Epidermis dendritic cells

In 1968, Paul Langerhans discovered the presence of a particular type of epidermal cell in humans, which is now named after him. The Langerhans cell (LC) is a mobile dendritic cell that has no desmosomes and is found in most multilayered malpighian epithelia. LCs make up 2-5% of epidermal cells. They come from the medulla and migrate towards and within the skin and stay within the epidermis for 2 weeks at the most. 

Melanocjdes are dendritic cells which are wedged between the basal cells of the epidermis. They derive from neural crest (i.e. at a high point of action of the nervous system) and have complex structures mainly made up of protein and various oxydase systems, including tyrosinase (160). 

Psoriasis is an inflammatory skin disease characterized by red, scaly, raised plaques. Usually, the psoriasis lesions are several centimeters in diameter and separated by normal-appearing skin [239, 240]. Psoriasis involves a chronic cutaneous pathologic process, driven by interactions between infiltrating leukocytes (Tcells, dendritic cells, macrophages, neutrophils), cytokines, chemokines and keratinocytes, the cells from the epidermis. The disease is initiated or exacerbated by infections, physical and/or emotional stress, antigenic stimuli and various medications (e.g., lithium, P-block-ers [241, 243]). Psoriatic plaques can revert back to symptomless skin spontaneously or after treatment vdth selective immune-targeted agents [239, 240, 242-244]. 

When MCP-V JE expression was limited to the keratinocy te, an interesting phenotype was observed. The constitutive expression in transgenic animals (as might be expected from the results described above) did not have a basal infiltrating monocyte population. However, an increase was observed in the skin dendritic-cell population in the basal layers of the epidermis. When examined after skin contact sensitization, a richer infiltrate of dendritic cells and monocytes were observed in the transgenic lines. 

Psoriasis is another chronic and autoimmune skin disorder characterized by raised, scaly, and reddened patches (or plaques), which result from hyperproliferation of the epidermis and inflammation of both epidermal and dermal layers [126]. This disease is also characterized by periods of remissirMi and relapse. Psoriasis is mediated by activated T cells [127] and activated dendritic cells located in psoriatic plaques. These cells release proinflammatory cytokines, including both TNF-a and IFN-y, that increase blood vessel synthesis, vasodilatation, and keratinocyte proliferation [128]. 

Also present in the epidermis are embedded macrophage-like Langerhans cells as well as pigmented melanocytes, cells with highly branched dendrites, which lie just above the basal stem cell layer. Each melanocyte contains hundreds of pigmented organelles called melanosomes. They contain not only the black or reddish melanin pigments but also the enzymes needed to form them (Chapter 25).n o... 

Diagram of a dendritic melanocyte surrounded by satellite keratinocytes. The Golgi area (G), where the melanosomes are synthesized, is shown around the nucleus. The other branched cell, higher in the epidermis, is a Langerhans cell with its tennis racquet-shaped granules. Courtesy of Dr. W. Quevedo, Jr. From Montagna et al.m... 

The principal pigment of human skin, hair, and eyes is melanin, which is synthesized in specialized cells, the melanocytes. They lie between the epidermis (outer layer) and the dermis (inner layer) as shown in Box 8-F. Melanocytes originate from embryonic nervous tissue and migrate into the skin by the third month of fetal life. They retain the highly branched morphology of neurons. Persons of different races all have the same numbers of melanocytes but the numbers and sizes of the pigmented melanosomes (Box 8-F) vary as does the content and chemical composition of the melanin.a d Melanosomes not only are found in the dendrites of the melanocytes but are transferred from them into adjacent epithelial cells.6 1... 

Melanin is an insoluble, high-molecular-weight polymer of 5,6-dihydroxyindole, which is synthesized from tyrosine (Figure 17-23). It is produced by pigment cells (melanocytes) in cytoplasmic organelles (melanosomes). In the epidermis, melanocytes are associated with keratinocytes, which contain melanosomes supplied by melanocytes via dendritic processes. Color variation in human skin reflects the amount of melanin synthesized in melanosomes. Melanin synthesis is apparently under hormonal and neural regulation. 

Normal melanocytes arise from melanoblasts and undergo a series of differentiation events before reaching a Hnal end-cell differentiation state. Normal melanocytes can be arrested in their differentiation process at any given state of maturation without loss of their prohf-eration capacity. Melanocytes adhere to the basement membrane of the epidermis, and despite a resting state maintain a lifelong prohf-eration potential. Melanocytes synthesize melanin to protect various tissues such as the skin from ultraviolet radiation (UVR)-induced damage, and reach the kerantinocytes in the upper layers of the epidermis via dendrites. Tyrosinase is an essential enzyme used within the melanosomes to synthesize melanin. 

The epidermis is an epithelium consisting of inner viable epidermis, a living hydrophilic layer, and outer nonviable epidermis, a hydrophobic layer made from dead cells. It is differentiated into stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale In the direction of dermis (Fig. 16.2). The viable epidermal layer has a thickness of about 0.02 to 0.2 mm. It is composed of many layers of keratinocytes, a widespread distribution of melanocytes, Langerhans cells, dendritic T cells, epidermotropic lymphocytes and Merkel cells, and a number of catabolic enzymes such as esterases, proteases, phosphatases, nucleotidases and lipases [Walters and Roberts, 2002 Barry, 2001). The outer and nonviable epidermis, namely, stratum corneum, is about 10 to 20 pm thick [Gregor and Ulrich, 2010). The stratum corneum Is deemed to be the major obstacle of drug permeation. 

These cells exit from the neural crest and migrate toward the epidermis where their dendritic extensions are in contact with the surrounding cells. On... 

Langerhans, P. Uber die Nerven der menschUchen Haul. Virchows Arch Pathol. 1868 44 325-37. In this paper, entitled On the nerves of the human skin , Paul Langerhans reported for the first time the presence of large cells with long dendrites in the epidermis of human skin. 

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