FLATTENED CELLS: CHARACTERISTICS, TYPES, FUNCTIONS AND EXAMPLES - BIOLOGY - 2025

The flattened cells are one of the 200 types of cells that are organized together to form the various tissues that exist in the human body. They are found predominantly in epithelial tissues, in conjunction with other cells of different morphologies.

Epithelial tissues, connective tissues, muscle tissues, and nervous tissues are the four types of tissues described in the human body. All of them are grouped, in turn, to form organs, specialized structures that are part of the body's systems.

Flattened endothelial cells that line the inner wall of blood capillaries (Source: Internet Archive Book Images via Wikimedia Commons)

Epithelial tissue is classified according to the number of cell layers that exist between the basal lamina and the free surface, and two types are known: simple and stratified epithelia. The former are made up of a single layer of cells, while the latter may have two or more layers.

A particular characteristic of epithelial tissues is that they present cells with different morphologies. They may have squamous cells, also called flattened cells, cuboidal cells, or cylindrical cells.

Accordingly, simple and stratified "squamous" epithelia, simple and stratified "cuboidal" epithelia, and so on can be described. Squamous epithelia include non-keratinized stratified squamous epithelia and keratinized stratified squamous epithelia.

Flattened cells can be identified as part of simple squamous epithelia, non-keratinized stratified, and keratinized stratified.

characteristics

Flattened cells are very thin polygonal cells. When viewed from the top, it can be seen that they have a very wide surface and a very thin profile if a cross section of these is seen. They are so thin that their nucleus protrudes or protrudes from the surface.

Thanks to these characteristics, when they are part of some stratified epithelia, flattened cells can feed by diffusion of nutrients that come from deeper cell layers, since sometimes the more superficial layers do not have blood supply.

In simple epithelia these cells are densely packed together or "packed" in such a way that when viewed from above, the epithelial surface resembles a mosaic of cells with protruding central nuclei.

The flattened cells in stratified epithelia may have nuclei and form part of the external, moist surface of some tissues and constitute what is called non-keratinized stratified squamous epithelium.

In other organs, the superficial layer of the stratified epithelium is made up of flattened dead cells, so they have lost their nucleus and are filled with keratin. It is for this reason that this type of epithelium is called keratinized stratified squamous epithelium.

Flattened cell types

Flattened cells can be classified into two groups:

- Flattened cells with nuclei.

- Flattened cells without nuclei.

An example of flattened cells without a nucleus are cells in the epidermis of the skin. However, they are considered dead cells that will be shed and eliminated, as is the case with keratinocytes.

The flattened cells with a nucleus, on the other hand, receive their own names according to the organ where the epithelium of which they are part is located. For example, those that make up the alveolar wall are called pneumocytes, and those that line blood and lymphatic vessels are called endothelial cells.

Functions and examples

Flattened cells as part of simple squamous epithelia

In the lungs

Simple squamous epithelia formed by flattened cells are found in the pulmonary alveoli, where they provide a wide contact surface between alveolar air (on one side) and capillary blood on the outer wall of the alveolus (on the other).

Due to the thin structure of the flattened cells, they facilitate the diffusion of gases from the alveolus to the capillary blood and vice versa, allowing the blood to balance with the alveolar gas as it leaves the alveolus, transforming the venous blood into arterial blood.

In the kidneys

Different structures within the nephrons of the kidney are also composed of simple squamous epithelia. In them, the flattened cells participate in filtering the blood that enters the kidney and in the formation of urine.

Simple squamous epithelia are also found in the pleural and peritoneal cavity, where they have a lubricating function that reduces friction and favors the movement of the viscera and the pleural layers with each other.

In the vascular system

The vascular and lymphatic endothelium is also composed of squamous epithelium, which provides a smooth surface for blood and lymphatic circulation, and at the capillary level it allows the exchange of liquids, gases and nutrients, as well as gaseous and metabolic waste from other tissues.

In the ear

Lining the middle ear and the inner ear, there are also simple squamous epithelia made up of flattened cells.

Flattened cells as part of stratified squamous epithelia

The flattened cells that are part of the non-keratinized stratified epithelium line the mouth, epiglottis, esophagus, vocal cord folds, and vagina. In these areas, the epithelia keep the surfaces moist and perform protective functions in these organs.

On the skin

The flattened cells that are part of the keratinized stratified epithelium (the keratinocytes) constitute the epidermis of the skin (the outermost layer).

Its function is mainly protective, since the skin is the largest organ in the human body and protects it from the environment, collaborates with the water balance and helps maintain body temperature.

The epidermis is not composed solely of keratinocytes, but these are undoubtedly some of the most abundant cells in this tissue. They have a life cycle between 20 and 30 days, so they are in constant renewal, something evident in their "progenitor" cells with high mitotic (division) rates.

When cells in the basal layers divide, they push the new cells to the surface, and the replacement takes place gradually as the more superficial ones accumulate keratin, lose their nuclei, die and "shed".

References

1. Despopoulos, A., & Silbernagl, S. (2003). Color Atlas of Physiology (5th ed.). New York: Thieme.
2. Dudek, RW (1950). High-Yield Histology (2nd ed.). Philadelphia, Pennsylvania: Lippincott Williams & Wilkins.
3. Gartner, L., & Hiatt, J. (2002). Text Atlas of Histology (2nd ed.). Mexico DF: McGraw-Hill Interamericana Editores.
4. Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Maryland: The National medical series for independent study.
5. Kuehnel, W. (2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy (4th ed.). New York: Thieme.