12):
Structure:
This epithelium consists of 4 to 6 layers of cells. The cells of deepest (= basal) layer are columnar or cuboidal. The cells of middle layer are polyhedral or pear shaped. The cells of the surface layer are large and globular or umbrella shaped. When this epithelium is stretched all the cells become flattened.
Location:
This epithelium is found in the renal calyces, renal pelvis, ureters, urinary bladder and part of the urethra. Because of its distribution, transitional epithelium is also called urothelium (epithelium present in the urinary system).
Function:
It permits distention. The transitional epithelium of the urinary bladder can be stretched considerably without being damaged. When stretched it appears to be thinner and the cells become flattened or rounded. Thus this epithelium is stretched or relaxed.
It is also protective in function. The cells of the basal layer of the transitional epithelium show occasional mitosis, but it is much less frequent than that of stratified squamous epithelium, as there is normally little erosion of the surface.
7. Role of Epithelial Tissue in Glands:
Some of the epithelial cells get specialised for secretion and are called glands.
Types of Glands:
The glands are classified in different ways:-
1. Based on Site of Secretion:
According to the site where secretion is released, the glands are of three types: exocrine, endocrine and heterocrine.
(A) Exocrine Glands:
The glands which pour their secretions on the epithelial surface, directly or through ducts are called exocrine glands. These include salivary glands, gastric glands, intestinal glands, tear glands, sweat glands, oil glands, mammary glands, etc.
The exocrine glands may be unicellular or multicellular. When unicellular glands secrete mucus, they are called mucous cells (= goblet cells or mucocytes).
A multicellular gland consists of a duct and secretory portion, both formed of epithelial cells, e.g., Crypts of Lieberkuhn (= a type of intestinal glands) in the human ileum, sweat (= sudoriferous) glands of mammalian skin, Brunner’s glands (= a type of intestinal glands) of human intestine, gastric glands and uterine glands, milk glands of egg laying mammals, and salivary glands, oil (sebaceous) glands, human milk glands, sublingual and sub mandibular salivary glands and exocrine part of pancreas, Cowper’s glands in males and Bartholin’s glands in females.
(B) Endocrine Glands (= Ductless Glands):
They lack ducts. Their secretions are called hormones which are poured directly into the blood and lymph. The blood and lymph carry hormones to the target organs. Examples of endocrine glands are thyroid, parathyroids, hypothalamus, pituitary, adrenals, thymus, etc.
(C) Heterocrine Glands:
They have both exocrine part and endocrine part. The former sends its secretion by way of a duct and the latter releases its secretion directly into the blood and lymph. Pancreas and gonads (testes and ovaries) are heterocrine glands.
2. Based on Mode of Secretion:
On the basis of the mode of releasing the secretion, the glands are of three types: merocrine, apocrine and holocrine.
(i) Merocrine glands:
In these glands, the secretion is discharged by the cells by simple diffusion, so that there is no loss of cells or their parts, e.g., goblet cells, most sweat glands, salivary glands, intestinal glands.
(ii) Apocrine glands:
In these glands, secretory products accumulate in apical part of the cells. Later, this part breaks off from the cell and is discharged as secretion, e.g., mammary glands and some sweat glands.
(iii) Holocrine glands:
In these glands, an entire cell, when filled with secretory products, disintegrates and is discharged as a part of secretion, e.g., sebaceous glands.
'
Sometimes holocrine glands are described as those endocrine glands which secrete only hormones, e.g., thyroid, parathyroids, adrenals, pituitary and hypothalamus.
Zymogen granules appear in the cytoplasm of the secretory cell.
'
No comments:
Post a Comment