Physiology and histology of the skin

• Hypodermis


  Subcutaneous layer, composed of loose connective tissue or subcutis tissue (also known as adipose tissue)
• Hypodermis


  • 80% fat
  • Provides protective cushion and contour to the body
  • Source of energy for the body
  • Contains vessels, nerves, fibers, adipose cells and fibroblasts
• Dermis


  Support layer of connective tissues above the hypodermis
• Dermis


  • About 25 times thicker than the epidermis
  • Consists of two layers: the reticular layer below and the papillary layer above
  • Primarily comprises connective tissues made of collagen protein and elastin fibers
  • Supplies the skin with oxygen and nutrients through a network of blood vessels and lymphatic channels
• Reticular layer


  Denser and deeper layer of the dermis, comprised mainly of collagen and elastin
• Papillary layer


  Connects the dermis to the epidermis, contains dermal papillae and capillaries that nourish the epidermis
• Collagen


  Protein substance of complex fibers that gives skin its strength and is necessary for wound healing
• Elastin


  Fibrous protein that forms elastic tissue and gives skin its elasticity
• Glycosaminoglycans (GAGs)


  Large protein molecules and water-binding substances found between the fibers of the dermis, help maintain and support collagen and elastin
• Hyaluronic acid


  A GAG that is a beneficial hydrating fluid in the dermal substance
• Dermal/epidermal junction (DEJ)


  Connects the dermis to the epidermis, consists of layers of connective collagen tissue with pockets and holes
• Epidermis


  Outermost layer of the skin, composed of 5 layers: stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum
• Keratinocytes


  Cells that comprise 95% of the epidermis, contain proteins and lipids
• Keratin


  Fibrous protein that provides resiliency and protection, found in all layers of the epidermis
• Stratum germinativum


  Basal cell layer, where stem cells undergo continuous cell division to replenish skin cells
• Melanocytes


  Cells in the basal layer that produce pigment granules called melanosomes, which produce melanin
• Stratum spinosum


  Spiny layer above the stratum germinativum, where cells continue to divide and change shape, and enzymes create lipids and proteins
• Langerhans cells


  Immune cells in the stratum spinosum that protect the body from infections
• Stratum granulosum


  Granular layer where production of keratin and intercellular lipids takes place, and enzymes dissolve structures holding cells together
• Natural moisturizing factors (NMFs)

  Water soluble compounds like triglycerides, ceramides, waxes, and fatty acids made in the stratum granulosum that hydrate the lipid layer and prevent water loss
• Stratum lucidum


  Thin, clear layer of dead skin cells under the stratum corneum, thickest on palms and soles
• Keratin


  Protein that forms the structural component of the skin, hair, and nails
• Intercellular lipids


  Lipids that are excreted from cells to form the skin's waterproofing barrier function
• Formation of keratin and intercellular lipids

  1. Enzymes dissolve structures (desmosomes) that hold cells together

  2. Cells become keratinized and move to the surface to replace shed cells
• Natural moisturizing substances (NMFs)

  Water soluble compounds like triglycerides, ceramides, waxes, fatty acids that hydrate the lipid layer, absorb water, and prevent water loss
• Stratum lucidum


  • Thin, clear layer of dead skin cells under the stratum corneum
  • Translucent layer made of small cells that let light pass through
  • Thickest on palms and soles
  • Keratinocytes contain clear keratin
  • Releases lipids forming bilayers of oil and water
• Stratum corneum


  • Top, outermost layer of the epidermis
  • Waterproof and permeable
  • Regenerates itself
  • Detoxifies the body
  • Responds to stimuli
  • Composed of hardened corneocytes (dead protein cells)
• Desquamation


  1. Continuous shedding of keratinocytes from the skin surface

  2. Replaced by new cells coming from lower layers

  3. Cell turnover rate slows with age
• Bilayers


  Thin polar membranes made of two layers of lipid molecules that form a continuous barrier around cells
• Melanin


  Pigment that protects skin from sun damage
• Melanocytes


  • Pigment-producing cells
  • All people have the same number of melanocytes
• Melanin production


  1. Stimulated by sun exposure

  2. Melanocytes make melanosomes which are transferred to keratinocytes

  3. Melanosomes carry pigment granules that provide skin color
• Tyrosinase


  Enzyme that stimulates melanocytes and produces melanin
• Pheomelanin


  Red to yellow melanin
• Eumelanin


  Dark brown to black melanin
• Brightening agents


  Products that suppress melanin production by interrupting biochemical processes
• Components of the skin

  • Subcutaneous layer (hypodermis)
  • Dermis (reticular and papillary layers)
  • Epidermis (5 layers)
• Hair


  • Appendage of the skin
  • No nerves
  • Different growth patterns in males and females
  • Growth rate slows with age
• Alpha keratin


  Softer keratin
• Beta keratin


  Harder keratin