Anatomy and Physiology of the Skin
Layers, outside in
The epidermis is avascular and nourished by diffusion. Its layers, deep to superficial, are basale (mitosis and melanocytes), spinosum, granulosum, lucidum (thick skin of palms and soles only), and corneum (dead keratinized cells that form the barrier). Cell turnover averages about 28 days. The dermis has a papillary and a reticular layer containing collagen, elastin, blood vessels, nerves, and glands. The subcutaneous layer stores fat for insulation and shock absorption.
Functions and chemistry
Skin protects, senses, regulates heat, excretes, secretes, absorbs minimally, and synthesizes vitamin D. The acid mantle (sebum plus sweat, roughly pH 4.5 to 5.5) inhibits bacteria and keeps the barrier intact; alkaline cleansers disrupt it.
Glands and pigment
Sebaceous glands attach to hair follicles and exist everywhere except palms and soles; overactivity contributes to acne. Eccrine glands cool the body; apocrine glands in the axillae and groin produce odor when bacteria break down their secretion. Melanocytes in the basal layer produce melanin for UV protection; the Fitzpatrick scale reflects that activity.
Bones, muscles, nerves
Facial bones include frontal, parietal, temporal, occipital, zygomatic, maxillae, mandible, and nasal. Key muscles: frontalis, temporalis, masseter (mastication), orbicularis oculi and oris, zygomaticus, buccinator, mentalis, and platysma. The trigeminal nerve (cranial V) carries sensation; the facial nerve (cranial VII) carries motor control.
Trap watch
Questions claim the epidermis contains blood vessels, that sebaceous glands cover the palms and soles, or that melanin is made in the dermis. All are false.
Rule of thumb
Superficial to deep: corneum, granulosum, spinosum, basale - memorized as Come, Go, Somewhere, Basal.