A. Epidermis
The epidermis is composed of keratinized stratified squamous epithelium . It contains 4 cell types: a) Keratinocytes (90%): 4-5 layers of cells that produce the protein keratin and lamellar granules which release a water-repellent sealant that decreases water entry and loss and inhibits the entry of foreign materials. **Keratin: is a tough, fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemical** b) Melanocytes (8%): develop from ectoderm and are cells that produce the pigment melanin and transfer it to keratinocytes. Once inside the keratinocytes, the melanin granules cluster to form a protective veil over the nucleus on the “skin” side shielding the nuclear DNA from UV damage. **Melanin: a yellow-red or brown-black pigment that contributes to skin colour and absorbs UV light** c) Langerhans cells (<2%): arise from red bone marrow and migrate to the epidermis. They are cells that perform immune responses against pathogens (they help other cells of the immune system recognize the invader and destroy it) They are easily damaged by UV light. d) Merkel cells (fewest): cells found only in the deepest layer of epidermal cells that detect tactile sensations (they contact a sensory neuron structure called a Merkel (tactile) Disc . Merkel cells and the associated sensory discs detect touch sensations. Within the epidermis, there are several distinct layers of keratinocytes. Most areas of skin have 4 layers of epidermal cells. Areas of high friction have 5 layers (e.g. muzzle, withers). A horse will shed its epidermis every 17 days.
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