Structure and Functions of the Human Skin
The skin is the largest organ in the human body. The skin of an average adult weighs about 7% of the body weight. This amounts to about 4-5 kg for an average adult, and spreads over an area of about 1.5-2 m2.
The human skin consists of two layers, i.e., dermis and epidermis (above the dermis). This is schematically shown in Figure 1. The tissue below the dermis (often referred to as hypodermis) is not a part of the skin.
Epidermis:(~60-250 µm in thickness)—Cytologically the most active part of the human skin. Dermis: (~1000 µm in thickness)—The thickest part of the human skin, below the epidermis. Hair follicle—location where the hair growth begins. Sebaceous gland—which exudes sebum, which lubricates the hair out of the skin (follicle). Without sebum lubrication, hair growth out of the follicle can be itchy, because of the rubbing of the scales (like a wool sweater). Arrector muscle—which connects the hair follicle to the nerve endings to generate tactile sensation on contact with the hair. Sweat glands—produce and exude sweat to the skin surface to maintain the body temperature. The dermis also contains a network of blood vessels and collagen fibers that strengthen the skin.
Hypodermis— the layer below the dermis which connects the skin to the underlying structures. The main cell types here are fibroblasts and adipocytes (fat cells). This layer provides a padding which absorbs and dissipates mechanical stresses without causing any damage. It connects the dermis to the muscles, tendons and bone below and supports blood vessels and nerves (this is not shown in the figure).
The complexity of the structure and function of skin can be realized from the following information: 1 square inch (6.5 cm2) of skin has 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1000 nerve endings. The average skin cell (keratinocyte) is ~30 µm in diameter and ~0.5 µm in thickness.
Biological Functions of the Human Skin
The most important function of the skin is to protect the body from disease-causing micro-organisms. Sweat glands positioned in the dermis maintain the body temperature. The nerve endings protect the skin from mechanical and thermal threats. The tactile function of the skin is further enhanced by the arrector muscles attached to the hair follicles, so that the body senses even the slightest contact with the hair. In humans, the skin plays an important aesthetic role, especially, with regard to the smoothness and shine of its surface and its color.
The epidermis is the outermost layer of the human skin. It contains no blood vessels, and therefore, is nourished exclusively by diffused oxygen from the environment. A schematic of the epidermis is shown in Figure 2. It is stratified into five sublayers (top –> bottom):
Crowding of these cells and the adhesive strength of the stratum corneum generates a pressure which compresses the cells into oblate ellipsoids. The cells become flatter and thinner as they move closer to stratum corneum. In this process the cells die and as the cytoplasm of the cells is squeezed out, the cells are filled with a protein called keratin, a process called keratinization. This process is slow and takes place over weeks. After reaching the stratum corneum the keratinocytes (the dead cells containing the protein keratin, often called corneocytes) are systematically sloughed off from the surface, a process called desquamation. The stratum corneum contains about 25-30 layers of dead cells, which provide protection to the active layers of the epidermis. Apart from keratinocytes, which form the largest fraction of the cells of the epidermis, it also contains other cells like melanocytes (mainly responsible for the color of the skin and its protection from sunlight, also the cause of melanoma, a type of skin cancer), Langerhans cells (responsible for protection from pathogens, part of the immune system) and Merkel cells. The latter are connected to the nerve endings and form part of the tactile sensory system. They are located in the fingertips, or other plantar regions of the skin. Rarely, these cells are also prone to cancer.
As an anatomical barrier protecting the body from external pathogens. Perspiration contains lysozyme which destroys bacteria by breaking their cell walls.
Sensation: Preventing mechanical and thermal damage to the body in conjunction with the nerve endings.
Forms a moisture barrier controlling the amount of water lost from the body. It is achieved by a combination of lipids and natural moisturizing factors (NMFs) formed by degradation of the contents of the cells which ultimately form keratinocytes in the stratum corneum.
Vitamin D synthesis by the skin exposed to sunlight.
Absorption: This is of importance in connection with delivery of drugs through skin patches. These formulations often contain actives that swell the skin to promote easy and faster diffusion.
As a water-resistant barrier which prevents the loss of essential nutrients from the body. This is achieved by the lipid monolayer covering the keratinocytes in the stratum corneum, as well as free lipids generated by hydrolysis of glycerides and breakdown of cell membranes.
Aesthetics: This is mostly related to the uppermost layer of the stratum corneum, especially its smoothness and shine. Often, the appearance of this layer is affected by dryness and underlying skin diseases, such as acne, psoriasis.
This layer of the skin is immediately beneath the epidermis. It is supplied with blood vessels. The upper layer of the dermis called the papillary region, interdigitates with the epidermis, as can be seen in Fig.1. The lower layer, known as the reticular region, is much thicker and has a dense concentration of collagenous elastic fibers which provide strength and resiliency to the skin. It also contains hair follicles, sebaceous glands, eccrine and apocrine sweat glands. Its important function is the maintenance of body temperature by activating sweat glands. The dermal nerve fibers pass through this layer to the dermal-epidermal junction.
This layer is not a part of the skin. This layer connects the dermis to the muscle and bone. It supplies the dermis with blood vessels. It consists of connective tissue, fat cells, and the protein elastin. This layer provides padding to absorb mechanical stresses and insulation for the body.
Further details of defensive functions of the skin can be found in: P. M. Elias, Stratum Corneum Defensive Functions: An Integrated View, J. Invest. Dermatol, 125, 183-200, 2005.