EPITHELIAL TISSUE: CHARACTERISTICS, DESCRIPTION, FUNCTIONS, TYPES - BIOLOGY - 2025

The epithelial tissue or epithelium is a membranous tissue that covers the surface of the body and the external surface of the organs, and that is formed by a set of closely united cells, which form tight sheets. The tissue lacks intracellular products.

The epithelium, together with the connective, muscular and nervous tissues, form the four basic tissues of animals. Its origin is embryonic, and they are formed from the three embryonic sheets or layers (ectoderm, mesoderm and endoderm).

Cylindrical or columnar stratified epithelial tissue from the urethra of the Mus musculus mouse. Taken and edited from: Department of Functional Biology and Health Sciences. University of Vigo. Galicia. Spain

There are several types of epithelial tissues, with different characteristics and specific functions. Histologists and physiologists recognize and classify these tissues according to the shape of the cell, the number of cell layers that make up said tissue and also according to the function it performs.

Features and Description

In general, the epithelial tissue is characterized by having individual cells closely united, forming continuous sheets or layers. They are non-vascularized tissues, so they do not have veins, vessels or arteries. This implies that the tissue is nourished by diffusion or absorption from underlying tissues or from the surface.

Epithelial tissues are closely related to connective tissue, in fact all epithelia grow or form on underlying vascularized connective tissue, separated by the basement membrane (an extracellular support layer).

The epithelium covers the entire external surface (the epidermis) and from there it covers all the pathways or passages that lead to the outside of the body; the digestive tract, respiratory tract, and urinary tract are examples of this.

Cells

In particular, the cells of epithelial tissue have a polarized distribution of organelles and proteins that are bound to the membrane between their basal and apical surfaces. Other structures found in epithelial cells respond to adaptations to clearly specific functions of the corresponding tissue.

Sometimes epithelial cells have extensions of the cell membrane, called cilia. Cilia, when present, are found exclusively on the apical surface of the cell and facilitate both the movement of fluids and trapped particles.

Epithelial cells are classified as follows:

Flaky

Squamous cells are those that, seen from above, are shaped like fish, snake or cobblestone scales, that is, flat, thin and polygonal (although fish and snake scales are not necessarily polygonal).

Cylindrical

Cylindrical or columnar cells are characterized by being taller than they are wide, they can be up to four times as long as they are wide. They have ciliary projections (cilia) and the nuclei are oval in shape.

Cuboidal

As the name implies, these cells are cube-shaped in cross section. They have a large, central and spherical nucleus.

Features

Protection

It is the tissue responsible for covering all body surfaces. It works as a kind of protective shield for the tissues that are under it, that is, it protects against mechanical damage, prevents the entry of microorganisms and dehydration by evaporation.

Absorption

On some surfaces of the body, the epithelial tissue is capable of absorbing substances and / or materials, especially nutritional ones.

Secretion

Certain parts of the body specialize in secreting products or substances such as intestinal mucus. These secretions are often made from the synthesis of simple molecules.

Excretion

As part of the functions of epithelial tissues is excreting waste substances that can be harmful to the body.

Surface transportation

Hair cells are not as common as others in epithelial tissue, but when they are present they are capable of moving and / or facilitating the movement of products, materials and substances.

Sensory functions

Epithelial tissue is capable of performing sensory functions capable of perceiving touch, heat, cold, or chemical information (such as perceiving flavors through the taste buds of the mouth).

Types

Some types of epithelia. Taken and edited from: Illu_epithelium.jpg: Arcadianderivative work: Ortisa.

According to most researchers and the available bibliography, the types of epithelial tissues are classified into two large groups: lining (which is subdivided into three types, depending on the number of layers it presents) and glandular (divided into several according to the secretion product):

-Lining epithelium

These are recognized by the number of layers they present and according to the type of cells (see characteristics and description) that make them up:

Simple

This epithelium is made up of a single layer of cells. When the epithelium is made up of squamous cells, it is called simple flat epithelium.

If instead it presents cubic or columnar cells, it is called cubic or simple columnar epithelium, depending on the type of cell of which it is composed. They are found in areas of the body where there is absorption and secretion of substances.

Stratified

Epithelium that has two or more layers of cells. According to the type of cells, it is divided into flat, columnar and cuboidal stratified epithelia. This type of tissue is fulfilling protective functions, although some authors generally mention that they are scarce and fulfill secretory functions of substances.

Pseudostratified

It is an epithelium made up of at least 2 different types of cells. It was named pseudostratified because when histologists made the cuts at the end of the 19th century, they observed nuclei in a row at different heights that made them believe that they were several layers.

Some time later it was discovered that these nuclei located at different heights did not represent several layers, but the same layer with different types of cells, for which they called it pseudostratified.

Some authors do not recognize this type of tissue since it is a single layer, it must be classified as simple, naming it as simple pseudo-stratified columnar epithelium, because it is made up of columnar cells that are all in contact with the basement membrane, although not all they reach the epithelial surface.

-Glandular epithelium

The glands originate from the epithelial lining tissue, an origin that occurs during embryonic development. The glands are generally closely related to secretory functions (although this is not always the case), and depending on where the secretions are directed they are called endocrine or exocrine.

Endocrine

The endocrine glandular epithelium (endocrine glands) secretes substances such as hormones and proteins. They do this through extracellular spaces from where it reaches the circulatory system (bloodstream), which serves as a highway to carry the product throughout the body.

The secreted product is generally not released freely or uninterruptedly. Once this has been synthesized it is stored, waiting for a body signal to be secreted.

Exocrine

The exocrine epithelial tissue (exocrine glands) is responsible for secreting substances or products to the external surface of the body and even to its internal cavities.

This tissue is recognized as highly complex, which is composed of a secretory unit that releases products (proteins, hormones, etc.) into an internal cavity that is directly connected to excretory ducts. This involves close communication between the secretory unit and the lining epithelial tissue.

Glandular epithelium. Sebaceous glands. Taken and edited from: OpenStax College.

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