Let’s know what Epithelial Tissue is
The human body consists of four types of tissue:
The epithelial tissue covers the body, the lines of all the cavities, and makes up the glands.
Most epithelial tissues are essentially large sheets of cells that cover all surfaces of the body exposed to the outside world and that line the outside of the organs.
The epithelium also forms a large part of the body’s glandular tissue. The skin is not the only area of the body exposed to the outside.
Other areas include the respiratory tract, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium.
Hollow organs and body cavities that do not connect to the outside of the body, which includes blood vessels and serous membranes, are lined with endothelium (plural = endothelial), which is a type of epithelium.
The epithelial cells are derived from the three main embryonic layers. The epithelium that lines the skin, parts of the mouth and nose, and the anus develop from the ectoderm.
The cells that line the airways and most of the digestive system originate in the endoderm.
The epithelium that lines the vessels in the lymphatic and cardiovascular system is derived from the mesoderm and is called the endothelium.
The epithelial tissues are almost completely avascular.
For example, no blood vessel goes through the basement membrane to enter the tissue, and the nutrients must come by diffusion or absorption from the underlying tissues or surface.
Many epithelial tissues are capable of rapidly replacing damaged and dead cells.
Detachment of damaged or dead cells is a characteristic of the superficial epithelium and allows our respiratory tract and digestive tracts to rapidly replace damaged cells with new cells.
The epithelial tissue is made up of cells placed together in sheets with the cells tightly bound together. The epithelial layers are avascular, but innervated.
The epithelial cells have two surfaces that differ both in structure and function.
The glands, such as exocrine and endocrine, are composed of epithelial tissue and are classified according to how their secretions are released.
Epithelial tissue forms boundaries between different environments, and almost all substances must pass through the epithelium. In its role as an interface tissue, the epithelium fulfills many functions, including:
- Protection of underlying tissues by radiation, desiccation, toxins and physical trauma.
- Absorption of substances in the lining of the digestive tract with different modifications.
- Regulation and excretion of chemical substances between the underlying tissues and the body cavity.
- The secretion of hormones in the blood vascular system. The secretion of sweat, mucus, enzymes, and other products that are delivered through the ducts come from the glandular epithelium.
- The detection of sensation.
Characteristics of the Epithelial Layers
The epithelial tissue is composed of cells presented in sheets with strong cell-to-cell attachments.
These protein connections hold the cells together to form a tightly connected layer that is avascular.
The epithelial cells are nourished by substances that diffuse from the blood vessels in the underlying connective tissue.
One side of the epithelial cell is oriented towards the surface of the tissue, body cavity or external environment and the other surface is attached to a basement membrane.
Epithelial tissues have five main characteristics:
Polarity: all the epithelia have an apical surface and a united lower basal surface that differ in structure and function. For this reason, the epithelia are described as exhibiting apical basal polarity.
Most apical surfaces have microvilli (small extensions of the plasma membrane) that increase the surface area.
For example, in epithelia that absorb or secrete substances, the microvilli are extremely dense, giving the cells an appearance of fuzepitlic tissue called brush edge.
Examples of this include epithelia lining the intestine and renal tubules.
Other epithelia have movable cilia (projections similar to a hair) that push substances along their free surface.
Next to the basal surface is the basal lamina (thin support sheet). The basal lamina acts as a filter that allows and inhibits the passage of certain molecules to the epithelium.
Specialized contacts: the epithelial cells fit together and form continuous leaves (except in the case of glandular epithelia). They do it with narrow junctions and desmosomes.
Narrow junctions form the closest contact between cells and help maintain the proteins in the apical region of the plasma membrane.
Desmosomes connect the plasma membrane to intermediate filaments in the cytoplasm.
With the support of the connective tissue, all the epithelia are supported by connective tissue.
For example, deep to the basal lamina is the reticular lamina (extracellular material that contains the fiber of the collagen protein) that forms the basement membrane.
The basement membrane strengthens the epithelium and helps resist stretching and tearing.
Avascular and innervated, although the epithelium is avascular (does not contain blood vessels), it is still innervated (supplied by the nerve fibers).
Epithelial regeneration has a high regenerative capacity and can reproduce rapidly as long as it receives adequate nutrition.
Types of Epithelial Tissue
The epithelial tissues are identified both by the number of layers and by the shape of the cells in the upper layers.
There are eight basic types of epithelium: six of them are identified based on both the number of cells and their shape; two of them are named for the type of cell (scaly) found in them.
There are three main cellular forms associated with epithelial cells: squamous epithelium, cuboidal epithelium and columnar epithelium.
There are three ways to describe stratification of the epithelium: simple, stratified and pseudostratified.
The pseudostratified epithelium has fine extensions similar to the hair called cilia and unicellular glands called goblet cells that secrete mucus.
This epithelium is described as ciliated pseudostratified epithelium.
The stratified epithelium differs from the simple epithelium in that it is multilayer. Therefore, it is found where body linings have to withstand mechanical or chemical insults.
In keratinized epithelia, the apical (outer) layers of the cells are dead and contain a resistant and resistant protein called keratin.
An example of this is found in the skin of mammals that makes the epithelium impermeable.
Transitional epithelia are found in tissues such as the urinary bladder where there is a change in the shape of the cell due to stretching.