Human Skin Structure and Functions

Posted on Nov 18, 2024 in Biology

Human Integumentary System

The integumentary system, or skin, weighs over 3-4 kg and covers about 1.5-2 m2. It’s the body’s largest organ, composed of distinct layers.

Skin Histology

The skin has three main layers:

  • Epidermis: Squamous epithelial tissue made of flat cells forming a continuous layer. It’s stratified with multiple layers that keratinize.
  • Dermis: Connective tissue providing mechanical support and protection.
  • Hypodermis: Subcutaneous adipose tissue (not always considered a skin layer).

Epidermis Layers

  • Basal Layer: Single row of cuboidal cells in constant division (stem cells).
  • Spinous Layer: 8-10 layers of flattened cuboidal cells.
  • Granular Layer: 3-5 layers of flat cells with keratohyalin granules.
  • Lucid Layer: Clear layer found only in palms and soles, made of dead cells.
  • Corneal Layer: Up to 30 layers of keratinized dead cells.

Keratinization involves cell migration from the basal layer to the surface, producing keratin for protection. Apoptosis, or programmed cell death, occurs as cells migrate.

About 10% of epidermal cells in the basal layer produce melanin, which gives skin color and protects against UV radiation. The epidermal-melanin unit transfers melanin to keratinocytes, forming a protective veil around the nucleus.

Immune Cells in the Epidermis

  • Langerhans Cells: Antigen-presenting cells that capture antigens and migrate to lymph nodes.
  • Merkel Cells: Mechanoreceptors in the basal layer, specialized for touch sensation.

Dermis

The dermis is located between the epidermis and the basal membrane. It includes:

  • Papillary Dermis: Cells with collagen.
  • Reticular Dermis: Collagen fibers providing resistance.
  • Immune Cells: Macrophages, lymphocytes, and mast cells.

Skin Appendages

Sweat Glands

  • Eccrine Glands: Secrete sweat for thermoregulation, opening into skin pores.
  • Apocrine Glands: Open into hair follicles, active during puberty, found in axillary, pubic, and inguinal regions.

Sebaceous Glands

Secrete sebum (oil) into hair follicles, lubricating and protecting hair.

Hair Follicles

Invaginations of epidermal cells with a matrix responsible for hair growth.

Sensory Innervation

  • Pacinian Corpuscles: Detect pressure and vibration.
  • Meissner’s Corpuscles: Detect touch.
  • Ruffini Corpuscles: Perceive movement.
  • Krause Corpuscles: Detect thermal stimuli.

Skin Functions

  1. Thermoregulation: Vasoconstriction and vasodilation.
  2. Protection: Of internal organs.
  3. Physical Barrier: Prevents microorganism entry.
  4. Absorption: Of topical substances.
  5. Immunity: Langerhans cells and lymphocytes.
  6. Sensory: Nerve endings for sensation.
  7. Excretion: Salts and minerals through sweat.
  8. UV Protection: Melanin.
  9. Mechanical Protection: Stratum corneum.
  10. Metabolic: Vitamin D synthesis.