Integumentary System
Your skin, hair, nails, and glands. The largest organ, the first barrier, and a surprisingly active endocrine participant.
Skin is not wrapping paper. It is a metabolically active, sensory, immune-surveilling, thermoregulating organ covering about 1.5-2 square meters, weighing roughly 15% of your body mass, and doing more of the work keeping you alive than most people credit.
At a glance
What it does
Keeps the inside in and the outside out. Barrier function blocks water loss, prevents pathogen entry, and filters UV radiation. Thermoregulation works through sweat evaporation and subcutaneous blood flow (dilate to dump heat, constrict to hold it). Sensation — touch, pressure, temperature, pain, itch — is mediated by dense networks of specialized nerve endings.
Skin also synthesizes vitamin D from cholesterol when hit with UVB, houses a large population of immune cells (Langerhans cells, resident T cells), and responds to and produces a long list of hormones. It is one of the most hormonally reactive tissues in the body.
Structure
Three main layers. The epidermis is the outermost, avascular, made mostly of keratinocytes that push up from a basal layer, keratinize, and eventually flake off. Contains melanocytes (pigment) and Langerhans cells (immune). The dermis is the middle layer — collagen and elastin scaffold, blood vessels, nerve endings, hair follicles, sweat and sebaceous glands. The hypodermis (subcutaneous) below that is fat and connective tissue for insulation, cushioning, and energy storage.
Hair grows from follicles rooted in the dermis and has complex hormonal sensitivity — scalp hair miniaturizes under DHT, body hair thickens under androgens, and the pattern flips between sexes. Nails are modified keratinized epithelium. Sebaceous glands are androgen-sensitive oil producers; eccrine sweat glands are for thermoregulation; apocrine glands in armpits and groin produce the fluid that actually smells once bacteria metabolize it.
Hormonal effects on skin
- Androgens (testosterone, DHT) — drive sebum production, coarsen hair, thicken dermis. Puberty-onset acne is androgen-driven. Pattern hair loss is DHT-driven.
- Estrogen — maintains dermal collagen, hydration, and elasticity; its loss at menopause drives rapid skin thinning and wrinkling.
- Cortisol — chronic elevation thins skin, slows wound healing, promotes striae (stretch marks), and suppresses the skin's immune function.
- Growth hormone and IGF-1 — support collagen synthesis; decline with age contributes to slower healing.
- Thyroid hormone — sets skin metabolic rate; hypothyroidism causes dry, cool, puffy skin, hyperthyroidism causes moist warm skin.
- Insulin and IGF-1 — dietary-driven spikes exacerbate acne through androgen signaling.
When it goes wrong
Acne is a combination of hormonal (androgen-driven sebum), follicular (hyperkeratinization blocking pores), and microbial (Cutibacterium acnes). Hits 85% of teenagers and often drags into adulthood. Eczema and psoriasis are immune-mediated — psoriasis is a genuine autoimmune disease, and biologics targeting IL-17 and IL-23 have transformed severe cases.
Skin cancer is the most common cancer by far. Basal cell and squamous cell carcinomas are usually UV-driven and curable. Melanoma is the dangerous one — lower incidence, far higher mortality if not caught early. Cumulative UV exposure is the main modifiable risk factor, along with avoiding tanning beds.
Hair loss is mostly androgenetic (male and female pattern) and tracks DHT sensitivity. Telogen effluvium — diffuse shedding triggered by stress, illness, postpartum, or rapid weight loss — reverses within months and is frequently misdiagnosed as permanent loss.
Honest take
The skincare industry is roughly 80% overpromise and 20% things that work. The evidence-backed short list: sunscreen, retinoids, and (for women past 40) topical or systemic estrogen in the right clinical context. Everything beyond that is incremental at best. For acne, diet actually matters — high glycemic load and skim dairy both push sebum production in controlled trials — though not as much as the "clear skin via diet" influencers claim. For hair loss, finasteride and minoxidil work, and the topical options being studied now are approaching the same efficacy without the systemic exposure.
Sources
- Bolognia, Dermatology, 5th ed. — the reference textbook.
- Kang et al., Fitzpatrick's Dermatology — standard reference on skin biology and disease.
- Smith et al., Journal of the American Academy of Dermatology — dietary influences on acne.