Anatomy, Physiology, and Skin Science
Layers and cells
Five epidermal layers from the bottom: basale, spinosum, granulosum, lucidum (palms and soles only), corneum. The epidermis is avascular - nutrients arrive by diffusion from the dermis. Key cells: keratinocytes (structure, keratin), melanocytes in the basal layer (melanin), Langerhans cells (immune defense), Merkel cells (touch). Turnover is roughly 28 days in young skin and slows with age.
The dermis has a papillary layer (capillaries, pain and touch receptors) and a reticular layer (collagen, elastin, glands, follicles, nerves). The hypodermis is subcutaneous fat for insulation and shock absorption.
Functions and chemistry
Protection, sensation, thermoregulation, excretion, secretion, vitamin D synthesis, and limited absorption. The acid mantle sits near pH 4.5–5.5, supported by sebum, ceramides, and natural moisturizing factor.
Muscles and nerves
Frontalis, orbicularis oculi and oris, zygomaticus (smiling), masseter and temporalis (chewing), platysma (neck). Motor control of the face is the facial nerve (VII); sensation is the trigeminal nerve (V). Naming the wrong cranial nerve is a favorite distractor.
Fitzpatrick scale
Types I–VI describe burning and tanning response. Low types burn easily and need conservative peel and treatment plans; types IV–VI carry higher risk of post-inflammatory hyperpigmentation.
Traps
- Calling the epidermis vascular.
- Mixing up sebaceous (sebum, acne) and sudoriferous (sweat) glands.
- Believing pores open and close with hot or cold water - they do not; heat only softens sebum.
Rule of thumb
Barrier repair means lipids plus NMF; acne means sebum plus follicle keratinization plus bacteria.