Human Anatomy: Skin Structure

Skin Overview

• Skin is the largest organ, covering the body's external surface.
• Composed of three layers: epidermis (outer), dermis (middle), and hypodermis (deepest).
• Functions as a barrier against pathogens, UV light, chemicals, and mechanical injury.
• Regulates temperature and water loss, with thickness varying by body region.

Epidermis

• Outermost layer consists of multiple strata: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.
• Stratum basale contains mitotically active keratinocyte stem cells and melanocytes, providing pigmentation.
• Stratum spinosum contains irregular cells linked by desmosomes and houses dendritic cells (Langerhans).
• Stratum granulosum has keratinocyte cells with keratohyalin and lamellar granules, contributing to the water barrier.
• Stratum lucidum found in thick skin (e.g., palms, soles) is composed of translucent cells (eleidin).
• Stratum corneum is the outermost layer, made of dead keratinocytes, providing a protective, variable thickness layer.

Cell Types in the Epidermis

• Keratinocytes: Predominant cells producing keratin and lipids, contributing to the water barrier and UVB absorption for vitamin D synthesis.
• Melanocytes: Synthesize melanin, protecting against UV radiation; located between stratum basale cells.
• Langerhans Cells: Dendritic immune cells in the stratum spinosum; involved in antigen presentation and immune response.
• Merkel Cells: Mechanoreceptors found in the stratum basale, responsible for light touch sensation, particularly in fingertips.

Dermis

• The dermis connects to the epidermis via the basement membrane and consists of two layers: papillary and reticular.
• Papillary Layer: Superficial, composed of loose connective tissue, interdigitating with the epidermis.
• Reticular Layer: Deeper, thicker layer made of dense connective tissue, housing sweat glands, hair follicles, and blood vessels.

Hypodermis

• Known as subcutaneous fascia; the deepest skin layer containing adipose tissue, neurons, and blood vessels.
• Provides insulation, energy storage, and shock absorption.

Skin Functions

• Barrier Function: Protects against water loss, microbial invasion, and physical or chemical trauma.
• Immunological Defense: Plays a role in both adaptive and innate immunity; Langerhans cells assist in antigen recognition and presentation.
• Homeostasis Regulation: Maintains body temperature and water balance through blood circulation and sweat secretion.
• Endocrine and Exocrine Functions: Synthesizes vitamin D and produces sebum for skin hydration; sweat glands help with thermoregulation.
• Sensory Functions: Contains nociceptors for touch, heat, cold, and pain, aiding in environmental interaction.
• Health Indicator: Skin characteristics provide insights into overall health, aiding in medical assessments.

Connective Tissue Overview

• Connective tissue, developed from mesenchyme, comprises elastic and collagen fibers, ground substance, and various cells.
• Specialized fluid connective tissues include blood and lymph.
• Components:
  • Cells: Fibroblasts, adipocytes, macrophages, mast cells, and leukocytes.
  • Types: Loose connective tissue (more ground substance) and dense connective tissue (more fibrous).
• Dense Regular Tissue: Arranged collagen fibers, providing tensile strength (e.g., tendons, ligaments).
• Dense Irregular Tissue: Fiber bundles arranged in various directions, providing multi-directional strength.

Epithelial Tissue

• Characterized by polarized organelles and membrane proteins, adapted for specific functions.
• Types of Epithelia: Classifications based on cell shape (squamous, cuboidal, columnar) and number of layers (simple, stratified).
• Special Features: Ciliated epithelium moves fluids and particles; found in respiratory tract and brain ventricles.

Epithelial Cell Shapes

• Simple Squamous: Thin, allowing rapid diffusion (e.g., alveoli, capillaries).
• Cuboidal: Secretion and absorption (e.g., kidney tubules, gland ducts).
• Columnar: Active in absorption/secretion, found in digestive and reproductive tracts.
• Ciliated Columnar: Moves particles via cilia, located in respiratory and reproductive systems.
• Pseudostratified Columnar: Appears stratified but is a single layer; found in respiratory tract, usually ciliated.

Summary of Functionality

• Epithelial tissues exhibit specialized structures adaptable to their roles in secretion, absorption, and protection, playing critical roles in overall body function and health.