Your skin is like a superhero suit, protecting you from harm and regulating your body. It's made up of layers, each with a special job. The outermost layer, the epidermis, is your first line of defense against the world.

Underneath, the dermis gives your skin strength and flexibility. It's packed with blood vessels, nerves, and glands that help you stay cool and feel sensations. The hypodermis, the deepest layer, acts like a cushion and keeps you warm.

Skin Structure and Function

Epidermis and dermis

• Epidermis forms outermost layer of skin composed of stratified squamous epithelium provides protective barrier against external factors (UV radiation, microorganisms, chemicals) consists of five distinct layers (stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, stratum basale) undergoes continuous renewal through keratinization (process of cell differentiation and maturation)
• Dermis located beneath epidermis composed of connective tissue (collagen and elastin fibers) provides mechanical support and elasticity to skin contains blood vessels (supply nutrients and oxygen, regulate temperature), lymph vessels (aid in immune function and fluid balance), nerve endings (responsible for sensory perception), hair follicles (produce hair), and sweat glands (regulate body temperature) plays crucial role in temperature regulation (blood vessel dilation or constriction) and sensory perception (various receptors for touch, pressure, temperature, pain)

Layers of the epidermis

• Stratum corneum (horny layer) outermost layer of epidermis consists of dead, flattened keratinocytes filled with keratin (tough, fibrous protein) provides waterproof barrier and protection against external factors (physical, chemical, biological)
• Stratum lucidum (clear layer) present only in thick skin (palms and soles) composed of dead, flattened keratinocytes aids in reducing friction and shear forces (prevents skin damage from mechanical stress)
• Stratum granulosum (granular layer) contains keratinocytes with keratohyalin granules (precursors to keratin) plays role in formation of stratum corneum produces lipids (ceramides, cholesterol, fatty acids) that contribute to skin's barrier function
• Stratum spinosum (spinous layer) consists of several layers of keratinocytes connected by desmosomes (cell junctions) provides mechanical strength to epidermis involved in synthesis of keratin and other structural proteins (involucrin, loricrin)
• Stratum basale (basal layer) deepest layer of epidermis contains stem cells that continuously divide to replenish epidermis anchored to dermis via hemidesmosomes (specialized cell junctions) produces melanin (pigment that protects against UV radiation) via melanocytes (pigment-producing cells)

Dermis and Hypodermis

Composition and functions of dermis

• Composition of dermis:
  1. Papillary dermis: uppermost layer, contains loose connective tissue and capillary loops (supply nutrients to epidermis)
  2. Reticular dermis: deeper layer, composed of dense irregular connective tissue (collagen and elastin fibers) provides strength and elasticity
• Functions of dermis:
  1. Provides mechanical support and elasticity to skin
  2. Contains blood vessels that nourish epidermis and regulate body temperature (vasodilation or vasoconstriction)
  3. Houses lymph vessels that aid in immune function and fluid balance (remove waste products and excess fluid)
  4. Contains nerve endings responsible for sensory perception (touch, pressure, temperature, pain)
  5. Includes hair follicles (produce hair), sweat glands (produce sweat for temperature regulation), and sebaceous glands (produce sebum for skin lubrication)
• Role in temperature regulation:
  1. Blood vessels in dermis dilate (increase in diameter) to increase heat loss or constrict (decrease in diameter) to retain heat
  2. Sweat glands produce sweat, which evaporates and cools skin surface (thermoregulation)
• Role in sensory perception:
  1. Meissner's corpuscles detect light touch and texture (located in papillary dermis)
  2. Pacinian corpuscles respond to deep pressure and vibration (located in reticular dermis)
  3. Ruffini endings sense sustained pressure and stretching of skin (located in reticular dermis)
  4. Free nerve endings detect temperature, pain, and itch sensations (located throughout dermis)

Hypodermis and dermatologic disorders

• Hypodermis (subcutaneous layer) deepest layer of skin, located beneath dermis composed of loose connective tissue and adipose tissue (fat cells) provides insulation (retains body heat), energy storage (triglycerides), and cushioning for underlying structures (muscles, bones)
• Relationship between hypodermis and dermatologic disorders:
  1. Disorders affecting hypodermis can alter skin appearance and function
  2. Cellulite: dimpling of skin due to uneven distribution of fat in hypodermis (more common in women)
  3. Lipodystrophy: abnormal distribution or loss of body fat, which can be caused by certain medications (antiretroviral drugs) or medical conditions (insulin resistance, autoimmune disorders)
• Impact on drug absorption and distribution:
  1. Hypodermis acts as reservoir for lipophilic drugs (drugs that dissolve in fat)
  2. Drugs administered via subcutaneous injection are absorbed through blood vessels in hypodermis (slower absorption compared to intramuscular or intravenous routes)
  3. Factors such as blood flow (affected by exercise, temperature), body fat composition (varies among individuals), and injection site (abdomen, thigh, upper arm) can influence drug absorption and distribution
  4. Certain medications (insulin, heparin, growth hormone) are specifically designed for subcutaneous administration to ensure proper absorption and therapeutic effect (sustained release, localized action)