Scientific Concepts: Skin, Anatomy, and Chemistry
Skin structure and function
Esthetics rests on three sciences: skin anatomy, chemistry, and body anatomy. Know the epidermal layers in order - stratum basale (where keratinocytes divide and melanocytes sit), spinosum, granulosum, and corneum, with the stratum lucidum appearing only on palms and soles. The dermis holds collagen, elastin, and fibroblasts; the subcutaneous layer cushions and insulates. Glands trip up candidates: sebaceous glands attach to hair follicles and make sebum, eccrine glands open directly onto skin and produce sweat nearly everywhere, and apocrine glands are limited to axillae and genital areas. Melanocytes make pigment; Langerhans cells support immunity.
Chemistry
Understand the pH scale (0–14, lower is more acidic), the acid mantle at roughly 4.5–5.5, and how surfactants lift oil so water can rinse it away. An emulsion pairs oil and water with an emulsifier. AHAs such as glycolic and lactic acid are water-soluble; salicylic acid is an oil-soluble BHA. Oxidation and reduction explain product degradation and some active ingredients.
Body anatomy
Name the facial bones (zygomatic, maxillae, mandible) and muscles (frontalis, orbicularis oculi, orbicularis oris, masseter). The fifth cranial nerve carries sensation; the seventh carries motor control.
Traps and a worked example
Questions swap muscle for bone, place five epidermal layers on every body site, or call sebaceous glands sweat glands. Rule of thumb: if a client is oily through the T-zone and tight on the cheeks, classify combination skin and treat each zone rather than labeling the whole face oily.