RT Book, Section A1 Mescher, Anthony L. SR Print(0) ID 1184201385 T1 Skin T2 Junqueira's Basic Histology Text and Atlas, 16e YR 2021 FD 2021 PB McGraw Hill PP New York, NY SN 9781260462982 LK accessmedicine.mhmedical.com/content.aspx?aid=1184201385 RD 2024/03/28 AB Skin SUMMARY OF KEY POINTSSkin consists mainly of a superficial stratified squamous epithelium, the epidermis, and a thicker layer of connective tissue, the dermis, which overlies a subcutaneous hypodermis.EpidermisThe epidermis consists of keratinocytes that undergo a terminal differentiation process called keratinization in a series of steps that form distinct epidermal strata or layers.The stratum basale is one layer of mitotically active cuboidal cells attached by hemidesmosomes and integrins to the basement membrane and to each other by desmosomes.The stratum spinosum has several layers of polyhedral cells attached to each other by desmosomes at the tips of short projections containing bundled keratin, or tonofibrils.The stratum granulosum is a thinner layer of keratinocytes, now flattened and filled densely with keratohyaline granules containing filaggrin and other proteins binding the tonofibrils.The superficial stratum corneum protects against water loss, friction, and microbial invasion, and consists of flattened, terminally differentiated cells, or squames, which are slowly lost.The epidermis-dermis interface is enlarged and strengthened by interdigitating epidermal ridges or pegs and dermal papillae in which microvasculature also supplies nutrients and O2 for the epidermis.Melanocytes in the basal epidermis synthesize dark melanin pigment in melanosomes and transport these to adjacent keratinocytes, which accumulate them to protect nuclear DNA from UV damage.Antigen-presenting cells called Langerhans cells form a network through the epidermis, intercepting and sampling microbial invaders before moving to lymph nodes in an adaptive immune response.DermisThe dermis has two major layers: a superficial papillary layer or loose connective tissue with a microvascular plexus, and a thicker dense irregular reticular layer containing larger blood vessels.Cutaneous Sensory ReceptorsSensory receptors in the epidermis include free nerve endings, which detect pain and temperature extremes, and basal tactile or Merkel cells, light-touch receptors associated with sensory nerve fibers.Other cutaneous sensory structures include Meissner corpuscles, encapsulated elliptical mechanoreceptors that surround sensory axons and detect light touch.Deeper in the dermis and subcutaneous layer are lamellated or pacinian corpuscles, which are ovoid and much larger than Meissner corpuscles, for detection of pressure or firm touch.Epidermal AppendagesHairs form in hair follicles, in which keratinocytes comprising the matrix of the deep hair bulb proliferate rapidly and undergo keratinization to form the medulla, cortex, and cuticle of a hair root.A large dermal hair papilla penetrates the base of the hair bulb, and its vasculature supplies nutrients and O2 for proliferating and differentiating cells.The growing hair root is surrounded by internal and external root sheaths continuous with the epidermis, a glassy membrane formed in part by the basal lamina, and a connective tissue sheath.Nails are formed in a manner similar to hairs: keratinocytes proliferate in the matrix of the nail root and differentiate with the formation of hard keratin as a growing nail plate with edges covered by skin folds.Sebaceous glands produce sebum by terminal differentiation of sebocytes, the classic example of holocrine secretion, secreting this oily substance onto hair in the follicles or pilosebaceous units.Eccrine sweat glands in the dermis produce sweat that is mostly water onto the skin surface, where its evaporation provides an important mechanism for cooling the body.Apocrine ...