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```markdown ## Chapter 5 ### The Integumentary System #### Definition The skin and its accessory structures. I. Layers A. Epidermis B. Dermis - a vascular layer C. Hypodermis = Subcutaneous (Fat) Layer #### 1. Epidermis: A. made up of keratinized stratified squamous epithelium...

```markdown ## Chapter 5 ### The Integumentary System #### Definition The skin and its accessory structures. I. Layers A. Epidermis B. Dermis - a vascular layer C. Hypodermis = Subcutaneous (Fat) Layer #### 1. Epidermis: A. made up of keratinized stratified squamous epithelium 1. Cell Types: a. Keratinocytes - Produce the water insoluble protein keratin which hardens structures b. Melanocytes - Approx. 90% of Epidermal cells - Produce melanin, a brown/black pigment which absorbs UV light and is the primary skin pigment - Approx. 8% of epidermal cells c. Langerhans cells - found in the bone marrow - help provide immunity to the epidermis d. Merkel cell (disc) - nerve ending for touch #### 2. Layers of the Epidermis (deep to superficial): A. Stratum Basale - a single layer of cuboidal shaped cells - mitotically active B. Stratum Spinosum - 8 to 10 cell layers thick - cells start to flatten as they reach the upper part of this layer C. Stratum Granulosum - 3 to 5 cell layers thick - cells continue to flatten out - cells begin to die forming a water resistant barrier, i.e. keratinization D. Stratum Lucidum - only present on the finger tips, palms and soles of the feet - 3 to 5 layers of cells E. Stratum Corneum - 20 to 30 cell layers in thickness - flattened dead keratinocytes, no nuclei - outer layers are continually shed needing to be replaced ```