The levels of chemical peels according to skin histology

In order to understand the function of different peeling agents it is important to have a good knowledge of the skin histology. Human skin consists of two main layers:

Epidermis

Epidermis is the outermost layer. It serves as a protective barrier to external environment. It is waterproof, it keeps the water in the body and prevents infections. It also plays a role in regulation of the body temperature. The epidermis consists of 5, well distinguished layers, called stratums:

• Stratum corneum
• Stratum lucidum (only present in palms and soles)
• Stratum granulosum
• Stratum spinosum
• Stratum basale (germinativum)

95% of epidermis is represented by keratinocytes; other cells to be found in epidermis are melanocytes, Merkel cells (“touch cells” with possible neuroendocrine role) and Langerhans cells (antigen-presenting cells). Keratinocytes proliferate in the stratum germinativum and further move up to more outside layers when they undergo cell differentiation; they change the shape and composition. Stratum corneum consists of cells without nuclei. During these changes keratinocytes form desmosomes (junctions between cells), they produce and secrete lipids as well as keratin which form extracellular matrix. Through desquamation keratinocytes from the outermost layer (stratum corneum) shed from the surface. Epidermis does not have blood vessels; nourishment comes via diffusion from capillaries from the upper dermis. Epidermal appendages (hair follicles, sebaceous and sweat glands) play an important role in re-epithelialization after epidermis is destroyed as they are the source of the epithelial cells.

Between epidermis and dermis lies basement membrane, which role cannot be underestimated. It is a thin sheet made from fibers. It holds cytokines, growth factors which are important in repair processes.

Dermis lies under the epidermis. It is built from connective tissue and provides elasticity as well as the strength, extensibility of the skin. It is composed of cells and extracellular matrix. Matrix consists of collagen and elastic fibers as well as microfibrils (very fine fiber-like strands which are built from glycoproteins and cellulose). In dermis one can also find nerve endings (touch, pain, temperature receptors), blood and lymphatic vessels, hair follicles, sebaceous glands and sweat glands (apocrine and eccrine).

Dermis is further divided into two regions:

• Papillary
• Reticular

Papillary region is the more superficial area, tightly connected to the basement membrane. It consists of projections which are called papillae and they extend into epidermis. Reticular area lies deeper, under the papillary region and it is much thicker. It consists of connective tissue rich in fibers: collagen, elastin and microfibrils embedded in proteoglycans as well as hair follicles and sebaceous, sweat glands, nails, vessels and receptors.

Under the skin there is a layer called hypodermis (subcutaneous layer) which mostly consists of adipose and connective tissues. It attaches the skin to deeper structures like bones or muscles.
The main purpose of chemical peels is controlled destruction of the outer damaged skin layers. As it was mentioned earlier the epidermis regenerates from the epidermal appendages. It is a quick process which usually takes up to 10 days. The new epidermis is better organized, it has vertical polarity and less dyschromia. Dermis takes longer to regenerate (even up to few months) but the results show improvement in solar elastosis, better organized bundles of collagen (compacted horizontally) and decreased ground substance.

One of many chemical peelings classifications divides chemical peels into 4 groups depending of the depth of the injury created by the peeling agent:

1. Very superficial peels which cause necrosis at the stratum corneum level. Agents that are used in this category are: 10% TCA, 30-50% GA, 20-30% Salicylic Acid, 1-3 coats of Jessner’s solution, 1-5% Tretinoin
2. Superficial peels which cause necrosis of all the layers of epidermis right up to the basal layer. Chemicals used are: 20-30% TCA, 50% GA, 4-7 coats of Jessner’s solution.
3. Medium- depth peels cause necrosis up to the upper reticular dermis, mostly up to papillary dermis. Agents used are: 35-50% TCA, 70% GA combined with 35%TCA, Jessner’s solution combined with 35% TCA, solid CO2 plus 35% TCA, un-occluded 88% phenol.
4. Deep peels penetrate to the midreticular dermis. The agent used is Baker-Gordon phenol peel.

The depth of the wound created by the peel depends on many factors:

• Type of chemical agent used
• Concentration of the chemical
• Number of coats applied
• Way of application, for example vigorous rubbing
• Contact time of the peeling agent with the skin
• Skin type
• Skin condition- presence of hyperkeratotic lesions, thickness of the epidermis and dermis
• Presence of oils on the skin surface
• Skin water content
• Temperature of the room, of the skin as well as of the solution applied
• Humidity of the air in the room where the procedure takes place
• Occlusion or non-occlusion
• Skin preparation before peels such as using topical tretinoin or alpha-hydroxy acid preparations

The deeper peel penetration creates:

• More dramatic results
• Longer healing time
• Higher risk of side effects and complications
• More uncomfortable patient’s experience during as well as after the procedure

One needs to remember that more treatments with superficial peels will not bring the same result as the one achieved after the deeper peels as the superficial peels do not reach the affected histological level.

Because the healing time after medium-depth peels is longer, at least 6 months intervals in between treatments are recommended. The superficial peels can be repeated after 2-3 weeks.
As a general rule light peel needs 5 days of healing, medium peel 7 days and medium/heavy peel up to 14 days.

The type of chemical peel to be used depends on the skin type and the problem which needs to be treated. Patients with Glogau type IV will notice minimal improvements in rhytides after superficial peels while deep peels are contraindicated in patients in Glogau type I. Epidermal defects, such melasma or post acne pigmentation can be treated with superficial peels, but deeper problems like deeper wrinkles, elastosis, collagen distortion will need deep peeling agent.

Mild to moderate skin photoaging damage might improve after medium- depth peels.

Superficial peels cause destruction of the epidermis only therefore the skin heals rapidly and without scarring. It is a safe procedure but often lacks in dramatic improvement.

Better results are achieved in wounding papillary and even reticular regions of the dermis, but this can lead to certain degree of eradication of epidermal appendages thus causing slower healing and possibility of scarring.

There are 3 main clinical phases indicating the level of depth penetration of the peeling agent:

1. Diffuse homogenous erythema indicates epidermal penetration
2. White frost indicates coagulative necrosis and it means of destruction of papillary dermis
3. Grey-white frost indicates coagulative necrosis of the reticular dermis

Very superficial and superficial skin peels include:

• Alpha-hydroxy acids (AHAs) like glycolic, lactic, malic, citric acids. The depth of penetration of AHA is time dependent therefore neutralization of the agent is important in order to stop the process. If one wants to achieve the desired result in shorter time a higher concentration of the peeling agent should be used. Low concentrations will cause reduction of cohesiveness of stratum corneum, while higher will lead to complete epidermolysis. White patches on the skin observed during the procedure indicate deeper than epidermis penetration and might not be desirable. The most commonly used AHAs are 50-70% Glycolic acids
• Beta-hydroxy acid (salicylic acid, SA) is also a keratolytic agent and well tolerated. It creates a pseudofrost once the salicylic acid crystalizes. It is a peel of choice for acne because as lipid soluble well penetrates the comedones. The most commonly used concentrations are 20-30%. Beta-lipohydroxy acid (LHA) is a new peel which is salicylic acid derivative. It has stronger lipophilicity than SA as well as antibacterial and anticomedonic properties.
• Jessner solution if applied alone belongs to superficial peels. This formula is made from salicylic acid, lactic acid and resorcinol, each 14g mixed with 95% ethanol to total volume of 100 ml solution.
• 10-30% trichloroacetic acid (TCA). TCA is neutralized by tissue fluids therefore does need to be neutralized by any alkaline agent. It leads to coagulation of proteins in the skin causing formation of frost.

Medium-depth peels include:

• 35% trichloroacetic acid (TCA). Higher concentrations up to 50% will cause deeper injury (up to reticular dermis), but due to high risks of scarring are not recommended.
• Combination of 35% TCA with solid CO2 (Brody’s peel)
• Combination of 35% TCA with Jessner solution (Monheit’s peel)
• Combination of 355 TCA with 70% Glycolic acid (Coleman’s peel)

Deep skin peelings are achieved by using pure phenol (88%) or phenol mixed with soap, water, croton oil or even olive oil. There are few formulas such as Baker-Gordon, Maschek-Truppman or Venner-Kellson. The most popular is Baker-Gordon formula which consists of 3 ml of phenol, 2 ml of water, 8 drops of liquid soap and 3 drops of croton oil. Deep peels lead to keratolysis and keratocoagulation. Deep peels have the most dramatic results but due to high risk of complications are often abandoned and replaced by other methods of skin resurfacing like lasers.
In summary one needs to stress the role of a good knowledge of skin histology as well as mode of action of different peeling agents. Peels can be divided into 3 categories depending on the level of their skin penetration. The depth of the peel is proportional to the results achieved but sadly also to the potential complications and time of healing.

References

1. “Chemical Peelings” in June 2015 Advanced Diploma in Aesthetic Medicine Module 3, Foundation for Professional Development, Edition 1
2. Fabbrocini G., “Chemical Peels” 2016 www.emedicine.medscape.com Retrieved 17/08/2016
3. Rendon M.I., Berson D.S., Cohen J.L., Roberts W.E.,2010 Evidence and Consideration in the Application of Chemical Peels in skin Disorders and Aesthetic Resurfacing The Journal of Clinical and Aesthetic Dermatology Jul (397):32-43
4. Khunger N., Standard guidelines of care for chemical peels 2008 Indian Journal of Dermatology, Venereology and Leprology 74(7):5-12