SIMPLE FLAT EPITHELIUM: CHARACTERISTICS, FUNCTIONS AND TYPES - BIOLOGY - 2024

A simple or squamous flat epithelium is a non-vascularized tissue that lines, covers or covers the surfaces of some body regions, mostly internal. Typically it is simply constituted by only one cell layer thick, which is why it receives this name, being a monolayer epithelium.

In general, in epithelial tissues the cells that make them up are intimately united with each other, keeping the intercellular material practically to a minimum and this compact cell layer resting on a basement membrane.

Source: Kamil Danak

Types

Depending on the individual cell forms that make up a simple epithelial tissue, various designations can be attributed to the particular tissue.

Simple flat epithelia

When the cells are obviously wider and deeper than their height, it is considered a simple flat epithelium. Due to the superficial shape of this tissue, it has also been given the name of simple squamous epithelium.

However, there are other types of simple epithelia, whose shape of the only layer of cells that compose it, are cubic or cuboid in shape, the width, height and cell depth being approximately equal.

Simple columnar epithelia

On the other hand, simple columnar epithelia are those in which the height of the epithelial cells is considerably higher than the other dimensions considered, giving them a columnar appearance.

Knowing this, it is easy to identify the type of epithelium in question. However, in special cases, the apical region of the cells that make it up are specialized and have cilia constituting, for example, simple cylindrical ciliated epithelia.

Cellular characteristics of a simple flat epithelium

The cells that are part of the epithelia share a set of typical characteristics among themselves regardless of their shape:

- Cells are closely arranged to one another, adhering to each other by means of specific cell-cell adhesion molecules. With this, they form highly specialized intercellular adhesions. Being so thin cells, their nuclei make prominence towards the lumen or cavity of the organ that they line

- In general they have morphological polarity and, as a consequence, also in their functionality. Each cell thus has three functional regions of different morphology: an apical, a lateral and a basal region.

- Its basal surface (basal region) is attached to a basement membrane that is an acellular layer with a great variety of polysaccharides and proteins.

- It is a non-glandular lining epithelium, the metabolic maintenance (oxygen and nutrients) of its cells is mediated by the surrounding connective tissues.

Where can we find simple flat epithelium?

Some epithelia, due to their characteristics or location, are given special names. Such is the case of the endothelium and the mesothelium. Both are characterized by being monolayer (simple) and flat epithelia, which line the vessels and body cavities respectively.

These epithelial tissues have their origin in the mesoderm, however, other epithelial tissues have their ectodermal or endodermal origin.

In general, endothelium constitutes the endothelial lining of the cardiovascular system (blood vessels and cavities of the heart) and the lymphatic vessels. On the other hand, the mesothelium is the epithelium that covers the cavities and the content of the closed cavities present in the body. Among them, the abdominal cavity, the pericardial cavity and the pleural cavity.

In both cases, the epithelia are mostly simple flat epithelia. There are exceptions in certain capillary veins of the lymphatic system, in which the epithelium (although it is still simple) is made up of cubic cells.

This type of epithelium can also be identified in the pulmonary alveoli and in Bowman's capsules and the loop of Henle in the kidney.

Features

The epithelia, despite their morphological diversity, usually perform varied functions in the different organs of the body, they may even have one or more functions, each of them.

It is typical for simple epithelia to have secretory or absorption functions, the height of the cells in the case of simple columnar or cuboidal epithelia being indicative of the degree of secretory or absorptive activity. For example, simple flat epithelia are highly compatible with a high transepithelial transport index.

On the other hand, this type of epithelium fulfills functions of exchange in the endothelium (vessels), a barrier in the central nervous system, exchange and lubrication in the body cavities (endothelium). In addition, it acts as a barrier in Bowman's capsules and urine filtering in the kidney, as well as in the exchange in the respiratory alveoli of the lung.

In all these locations, through the simple flat epithelium, abundant and rapid transports of substances take place, these materials being essentially gases, water and ions. As the junctions between cells are so tight, all the substances carried by the flat epithelium necessarily pass through the cells that compose it.

Medical relevance

The vulva, vagina, and the portion of the cervix projected into it, called the vaginal portion, ectocervix, or exocervix, are lined by a stratified flat epithelium. On the other hand, the endocervical canal of the cervix is ​​lined by a simple flat mucosecretory epithelium.

The stratified flat epithelium of the exocervix is ​​made up of three layers: a superficial layer, an intermediate layer, and a basal and parabasal zone, with cells that share similar cytomorphological features.

These epithelia unite in an area called the junctional or transformation zone, where a sudden transition occurs between the simple stratified epithelium of the ectocervix and the simple flat epithelium of the endocervix. During menstruation, these epithelia undergo proliferative and remodeling changes, caused by steroid hormones.

Alterations in the transformation between these epithelia constitute precancerous lesions of the cervix. In these conditions, the cells of the cervix constantly project towards the vagina.

One of the most important and common ways for the detection of these alterations and the diagnosis of precancerous and cancerous lesions of the cervix, is the Papanicolaou stain of the cervico-vaginal cytology, where the colored cells of the cervix are detected in the vaginal area.

On the other hand, because epithelial tissues present high rates of cell substitution by cell division, asymptomatic lesions such as mesothelial cysts can be generated. Histologically, they have a coating of flat epithelial cells and, in addition, they can have both cubic and columnar cells.

References

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