Skin Anatomy, Physiology, and Histology
Layers and what lives in them
The epidermis is the avascular outer layer; its deepest layer, the stratum germinativum (basal layer), is where mitosis produces new cells that migrate outward and keratinize. The stratum corneum is the outermost, fully keratinized barrier. The dermis holds collagen and elastin (fibroblasts), blood vessels, nerves, glands, and hair follicles. The subcutaneous layer is adipose and connective tissue providing insulation and cushioning.
Glands, follicles, and pigment
Sebaceous glands secrete sebum and are attached to hair follicles. Eccrine (sweat) glands regulate temperature; apocrine glands are associated with hair follicles in axillae and genital areas. Melanocytes in the basal layer produce melanin, transferred to keratinocytes via melanosomes - this determines color and protects from UV. Langerhans cells are immune; Merkel cells are sensory.
Physiology worth naming
Skin functions include protection, sensation, heat regulation, excretion, secretion, and absorption. Absorption matters for product selection - ingredients can pass through follicles and intercellular lipids.
Traps
- Confusing stratum lucidum (palms and soles only) with a universal layer.
- Assuming the dermis regenerates like the epidermis - dermal damage (deep scarring) does not fully regenerate.
- Mixing up sebaceous (sebum, follicle-attached) and sudoriferous (sweat) glands.
Worked example
A question asks where a product's active ingredient most easily penetrates. Answer: through hair follicles and sebaceous glands (the follicular route), plus intercellular pathways in the stratum corneum - not through the basal layer directly.