Skin Anatomy: A Comprehensive Overview

Chapter 5: Integumentary System - Skin Anatomy

General Characteristics of Skin

• Skin thickness varies significantly across the body:
  • Thickest: Found in areas subject to constant wear and tear, such as the palms of hands and soles of feet.
  • Thinnest: Located over delicate structures like the eyelids, eardrums, and genitalia.

Main Layers of the Skin

• The skin is primarily composed of two main layers:
  • Epidermis: The thinner, outer layer, which is made of epithelium.
  • Dermis: The thicker, inner layer, located beneath the epidermis.

The Epidermis

• Composition & Nature:
  • It is an epithelial tissue.
  • Avascular: Lacks blood vessels, meaning it receives nutrients by diffusion from the underlying dermis.
• Key Protein:
  • Keratin: A tough, fibrous protein that provides protective qualities to the skin, hair, and nails.
• Layers (Strata) of the Epidermis:
  • The epidermis typically consists of 4 layers in thin skin and 5 layers in thick skin. These layers, from innermost to outermost, are:
    • Stratum Basale (Germinativum): The innermost, deepest layer, responsible for cell division and producing new keratinocytes.
    • Stratum Spinosum: Lies above the stratum basale.
    • Stratum Granulosum: Characterized by granular cells that produce keratin.
    • Stratum Lucidum: Present only in thick skin (e.g., palms and soles); it is absent in thin skin.
    • Stratum Corneum: The outermost layer, composed of dead, flattened keratinocytes that provide a protective barrier.
• Specialized Cells within the Epidermis:
  • Melanocytes: Primarily located in the stratum basale; these cells produce melanin, the pigment responsible for skin color and protection against UV radiation.
  • Dendritic Cells (Langerhans Cells): Derived from monocytes (a type of white blood cell), these cells are part of the immune system, acting as antigen-presenting cells to initiate immune responses.
• Cell Turnover: The process of new skin cells being produced in the stratum basale and migrating to the surface (stratum corneum) takes approximately 7 to 10 days.

The Dermis

• The dermis is the major portion of the skin and is divided into two sub-layers:
  • Papillary Layer (Outer):
    • Contains tactile corpuscles (Meissner's corpuscles), which are touch receptors sensitive to light touch.
    • Features dermal papillae, upward projections that interdigitate with the epidermis. These papillae are responsible for forming fingerprints, enhancing grip on surfaces.
  • Reticular Layer (Inner):
    • This layer constitutes the majority of the dermis, approximately 80\% to 85\% of its thickness.
    • Contains lamellar corpuscles (Pacinian corpuscles), which are pressure receptors responsible for detecting deep pressure and vibration.

Subcutaneous Tissue (Hypodermis)

• Located beneath the dermis.
• Primarily composed of adipose tissue (fat), which provides insulation, energy storage, and shock absorption.