Skin is a complex organ covering the entire surface of the body. Aged skin is characterized by appearance of wrinkles, laxity, and pigmentary irregularities. These changes occur under the influence of intrinsic and extrinsic factors, with sun exposure being the most deleterious to the skin. Skin changes associated with aging are the focus of many surgical and nonsurgical procedures aimed to improve the appearance of skin. Knowledge of skin histology and physiology will deepen the understanding of cutaneous changes associated with aging and will promote optimal cosmetic and functional patient outcomes.
Skin is a complex organ covering the entire surface of the body. It provides a protective physical barrier between the body and the environment, preventing losses of water and electrolytes, reducing penetration by chemicals, and protecting against pathogenic microorganisms. The skin is important in regulation of body temperature and provides immunologic surveillance. It contains sensory and autonomic nerves and sensory receptors, which detect incoming stimuli of touch, vibration, pressure, temperature, pain, and itching.
Skin is an important component of outward beauty and is the focus of various cutaneous surgical and nonsurgical procedures. Skin changes associated with chronologic aging or photoaging, such as wrinkling, laxity, and changes in pigmentation, prompt patients to seek cosmetic procedures to improve the appearance of their skin. This article reviews skin histology and physiology as well as changes associated with cutaneous aging.
Normal skin architecture
Skin is organized into 3 layers: the epidermis, the dermis, and the hypodermis ( Fig. 1 ). The fine structure of the skin shows considerable regional variations in epidermal and dermal thickness, distribution of epidermal appendages, and melanocyte content. Skin is glabrous or non–hair-bearing on the palms and soles, whereas hair-bearing skin covers the rest of the body.
Embryologically, the epidermis and its appendages develop from the surface ectoderm, and the dermis and hypodermis arise from mesoderm. During the fourth week of embryonic development, a single layer of ectoderm surrounds the embryo. This simple epithelium overlies a loosely organized layer of undifferentiated mesoderm known as mesenchyme. At about 6 weeks, the ectoderm and the underlying mesoderm begin to proliferate and differentiate. Hair follicles, nails, and glands begin to develop in the third month. By the end of the third month, regular bundles of collagen appear in the dermis. The embryonic connective tissue below the dermis develops into the subcutaneous layer of loose connective tissue characterized by fat islands.
The Epidermis
The normal epidermis is a stratified squamous epithelium undergoing continuous renewal. The major cell in the epidermis is the ectodermally derived keratinocyte, making up approximately 95% of the epidermal cells. As the keratinocyte progressively moves from its attachment to the basement membrane to the skin surface, it forms several morphologically distinct epidermal layers: stratum basale or stratum germinativum, stratum spinosum, stratum granulosum, and stratum corneum ( Fig. 2 ). On the palms and soles, an additional layer, stratum lucidum, can be identified between the stratum corneum and stratum granulosum. Other cell types found in the epidermis include melanocytes, Langerhans cells, and Merkel cells.
The basal layer is composed of a single layer of cuboidal cells, which rest on the basement membrane. Melanocytes can be seen between the basal cells of the epidermis. The basal cells divide, giving rise to the next layer: the prickle cell layer or stratum spinosum. This layer is usually 3 to 4 cells thick and is composed of polygonal cells with preformed keratin. The desmosomal attachments between the cells appear as small spines, giving rise to the name stratum spinosum. The stratum spinosum is succeeded by stratum granulosum, which is usually 1 to 4 cells thick and derives its name from cytoplasmic keratohyalin granules. The outermost layer of the epidermis is stratum corneum composed of flattened keratinocytes that have lost their nuclei and cytoplasmic organelles. These keratinocytes are shed in the process of epidermal turnover.
Melanocytes are derived from neural crest cells and migrate into the epidermis where they produce melanin. They are distributed among the basal keratinocytes with the ratio of 1 melanocyte to 4 to 10 basal cells. This ratio varies with anatomic location, with maximal density of melanocytes on genital skin. Melanin is produced from tyrosine by the enzymatic activity of tyrosine kinase and is stored in melanosomes. Melanosomes are transported along the dendritic processes of melanocytes to adjacent keratinocytes, where they form an umbrella-like cap over the nucleus, protecting it from the injurious effects of UV light. The ethnic variations in pigmentation are attributable to the different activity of melanocytes, not the difference in number of melanocytes. With age, melanocyte density decreases by 6% to 8% per decade, with higher density of melanocytes in sun-exposed skin than in non-exposed skin at all ages. This explains the generalized increase in pigmentation and simultaneous decrease in melanocyte density that often accompany aging.
Langerhans cells are antigen-presenting cells derived from bone marrow. They make up 3% to 6% of all cells in the epidermis and are found mainly within the spinous layer. The dendritic processes of Langerhans cells uptake antigens deposited on the skin, process them, and present them to T lymphocytes for activation of the immune response. The number of Langerhans cells increases during allergic reactions, such as contact hypersensitivity. With aging and chronic sun exposure, the number of Langerhans cells decreases, which may play a permissive role in the development of cutaneous carcinoma in individuals who are elderly with sun-damaged skin.
Merkel cells are found in the basal layer of the epidermis and in the epithelial sheath of hair follicles. Merkel cells migrate from neural crest to the skin and have similar chemical and structural properties to an amine precursor uptake decarboxylation (APUD) cell. Merkel cells are associated with sensory nerve endings in the skin and may function as mechanoreceptors.
Epidermal appendages
Epidermal appendages are specialized epithelial structures located mainly in the dermis and hypodermis, but connected to the epidermis. They include pilosebaceous follicles, sweat glands, and apocrine glands. They play an important role in the epithelialization phase of wound healing.
Pilosebaceous unit
The pilosebaceous unit consists of hair, hair follicle, sebaceous gland, and arrector pili muscle ( Fig. 3 ). It is distributed throughout the integument, absent only in palms and soles and portions of the genitalia.
