Structure of Human Skin

Overview

• Human skin, also known as the integument or the cutaneous membrane, consists of two main layers:
  1. Epidermis
  2. Dermis
• There is a subcutaneous layer beneath the dermis, not technically part of the skin.

Epidermis

• Layers: 4 to 5 layers depending on skin type.
  • Thick skin: 5 layers (found on palms & soles)
  • Thin skin: 4 layers (covers most of the body)

Layers of the Epidermis

1. Stratum Basale

   • Deepest layer, single-cell thick, attached to the basement membrane.
   • Cell types:
     • Keratinocytes: Most common, undergo division, produce keratin.
     • Melanocytes: Produce melanin, protect DNA from UV radiation.
     • Tactile cells: Serve as light touch receptors.

2. Stratum Spinosum

   • Contains daughter keratinocytes and epidermal dendritic cells.
   • Cells connected by desmosomes, appear prickly.
   • Dendritic cells: Immune cells fighting infections.

3. Stratum Granulosum

   • 3-5 layers of keratinocytes.
   • Keratinization process: Keratinocytes fill with keratin, lose nucleus and organelles, die.

4. Stratum Lucidum

   • Present only in thick skin.
   • Clear, flat keratinocytes without nucleus or organelles.
   • Contain eleidin, precursor to keratin.

5. Stratum Corneum

   • Top layer, consists of dead keratinized cells.
   • New keratinocytes reach this layer in ~2 weeks, shed after ~2 weeks.

Dermis

• Composition: Connective tissue proper, mainly collagen fibers.
• Structures housed:
  • Blood vessels, hair follicles, sweat and sebaceous glands, sensory nerve endings, nail roots, arrector pili muscles.
• Layers:

  1. Papillary Layer

     • Superficial, loose connective tissue.
     • Forms dermal papillae for increased contact with the epidermis.
     • Contains capillaries and sensory nerve endings.

  2. Reticular Layer

     • Dense connective tissue with collagen and elastic fibers.
     • Extends to subcutaneous layer.

Subcutaneous Layer (Hypodermis)

• Not part of the skin, located below the dermis.
• Composition: Loose connective and adipose tissue (fat).
• Functions:
  • Binds skin to underlying structures.
  • Acts as a cushion and insulator.
  • Provides energy storage.