Accessory Structures Provide Protection
and Help Regulate Body Temperature 96
• Hair, glands, and nails develop from the embryonic
epidermis. Hairs consist of threads of fused, keratinized
cells. A hair has a shaft above the surface, a root below,
and a hair follicle in the subcutaneous layer. Smooth
muscle bundles called arrector pili and oil glands are as-
sociated with hairs.
• Oil glands secrete sebum (oil) that softens the skin, prevents
water loss, and inhibits bacterial growth. Eccrine sweat
glands secrete sweat and participate in temperature regula-
tion, while apocrine sweat glands open into hair follicles and
secrete sweat during emotional states (for example, fear,
embarrassment, sexual excitement). Ceruminous glands,
which are found in the external ear canal, are modified
sweat glands that secrete earwax (cerumen).
• As shown, nails are plates of dead, hard, keratinized cells
that cover the ends of the fingers and toes. A nail has a nail
body, free edge, root, lunula, cuticle, and nail matrix. The
nail matrix is the portion of the nail root that contains the
living, dividing nail cells.
Parts of the nail • Figure 4.5
The Skin Plays a Number of Roles
in the Body 100
• The skin covers a large surface area (~2 m2, or 22 ft2) and
acts as a large reservoir of blood (8%-10% total circulation)
that makes it ideal for radiating heat. The skin removes ex-
cess body heat through radiation and by evaporating sweat.
It can also conserve heat by restricting blood flow to the
• Throughout the skin, specialized nerve endings provide cu-
taneous sensations of touch, pressure, pain, heat, tickling,
stretching, and so on. The skin also makes vitamin D upon
exposure to sunlight. The vitamin D gets processed else-
where and ultimately stimulates calcium and phosphorus
absorption from the intestine, so the skin plays an important
role in calcium homeostasis.
• Within limits, the skin can repair cuts and abrasions. In epi-
dermal wound healing, basal cells migrate into the wound
area and begin to enlarge and grow, while adjacent stem
cells divide to fill in the gaps. In deep wound healing, as
shown, blood clots form to seal the wound, immune cells
migrate into the area to fight bacteria and ingest foreign
material, fibroblasts emerge to lay down collagen fibers that
will form scar tissue, and epithelial cells move in to repair
the epidermis. Eventually, the clot becomes a covering scab
that sloughs off, the epidermis gets reformed, and tougher
scar tissue fills the gap.
Deep wound healing • Figure 4.7
Blood clot forms to seal the wound.
^ Clot becomes
fills the gap.