Layers of the epidermis: structure, functions and effective anti-aging procedures

In the human body, the skin covers complex and vital functions, the main of which can be divided into three groups: protective, regulatory and sensory. Absolutely all layers of the skin are involved in their implementation – epidermis, dermis and hypodermis (subcutaneous fatty tissue). In this article, Dr. Ahsan Ullah talks about the main layers of the epidermis, its functions and aesthetic procedures that aim to improve the quality of the skin by acting on the epidermal layer.

Layers of the epidermis: basal, spiny, granular, lustrous, horny

The epidermis consists of four main layers:

• basal;
• spiky;
• grainy;
• horny.

Also, such parts of the body as the palms and soles of the feet are characterized by the presence of the fifth – shiny layer of the epidermis.

The above layers consist mainly of keratinocytes that produce the protein keratin, the maturation process of which can be traced throughout the entire thickness of the epidermis.

1. Basale (stratum basale)

Keratinocytes make up the majority (up to 95%) of the cells of the basal layer of the epidermis and are characterized by a highly organized structure. They contain intercellular proteins - tonofilaments – components of the cytoskeleton. These structures are interconnected by desmosomes and are connected to the basal lamina by means of hemidesmosomes. This structure ensures the strength and stability of the epidermis, prevents its complete exfoliation and is the basis for the barrier function.

A significant role in the barrier function of the epidermis is played by:

• melanocytes that secrete melanin and provide protection from solar radiation;
• Merkel cells that perform a receptor function;
• Langerhans cells – dendritic cells that initiate an immune response in case of skin damage.

2. Spinous layer (stratum spinosum)

As they divide (mitosis), keratinocytes move upward and form a spiny layer, consisting predominantly of squamous cells. This is the thickest layer of the epidermis, providing its strength and flexibility due to the flatness and increased density of keratinocytes. It is in this layer that the keratinization process begins.

3. Granular layer (stratum granulosum)

As cells differentiate further, keratinocytes shrink and flatten more and more, forming a layer of epidermis consisting of 3 to 5 rows of cells. These granular cells contain keratohyalin, which looks like dark granules under a light microscope and contains two proteins – profilagrin and involucrin, which play an important role in the barrier function of the epidermis. Profilagrin contributes to the accumulation of keratin in the stratum corneum, and involucrin – the formation of a cell wall that protects the keratin in the stratum corneum. Lamellar granules contain lipids and glycoproteins that act as binders in the stratum corneum of the epidermis.

4. Shiny Layer (stratum lucidum)

This layer, which is 3-5 cell rows thick, is formed by superdense keratinocytes that form an additional barrier to protect the skin from external stressors.

5. Corneous layer (stratum corneum)

The stratum corneum of the epidermis contains densely packed dead cells, rich in keratin, approximately 15-20 layers thick. As stated above, the lamellar granules from the granular layer help to retain cells in the stratum corneum, providing a semi-impermeable layer – major part of the physical skin barrier. For dead cells of the stratum corneum, the process of exfoliation is characteristic. With aging, this layer of the epidermis becomes more dense and therefore the process of its desquamation and, as a result, the renewal of cells in the deeper layers of the epidermis slows down, which ultimately leads to age-related changes such as skin dullness.

There are various aesthetic procedures that help improve skin condition through controlled damage to the epithelium.

Skin rejuvenation by acting on different layers of the epidermis

The keratinization process, affecting the different layers of the epidermis, usually takes about 4 weeks, depending on the quality of the patient's skin. With aging and exposure to environmental factors, keratinization slows down due to a decrease in the efficiency of the desquamation process. This leads to the accumulation of dead cells and a decrease in the normal rate of skin regeneration and, as a result, a deterioration in its quality, namely:

• dullness;
• flabbiness;
• thickening of the stratum corneum.

Thus, stimulation of the desquamation process is the main goal of certain aesthetic procedures. There are various techniques for doing this, including:

• chemical peels;
• mesotherapy;
• Microdermabrasion.

Influence of chemical peels at the level of different layers of the epidermis

The essence of chemical peels is the topical application of various chemicals to destroy the epidermal barrier, which leads to exfoliation and subsequent cell regeneration. This process contributes to the exfoliation of keratinized layers of the epidermis with their subsequent self-healing due to the activation of the keratinization process in the basal layer.

The effect of peels is based on the ability of the skin to heal itself; this type of controlled damage triggers cell regeneration and epidermal healing.

There are three main types of chemical peels:

• surface;
• middle;
• deep.

At the same time, the action of superficial peels is directed to the epidermis layer.

Alpha hydroxy acids

Alpha hydroxy acids (ANA) – hydrophilic (water-soluble) acids derived from natural sources such as fruits, nuts, and milk. These substances penetrate into the stratum corneum of the epidermis and lead to destabilization of the desmosomes and, accordingly, desquamation of the cells. As a result, the epidermal barrier is destroyed with its subsequent regeneration. Therefore, ANA peels are recommended for patients with thickened and photodamaged skin.

A study by Okano et al. looked at the effects of glycolic acid on the skin and demonstrated accelerated collagen synthesis through degradation of keratinocytes. As keratinocytes degrade, the epidermal barrier is destroyed and exfoliated, which contributes to its accelerated recovery. Due to this, the keratinization process is accelerated, and the skin becomes more youthful.

Beta-hydroxy acids

VNA – lipophilic (fat-soluble) acids penetrate the epidermal barrier, dissolving sebum and promoting exfoliation of the epidermal layer. For example, peels based on salicylic acid & nbsp; recommended for patients with oily skin prone to acne.

Jessner Peel

Jessner Peel consists of a combination of alpha and beta hydroxy acids in low concentrations. The purpose of such peeling – general chemical exfoliation of the epidermal and deeper layers of the skin. Such peelings allow to achieve a general aesthetic improvement in the skin condition. Jessner peels are said to break down the desmosomes as well as the sebum, resulting in effective skin exfoliation.

Possibilities of mesotherapy in skin rejuvenation at the level of the epidermis layer

The procedure of collagen-inducing therapy (CIT, or mesotherapy) consists in exposing the skin with microneedles that penetrate the stratum corneum of the epidermis and cause microdamage to local tissues. In response to such damage, the process of release of growth factors is triggered, stimulating the synthesis of collagen and elastin in the papillary dermis.

This technique also allows for more efficient delivery of topical cosmeceuticals such as vitamins A and C, as well as their penetration into the epidermal barrier, absorption by cells in deeper layers, which in turn stimulates neocollagenesis and neovascularization. Regenerated skin becomes more toned and firm.

For mesotherapy, needles of various lengths are used:

• smaller needles – smoothing fine lines and wrinkles;
• needles larger – post-acne correction.

Microdermabrasion – effective procedure to renew the epidermis layer

This procedure uses mechanical agents such as exfoliating crystals or diamond particles that damage the desmosomes and hemi-desmosomes.

The disruption of the epidermal barrier after microdermabrasion ensures the removal of dead cells from the stratum corneum.

This aesthetic procedure is minimally invasive and achieves excellent aesthetic results with the right choice of the patient. This technique is great for:

• aging skin;
• photodamaged skin;
• in case of insufficiently effective exfoliation of the stratum corneum, which prevents the migration of keratinocytes from the basal to the more superficial layers of the epidermis.

The purpose of microdermabrasion – enhance the keratinization process and make the skin more "fresh"; and smooth.

Read also: Anatomy of facial aging: where does youth go and how to get it back

Thus, chemical peels, mesotherapy and microdermabrasion work on the epidermal layer to improve skin quality by activating the skin's natural ability to heal itself in response to controlled damage.