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SUMMARY OF KEY POINTS
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Skin SUMMARY OF KEY POINTS
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.
Epidermis 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.
Dermis 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 ...