뒤로The Epidermal Layers of the Skin: Structure and Function
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The Integumentary System
The Epidermis: Structure and Layers
The epidermis is the outermost layer of the skin, composed of stratified squamous epithelium. It provides a protective barrier and is organized into distinct layers, or strata, each with specialized cells and functions. The structure of each layer is closely related to its role in skin health and protection.
Stratum | Description |
|---|---|
Basale | Deepest layer, composed of a single row of cuboidal or columnar keratinocytes that contain scattered keratin intermediate filaments (tonofilaments); stem cells undergo cell division to produce new keratinocytes; melanocytes and tactile epithelial cells associated with tactile discs are scattered among keratinocytes. |
Spinosum | Eight to ten rows of many-sided keratinocytes with bundles of keratin intermediate filaments; contains projections of melanocytes and intraepidermal macrophages. |
Granulosum | Three to five rows of flattened keratinocytes, in which organelles are beginning to degenerate; cells contain the protein keratohyalin (converts keratin intermediate filaments into keratin) and lamellar granules (release lipid-rich, water-repellent secretion). |
Lucidum | Present only in skin of fingertips, palms, and soles; consists of four to six rows of clear, flat, dead keratinocytes with large amounts of keratin. |
Corneum | Few to 50 or more rows of dead, flat keratinocytes that contain mostly keratin. |
Key Features and Functions of Each Epidermal Layer
Stratum Basale: The site of active cell division, providing new keratinocytes to replace those shed at the surface. Melanocytes here produce melanin, contributing to skin color and UV protection. Tactile cells (Merkel cells) are involved in sensory perception.
Stratum Spinosum: Provides strength and flexibility to the skin. The presence of immune cells (intraepidermal macrophages or Langerhans cells) helps defend against pathogens.
Stratum Granulosum: Cells here undergo apoptosis (programmed cell death) as they move toward the surface. Keratohyalin and lamellar granules are crucial for forming the tough, water-resistant barrier of the skin.
Stratum Lucidum: Found only in thick skin (palms, soles, fingertips), this translucent layer adds an extra barrier of protection and durability.
Stratum Corneum: The outermost layer, consisting of many layers of dead, keratinized cells. It provides a tough, protective barrier against environmental damage, pathogens, and water loss.
Clinical Relevance
Damage to the stratum basale can impair skin regeneration and healing.
Thickening of the stratum corneum (hyperkeratosis) can occur in response to friction or pressure, forming calluses.
The presence or absence of the stratum lucidum distinguishes thick skin (palms, soles) from thin skin (most of the body).
