Epidermis

1. Most superficial layer of skin

2. Avascular

   1. Cells receive nutrients and excrete waste through diffusion of substances through epidermis and dermis capillaries

3. Separated from dermis by basement membrane

5 Layers of the Epidermis:

1. Basale

   1. Deepest layer of epidermis

   2. Simple cuboidal or columnar layer

   3. Attached to the basement membrane by hemidesmosomes

   4. Where keratinocyte stem cells undergo mitosis every 19 days and form keratinocytes

   5. Where melanocytes package melanin into melanosomes

   6. Hemidesmosome/desmosome connections in this layer provide structural support to the skin

2. Spinosum

   1. 8-10 layers of many sided cells

   2. Keratinocytes begin to flatten

   3. Desmosomes break and new ones form

   4. Keratinocytes phagocytize melanosomes

   5. Lamellar Bodies and more keratin fibers form inside keratinocytes

      1. Lamellar Body - Lipid-filled membrane organelle

3. Granulosum

   1. 2-5 layers of diamond-shaped, semi-flat cells

   2. Cells are oriented parallel to the skin’s surface

   3. Keratohyalin accumulate in keratinocyte’s cytoplasm

   4. Lamellar Bodies move into the plasma membrane and release lipid contents into the extracellular space

   5. Organelles degenerate and keratinocyte dies in more superficial layers

      1. Keratohyalin and keratin fibers don’t degenerate

4. Lucidum

   1. Thin, clear zone

   2. Only located in thick skin areas

   3. Multiple layers of dead skin cells

   4. Keratohyalin disperses around Keratin fibers

5. Corneum

   1. Most superficial layers

   2. 25 or more layers of cornified cells overlapping

      1. Dead keratinocytes full of keratin

   3. squamous cells joined by desmosomes

   4. Desmosomes break and cells slough off body

Thick Skin:

1. Contains all 5 strata layers

2. Lucidum

3. Found in areas subject to friction/pressure

   1. Fingertips, feet soles, palms

Thin Skin:

1. No Lucidum

2. More flexible than thick skin

3. Each strata is a few layers less

4. Only place hair is found