Like macrophages, Langerhans’ cells express both MHC I and MHC II molecules, as well as Fc receptors for immunoglobulin G (IgG). Langerhans’ cells also express complement C3b receptors as well as fluctuating quantities of CD1a molecules. As an antigen-presenting cell, the Langerhans’ cell is involved in delayed-type hypersensitivity reactions (e.g., contact allergic dermatitis and other cell-mediated immune responses in the skin) through the uptake of antigen in the skin and its transport to the lymph nodes. Skin biopsy specimens from individuals with AIDS or AIDS-related complex reveal that Langerhans’ cells contain HIV in their cytoplasm. Langerhans’ cells appear to be more resistant than T cells to the deadly effects of the HIV and may, therefore, serve as a reservoir for the virus.
In addition, a malignant transformation of Langerhans’ cells is responsible for histiocytosis X (Langerhans’ cell histiocytosis), a group of immune diseases that are characterized by abnormal increase and spread of Langerhans’ cells. Accumulation of these abnormal cells may form tumors, which can affect various parts of the body, including the bones, lungs, skull, and other areas and organs.
Merkel’s cells are epidermal cells that function in cutaneous sensation.
Merkel’s cells are dendritic cells located in the stratum basale. The origin of Merkel’s cells is unknown; they possess antigenic markers of both epidermal and neural type. They are most abundant in skin where sensory perception is acute such as the fingertips. Merkel’s cells are bound to adjoining keratinocytes by desmosomes and contain intermediate (keratin) filaments in their cytoplasm. The nucleus is lobed, and the cytoplasm is somewhat denser than that of melanocytes and Langerhans’ cells. They may contain some melanosomes in their cytoplasm, but they are best characterized by the presence of 80-nm dense-cored neurosecretory granules that resemble those found in the adrenal medulla and carotid body (Fig. 15.10). Merkel’s cells are closely associated with the expanded terminal bulb of afferent myelinated nerve fibers. The neuron terminal loses its Schwann cell covering and immediately penetrates the basal lamina, where it expands into a disc or platelike ending that lies in close apposition to the base of the Merkel’s cell. The combination of the neuron and epidermal cell, called a Merkel’s corpuscle, is a sensitive mechanoreceptor.
FIGURE 15.10 • Electron micrograph of a Merkel’s cell. The cell has small neurosecretory granules in the cytoplasm and makes contact with a peripheral terminal (NT ) of a neuron. The dermis (D) is in the lower part of the micrograph. 14,450. (Courtesy of Dr. Bryce L. Munger.)
Merkel cell carcinoma (MCC) is a rare but highly aggressive type of skin cancer that develops when Merkel cells undergo uncontrolled proliferation. It starts most often in areas of skin exposed to the sun, such as the head, neck, and upper and lower limbs. MCC tends to grow quickly and to metastasize via lymph vessels at an early stage.
The skin is endowed with sensory receptors of various types that are peripheral terminals of sensory nerves (Fig. 15.11). It is also well supplied with motor nerve endings to the blood vessels, arrector pili muscles, and sweat glands.
Free nerve endings are the most numerous neuronal receptors in the epidermis.