Here's a breakdown:
* Epidermis (Outer Layer):
* Stratum Corneum: This is the main target. It's made of dead skin cells (corneocytes) held together by lipids. Moisturizers work by hydrating these cells, filling in gaps between them, and creating a protective barrier.
* Other Layers of Epidermis: While some small molecules in moisturizers *might* reach deeper layers of the epidermis (like the stratum granulosum or stratum spinosum), it's not their primary function. The stratum corneum acts as a significant barrier.
* Dermis (Middle Layer): Moisturizers generally don't penetrate the dermis. The dermis contains collagen, elastin, blood vessels, and nerve endings. Substances that target the dermis (like some anti-aging ingredients) require more specialized delivery systems (like prescription retinoids or professional treatments).
* Hypodermis (Deepest Layer): Moisturizers don't reach the hypodermis (subcutaneous fat layer).
In summary:
Moisturizers work mainly on the stratum corneum of the epidermis, providing hydration and a protective barrier. Their effects are primarily on the surface of the skin.