Cover me! Understanding dermatology in Sports and Exercise Medicine – part I

Skin anatomy

Mature skin consists of the epidermis and its appendages, the dermis, and subcutaneous fat (see figure 1)^(5,6).

Epidermis

Keratinocytes 
The keratinocytes, or squamous cells, are five layers in the epidermis. They divide, flatten and lose their nuclei while synthesizing keratin, a tough and pliable protein composed of highly protective tonofibrils. Although the average epidermal transit time for a keratinocyte is 28 days, it is much less for some diseases, such as psoriasis, which is only five days.

Melanocytes
Melanocytes are found in the basal layer and are specialized skin cell that produces the protective skin-darkening pigment melanin.

Langerhans cells
Langerhans cells are the pale dendritic cells found in the suprabasal layer. They are the antigen-presenting cells in the epidermis and play an essential role in immune function.

Merkel cells
Merkel cells are oval-shaped mechanoreceptors essential for light touch sensation.

Figure 1: Skin cross-section

Epidermal appendages

Pilosebaceous unit
These are units formed by a hair with a sebaceous gland opening into the hair canal and lubricating skin and hair. The hair follicle is an invagination of the epidermis into the dermis, and it terminates around a richly vascular dermal papilla. The follicle part around the papilla actively divides and is the hair root. Three main varieties of human hair occur in different body parts^(5,6).

The arrector pili muscles, responsible for ‘goose-flesh,’ attach to the follicle and are under autonomic control. These units occur everywhere on the body except for palms and soles. Hence, the fallacy that it is benign if a mole has hair growing out of it, as malignant lesions also have hair follicles^(5,6).

Apocrine sweat glands
These are modified sebaceous glands localized to the axillae, nipples, perineum, and genitalia. Secretions are odiferous and are under endocrine and sympathetic control^(5,6).

Eccrine sweat glands
These glands are distributed over the entire skin surface, secrete clear fluid containing electrolytes, and are under sympathetic control. Therefore, they are vital in thermoregulation and play a critical role during strenuous exercise^(5,6).

Dermis

The epidermis is entirely dependent on the underlying dermis for support and nutrition. Collagen, which forms the bulk of the dermis, provides support. The other two proteins, elastin and reticulin are present in smaller amounts and contribute to extensibility and tensile strength. The sun causes accelerated aging in these proteins, also known as photoaging. This results in the skin sagging and wrinkling, a common sign in athletes who train and compete outdoors due to prolonged sun exposure. Although it is a cosmetic inconvenience, for the most part, it indicates the damage caused and, thus, the increased risk of developing skin cancer^(5,6).

Fibroblasts are the most numerous cells in the dermis and are responsible for synthesizing all three proteins. Macrophages, mast cells, and lymphocytes are also typical dermal cellular components, but certain pathological states increase their numbers, some of which are aggravated or induced by exercise^(5,6).

The ground substance contains the cutaneous vasculature, nerve supply, and skin appendages. These structures are vital in thermoregulation which is essential for athletes as the sun can interfere with their proper functioning during strenuous exercise^(5,6).

Nails

The nail is formed from an epidermal invagination and is predominantly hard keratin. Nails assist in grasping small objects and protect the fingertip from trauma. The nail plate grows out from the nail matrix and rests on the underlying nail bed (see figure 2). Fingernails grow an average of 0.1 mm per day, requiring four to five months to replace a complete nail plate. The growth is much slower for toenails, with 12 to 18 months needed to replace the great toenail^(5,6).

Figure 2: Nail unit cross-section

Subcutaneous Fat

This layer is also known as the hypodermis, and it provides insulation for warmth and shock absorption for the surrounding vital organs^(5,6)_.

Table 1: Summary of the characteristics, structure, and function of the skin^(5,6).

Conclusion

Pathology of the skin can interfere with localized and global functions. Athletes are prone to more skin conditions than the general population, and prolonged exposure to environmental factors exacerbates the risk of pathology. The three layers of the skin form an effective barrier to the external environment, allow the transmission of sensory information and serve a significant role in maintaining homeostasis. Collagen and elastic filaments of the dermal layer provide the underlying tensile strength of the skin. The high rate of cell proliferation in the epidermis and epithelial tissue and the frequent exposure to physical and chemical damage results in the exceedingly high incidence of skin cancers in humans.

Skincare is essential for physical health, specifically for the athlete. Preventing, treating, and detecting pathology is paramount to ensuring peak physical condition. In addition, skin health knowledge will improve the holistic treatment of athletes recovering from an injury, surgery, or participating in rehabilitation. In the remainder of the series, we will uncover skin pathologies experienced by athletes and provide clinicians with an understanding of the prevention, identification, and management of these conditions in the active population.

Reference list

1. Arch Dermatol 2010; 146(3):279-282.
2. Curr Dermatol Rep 2014; 3(1):40-45.
3. JAMA Dermatol 2015; 151(10):1081-1086.
4. Photoimmunology and Photomedicine 2016; 32(4)
5. MacKie, Rona. Clinical Dermatology: An Illustrated Textbook. 2nd ed., Oxford University Press, 1986.
6. Md, Berger Timothy, and Odom Richard Md. Andrews’ Diseases of the Skin: Clinical Dermatology. 9th ed., Saunders, 2000.