Accessory Structures

Hair and follicles. A hair is a flexible strand of dead, keratinized cells produced by a hair follicle that extends from the epidermis down into the dermis. The living, dividing part is the hair bulb at the base; the visible shaft is dead keratin (which is why a haircut doesn't hurt). Hair helps with protection (guarding the scalp from sun and injury, filtering particles in the nose, shielding the eyes with lashes) and sensation (follicles are wrapped in nerve endings that detect light touch). Attached to each follicle is a small smooth muscle, the arrector pili, which contracts when you're cold or startled, pulling the hair upright and producing goosebumps — a vestige of fluffing fur for warmth.

Nails. A nail is a scale-like plate of hard keratin covering the tips of fingers and toes. It protects the sensitive tips, aids in grasping and fine manipulation, and lets you scratch. Nails grow from the nail matrix at the base; the pink color comes from underlying capillaries, and the pale lunula ("little moon") is the visible part of the matrix. Nail changes (color, shape, thickness) can reflect systemic conditions, making them a quick clinical clue.

Glands: oil vs sweat. The skin has two main gland families:

• Sebaceous (oil) glands secrete sebum, an oily mixture usually released into hair follicles. Sebum lubricates hair and skin, slows water loss, and has mild antibacterial and antifungal action. These glands become very active at puberty; when a follicle clogs and sebum and bacteria accumulate, the result is acne.
• Sweat (sudoriferous) glands come in two types:
  • Eccrine (merocrine) glands are the most numerous, found nearly everywhere (especially palms, soles, forehead). They produce watery sweat directly onto the skin surface for evaporative cooling — the body's main thermoregulation gland — and also excrete small amounts of waste.
  • Apocrine glands are located mainly in the axillary (armpit) and genital regions, become active at puberty, and release a thicker secretion into hair follicles. Their sweat is odorless until skin bacteria break it down, producing body odor. Modified apocrine glands include ceruminous glands (earwax) and mammary glands (milk).

How it works

Temperature regulation via sweat:

Body temperature rises
   → hypothalamus signals eccrine sweat glands
   → watery sweat reaches the skin surface
   → sweat evaporates, carrying away heat
   → body cools toward the set point

This ties directly back to the homeostasis and thermoregulation loop from Unit 1.

Comparisons

Common confusions

• Sebaceous vs sweat glands. Sebaceous make oily sebum (lubrication); sweat glands make watery/odor sweat (cooling).
• Eccrine vs apocrine. Eccrine = everywhere, watery, cooling; apocrine = armpit/groin, thicker, body odor after bacteria.
• Hair and nails are mostly dead keratin — the living growth zone is the bulb/matrix at the base.
• Sweat itself doesn't smell. Body odor comes from bacteria digesting apocrine secretions.

Memory aids

• Sebaceous = "Sebum = oil = Slick."
• Eccrine = "Everywhere, cooling."
• Apocrine = "Armpit, activates at puberty."
• Arrector pili = "raises the hair — goosebumps."

Quick review

• Hair/follicles protect and sense; the arrector pili muscle makes goosebumps. Nails (hard keratin) protect fingertips and aid grasping.
• Sebaceous glands make sebum (lubricates, waterproofs, mildly antibacterial); clogging causes acne.
• Eccrine sweat glands (everywhere) produce watery sweat for cooling; apocrine glands (axilla/groin) make thicker sweat that bacteria turn into body odor.
• Heavy sweating causes fluid/electrolyte loss — a key clinical point tied to thermoregulation.