Epidermis

Epidermis

1. 5 Layers

   • 1- Stratum Basale
     • Deepest layer of the epidermis and lies closest to the dermis
     • Contains the most adequately nourished of the epidermal cells because nutrients from dermis reach to the first
     • Also called Stratum germinativum because cells are constantly dividing and millions are being produced each day
       • The daughters cells are pushed upward, away from the source of nutrients, to become part of the layers closer to the skin surface
         • Become part of spinosum and granulosum and then flatter and keratinized
     • Melanocytes
       • spider-shaped cells that produce melanin
         • pigment that ranges from yellow to brown to black
         • Freckles and moles are seen where melanin is concentrated in one spot
       • when the skin is exposed to sunlight, which stimulates the melanocytes to produce more melanin, tanning occurs
         • Melanin accumulates within melanocytes in membrane-bound granules called melanosomes
           • These granules then move to the ends of the melanocytes where they are taken up by keratinocytes
             • Inside the keratinocytes, the melanin forms a pigment umbrella over the superficial side of the nuclei and shields their genetic material (DNA) from UV radiation
   • Stratum spinosum
   • Stratum Granulosum
   • Stratum Corneum
     • The outermost layer is 20 to 30 cell layers thick and accounts for about three-quarters of the epidermal thickness
     • the shinglelike dead cells are completely filled with keratin and are referred to as cornified or horny cells
       • Keratin is a tough protein and its abundance in the layer allows that layer to provide a durable overcoat for the body
         • Which protects deeper cells from hostile external environment and from water loss, and helps the body resist biological, chemical, and physical assaults.
     • rubs and flakes slowly as dandruff
       • Average person sheds about 18kg of these flakes
         • This layer is replaced by cells produced by the division of the deeper basale cells
           • new epidermis every 25 to 45 days
   • Stratum Lucidum
     • Occurs only where skin is hairless and extra thick
       • Palm of the hands and sole of the feet
     • The accumulating keratin inside them, secreting a water-repellent glycolipid into the extracellular space, and their increasing distance from the blood supply in the dermis, effectively dooms the stratum lucidum cells because they're unable to get adequate nutrients and oxygen

2. Avascular

   • No blood supply of its own

3. Most cells are keratinocytes

   • Produce keratin
     • fibrous proteins that make the eoidermis a tough protective layer

4. Epidermal dendritic cells

   • important in alerting and activating immune system cells to a threat such as bacterial and viral invasion

5. Merkel cells

   • At the epidermis-dermis junction, are associated with nerve endings and serve as touch receptors called merkel discs