CPD: Understanding Skin Layers and Injectables

Understanding Skin Layers and Injectables

Miss Priyanka Chadha, Miss Lara Watson and Dr Catrin Wigley provides an overview of the skin’s function and anatomy and its relevance for successful aesthetic treatments

The skin is the body’s largest organ, comprising around 16% of a person’s total body weight.1 Its complex structure and range of functions reflect its embryological origin from both ectoderm and mesoderm germ cell layers.2 Its functions are broad, performing a vital role in thermoregulation and water homeostasis, and providing a barrier from harmful agents such as UV rays, pathogens and mechanical trauma, as well as being a key endocrinological organ in vitamin D production.2 Skin also plays an integral role in our everyday psychosocial wellbeing and how we interact with one another, offering information on heritage, age and general health.2 As the skin provides the canvas for almost all aesthetic medicine, a working knowledge of its function and anatomy is essential for clinicians to provide safe treatments, as well as knowing how to optimise results. This article will therefore cover the functional anatomy of the skin in relation to injectable practice, as well as look at the emerging trends that exploit the properties of the integumentary system.

The epidermis The skin can be broadly classified into three layers: the epidermis, dermis and subcutaneous layer, which is anchored to underlying structures by connective tissue.3 The outermost layer, the epidermis, is largely composed of keratinocyte cells arranged into a stratified squamous epithelium.3 The epidermal keratinocytes function to make keratin, a protein that provides the epidermis with its main properties as a barrier. The different stages of maturation of the keratinocytes are arranged into layers; the basal cell layer, suprabasal cell layer, granular cell layer and, most superficially, the horny cell layer.4 As keratinocytes migrate through the layers of the epidermis, they become less lipophilic and denser in keratin.3 The epidermis contains no blood vessels, lymphatics or nerves, thus cells in the basal cell layer are supplied by diffusion from the upper portion of the dermis.5 The horny cell layer is shed through a process called desquamation, and the entire process from the basal cell layer takes around 28 days.3 Around 95% of the epidermal cells are keratinocytes, and the remaining minority consists of melanocytes, merkel cells and langerhans cells.6 Melanocytes are responsible for the production of melanin, the pigment responsible for UV protection and in varying skin complexions.6 It is the overproduction of melanin by melanocytes that causes the characteristic facial hyperpigmentation seen in melasma.7 Merkel cells are tactile cells that interact with sensory nerves within the basal cell layer of the epidermis. These cells are found in higher concentrations in skin regions that require more touch feedback, such as in the fingertips.4 Langerhan cells, on the other hand, are involved in a variety of T-cell responses and immunity.4 The interface between the epidermis and dermis contains a porous basement membrane that allows the exchange of nutrients such as peptides between the layers, as well as providing structural adherence.8 With age, the epidermis has been observed to become atrophied and so many mild chemical peeling agents, such as glycolic acid, target the epidermis to accelerate the process of desquamation, improving the skin texture and reducing the appearance of fine lines or uneven pigmentation.9

The dermis The dermis is arguably the most clinically relevant skin layer to aesthetic medicine and is where most treatments target their technologies.10 The dermis is the intermediate layer of skin sitting between the epidermis and subcutaneous layer. The role of the dermis is to sustain and support the epidermis, as well as providing elasticity and tensile strength.8 Being connective tissue, the dermis contains ground substance, fibres and cells such as fibroblasts, mast cells and histiocytes.4 The dermis also accommodates vascular and lymphatic channels, nerves and appendages such as hair follicles, sweat glands and sebaceous glands.4,11

Hair Pain receptor Capillary network

Sebaceous gland Erector pili muscle Epidermis

Heat receptor (Ruffini endings)

Hair follicle Cold receptor (Krause muscle) Dermis

Motor nerve

Hair bulb Vein Artery Subcutaneous layer

Subcutaneous fat Subdermal muscle layer Nerve endings Deep fascia Sweat (eccrine gland) Pacinian corpuscle (pressure receptor)

Touch receptor (Meissner’s corpuscle)

The dermis can be subdivided histologically into the more superficial papillary dermis and the deeper reticular dermis, the latter forming the majority of the layer.4 The papillary layer consists of loose connective tissue containing small capillaries, elastin and some collagen fibres, whereas the arrangement of the reticular dermis is of dense connective tissue, larger blood vessels and thick bundles of collagen.4 Medium-strength chemical peels such as 50% TCA work by penetrating the papillary dermis, whereas deep chemical peels (not as commonly used in practice) penetrate the full thickness of the dermis.12 Collagen provides the majority of the dry weight of the dermis and gives the layer its pale colour.4 Collagen is produced by fibroblast cells and provides the skin with its stress-resistant properties.1 With age, the production and quality of collagen types I and III reduces, playing an integral part of the intrinsic ageing process of skin.9 Topical retinoids and peptides, for example, are a popular treatment for upregulating the synthesis of collagen within the dermis.9 Non-ablative laser resurfacing technologies also target the dermis to generate thermal injury and encourage collagen production.13 Elastin fibres are not as tough as collagen fibres but provide the skin with elasticity, although they do little in the way of shape deformation.4

The ground substance is an amorphous gelatinous matrix largely made up of sugar and protein rich molecules called glycosaminoglycans.1 Hyaluronic acid is an example of a glycosaminoglycan found in the connective tissue, although it is a minor component of the dermal ground substance. Fractional laser has gained popularity over recent years, offering encouraging results.14-16 These lasers, such as C02 lasers, work by evaporating the epidermis and stimulating matrix remodelling within the ground substance of the dermis.13,17

Although a minor constituent of the dermal ground substance, hyaluronic acid plays an integral role in the skin ageing process as it becomes the most depleted glycosaminoglycan with age.18 As it naturally occurs in the dermis, the hypoallergenic and hydrophilic properties of hyaluronic acid have made it a safe, effective and reversible substance for dermal fillers. It is therefore no surprise that these products have become so commercially appealing.19,20 The dermis hosts a two-tier vascular supply made of two interconnecting plexuses. Most superficially lies the superficial plexus within the papillary dermis, followed by the larger deep plexus that sits within the lower reticular dermis surrounding adnexal structures such as sweat glands and hair follicles.8 The nerves of the skin are found in various forms and also lie within the dermis. Sensory afferent signalling within the skin is very complex and includes touch, pain, vibration and pressure.1 Of most relevance, however, non-myelinated free nerve endings responsible for pain transmission are found in abundance within the papillary dermis.4

The most common commercial preparations of dermal filler contain local anaesthetic, such as 1% lidocaine which act on these free nerve endings to make the procedure more tolerable for patients.21 The application of vibration devices in conjunction to dermal filler and muscle relaxants have recently gained impetus.22 The concept is that the devices can influence simulation-induced analgesia making treatments more tolerable, with provisional results appearing promising.22,23 Subcutaneous fat The deepest layer of the skin is the subcutaneous fat, which sits between the dermis and fascia of the facial muscles. Its role is to provide a cushion to external force, preserve body heat and retain moisture.4 The subcutaneous layer largely consists of fat cells arranged into lobules. The thickness of the subcuticular fat varies according to anatomical site, body habitus and age.24 For example, with age the density of subcutaneous fat in the cheeks is disproportional to other anatomical sites.25 Over time, distinct ‘pockets’ of fat have been noted to become more discrete than other areas, giving rise to characteristic signs of ageing such as the nasolabial fold and jowls.26 Historically, autologous fat was the first commercial dermal filler and is still widely used for longer term volume correction, however its use is generally restricted to the surgical world as the procedure is more technically challenging.27

Applications to practice A working knowledge of the skin’s histology not only provides a framework for optimising results in a broad range of skin types, but also allows the anticipation of potential procedural complications. Atopic dermatitis is a complex condition that largely describes an immune-mediated impairment of the skin barrier function, specifically affecting its keratinisation.28 Subsequently, water is lost more readily from the skin’s surface and has an increased susceptibility to microbes. Clinicians must therefore assess the suitability of patients for dermal filler by the severity of the dermatitis, its proximity to the proposed treatment area, and the risk of complications such as infection.29 Rosacea also affects many patients seeking aesthetic treatments and is a relatively common condition characterised by flushing, telangiectasia, and erythema. Its pathophysiology is complex and in part due to the dysregulated release of peptides from the epidermis.30 Patients with rosacea are at higher risk of postinjection erythema and should be warned about this prior to treatment.31 Another common skin condition affecting the epidermis is psoriasis, a multifactorial immune-mediated reaction resulting in epidermal thickening and hyperproliferation.32 Dermal fillers can precipitate psoriatic flareups via the Koebner response, and may impact how the product distributes throughout the skin layers.29 Other inflammatory skin reactions such as hypertrophic and keloid scarring, and post-inflammatory hyperpigmentation can be initiated by dermal filler.29,33,34 A detailed patient history and thorough assessment of abnormal scarring, hyperpigmentation, and Fitzpatrick skin types must therefore be conducted in all patients to assess for treatment suitability and advise on potential complications. Another such example is in cases of active infection, such as that of herpes simplex virus (HSV). Any active skin infections should defer treatment and, in the case of HSV, there is some evidence to support prophylactic antiviral therapy in patients seeking treatments with a known history.29,35 On assessment of patients, skin thickness and quality must also be considered in patient selection, and in the selection of the dermal fillers used. An example of this significance is in patients with skin atrophy following long-term steroid use, whereby superficial or medium-depth dermal filler would not be suitable, and places patients at increased risk of complications such as the Tyndall effect.29,36

Consider the skin anatomy In this article we have considered the key constituents of the three layers of the skin and their relation to aesthetic practice. An awareness of how therapies such as dermal fillers target specific properties of the skin remains central to providing safe, up-to-date, and optimised cosmesis for patients.

Aesthetics Clinical Advisory Board Member Miss Elizabeth Hawkes says…

A sound understanding of skin anatomy is crucial for all aesthetic practitioners and cosmetic surgeons. Success or failure is often determined by skin remodelling, whether it be following injectable treatments, medical-grade skincare, peels, laser or cosmetic surgery. With a thorough knowledge of skin anatomy and physiology, aesthetic practitioners can offer safe and effective treatments. This CPD article discusses the anatomy of the skin with an in-depth description of its layers and cellular subtypes. It continues by applying the scientific knowledge to clinical practice, with descriptions of common skin diseases. Some of the basic principles of aesthetic medicine and ageing are described in this paper, with further reading suggestions in the references.

REFERENCES

1. Venus M, Waterman J, McNab I. Basic physiology of the skin. Surgery. 2011. 2. McGrath JA, Uitto J. Anatomy and Organization of Human Skin. In: Rook’s Textbook of Dermatology:

Eighth Edition. 2010. 3. Chu D. Overview of biology, development, and structure of skin. Fitzpatrick’s Dermatology Gen Med. 2012. 4. Shimizu H. Shimizu’s Dermatology. Shimizu’s Dermatology. 2017. 5. Cross SE, Roberts MS. Importance of dermal blood supply and epidermis on the transdermal iontophoretic delivery of monovalent cations. J Pharm Sci. 1995. 6. Haake A, Scot GA, Holbrook KA. Structure and function of the skin: overview of the epidermis and dermis. In: The Biology of the Skin. 2001. 7. Ogbechie-Godec OA, Elbuluk N. Melasma: an Up-to-Date Comprehensive Review. Dermatology and

Therapy. 2017. 8. Kolarsick P, Kolarsick M, Goodwin C. Anatomy and Physiology of the Skin. J Dermatol Nurses Assoc. 2011;3(4):203–13. 9. Sadick NS, Karcher C, Palmisano L. Cosmetic dermatology of the aging face. Clin Dermatol. 2009. 10. Micheels P, Besse S, Sarazin D, Vincent AG, Portnova N, Diana MS. Quantifying depth of injection of hyaluronic acid in the dermis: Data from clinical, laboratory, and ultrasound settings. J Drugs

Dermatology. 2016. 11. Burns DA, Breathnach SM, Cox NH, Griffiths CEM. Rook’s Textbook of Dermatology: Eighth Edition.

Rook’s Textbook of Dermatology: Eighth Edition. 2010. 12. Soleymani T, Lanoue J, Rahman Z. A practical approach to chemical peels: A review of fundamentals and step-by-step algorithmic protocol for treatment. Journal of Clinical and Aesthetic Dermatology. 2018. 13. Atiyeh BS, Dibo SA. Nonsurgical nonablative treatment of aging skin: Radiofrequency technologies between aggressive marketing and evidence-based efficacy. Aesthetic Plastic Surgery. 2009. 14. Rahman Z, MacFalls H, Jiang K, Chan KF, Kelly K, Tournas J, et al. Fractional deep dermal ablation induces tissue Tightening. Lasers Surg Med. 2009. 15. Shin MK, Lee JH, Lee SJ, Kim NI. Platelet-rich plasma combined with fractional laser therapy for skin rejuvenation. Dermatologic Surg. 2012. 16. Rahman Z, Alam M, Dover JS. Fractional Laser treatment for pigmentation and texture improvement.

Skin Therapy Lett. 2006. 17. Shin JW, Kwon SH, Choi JY, Na JI, Huh CH, Choi HR, et al. Molecular mechanisms of dermal aging and antiaging approaches. Int J Mol Sci. 2019. 18. GHERSETICH I, LOTTI T, CAMPANILE G, GRAPPONE C, DINI G. HYALURONIC ACID IN CUTANEOUS

INTRINSIC AGING. Int J Dermatol. 1994. 19. John HE, Price RD. Perspectives in the selection of hyaluronic acid fillers for facial wrinkles and aging skin. Patient Preference and Adherence. 2009. doi: 10.2147/PPA.S3183 20. Price RD, Berry MG, Navsaria HA. Hyaluronic acid: the scientific and clinical evidence. Journal of

Plastic, Reconstructive and Aesthetic Surgery. 2007. 21. Allemann IB, Baumann L. Hyaluronic acid gel (JuvédermTM) preparations in the treatment of facial wrinkles and folds. Clinical Interventions in Aging. 2008. 22. Kuwahara H, Ogawa R. Using a Vibration Device to Ease Pain During Facial Needling and Injection.

Eplasty. 2016. 23. Guney K, Sezgin B, Yavuzer R. The Efficacy of Vibration Anesthesia on Reducing Pain Levels during

Lip Augmentation: Worth the Buzz? Aesthetic Surg J. 2017. 24. Mueller WH, Wohlleb JC. Anatomical distribution of subcutaneous fat and its description by multivariate methods: How valid are principal components? Am J Phys Anthropol. 1981. 25. Raskin E, LaTrenta GS. Continuing medical education article-facial aesthetic surgery. Why do we age in our cheeks? Aesthetic Surg J. 2007. 26. Rohrich RJ, Pessa JE. The fat compartments of the face: Anatomy and clinical implications for cosmetic surgery. Plast Reconstr Surg. 2007. 27. Groen JW, Krastev TK, Hommes J, Wilschut JA, Ritt MJPF, Van Der Hulst RRJW. Autologous fat transfer for facial rejuvenation: A systematic review on technique, efficacy, and satisfaction. Plastic and

Reconstructive Surgery - Global Open. 2017. 28. Pelc J, Czarnecka-Operacz M, Adamski Z. Structure and function of the epidermal barrier in patients with atopic dermatitis - Treatment options. Part one. Postepy Dermatologii i Alergologii. 2018. 29. De Boulle K, Heydenrych I. Patient factors influencing dermal filler complications: Prevention, assessment, and treatment. Clin Cosmet Investig Dermatol. 2015. 30. Yamasaki K, Gallo RL. The molecular pathology of rosacea. Journal of Dermatological Science. 2009. 31. Funt D, Pavicic T. Dermal fillers in aesthetics: An overview of adverse events and treatment approaches. Clinical, Cosmetic and Investigational Dermatology. 2013. doi: 10.2147/CCID.S50546 32. Roberson EDO, Bowcock AM. Psoriasis genetics: Breaking the barrier. Trends in Genetics. 2010. 33. Heath CR, Taylor SC. Fillers in the skin of color population. J Drugs Dermatology. 2011. 34. Taylor SC, Burgess CM, Callender VD. Safety of nonanimal stabilized hyaluronic acid dermal fillers in patients with skin of color: A randomized, evaluator-blinded comparative trial. Dermatologic Surg. 2009. 35. De Boulle K. Management of complications after implantation of fillers. J Cosmet Dermatol. 2004. 36. Luebberding S, Alexiades-Armenakas M. Critical Appraisal of the Safety of Dermal Fillers: A Primer for

Clinicians. Current Dermatology Reports. 2013.

Miss Priyanka Chadha currently works as a plastic surgery registrar in London and is co-director of Acquisition Aesthetics training academy. Her academic CV comprises national and international prizes and presentations, as well as higher degrees in surgical education and training. Miss Chadha is a key opinion leader for Galderma. Qual: MBBS(Lond), BSc(Hons), DPMSA(Lond), MRCS(Eng), MSc(Lond)

Miss Lara Watson is dual-qualified in medicine and dentistry and works as a registrar in oral and maxillofacial surgery. She is a faculty member for Galderma and is also a co-founding director of Acquisition Aesthetics with a strong background in anatomy and scientific research. Qual: BM, BMedSci, BSc, MRCS(Eng), BDS(Hons)

Dr Catrin Wigley is a plastic surgical trainee in Central London. She was previously an academic trainee in the West Midlands and has held several lecturing roles. Dr Wigley practises aesthetics alongside her training and research interests. Qual: BMedSc, Hons MBChB

Test your knowledge! Complete the multiple-choice questions below and email editorial@aestheticsjournal.com to receive your CPD certificate!

Questions

1. The epidermis is primarily made of what cell type?

2. What is the main constituent of the dermis?

3. What type of substance is hyaluronic acid?

4. In what layer of the skin do the sebaceous glands exist?

5. The reduction of which collagen is most associated with the ageing process?

Possible answers

a. Keratinocytes b. Fibroblasts c. Melanocytes d. Langerhans cells

a. Sweat glands b. Elastin c. Collagen d. Hair follicles

a. Glycophosphatidylinositol b. Glycosaminoglycan c. Glucosamine d. Glycocyamine

a. Dermis b. Suprabasal epidermis c. Subcutaneous fat d. Granular cell epidermis

a. II and III b. I and IV c. III and IV d. I and III

Answers: 1. A, 2. C, 3. B, 4. A, 5. D