4.2 The Epidermis

• The epidermis is the superficial layer of the skin, composed of stratified squamous epithelium
• Four cell types in epidermis:

  • keratinocytes
  • melanocytes
  • merkel cells
  • langerhan cells
• Keratinocytes: are the most numerous
• Melanocytes are pigment producing cells
• Merkel cells help detect sensation
• Langerhan cells are scattered among keratinocytes
• Layers of the epidermis:

  • stratum basale
  • stratum spinosum
  • stratum granulosum
  • stratum lucidum
  • stratum corneum
• Stratum basale is the deepest epidermal layer, also known as stratum germinatum, single layer of cells
• The stratum basale is firmly attached to the basal lamina, separating the epidermis from loose connective tissue of the adjacent dermis
• Basal cells are large stem cells that dominate the stratum basale
• Melanocytes are pigment producing cell
• Melanocytes are scattered among basal cells
• Cytoplasmic process injects melanin into basal cells of the layer and keratinocytes of superficial layers
• Melanin is a black, brown, or yellow brown pigment
• Albinism is a disorder characterized by low melanin production
• Skin surfaces which lack hair contain specialized epithelia cells known as merkel cells
• Merkel cells are common in fingertips and lips
• When compressed, merkel cells release chemicals to stimulate sensory nerve endings
• The stratum spinosum is when a basal cell divides, one new cell is pushed into the next superficial layer which is several cells thick
• Tonofibrils are bundles of protein filament which extend rom side to side of cell in keratinocyte
• Tobofibrils connect keratinocytes to neighbors and support cell junctions
• Langerhan cells are common but cannot be seen, trigger an immune response
• Stratum granulosum is superficial to the stratum spinosum, the most superficial layer of epidermis where all cells still posses a nucleus
• Keratinocytes move to the stratum granulosum from the stratum spinosum, then make keratohyalin and keratin
• The stratum lucidum is on thick cells, a clear layer superficial to stratum granulosum
• Stratum lucidum

  • lack organelles and nuceli
  • flattened and densely packed
  • filled with keratin filaments
• The stratum corneum is the most superficial layer, consisting of flattened dead cells
• The cells of the stratum corneum have thick plasma membranes, lack organelles and a nucleus, and contain many keratin filaments
• The process of keratinization (containing keratin) occurs nearly everywhere on the skin
• 15-30 days for cells to move from bottom to top, and remain there for about two weeks
• Thin skin is most of the body. Only four layers, so no stratum corneum
• Thick skin on the palms and soles. Five layers with about 30 keratinized cell layers
• Dermal ridges are formed by the stratum basale of the epidermis and extends to the dermis
• Dermal papillae are projections from dermis to epidermis
• Ridges on the skin allow for friction and grip
• Dermal blood supply and epidermal pigments affect skin color
• When vessels dilate, red is more visible. This is blushing
• When circulation is reduce, skin becomes pale
• Cyanosis when skin takes on bluish color
• Two pigments determine skin color: carotene and melanin