Scientific Concepts: Skin Histology, Disorders, and Anatomy
Structure first
Skin has three main layers: epidermis, dermis, and hypodermis. In the epidermis, keratinocytes are born in the stratum basale, flatten upward through the spinosum and granulosum, and end as corneocytes in the stratum corneum. The stratum lucidum appears only on palms and soles - a favorite trick answer. Melanocytes sit in the basale and transfer melanin through melanosomes; Langerhans cells are immune sentinels; Merkel cells are touch receptors. The dermis (papillary, then reticular) holds collagen, elastin, follicles, sebaceous and eccrine glands, and vessels. Sebaceous glands attach to follicles, so palms and lips produce no sebum.
Function and chemistry
The acid mantle (roughly pH 4.5–5.5) resists bacteria and limits water loss, which is why harsh alkaline cleansers strip and irritate. Know the Fitzpatrick scale (I–VI) for UV risk and treatment planning, and separate skin type (oily, dry, combination, normal, sensitive) from skin condition (acne, rosacea, melasma, dehydration). Anatomy items focus on facial bones, muscles of expression (frontalis, orbicularis oculi, zygomaticus, masseter), and cranial nerves: facial nerve VII moves the face, trigeminal V carries sensation.
The trap to watch
Questions blur contagious and non-contagious findings. Herpes simplex, impetigo, conjunctivitis, ringworm, and lice are contagious - no service, refer out. Acne, rosacea, seborrheic dermatitis, melasma, and keratosis pilaris are not contagious but may still be contraindications during a flare.
Rule of thumb: if a lesion is new, changing, bleeding, painful, or you cannot name it with confidence, do not treat it - document and refer. Example: a client arrives with a tingling cluster of vesicles on the lip; that is a cold sore, so the service is refused and the client is rescheduled after healing.