Chapter 60 - Structure & Function of the Skin

• functions of the skin:

  • prevent body fluids from leaving the body
  • protect the body from potentially damaging environmental agents
  • serve as an area for heat exchange
  • provide protection against invading microorganisms
  • endocrine organ (inactive form of vitamin D produced)
  • somatosensory
• the skin is somatosensory and relays pressure, temperature, and pain to the CNS for localization and discrimination
• the outer epidermis is an avascular layer composed of 4 to 5 layers of stratified squamous keratinized epithelial cells formed in the deepest layer of the epidermis
• stratified squamous keratinized epithelial cells migrate to the skin surface to replace cells lost during normal shedding
• the thickness of the outer epidermis varies but is thicker on palms
• the basement membrane is a thin adhesive layer cementing the epidermis to the dermis
• the basement membrane is the layer involved in blister formation
• the inner dermis is a connective tissue layer that separates the epidermis from the underlying subcutaneous fat layer
• the inner dermis contains blood vessels and nerve fibers that supply the dermis
• the keratinocyte is the major cell of the epidermis, comprising 95% of the cells of this layer
• the keratinocytes that originate in the basal layer change morphologically as they are pushed toward the outer layer of the epidermis; this is called keratinization
• the migration time of a keratinocyte from the basal layer to the striatum corneum is 20 to 30 days
• the rate of production of new keratinocytes needs to be consistent with the rate of shedding of old keratinocytes
• when the rate of production of new keratinocytes are not in balance, skin abnormalities occur
• cells of the epidermis:

  • keratinocytes
  • melanocytes (pigment production)
  • Merkel cells (sensory touch receptors)
  • Langerhans cells (immunologic cells recognizing foreign antigens)
• the papillary dermis is a thin, superficial layer that lies adjacent to the epidermis
• the papillary dermis consists of collagen fibers and ground substance and is highly vascularized
• the papillary dermis is densely covered with conical projections called dermal papillae
• the papillary dermis functions to:

  • minimize the separation of the dermis and the epidermis
  • nourish the epidermal layers of the skin
• the reticular dermis is the thicker area of the dermis and forms the bulk of the dermal layer
• the reticular dermis is composed of a complex meshwork of three-dimensional collagen bundles interconnected with large elastic fibers and ground substance
• the reticular dermis is composed of a viscid gel that is rich in mucopolysaccharides
• immune cells found in the dermis include macrophages, T cells, and mast cells
• scleroderma is an autoimmune disease of connective tissue characterized by excessive collagen deposition in the skin and internal organs, resulting in skin thickening through fibrosis
• scleroderma is either diffuse or generalized
• skin changes in scleroderma involve the trunk and proximal extremities that are severe and progressive
• CREST variant of scleroderma stands for:

  • calcinosis
  • Raynaud phenomenon
  • esophageal dysmotiliyu
  • sclerodactyl
  • telangiectasia
• eccrine sweat glands are simple tubular structures that originate in the dermis and open directly to the skin surface
• there are several million eccrine sweat glands that vary in density and are located over the entire body surface
• eccrine sweat glands function to transport sweat to the outer skin surface to regulate body temperature
• apocrine sweat glands are less numerous than eccrine sweat glands, are larger, and located deep in the dermal layer
• apocrine sweat glands open through a hair follicle, even though a hair might not be present
• apocrine sweat glands are primarily found in the axillae and groin
• nails are hardened keratinized plates that are an end product of dead matrix cells and grow continuously
• nails function to protect the fingers and toes and enhance dexterity
• circumscribed, flat, nonpalpable changes in skin colour:

  • macule (small, up to 1 cm; e.g., treckle, petechia)
  • patch (larger than 1 cm; e.g., vitiligo)
• palpable elevated solid masses:

  • papule (up to 1 cm; e.g., elevated nevus)
  • plaque (a flat, elevated surface larger than 1 cm, formed by the coalescence of papules)
  • nodule (0.5 cm to 1 to 2 cm, often deeper and firmer than a papule)
  • tumour (larger than 1 to 2 cm)
  • wheal (a somewhat irregular, relatively transient, superficial area of localized skin edema; e.g., mosquito bite, hive)
• circumscribed superficial elevations of the skin formed by free fluid in a cavity within the skin layers:

  • vesicle (up to 1 cm, filled with serous fluid; e.g., herpes simplex)
  • bulla (greater than 1 cm, filled with serous fluid; e.g., second-degree burn)
  • pustule (filled with pus; e.g., acne, impetigo)
• pruritus is an itchy sensation that originates in free nerve endings in the skin with cellular mediators that are thought to initiate the process via irritation
• pruritus is carried by small myelinated type C nerve fibers (itch-specific neuronal fibers in the spinothalamic tract and CNS)