Overview
This lecture explains the structure and functions of the five layers of the epidermis, the outermost part of human skin, focusing on their unique features and roles.
Structure of Skin
- Skin consists of three main parts: epidermis (top five layers), dermis (next two layers), and subcutaneous tissue (hypodermis).
- The epidermis is the outermost, protective layer made entirely of dead cells at the top.
Layers of the Epidermis (From Deepest to Superficial)
- Stratum Basale (Basal Layer): Sits above the dermis; generates keratinocytes through rapid cell division.
- Contains melanocytes, which produce melanin, the pigment responsible for skin color (amount of melanin, not cell count, determines darkness).
- Stratum Spinosum (Spiny Layer): Characterized by desmosomes (cell connections) creating a spiny appearance.
- Includes Langerhans cells, which function in immune defense.
- Stratum Granulosum (Granular Layer): Contains keratohyalin granules (keratin-handling proteins) and releases lamellar bodies (lipids) forming a water-proof barrier.
- Stratum Lucidum (Clear Layer): Composed of dead, clear keratinocytes lacking nuclei and organelles; seen only in thick skin (palms/soles).
- Stratum Corneum (Coroner’s Layer): Top layer made of 15-20 layers of dead, flat keratinocytes that continuously slough off.
- In reptiles, the stratum corneum is shed all at once (molting).
Key Terms & Definitions
- Epidermis — Outermost layer of the skin, composed of five strata.
- Keratinocyte — Skin cell that produces keratin for strength and protection.
- Cytokeratin (Keratin) — Structural protein giving skin toughness.
- Melanocyte — Cell that produces melanin pigment.
- Melanin — Pigment determining skin color based on quantity, not number of melanocytes.
- Desmosome — Junction/connection between skin cells, providing structural integrity.
- Langerhans Cell — Immune cell in the epidermis, defending against pathogens.
- Keratohyalin Granule — Granule containing proteins for keratin processing.
- Lamellar Body — Structure releasing lipids to form a water-tight seal.
- Sloughing — Shedding or loss of the top layer of cells.
Action Items / Next Steps
- Review the functions and order of each epidermal layer.
- Prepare for discussion or quiz on the characteristics and cells found in each stratum.