……and what happens as we age

darker skins are more melanin competent than fairer skin types

The skin is an incredible organ that serves many vital purposes. It protects us from the outside environment, pathogens, physical injury, chemicals, and harmful ultra-violet light. Beyond protection, it plays a key role in thermoregulation, vitamin D and melanin production, and preventing water absorption and loss. Interestingly, the structure and skin function can vary depending on the body part it covers. For instance, sweat glands, hair follicles, nerve density, and layer thickness differ across various locations. Despite these differences, the fundamental properties of skin remain consistent. The skin is made up of three primary layers: the epidermis (outer layer), the dermis (inner layer), and the subcutaneous fat layer, also known as the hypodermis.

The Skin Microbiome

Within the structure of the skin is home to trillions of microorganisms collectively known as the skin microbiome. This microbiome serves multiple functions, including maintaining a hostile pH for harmful bacteria, alerting the immune system to potential threats, protecting against UV damage, controlling inflammation, and aiding wound healing.

The Epidermis

The thickness of the epidermis varies greatly, ranging from 0.1mm on the eyelids to 1mm on the palms and soles. This avascular (without blood supply), translucent layer is composed of stratified squamous epithelium (layers of scaly cells) . It is divided into five distinct layers:

  • Stratum Corneum: The outermost layer, which is non-viable and responsible for the initial immune response.
  • Stratum Lucidum: Blocks water absorption and loss.
  • Stratum Granulosum: Produces keratin.
  • Stratum Spinosum: Contains dendritic cells and ribonucleic acid.
  • Stratum Basale: Continuously produces new keratinized cells to replace those that shed (Yousef et al., 2022).
Detailed image of skin structure

Epidermal Cells

The skin function of the epidermis is reliant on its four main cell types:

  • Keratinocytes: Making up 95% of epidermal cells, these originate in the stratum basale and migrate outward, producing keratin to form a lipid-water barrier on the skin’s surface. Proper synchronization between their production and shedding is essential to avoid skin malformations (Grossman, 2014).
  • Melanocytes: Found in the stratum basale, these produce melanin, which scatters UV light and protects against radiation. UV exposure increases melanin production, providing additional protection. The absence of the enzyme tyrosinase, essential for melanin production, leads to conditions like albinism (Grossman, 2014).
  • Langerhans Cells: Immunologic cells in the basal layer that bind with antigens and alert the immune system via the lymphatic system. Stress can worsen skin conditions by triggering the sympathetic nerves (Grossman, 2014; Yousef et al., 2022).
  • Merkel Cells: Though not detailed above, these are mechanoreceptors for touch.

If the stratum corneum is compromised, immune cells like mast cells, T cells, macrophages, and fibroblasts from the dermis act as secondary defenders to protect the skin function.

The Dermis

The dermis, an elastic connective tissue layer, supports vasculature and most accessory structures like hair bulbs, sweat glands, nerves, and lymphatic vessels. It comprises of the following layers:

  • Papillary Dermis: A thin, vascularised layer containing papillae that connect to the epidermis, creating ridges for grip on fingers and feet. This layer also contains lymph vessels and nerve tissues.
  • Reticular Dermis: A thicker layer with collagen fibers arranged in bundles, providing strength, elasticity, and structure (Grossman, 2014; Scott, 2011).

The dermis’ extracellular matrix, made of collagen and elastin, is vital for maintaining the skin’s resilience and flexibility.

Thermoregulation

Thermoregulation is managed by the hypothalamus in the brain and executed through eccrine sweat glands and the cardiovascular system. Vasoconstriction reduces blood flow to conserve heat, while vasodilation increases flow to release heat. Additionally, sweat production helps cool the body through evaporation (Rishe, 2021).

Additional Immune Roles

If the stratum corneum is ruptured, immune cells like mast cells, T cells, macrophages, and fibroblasts, all found in the dermis, provide a second line of defense (Davies et al., 2021). Mast cells are placed near appendages that are close to surfaces, connecting to the outside environment as part of the defense system. They have roles in innate immunity by interacting with antigens, protecting against parasitic infestation, repairing tissue, and promoting angiogenesis (capillary repair) (Metcalf et al., 1997). Fibroblasts from the reticular dermis play a role in the proliferation stage of skin healing by providing granulation tissue to the matrix (Wilkinson & Hardman, 2020).

Melanin and UV Protection

Melanocytes in the stratum basale produce melanin. Which pigments the skin and provides protection against UV light by scattering and diffusing dangerous rays. The enzyme tyrosinase is essential for melanin production; its absence leads to albinism (Grossman, 2014). The structure and function of darker skin types means it has greater protection from UV rays than lighter skin types. This is because it contains more melanin producing cells.

Langerhans Cells and Stress

Langerhans cells which located in the basal layer of the epidermis, account for approximately 4% of cells. They serve immunologic functions by recognizing harmful substances. They bind with antigens and travel through the lymphatic system to lymph nodes. Their activity, stimulated by sympathetic nerves, can explain why skin conditions like acne worsen during stress (Yousef et al., 2022).

The Effects of Ageing on the Skin

As we age, our skin’s ability to renew itself slows down, the outer layer (epidermis) becomes thinner for both sexes. For women, a drop in oestrogen levels adds to the early signs of ageing. These internal changes, combined with environmental factors like sun exposure, weaken the skin, making it more prone to issues. (Farage et al., 2013). Fat pads under the skin shrink, and supporting ligaments become looser and more noticeable (Mendleson & Wong, 2013). Facial muscles, which are attached to the skin, also play a role in how the skin looks. When the skin’s deeper layers (the dermis) weaken, these muscles can cause exaggerated expressions, like a constant “tight-lipped” look. (Swift et al., 2021).

Sun exposure, particularly UVA and UVB rays, break down collagen and can even cause lasting damage to genes that protect against cancer. (Pandel et al., 2013). The skin’s extracellular matrix (ECM)—which includes collagen and elastin—plays a big role in skin repair. The ECM and makes up about 70% of the dermis. Most of the visible damage from sun exposure (photoaging) happens in the ECM, where the impact is most noticeable. (Widgerow et al., 2016). Natural signs of ageing, like seborrheic keratoses (small skin growths), are common, but photodamaged skin often shows other signs. Such as uneven pigmentation, wrinkles, dryness, sagging, and even precancerous spots or melanomas. (Pandel et al., 2013). Click on the following British Skin Foundation link for more information. What is skin cancer?

Free Radicals

Free radicals (FRs) are unstable molecules naturally produced during normal body processes but are also triggered by external factors like UV rays and cigarette smoke. As we get older, our body becomes less effective at fighting off their harmful effects which leads to oxidative stress. This is where the balance between oxygen production and protection gets disrupted, causing damage to cells and tissues. (Pizzino et al., 2017). Antioxidants are key to fighting oxidative stress by neutralizing free radicals and protecting cells. Studies show that getting antioxidants like vitamins A, C, and E from food can lower cancer risks. However, it is important to note taking them as supplements doesn’t have the same benefit (Eldridge, 2022).

Keeping skin looking and feeling its best involves several simple habits. Using a good broad-spectrum sunscreen with SPF 30 or higher is essential to protect against UV damage . Eating a diet packed with antioxidant-rich foods and staying active with regular exercise also go a long way in supporting overall skin health and slowing down signs of ageing. (Simioni et al., 2018; Pandey et al., 2022).

The skin’s complexity and multifunctional nature make it one of the most fascinating organs, playing crucial roles in protection, sensory input, and maintaining overall homeostasis. We now know how the structure and function change over time and the steps we can take to prevent premature ageing with lifestyle choices, and an active skin-care regime.

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02/01/2025

Laura Henshaw-Pullan (NMC 16D1705E)
BSc (hons), MSc, PGDip Injectable Therapies, Ind Nurse Prescriber 
Owner and Clinician LHP Aesthetics & Skincare Ltd, Yelverton, Devon