Skin PathologylEpidermis is composed of stratified squamous epithelium with the following layers:Stratum corneum (anucleate, cornified cells) Stratum granulosum (granular layer)squamous cells containing keratohyaline granules Stratum spinosum (prickle cell layer)squamous cells have prominent desmosomes Stratum basale (basal cell layer)stem cellsSkin PathologylMitotic activity in normal skin should be limited to the basal portion of the epitheliumlMelanin is synthesized in melanocytesDerived from neural crest and located in epidermisMelanosomes are the organelles that synthesize melaninSkin Pathology Melanin synthesislMelanin is formed when tyrosinase converts tyrosine to 3,4 dihydroxyphenylalanine (DOPA) which is then polymerized in the Golgi apparatus into membrane bound organelles (melanosomes)lSkin color is based on the number and size plus packaging of melanosomes in cells other than the one in which they were formedSkin Pathology-MelaninlMelanin granules are transferred by dendritic processes from melanocytes to keratinocytesthere the granules protect the skin from UV light effectslBlacks have the same number of melanocytes as whites, but their melanosomes are larger and more numerous and the cells have more dendritic processeslBoth sunlight and ACTH stimulate melanin synthesisSkin PathologylLangerhans cells:antigenprocessing histiocytic cells located in the epidermis that contain Birbeck granules (same as in eosinophilic granulomas)lMerkel cellsNeuroendocrine cells in the basal layer of the epidermis that have neuroendocrine granulesMay have tactile functionSkin Pathology Definitions lMaculeA circumscribed area of any size characterized by its flatness and usually distinguished from surrounding skin by its colorationlPapuleElevated solid area 5 mm or less acrosslNoduleElevated solid area greater than 5 mm acrosslPlaqueElevated flat-topped area, usually greater than 5 mm acrossSkin Pathology Definitions lVesicleA fluidfilled raised area 5 mm or less acrosslBullaFluidfilled raised area greater than 5 mm acrosslBlisterCommon term used for vesicle or bullalPustuleDiscrete, pusfilled raised areaSkin Pathology DefinitionslWheal Itchy, transient, elevated area with variable blanching and erythema formed as the result of dermal edemalScale Dry, horny, platelike excrescence, usually the result of imperfect cornificationlLichenification Thickened and rough skin characterized by prominent skin markings; usually the result of repeated rubbing in susceptible persons Skin Pathology DefinitionslExcoriationA traumatic lesion characterized by breakage of the epidermis, causing a raw linear area, i.e. a deep scratch. Such lesions are often self induced.lOnycholysisLoss of integrity of the nail substanceSkin Pathology DefinitionslHyperkeratosis Hyperplasia of the stratum corneum often associated with a qualitative abnormality of the keratin; clinically presents as a white patch (leukoplakia)lParakeratosis Keratinization characterized by retention of the nuclei in the stratum corneumlAcanthosis Epidermal hyperplasia usually due to hyperkeratosisSkin Pathology - DefinitionslRete Ridge HyperplasiaAccentuation of the basal cell layer into the underlying superficial papillary dermis giving a saw -tooth appearancelDyskeratosisAbnormal keratinization occurring prematurely within individual cells or groups of cells below the stratum granulosumlAcantholysisLoss of intercellular connections resulting in loss of cohesion between keratinocytes; may be due to immunologic destruction of the intercellular bridges as in pemphigusSkin Pathology DefinitionslPapillomatosisHyperplasia of the papillary dermis with elongation and/or widening of the dermal papillaelLentiginousA linear pattern of melanocyte proliferation within the epidermal basal cell layer; lentiginous melanocytic hyperplasia can occur as a reactive change or as a feature of a melanomaSkin Pathology DefinitionslSpongiosis Accumulation of fluid between keratinocytes that may lead to vesicle formation manifested as blisters on the skinlErosion Discontinuity of the skin, exhibiting incomplete loss of the epidermislUlceration Discontinuity, often excavative, or skin exhibiting complete loss of the epidermis and portions of the dermis and even subcutaneous fatSkin Pathology-DefinitionslGrentz zoneA zone of sparing beneath the epidermis (characteristic of lepromatous leprosy)lLiquefactive degenerationVacuolization of the keratinocytes in the basal cell layer (characteristic of immune destruction of the basement membrane in lupus involvement of skin)lNummularCoin-shaped (nummular eczema, tinea corporis)Skin Pathology-DefinitionslType IV collagen is in the basement membranelThe dermis is composed ofPapillary dermislDirectly below the epithelium; loose connective tissuelCommonly altered with epidermal diseases, particularly the vasculature (e.g. psoriasis)Reticular dermislC