Overview
This lecture covers the layers of the epidermis, the main cell types within these layers, differences between thick and thin skin, and some clinical relevance.
Epidermal Cell Types
- The four primary epidermal cells are keratinocytes (majority), melanocytes (produce melanin), Merkel cells (detect light touch), and Langerhans cells (immune function).
- All four cell types are present in the basal (deepest) layer of the epidermis.
Types of Epidermis: Thick vs Thin Skin
- Thick skin (palms, soles) has five layers: stratum basale, spinosum, granulosum, lucidum, and corneum.
- Thin skin lacks the stratum lucidum and has fewer layers overall.
Epidermal Layers (Deep to Superficial)
- Stratum Basale: Deepest, single cuboidal cell layer attached to basement membrane by hemidesmosomes; contains all cell types and stem cells; all cells are alive.
- Stratum Spinosum: 8-10 layers mainly of living keratinocytes connected by desmosomes and some gap junctions; present in both thick and thin skin.
- Stratum Granulosum: 3-5 layers of keratinocytes; apoptosis occurs here as cells become filled with keratin and die; keratohyalin protein aids in keratinization; lipid granules released.
- Stratum Lucidum: Present only in thick skin; clear layer of dead keratinocytes.
- Stratum Corneum: Outermost layer; 20-40 layers of dead keratinocytes; thickness varies between thick and thin skin.
Key Features & Review Points
- Mnemonic for layers (deep to superficial): Before Sex Get Latex Condoms (Basale, Spinosum, Granulosum, Lucidum, Corneum).
- Apoptosis (cell death) mainly occurs in the stratum granulosum.
- Layers composed entirely of dead cells: stratum lucidum (thick skin only) and stratum corneum.
- Cell layers move from deep (living) to superficial (dead) over about one month.
- Most cell connections are made via desmosomes.
Clinical Correlation: Epidermolysis Bullosa
- A genetic disease where faulty proteins in cell junctions cause the skin to blister, slough off, and become prone to infection.
Key Terms & Definitions
- Keratinocyte — Main cell producing keratin in the epidermis.
- Melanocyte — Cell that produces melanin, the skin pigment.
- Merkel cell — Sensory cell for light touch.
- Langerhans cell — Immune cell in the skin.
- Stratum basale — Deepest epidermal layer attached to basement membrane.
- Stratum spinosum — Layer of spiny-appearing, living keratinocytes.
- Stratum granulosum — Keratinization and apoptosis layer.
- Stratum lucidum — Clear, dead-cell layer in thick skin only.
- Stratum corneum — Outer layer of dead, keratinized cells.
- Hemidesmosome — Structure attaching cells to the basement membrane.
- Desmosome — Structure connecting adjacent cells.
Action Items / Next Steps
- Practice identifying each epidermal layer and its main features.
- Review the mnemonic for layer order.
- Research more on epidermolysis bullosa if interested.
- Complete any assigned worksheets or diagrams identifying layers and cell types.