Integumentary System - Comprehensive Study Notes (Chapter 6)

6.1 Composition and Functions of the Integument

  • Integument = skin and its derivatives (nails, hair, sweat glands, sebaceous glands).
  • Structure and scope:
    • Skin is the barrier to the outside world; subjected to trauma, harmful chemicals, pollutants, microbes, and damaging sunlight.
  • Key statistics (per square inch of skin):
    • Up to 20 ft20 \ \text{ft} of blood vessels, 650650 sweat glands, 100100 sebaceous glands, and over 1000 nerve endings.
  • General functions:
    • Barrier against the outside environment.
    • Provides sensation and information about touch, pressure, temperature, and pain.
  • Terminology:
    • Hypodermis (subcutaneous layer) lies deep to the dermis; adipose CT is not part of the integument but closely associated and supports it.
  • Relevant connections:
    • The integument is the largest organ and works in concert with the nervous and immune systems to maintain homeostasis and protect underlying tissues.

6.1a Epidermis

  • Epidermis = keratinized stratified squamous epithelium; consists of five strata (layers) from deep to superficial:
    • Stratum basale
    • Stratum spinosum
    • Stratum granulosum
    • Stratum lucidum (found only in thick skin)
    • Stratum corneum
  • Living cells occupy the first three layers; superficial layers contain dead cells.
  • Epidermis layers at a glance:
    • Stratum basale: deepest layer; a single layer of cuboidal cells attached to the basement membrane; cells include:
    • Keratinocytes
    • Melanocytes
    • Tactile (Merkel) cells
  • Function notes:
    • Epidermis provides strength, water resistance, and protection from abrasion and chemical damage.

6.1a Epidermis — Keratinocytes, Melanocytes, and Tactile Cells

  • Keratinocytes
    • Most abundant cell type; present in all epidermal layers.
    • Basal keratinocytes act as stem cells; they divide to produce new keratinocytes to replace dead ones shed from the surface.
    • Produce the protein keratin, which strengthens the epidermis and makes it water resistant.
    • In stratum basale, keratinocytes act as stem cells; in other layers they differentiate and move upward.
  • Melanocytes
    • Scattered among keratinocytes in the stratum basale.
    • Produce and store melanin in response to ultraviolet (UV) light; transfer melanin to keratinocytes in basal layers.
    • Melanin pigment shields and protects DNA from UV radiation, helping prevent mutations.
  • Tactile (Merkel) cells
    • Scattered among cells in the stratum basale.
    • Sensitive to touch; when compressed, they stimulate sensory nerve endings.
    • More numerous in sensitive areas (e.g., fingertips).

6.1a Epidermis — Stratum basale details

  • Stratum basale:
    • Deepest layer; a single layer of cuboidal cells attached to the basement membrane.
    • Three cell types:
    • Keratinocytes
    • Melanocytes
    • Tactile (Merkel) cells
  • Keratinocyte lifecycle:
    • Most abundant cell type; migrate from the basal layer toward the surface and are shed from the surface.
    • Produce keratin; keratin strengthens and water proofs the epidermis.
  • Melanocyte function (detailed):
    • Melanin production increases with UV exposure and is transferred to neighboring keratinocytes to protect DNA.

6.1a Epidermis — Stratum spinosum

  • Stratum spinosum:
    • Several layers of polygonal keratinocytes that rise from the basale.
    • Stem cells remain in the basal layer; cells differentiate into non-dividing, highly specialized keratinocytes.
    • Cells attach to neighbors via many cell junctions and provide structural support between epidermal cells.
  • 6.1a notes:
    • Stratum spinosum also contains Langerhans (epidermal dendritic) cells—immune cells that fight infection in the epidermis by phagocytic activity and initiating immune responses; protect against pathogens and some epidermal cancers.

6.1a Epidermis — Stratum granulosum

  • Stratum granulosum:
    • 3 to 5 layers of keratinocytes.
    • Keratinization begins in this layer.
    • Keratinocytes produce large amounts of keratin; accumulation causes nucleus and organelles to disintegrate; cells die.
    • Keratinization not complete until they reach superficial layers; fully keratinized cells are dead but structurally strong.

6.1a Epidermis — Stratum lucidum (thick skin only)

  • Stratum lucidum:
    • 2–3 layers of keratinocytes; thin, translucent layer.
    • Flattened, pale cells filled with translucent protein.
    • Found only in thick skin, such as the palms of the hands and soles of the feet.

6.1a Epidermis — Stratum corneum

  • Stratum corneum:
    • 20–30 layers of dead, scaly, interlocking keratinized cells that are eventually shed.
    • Dead keratinocytes are anucleate and tightly packed; contain large amounts of keratin.

6.1a Epidermis — Keratinization timeline

  • Keratinization = keratinocytes change as they move through epidermal layers; become a