SUMMARY OF KEY POINTS (FROM BOOK)

• • 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


  Consists of keratinocytes that undergo a terminal differentiation process called keratinization in a series of steps that form distinct epidermal strata or layers
• Stratum basale


  • One layer of mitotically active cuboidal cells attached by hemidesmosomes and integrins to the basement membrane and to each other by desmosomes
• Stratum spinosum


  • Several layers of polyhedral cells attached to each other by desmosomes at the tips of short projections containing bundled keratin, or tonofibrils
• Stratum granulosum


  • Thinner layer of keratinocytes, now flattened and filled densely with keratohyalin granules containing filaggrin and other proteins binding the tonofibrils
• Stratum corneum


  • Superficial layer that protects against water loss, friction, and microbial invasion, and consists of flattened, terminally differentiated cells, or squames, which are slowly lost
• Epidermis-dermis interface


  • 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

  • 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.
• Hair follicles


  Structures 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
• Hair


  • Formed in hair follicles
  • Surrounded by internal and external root sheaths continuous with the epidermis
  • Glassy membrane formed in part by the basal lamina
  • Connective tissue sheath
• Dermal hair papilla


  Penetrates the base of the hair bulb and supplies nutrients and O2 for proliferating and differentiating cells
• Nails


  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 skinfolds
• 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 sweat glands


  Restricted to skin of the axillae and perineum, have much wider lumens than eccrine glands, develop after puberty, and secrete protein-rich sweat onto the hair of hair follicles