Integumentary System Lecture Notes

Overview of the Integumentary System

Structure of the Skin

• Largest organ in the body.
• Consists of three primary layers:
  1. Epidermis
  • Composed of epithelial tissue.
  • Thin and avascular (lack of blood vessels).
  1. Dermis
  • Consists of two layers of connective tissue.
  • Thicker and vascular (contains blood vessels).
  1. Hypodermis
  • Not technically part of the skin.
  • Composed mainly of subcutaneous adipose tissue and some areolar connective tissue.
  • Vascular in nature.

Layers of the Dermis (from superficial to deep)

1. Papillary Dermis

   • Located closest to the stratum basale of the epidermis.
   • Composed of areolar connective tissue.
   • Contains:
     • Dermal Papillae: Projections that extend into the epidermis containing blood vessels and nerve endings.
     • Meissner's Corpuscles: Specialized touch receptors located here.
     • Free Nerve Endings: Sensory receptors for pain, warmth, cold, and itch sensations.

2. Reticular Dermis

   • Thickest and deepest layer of the skin.
   • Composed of dense irregular connective tissue enriched with collagen and elastic fibers.
   • Functions to connect and bind skin layers, providing resistance to stretch.
   • Contains:
     • Blood vessels
     • Nerves
     • Sebaceous glands (oil glands)
     • Hair follicles
     • Sudoriferous glands (sweat glands)

Specialized Nerve Endings in the Skin

• Lamelated Corpuscles: Located in the dermis and hypodermis; sensitive to deep pressure.

Epidermis

Description

• Composed of 4-5 thin layers of keratinized epithelial cells.

Main Cell Types in the Epidermis

1. Keratinocytes (approx. 90%)
   • Primary cells of the epidermis, responsible for producing keratin, a tough protective protein.
2. Melanocytes
   • Reside in the deepest layer of the epidermis.
   • Produce melanin in response to UV exposure, which acts to protect cellular DNA from mutations.
   • Notably sensitive to UV damage.
3. Macrophages (Dendritic Cells)
   • Synthesized in the bone marrow and migrate to the epidermis.
   • Function to protect against infection.
4. Tactile Cells (Merkel Cells)
   • Encapsulated nerve endings that detect touch.

Stratum Corneum

• Outermost layer of the epidermis, typically 25-35 layers thick.
• Characteristics:
  • Avascular (no blood cells present); nutrition provided through diffusion from deeper layers in the dermis.
  • Contains no cellular organelles.
  • Has very tight intercellular junctions.
  • Continuously sheds and replaces cells.
  • Main layer of protection for underlying tissues.
• Calluses: Abnormal thickening of the stratum corneum resulting from repeated abrasion.

Epidermal Growth Factor (EGF)

• A hormone-like protein that stimulates mitosis (cellular division) in the epidermal cells.

Unique Features of the Dermis

• Epidermal Ridges: Projections from the papillary dermis that create fingerprints.
  • Visible in thick skin areas (e.g., stratum lucidum).
  • Develop during the third month of fetal development.
  • Genetically determined and enhance friction and grip strength.
  • Each individual's fingerprints are unique.