LINING EPITHELIAL TISSUE: DEFINITION AND TYPES - ANATOMY AND PHYSIOLOGY - 2025

The epithelial lining tissue is that which covers the body surface of animals. Epithelial tissues, or epithelia, are those formed by one or more layers of cells that cover all the surfaces of the body.

Epithelia are groups of cells with a lot of connection between them through intercellular links. These tight junctions prevent the free circulation of substances thanks to the formation of protective and waterproof barriers. The epithelia are in continuous regeneration, as they are subjected to great wear and tear.

Each stem cell divides and one of the divisions survives, which in turn divides again, thus continuing the life cycle of the epithelia.

Epithelial tissues fulfill various functions: protection, segregation, absorption, sensory reception, excretion, and transport. In the protective function is the epithelial lining tissue, which controls the entry and exit of substances.

Segregating epithelia are capable of synthesizing and secreting molecules, depending on where it is in the body. Absorption epithelia, as their name indicates, have the functionality of absorbing molecules through microvilli.

The epithelia responsible for sensory reception have nerve endings in the sensory organs. Through the excretory epithelia, toxins and waste are released.

The transport epithelia move the cilia to transport substances. You may also be interested in reading about squamous epithelial cells: characteristics and diseases.

Characteristics of the lining epithelial tissue

The epithelial lining tissue is that which covers the body with cells closely attached to each other. It has little intercellular space, and to prevent the flow of molecules, it has an extracellular matrix.

The cells that make up the lining epithelial tissue age very quickly, as they are subjected to greater wear and tear than cells in other parts of the body. These cells wear out more by the free part that is in contact with the outside, and to regenerate it does so through its deep part, which has less wear.

These cells form an extracellular matrix, also known as the basal lamina or lamina propria. This sheet separates the lining tissue from the connective tissue. Connective tissue is what provides the lining tissue with nutrients and oxygen, since epithelial tissue does not have blood vessels or lymphatics.

To provide nutrients, connective tissue transports them through capillary beds, by transudation through the extracellular matrix. The transudate is basically a filtering of the extravascular fluid, which is not found in the capillaries. The lining fabric depends on this transudate in order to maintain its metabolism.

The basal lamina is a dense membrane made up mostly of electrodense material. Electromagnetic structures are easier to distinguish on a microscope, as they are darker. This depends on the amount of lipids and water, the more lipids it contains, the less electrodense it will be and the membrane will be clearer under a microscope.

A distinction is made of cells depending on their position in the lining tissue. Those that are more in contact with the surface or the exterior are called the apical pole. Those that are inside, or in contact with, the basal lamina are known as the basal pole.

Within the apical pole, which is in contact with the outside, we can find microvilli, stereocilia, cilia and flagella. Microvilli are cylindrical extensions that increase the absorption surface.

The stereocilia, which are pear-shaped, promote the transport and absorption of nutrients. On the other hand, the cilia resemble microvilli, although they are longer. The flagella, similar to cilia, are even larger.

In the basal pole, the part closest to the membrane, we find invaginations and hemidesmosomes. Invaginations are folds of the membrane, whereas hemidesmosomes are desmosomes that join the epithelium with the membrane.

Desmosomes are cellular structures that maintain cohesion between neighboring cells.

Classification of epithelial tissue

To classify the different types of epithelial tissue, we rely on the arrangement, the part of the body where they are found, and the morphology, that is, the number of layers between the surface and the lamina.

Simple or monostratified epithelium

This tissue is found in low wear areas, formed only by a layer of cells, and participates in diffusion, osmosis, filtration and absorption processes. We can in turn classify it into several categories.

• Simple squamous or squamous epithelium
• Simple cuboidal or cuboidal epithelium
• Simple cuboidal epithelium with microvilli
• Simple columnar or simple columnar epithelium
• Simple columnar secretory epithelium
• Simple columnar epithelium with absorbent cells
• Simple columnar epithelium with hair cells

Stratified epithelium

It is found in areas of wear or friction, and is made up of more than one layer of cells. It is perpendicular to the membrane. The classification of the stratified epithelium focuses only on the morphology of the cells and the upper layer, and can be:

• Non-keratinized stratified squamous epithelium
• Keratinized stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium
• Transitional epithelium
• Pseudostratified

Transitional or polymorphic epithelium

The transitional epithelium is made up of several layers of cells and was originally thought to be a transition between stratified columnar and stratified squamous. But after various investigations, he is considered a different type.

This is normally found in the urinary tract. The surface of this epithelium is domed, and for example, when the bladder is distended, these domes flatten, causing a narrowing of the epithelium.

Pseudostratified columnar epithelium

It resembles stratified epithelium, but only has one layer of cells, where the nuclei of these are found at different levels, making it appear stratified.

Only some of the cells that make up this epithelium come to touch the outside. Within this we find the following distinctions:

• Non-ciliated pseudostratified columnar epithelium
• Pseudostratified ciliated columnar epithelium
• Pseudostratified columnar epithelium with stereocilia

References

1. S. Becket (1976) Biology, A modern Introduction. Oxford University Press.
2. Johnstone (2001) Biology. Oxford University Press.
3. Byrum (2005) Cells. Popular Prakashan.
4. Lewin (2007) Cells. Jones & Bartlett Learning.
5. Ian Freshney, Mary G. Freshney (2002) Culture of Epithelial Cells. Wiley Publishers.
6. Andrew J. Shaw (1996) Epithelial Cell Culture. Oxford University Press.
7. Ashton Acton (2013) Epithelial Cells. Scholarly Editions.