Eduardo Zappi - Dermatopathology: Classification of Cutaneous Lesions

Diagrammatic representation of the development of human skin. The germinal layer is depicted in heavy gray , and the melanocytes interspersed in it are shown in black . The granular layer is represented as a dark gray dotted band while desmosomes are pictured as faint interstitial lines . The basement membrane appears as a heavy double line . For the sake of simplicity, Langerhans and Merkel cells, as well as nuclear contours, are omitted from the drawing. The time of gestation in weeks and months is indicated along the horizontal axis of the figure

Cellular components of the epidermis

Schematic drawing of a vertical section of the skin under light microscopy. The epidermal surface appears covered by loose keratin scales. The lower epidermal boundary is outlined by a double line corresponding to the basal membrane. This one appears irregularly woven due to the epidermal ridges projected downward. The dermis is molded by these, forming intervening mounds known as dermal papillae. Keratinocytes, the only nondendritic elements of the epidermis are represented in white , with the plasma membrane ( PM ) surrounded by intercellular bridges. Note the variability in size and shape of the keratinocytes, from small, low cylindrical in the basal layer to bulkier and progressively flattened at upper layers. Also note the progressive decrease of intercellular bridges to the point that at the upper epidermal layer, the cellular interstitia appear devoid of these. Simultaneously, within the keratinocytes at the midepidermal level appear keratohyalin granules, characteristic of the granular layer. Also, at this level, the PM of the keratinocytes begins to thicken by apposition of involucrin to the inside of the plasma membrane. The buildup of tonofilaments within keratinocytes and the thickening of their walls end up obliterating the cavity of keratinocytes, which terminate as homogeneous keratin plaques ready to be desquamated. Melanocytes, Merkel cells, and Langerhans cells, the three dendritic elements of the epidermis, are represented in black . At the epidermal basal layer in the figure, there are, from left to right, three melanocytes with profuse cellular projections. To their right appears an isolated Merkel cell with blunt dendritic projections, sitting on a synaptic plaque connected to a nervous fiber. In contrast to melanocytes, Merkel cells appear joined by intercellular bridges to neighboring keratinocytes. Beneath the granular layer are seen two Langerhans cells, endowed with long dendritic projections but devoid of intercellular bridges. The superficial or papillary dermis just below the basement membrane is depicted faintly gray . The deep or reticular dermis underneath may be distinguished from the papillary dermis by its deeper gray tone . Between the superficial and the deep dermis run the vessels of the subpapillary plexus. Additional details are found in the text