Porth's Essentials of Pathophysiology, 4e

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Integumentary Function

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increases the production of eumelanin, a brownish-black pigment, which causes tanning to occur. The primary function of such melanin is to protect the skin by absorbing and scattering harmful ultraviolet rays, which are implicated in skin cancers. Localized concentrations of eumelanin are also responsible for the formation of freckles and moles. Pheomelanin, a reddish-yellow pig- ment, is particularly concentrated in the lips, nipples, glans penis, and vagina. Both types of melanin are found in hair, particularly red hair. It has been suggested that the reason fair-haired individuals are more susceptible to skin cancers may be due to the enhanced photoreac- tivity of pheomelanin, as compared to eumelanin. Langerhans Cells Langerhans cells are dendritic cells that reside princi- pally in the stratum spinosum of the epidermis and play a major role in the functioning of the skin’s immune system (Fig. 45-4). They are antigen-presenting cells

that originate in the bone marrow and are part of the mononuclear phagocytic system. Like macrophages, Langerhans cells express both major histocompatibil- ity class (MHC) I and II molecules as well as receptors for immunoglobulins (see Chapter 15). Their major function is to phagocytose and process foreign anti- gens. They then migrate into regional lymph nodes in the dermis, where they present the processed anti- gens to T lymphocytes (see Fig. 45-4). As antigen- presenting cells, the Langerhans cells are involved in delayed-type hypersensitivity reactions such as contact dermatitis and other cell-mediated immune responses in the skin. Merkel Cells Merkel cells are epidermal cells that function in cutane- ous sensation. They are interspersed among the kerati- nocytes of the stratum germinativum of the epidermis, usually as single cells, and are particularly abundant in areas of the skin where sensory perception is acute, such as the fingertips. Myelinated sensory neurons tra- verse the basement membrane to approach the Merkel cell, forming a Merkel cell–neuron complex. These complexes may function as tactile mechanoreceptors. Merkel cells display distinctive, dense granules in their organelles and cytoplasm, suggesting that they possess neurosecretory function. These functions may include metabolic support of their associated neurons, neuron development, and regeneration after injury. These cells are also involved in a form of skin cancer called Merkel cell carcinoma. Dermis The dermis is the connective tissue layer that sepa- rates the epidermis from the subcutaneous fat layer (see Fig. 45-1). It supports the epidermis and serves as its primary source of nutrition. The main component of the dermis is collagen, a group of fibrous proteins. Collagen represents 70% of dry skin weight and serves as the major stress-resistant material of the skin. The hair follicles and glandular structures are embedded in the dermis and continue through the epidermis. Based on its tissue structure, the dermis can be divided into a superficial papillary dermis and a deeper reticular dermis. Both layers contain fibroblasts that synthesize the tissue’s ground substance and collagen, as well as immune cells, blood vessels, and nerves. Papillary Dermis The papillary layer of the dermis is a thin superficial layer that interdigitates directly with the epidermis, but is separated from it by the basement membrane zone. It consists of loosely arranged collagen fibers and ground substance. This layer is densely covered with conical projections called dermal papillae (see Fig. 45-1). The basal cells of the epidermis project into the papillary dermis, forming rete ridges. It is believed that the dense structure of the dermal papillae serves to maximize the adhesion of the dermis and the epidermis. This layer of

Antigen

Langerhans cell

Epidermis

Afferent lymphatic vessel

Dermis

Lymph node

T lymphocyte

T cell zone

FIGURE 45-4. Langerhans cells, showing a multistep-process that includes capture and processing of the antigen, movement into a lymph node, and presentation of the antigen to aT lymphocyte.