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  • Skin consists mainly of a superficial stratified squamous epithelium, the epidermis, and a thicker layer of connective tissue, the dermis, which overlies a subcutaneous hypodermis.

  • The 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.

  • The 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 Receptors
  • Sensory 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 Appendages
  • Hairs 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 ...

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