Stratified Squamous Epithelium: Structure, Functions, Examples

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The stratified squamous epithelium consists of several layers of cells, where the cells in the apical layer and several layers present deep to it are squamous, but the cells in deeper layers vary from cuboidal to columnar.

As the most important difference between the simple epithelium and the stratified epithelium is the number of the layer of cells, the functions of these layers also differ in the same respect whereas the structure is more or less similar. As a result of the continual cell division in the lower (basal) layers, the cells in the superficial layers are pushed towards the surface, where they are shed.

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Structure of the stratified squamous epithelium

The stratified squamous epithelium has two or more layers of cells. The cells in the apical layer and several layers deep to it are squamous while the cells in deeper layers vary from cuboidal to columnar.
In the stratified squamous epithelium, only one layer is directly attached to the basement membrane, and the rest of the layers are connected to one another to maintain the structural integrity.
As basal cells divide, daughter cells arising from cell divisions push older cells upward toward the apical surface.
The cells in the epithelium are attached firmly with very few intracellular spaces.
Unlike in simple epithelium, in the stratified epithelium, only the outermost layer of the tissue is exposed towards the lumen at the apical surface. All the other sides of the cells are attached to other cells through cell junctions and adhesions.
These cells have many desmosomes and other adhesins and become more irregular in shape and then flatten as they move towards the surface.
As they move toward the surface and away from blood supply in underlying connective tissue, they become dehydrated and less metabolically active.
Tough proteins predominate as cytoplasm is reduced, and cells become sturdy, hard structures that eventually die.
At the apical layer, after dead cells lose cell junctions, they are sloughed off, but they are continuously replaced as new cells emerge from basal cells.
Just like all other epithelial tissue, the stratified squamous epithelium also doesn’t have a direct blood supply but has a distinct nerve supply.

Based on the accumulation of keratin by the cells towards the surface, the stratified squamous epithelium is of two types:

1. Keratinized stratified squamous epithelium

Keratinized stratified squamous epithelium is a type of stratified squamous epithelium in which the cells have a tough layer of keratin in the apical segment of cells and several layers deep to it.
Keratin is a tough, fibrous intracellular protein that helps protect skin and underlying tissues from heat, microbes, and chemicals.
As the cells move upwards, they accumulate keratin in the process of keratinization, where they become thin, metabolically inactive pockets (squames) of keratin lacking nuclei.
The relative amount of keratin increases in cells as they move away from the nutritive blood supply and the organelles eventually die.
As a result of the keratinization, the cells on the top layer loses all its function and is only involved in providing protection against dehydration and other mechanical stress.

2. Non-keratinized stratified squamous epithelium

Non-keratinized stratified squamous epithelium is the epithelium in which the cells do not have a lot of keratin deposits within them, but rather are moisturized by mucus from the salivary or the mucus glands.
In this case, the flattened cells of the surface layer retain their nuclei and most metabolic functions.
Because non-keratinized stratified squamous epithelium doesn’t have as much keratin deposits, this epithelium is usually found in areas that need to be kept hydrated or are affected by dryness.
This epithelium does have some amount of keratin which, in turn, depends on the age of the layer of the tissue and the mechanical wear and tear it has been subjected to.
Some cells of this epithelium might even produce trace amounts of mucus and other lubricating agents to keep the surface from drying out.

Functions of the stratified squamous epithelium

The most important and crucial function of stratified squamous epithelium is protection.

1. Protection

The stratified squamous epithelium provides protection against mechanical stress, chemical abrasions, and even radiation.
The keratinized epithelium present on the surface of the skin blocks out the harmful radiation and prevents the exposure of internal tissues and organs to the radiation.
Similarly, it also prevents water loss due to heat and damage to the internal organs by any physical distress.
When damaged, this epithelium replaces the outermost layer, which also aids the effectiveness against any stress or invasion.
The non-keratinized stratified squamous epithelium prevents the entry of foreign invaders as the surface is kept consistently moist.
Both the types of the stratified squamous epithelium act as the first line of defense against microbial invasion or any physical harm.

2. Secretion

As mentioned earlier, the non keratinized stratified squamous epithelium consists of cells that produce some amount of mucus to keep the surface moist.
The mucus also helps to balance the pH of the surrounding, as in the case of the vaginal epithelium.
In addition, it also acts as a lubricating agent in a region frequently subjected to friction like the buccal cavity.

Location and Examples of Stratified squamous epithelium

The keratinized epithelium lines the region exposed to the external environment and continuously subjected to stress like the superficial layer of the skin.
It also forms the epidermis of the palm of the hand and the sole of the foot.
The non keratinized epithelium forms the lining of the internal surfaces and cavities, which commonly endure friction and physical wear and tear.
In the digestive system, the stratified squamous epithelium lines the surface of the tongue, the hard upper palate of the mouth, the esophagus, and the anus.
Similarly, it is also found in the female reproductive parts like the vagina, cervix, and the labia majora.
The upper respiratory tract that might come in contact with food is also lined with stratified epithelium.
The stratified epithelium covers the cornea of the eye in order to protect the delicate tissues present in the eye.
The epidermis of the urethra in the urinary system is also made up of stratified squamous epithelium.

References and Sources

Mescher AL (2016). Basic Histology. Fourteenth Edition. McGraw-Hill Education.
Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
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