Anatomy and Physiology of the Skin and Hair
Layers and structures
Know the epidermis (avascular, keratinized), the dermis (collagen, elastin, blood vessels, nerves, glands), and the subcutaneous layer (fat). The hair follicle is an invagination of the epidermis extending into the dermis, sometimes into the subcutis. Key follicle regions: the infundibulum (upper portion near the surface), the isthmus, and the bulge, which houses stem cells and is a critical target. The hair bulb surrounds the dermal papilla, which contains capillaries and nerves and supplies nourishment. The sebaceous gland typically attaches near the upper follicle.
Understand the arrector pili muscle, the apocrine and eccrine glands, and the acid mantle of the skin, which protects against pathogens and influences healing. Recognize skin types and conditions that change treatment: thin or sun-damaged skin, scarring, keloid tendency, and vascularity.
Traps in questions
- The epidermis contains no blood vessels; bleeding indicates deeper injury than intended.
- The papilla is dermal, not epidermal, and is essential to hair growth but not the only target.
- Sebaceous glands are not part of the follicle's germinative apparatus.
- Depth of insertion depends on follicle depth, which varies by body site.
Rule of thumb
Map the follicle to skin depth: shallow on the upper lip, deeper on the chin and legs. If a question describes a needle inserted too shallowly, the likely consequence is incomplete follicle destruction and regrowth.