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Stratified Squamous Keratinized Epithelium

  • Composed of several layers of keratinocytes.

  • Keratinocytes divide at the basal end and migrate, becoming flattened as they ascend.

  • Upper layer consists of thick keratinized cells that form a waterproof barrier.

Functions of the Skin

Protection

  • Acts as a barrier against microbes, abrasion, and dehydration.

  • Secretes substances like sweat and sebum.

  • Contains dermocidin with antibiotic properties.

  • Normal flora on skin help prevent pathogenic overgrowth.

Temperature Regulation

  • Regulates body temperature via sweating and blood vessel dilation/constriction.

  • Sweat evaporation cools the body.

  • Up to 500 mL of sweat produced daily, increasing to 1.5 L/hour during activity.

Nutrient Synthesis and Storage

  • Synthesizes Vitamin D (cholecalciferol) when exposed to UV light, essential for calcium and phosphorus absorption.

  • Deficiency can lead to rickets in children.

Sensory Reception

  • Contains various sensory receptors (e.g., free nerve endings, Meissner's corpuscles) that detect pressure, pain, and temperature.

  • Sensory receptors include:

    • Free Nerve Endings: Pain and temperature detection.

    • Meissner's Corpuscles: Light touch sensitivity.

    • Pacinian Corpuscles: Detect gross pressure and vibrations.

    • Ruffini's Corpuscles: Irregular pressure detection and stretch response.

    • Merkel Discs: Detect pressure and texture.

Excretion and Secretion

  • Skin secretes sweat (eccrine and apocrine glands) and may absorb certain substances.

  • Eccrine glands are widespread and secrete a watery sweat.

  • Apocrine glands are located in specific regions and secrete thicker sweat.

Composition and Structure of the Skin

Epidermis

  • Composed of keratinocytes (75% of skin cells) and melanocytes.

  • Has 5 layers (stratum basale, spinosum, granulosum, lucidum, corneum) in thick skin; 4 layers in thin skin.

Dermis

  • Composed of 2 layers: papillary (upper) and reticular (lower).

  • Contains blood vessels, nerves, hair follicles, and glands.

  • Collagen and elastin fibers provide strength and elasticity.

Skin Color and Pigmentation

  • Skin color driven by melanin production, influenced by UV exposure.

  • Eumelanin and pheomelanin contribute to the range of human skin tones.

  • Albinism results from a lack of melanin production.

Layers of the Epidermis

  • Stratum Basale: Mitotic layer, contains basal/stem cells, and melanocytes.

  • Stratum Spinosum: Spiny cells where keratin synthesis begins; this layer is thicker.

  • Stratum Granulosum: Grainy layer due to keratin formation, cells become flatter.

  • Stratum Lucidum: Present only in thick skin; smooth, translucent layer of dead cells.

  • Stratum Corneum: Outermost dead cell layer, protects against abrasion and microbial penetration.