Chapter 1: Introduction to Clinical Dermatology
Skin Structure and Function
In order to understand the basic pathophysiology of skin disease, it is important to begin with a review of the components of the skin and their function. The skin has three main parts: the epidermis, dermis, and subcutaneous tissue.
The epidermis is comprised of several layers. The innermost basal layer is attached to the dermis at the complex dermal-epidermal (DE) junction. The basal layer contains primarily keratinocytes, but has occasional pigment producing melanocytes. Keratinocytes progress upwards through the epidermis to the outermost layer, the stratum corneum, before sloughing off. The outer layer has a crucial role in maintaining the barrier function of the skin. Within the middle layers of the epidermis are Langerhans cells, an important part of the skin’s immune system that act as antigen presenting cells.
The dermis is the portion of the skin immediately under and connected to the epidermis. The dermis is primarily comprised of collagen with intervening elastic fibers, blood vessels, and nerves. The dermis provides structure and allows the skin to maintain elasticity and resist stress. Blood vessels within the skin provide nutrients and allow for transport of inflammatory cells to areas of infection or inflammation. Nerves within the skin mediate pain, touch, pressure, and itch sensation. The thickness and constitution of the dermis varies by body site, for example, with thinner dermis on the face and thicker on the back.
Beneath the dermis is the subcutaneous layer comprised of fat cells (adipocytes) held together in lobules separated by fibrous septa. The subcutaneous fat layer gives form and cushioning as well as functioning as an endocrine organ.
When the skin is affected by illness or injury, it is not able to perform these functions. Problems such as cancer, dehydration, hyperthermia, hypothermia, infarction, infection, and pruritus may result.
In addition, the skin is an important component of appearance, and healthy skin is frequently seen to convey beauty. Patients with skin disease, particularly when it is visible to others, may feel stigmatized. This psychological impact of skin disease can be out of proportion to what others expect and can even outweigh the physical impact of the disease.
Using standard terminology in the description of skin lesions allows for easy communication between health practitioners regarding the nature of the lesions being evaluated. This terminology is commonly referred to as morphology.
The skin has many important functions. The 5 basic functions of the skin are:
- Barrier formation
- Thermoregulation
- Photoprotection
- Cutaneous circulation
- Immunological protection
