Anatomy and Physiology of the Skin and Supporting Structures
The epidermis
Five strata, deep to superficial: basale, spinosum, granulosum, lucidum (palms and soles only), and corneum. The basale is the mitotic layer; it holds melanocytes (melanin) and Merkel cells (light touch). Spinosum hosts Langerhans cells, the immune sentries. Granulosum is where cells flatten and keratohyalin appears. Corneum is dead, keratinized, and forms the barrier. Cell turnover is roughly 28 days and slows with age.
The dermis and subcutis
The papillary dermis is thin, with capillaries and nerve endings; the reticular dermis is thick, with collagen and elastin bundles, fibroblasts, mast cells, sebaceous glands, and follicles. Ground substance holds water and supports the fibers. The subcutaneous layer stores fat for insulation and shock absorption.
Appendages
Sebaceous glands produce sebum and empty into a follicle; eccrine glands cool the body and are everywhere; apocrine glands sit in the axillae; arrector pili muscles give goosebumps. The nail has a plate, bed, matrix, lunula, and cuticle - the matrix is the growth site.
Functions and barrier
Protection, sensation, thermoregulation, excretion, secretion, vitamin D synthesis, and limited absorption. The acid mantle sits near pH 4.5–5.5, supporting friendly flora and barrier lipids.
Traps
Asking which collagen-rich layer is which; placing melanocytes in the dermis; confusing basale with corneum; giving the nail matrix the wrong job.
Rule of thumb
'Basale = birth, corneum = cover' - new cells are born at the base and end up protecting at the surface.