Skin

• Skin is the largest and heaviest organ in the body
• Skin is also known as the integument or cutaneous layer
• Functions of the skin:

  • Provides a physical barrier against thermal and mechanical insults
  • Dark pigment melanin protects against UV radiation
  • Acts as a permeability barrier against overhydration and dehydration
  • Serves sensory functions such as touch, vibration, temperature, and pain perception
  • Helps in thermoregulation to maintain body temperature
  • Supports metabolic functions like vitamin D synthesis
  • Plays a role in self-identification through fingerprints
• Layers of the skin:

  • Epidermis
  • Dermis
  • Hypodermis or Subcutaneous
  • Epidermis is of ectodermal origin and consists of keratinized stratified squamous epithelium
  • Dermis is of mesodermal origin and contains dense connective tissue
  • Hypodermis is not considered part of the skin and contains areolar and adipose connective tissue
• Epidermal layers:

  • Stratum corneum: superficial layer with 20-30 layers of dead and anucleate keratinized cells
  • Stratum lucidum: 2-3 layers of anucleate, dead cells, seen only in thick skin
  • Stratum granulosum: 3-5 layers of keratinocytes with distinct kerato-hyaline granules
  • Stratum spinosum: several layers of keratinocytes, larger than those of the stratum basale, with Langerhans cells present
  • Stratum basale or germinativum: single layer of cuboidal to columnar cells resting on the basal lamina, site of most mitoses, gives rise to keratinocytes
• Cells of the epidermis:

  • Keratinocytes: primary cells in the epidermis, most abundant, originate from the stratum basale and move up to the stratum corneum
  • Langerhans cells: 2-3% of epidermal cells, mainly found in the stratum spinosum, play a role in the immune response
  • Merkel cells: also known as epithelial tactile cells, mechanoreceptors for light touch, abundant in fingertips and soles
  • Melanocytes: located in the stratum basale, synthesize melanin to protect against UV rays, determine skin color
• Dermis:

  • Supports the epidermis and binds it to the hypodermis
  • Rich in nerve endings, blood and lymphatic vessels
  • Contains hair follicles, sebaceous glands, sweat glands
  • Has two layers: Papillary dermis (PD) and Reticular dermis (RD) with no distinct borders between them
• Sensory nerve receptors of the skin:

  • Epidermis contains free nerve endings and Merkel's corpuscles
  • Dermis contains several encapsulated nerve endings like Pacinian corpuscles, Meissner's corpuscles, and Ruffini's corpuscles
• Thick and thin skin:

  • Thick skin found on palms, soles, and fingertips, non-hairy, has 5 layers of epidermis, presence of sweat glands
  • Thin skin found elsewhere on the body, hairy, has 4 layers of epidermis without stratum lucidum, presence of hair follicles and sebaceous glands
• Nail:

  • Consists of the nail bed, nail plate, root, body, free edge, nail fold, nail groove, eponychium (cuticle), lunula, nail matrix, and hyponychium
• Hair:

  • Elongated keratinized structure growing within hair follicles, found everywhere on the skin except palms, soles, lips, genitalia
  • Components include bulb, papilla, root, shaft, and arrector pili muscle
• Glands:

  • Eccrine sweat gland: widely distributed, merocrine gland with small lumens
  • Apocrine sweat gland: confined to axillary and perineal regions, ducts open into hair follicles, larger lumen
  • Sebaceous gland: holocrine branched acinar gland, opens into hair follicle to secrete sebum
• Clinical applications:

  • Acne vulgaris: inflammatory disorder of the pilosebaceous unit
  • Psoriasis: abnormal keratinocyte proliferation and immune cell infiltration
  • Albinism: due to defective melanocytes
  • Alopecia areata: patchy baldness, autoimmune attack on hair follicles
  • Koilonychia: spoon nails, concave nail dystrophy