15. Skin & Regulation of Body Temperature

Skin

  • The skin is the largest organ of the body, with a total area of about 20 square feet. 
  • The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.

Skin has three layers:

  • The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
  • The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
  • The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.

The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis.

THE EPIDERMIS

  • The epidermis is composed of keratinized, stratified squamous epithelium. 
  • It is made of four or five layers of epithelial cells, depending on its location in the body. 
  • It does not have any blood vessels within it (i.e., it is avascular).
  • The cells in all of the layers except the stratum basale are called keratinocytes. 
  • A keratinocyte is a cell that manufactures and stores the protein keratin. 
  • Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties.

THE DERMIS

  • The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over”) and hypodermis (hypo- = “below”).
  • It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands.
  • The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts.

STRATUM BASALE

  • The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis.
  • The stratum basale is a single layer of cells primarily made of basal cells. 
  • A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis.
  • Two other cell types are found dispersed among the basal cells in the stratum basale. 
  • The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves.
  • The second is a melanocyte, a cell that produces the pigment melanin. 
  • Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage.
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