Skin Anatomy, Physiology, and Histology
Skin anatomy, physiology, and histology is the domain that explains why products behave as they do. The Colorado exam tests the layers of skin, the cells and appendages in each, the barrier and acid mantle, and the difference between epidermis and dermis when a product or lesion is discussed.
Epidermis
The epidermis is avascular and is built mainly of keratinocytes. From deep to surface: stratum basale (mitosis, melanocytes), stratum spinosum (Langerhans cells), stratum granulosum (keratohyalin), stratum lucidum only on palms and soles, and stratum corneum (dead keratin and barrier). Melanin is produced in the basale and transferred to keratinocytes; it is not a layer of dead cells. The acid mantle is about pH 4.5–5.5 and supports barrier lipids and natural moisturizing factor.
Dermis and hypodermis
The dermis has a papillary layer and a reticular layer. Fibroblasts make collagen and elastin; glycosaminoglycans hold water. Blood vessels, nerves, glands, and hair follicles sit here. The hypodermis is subcutaneous fat that cushions and insulates.
Appendages and receptors
- Sebaceous glands are attached to follicles and are dense on the face, scalp, chest, and back; they are absent on palms and soles.
- Eccrine glands cool the body; apocrine glands are in axillae and genital areas and connect to hair follicles.
- Sensory receptors: Merkel (pressure), Meissner (light touch), Ruffini (heat), Pacinian (deep pressure and vibration), free nerve endings (pain and temperature).
Worked example
A client asks whether a 10% glycolic acid treatment 'removes a layer of skin.' Glycolic acid is an AHA. At low pH it reduces cohesion between corneocytes in the stratum corneum, so dead surface cells shed and penetration improves. It does not remove the dermis. The barrier being manipulated is the stratum corneum, which is why overuse causes stinging, redness, and water loss.
Traps
- Saying the epidermis contains blood vessels.
- Placing melanocytes in the stratum corneum.
- Confusing sebaceous glands with eccrine glands.
- Claiming a cosmetic 'reaches the dermis' without a delivery rationale.
- Forgetting that the barrier is the stratum corneum, not the whole epidermis.