According to shape and layer of cells, epithelial tissue which are present in human body is divided in two main types: 1. Simple Epithelial Tissue 2. Compound Epithelial Tissue.
Type # 1. Simple Epithelium Tissue:
I. Pavement (Squamous) (Fig. 1.20A):
Description:
It is composed of a single layer of large flat cells placed on a thin basement membrane.
Distribution:
It is found in the alveoli of the lungs, the serous membranes (peritoneum, pleura etc.), Bowman’s capsule and Henle’s loop of the nephron, the inner lining of the heart, the lining epithelium of the blood vessels, lymphatics and Descemet’s membrane at the back of cornea, inner surface of the tympanic membrane, the membranous labyrinth, etc. In the lungs, serous membranes, blood vessels and lymphatics – the pavement epithelium has been given a special name – the endothelium (Fig. 1.21). In the heart, it is called endocardium.
Functions:
i. Has got a dialysing or filtering function.
ii. Helps easy passage of liquid and gases through it.
iii. Protective.
Some workers subdivide this class into three groups:
i. Endothelium:
Lining the heart, blood vessels, lymphatics, etc.
ii. Mesothelium:
Lining the serous cavities, e.g., pleura, pericardium, peritoneum.
iii. Mesenchymal Cells:
Lining the subdural and subarachnoids spaces, perilymphatic spaces of the internal ear and the chambers of the eyeball, etc. (The lining of the joint cavities is made up of flattened connective tissue cells – may be fibroblasts – and not the true mesenchymal cells.)
II. Cubical (Cuboidal):
Description:
It is composed of a single layer of cubical cells having same dimensions on each side and placed upon a basement membrane (Fig. 1.20, B, 1.22).
Distribution:
They are found in the small terminal respiratory bronchioles, in the inner parts of the digestive glands, the salivary glands, thyroid, covering of ovary, etc.
Functions:
i. Forming a protective layer on the surface.
ii. Often serves some other important functions, such as secretion, storage, etc.
III. Columnar (Cylindrical):
Description: Here the height of the cells is more than their breadth. Generally, it is composed of a single layer of cells arranged on a basement membrane (Fig. 1.23).
Distribution:
It is found in the stomach, whole of the small and large intestine, the alveoli and ducts of many glands etc. The free surface of the ovary and the convoluted parts of the renal tubules are believed to be lined by modified short columnar cells. In detail, the cells of the columnar epithelium vary in different places. In the alimentary canal and proximal convoluted tubule of nephron their free borders are longitudinally striated.
Hence, called brush border epithelium. Under the electron microscope, the brush border appears to be fingerlike projections termed microvilli. Both the intestinal and renal epitheliums are responsible for absorption and the microvilli increase the surface area to facilitate the absorption process.
Another type of columnar epithelium occurring in intestine, mainly large intestine, is responsible for the secretion of mucus. These cells are called goblet cells. Electron microscopically the non-secreting goblet-cells, but not the secreting ones, show microvilli.
Functions:
Columnar epithelium has got two chief functions absorption and secretion. The large ducts of the digestive glands (salivary glands) may show two incomplete layers of columnar cells. All the cells touch the basement membrane but all of them do not reach the surface, because, the deeper cells are shorter in length. This is sometimes called pseudostratified columnar.
IV. Ciliated (Fig. 1.20, D (i) & (ii), 1.24):
Description:
The cells are generally columnar in shape but at places may be cubical. The free surface has got hair-like processes – 20 to 30 on each cell. These processes are called the cilia or flagellae. That border of the cell upon which the cilia are set, is found to contain a row of particles – the basal particles (basal corpuscles).
To each of these basal particles one cilium is attached. These basal particles are believed to be the fragments of the centriole of the cell. The cilia seem to be prolonged through the basal particles into the protoplasm of the cell in the form of fine longitudinal filaments known as the rootlets.
They are very distinct in the large ciliated cells lining the alimentary canal of some Molluscas, but, are less distinct in the ciliated cells of the vertebrates. Their significance is not understood. All the cells in this epithelium may not be ciliated. Nonciliated cells and the mucus producing goblet cells remain scattered throughout the epithelium.
Distribution:
They are very widely distributed. They are found in the respiratory passages and the cavities that open into it – from the trachea downwards excepting the terminal bronchioles and the alveoli. It is also found in the Fallopian tubes, in the greater part of the body of the uterus and in the efferent tubules of the testes.
In the central nervous system it is also present as the lining epithelium (known as ependyma) of the ventricles, the central aqueduct and the central canal of the spinal cord. Here the cells are cubical in shape. In the trachea the ciliated epithelium has got two layers. Hence it is called pseudostratified columnar ciliated epithelium (Fig. 1.25). The deeper basal layer is non-ciliated. From this the ciliated cells develop.
Under the light microscope the cilia or flagellae appear as simple projections but under the electron microscope, they have a complex internal structure.
The membrane of a typical cilium or flagella is continuous with the cell membrane and represents an extension of it. A bundle of fibres, axoneme, is in the centre of the organelle. In the axoneme, most of the ATPase protein, dynein, is located. But these enzyme proteins possess certain differences from those found in muscle.
Ciliary movement, its cause and the factors affecting it (Fig. 1.26):
The remarkable property of the cilia is that they appear to move permanently so long as the cell is living. When the cell is dead, ciliary movement ceases. The ciliary movement consists of two phases – a sudden bending in the form of a sickle, known as the effective phase and then a sudden return to the original erect position- the return phase.
In the effective phase the cilia become rigid. In the return phase they are limp. The rate of movement is approximately ten to twenty times per second. There seems to be a functional co-ordination between all the cells of a ciliated epithelium.
The movement is passed on from cilium to cilium and from cell to cell always in the same direction. When looked upon from the surface, a series of waves are seen to pass in regular sequence over the cilia, just like the waves caused by a gentle wind over the paddy fields. These waves are known as the metachronal waves.
The movements of cilia depend upon the life of the cell and upon their connections with the basal particles. As a rule they are independent of nerves. But in some instances ciliary movement can be modified through the action of nerves. The movements are hastened by warmth and weak alkalies. They are slowed by cold and acids. Calcium ions are essential for their action.
Although the rate of oxygen consumption is directly proportional to the rate of ciliary action, yet the movements continue for some time even when oxygen is withheld. But ultimately anoxia stops the movements. Excess CO2 ether vapour, chloroform, etc., arrest the movement.
Mechanism of the Ciliary Movement:
The mechanism of ciliary movement is not known. Probably the underlying principle is same in all the ciliated bodies, e.g., the ciliated epithelium, the flagellated bacteria and parasites, the tail of the spermatozoa, etc. Formerly it was thought that displacement of water from the cell to the cilia and vice versa – is responsible for the ciliary movement. But later evidence indicates that the movement is generated in the cilium itself.
Gray suggests that it may be due to rhythmic displacement of the water content of the cilia from one side to the other. That side which holds more water becomes convex and the opposite face concave. Thus the cilium assumes a sickle-like attitude (effective phase). In the next moment water flows back to the opposite side and the cilium stands erect, thus effecting the return phase.
Functions of Ciliary Epithelium:
The function of ciliary movement is to maintain a flow of mucus or liquid and the suspended particles constantly in one direction. In the trachea it carries mucus, foreign particles and bacteria outside at the rate of 1-2 cm per minute. In the Fallopian tube it carries the ovum towards the uterus.
The function of the ciliated ependymal lining of the central nervous system is not properly understood. It is suggested that it helps to maintain the circulation of cerebrospinal fluid in the ventricles, central aqueduct and the central canal of the spinal cord.
V. Glandular (Fig. 1.20E):
Description: This epithelium lines the alveoli and portions of the ducts of the glands, e.g., the mammary glands, sweat (or sudoriferous) glands and sebaceous (or oil) glands, salivary glands, parts of intestinal glands, alveoli of the thyroid gland, etc. They are generally cubical, short columnar or polyhedral in shape and consist usually at one layer. Sometimes an incomplete second layer may be seen, such as in the salivary glands.
Function:
Manufacturing new substances and passing them out into their respective secretions.
Glands:
According to the mode of secretion, gland cells are divided into three classes:
i. Holocrine Type:
Here the secretion collects inside the whole of the cell. The cell ultimately dies and disintegrates and thus the secretion is discharged. The adjoining younger cells multiply and replace the lost one, e.g., sebaceous glands.
ii. Apocrine Type:
Here the secretion collects in the outer portion of the cell only, which gradually swells up and bursts. The rest of the cell remains intact and alive, and repeats the process again, e.g., mammary glands and possibly the goblet cells.
iii. Merocrine (Or Epicrine) Type:
Here no gross histological change is visible in the cell. The secretion is quietly liberated through the cell membrane, e.g. digestive glands, endocrine glands etc.
Goblet Cells (Chalice Cells):
These are mucus-secreting cells. They are widely distributed and remain scattered mostly in between the columnar and ciliated cells, e.g., trachea, gastro-intestinal tract, etc. The deeper part of the cell contains the nucleus and the cytoplasm, while the apical part contains mucinogen granules. They form the glycoprotein mucin (Fig. 1.28). The latter gradually distends and ultimately bursts, discharging the mucus outside. This process goes on repeatedly. In stained specimens the discharged cells look like white gaps.
This mucus serves many important functions:
i. Lubrication.
ii. Protective layer on the mucous membrane.
iii. Dilution of irritants.
iv. Neutralisation of acids, alkalies, etc.
v. Entangling bacteria, foreign particles, etc.
Type # 2. Compound Epithelium Tissue:
I. Transitional:
Description:
It consists of three or four layers of cells and thereby occupies an intermediate position between the single-layered simple epithelium and many-layered stratified epithelium. Hence the name ‘transitional’. (Fig. 1.20F).
The cells in the superficial layer are large, flat and irregularly quadrilateral, often containing two nuclei. The next layer consists to pyriform cells with the rounded ends outside and fitting into the cup-shaped depressions on the deep surface of the cells in the superficial layer. The next one or two layers consist of small polyhedral cells which remain packed in between the pointed ends of the pyriform cells of the second layer (Fig. 1.29). The cells are capable of deformation without any disturbance of functions.
Distribution:
This type of epithelium is found in the pelvis of the kidney, in the ureter, in the urinary bladder and in the upper part of the urethra. It is to be noted that the distribution is limited to the lower parts of the urinary system.
Functions:
i. Protective.
ii. Prevents reabsorption of the excreted material back to the system.
iii. Prevents in drawing of water from blood and tissues by the higher osmotic pressure of urine.
II. Stratified Squamous Cornified:
Description:
It is composed of many layers of the cells. Usually the superficial layers are horny due to disposition of Keratin (Fig. 1.30).
Distribution:
It is found in the skin. The hairs, nails, hoofs, horns, enamel of the teeth, etc., are modified epithelial tissue of this class. The skin affords a typical example of this type of epithelium. The most superficial layer is horny. In the next layer the cells are pressed down to flattened scales. Further down the cells are broader and polyhedral. Still deeper the cells appear to be short columnar in type. These deep cells are connected with each other by numerous intercellular fibrils and protoplasmic processes which look like thorns (Fig. 1.31).
Due to this thorny appearance these cells are called the prickle cells. With the help of these prickles, the cells in the epithelium are firmly tied together. Due to friction the superficial layers of cells are continuously falling off. These cells are being constantly replaced by the division of cells in the deeper layer (Fig. 1.20G).
Function:
This type of epithelium is always found in those places which are constantly exposed to atmosphere, mechanical pressure, friction and injury.
III. Stratified Squamous Non-Cornified:
Description:
It is same as above excepting that the superficial layer is not keratinized (Fig. 1.32).
Distribution:
It is found in the cornea, mouth, pharynx, oesophagus, and anal canal, lower parts of the urethra, the vocal cords, the vagina and the cervix.
Function:
It affords mechanical protection (Fig. 1.20H).
Stratified Columnar:
It is rare and found only in a few places, covering small areas, e.g., fornix of conjunctiva, some parts of pharynx, epiglottis, anal mucosa, cavernous part of male urethra, etc., (Fig. 1.33).
Stratified Columnar Ciliated:
This also is found only in small areas, e.g., nasal surface of the soft palate, some parts of larynx, etc.