Structures of the Integumentary System
SKIN SEBACEOUS GLANDS
epidermis sebaceous
dermis ducts
subcutaneous layer NAILS
SWEAT GLANDS cuticle
eccrine sweat glands nail
apocrine sweat glands nail bed
HAIR matrix
follicle (nail root)
shaft
Functions of the Integumentary System
The integument, from the Latin word for “cloak,”
covers the body. Its structures—the SKIN, HAIR, and
NAILS—form the image the body presents to the
outside world. Its functions—protective barrier,
tactile perception, temperature regulation, IMMUNE
RESPONSE—enable the body to survive in that out-
side world.
The foundation of the integumentary system is
the skin, which, as the body’s largest organ,
accounts for 15 percent of the body’s weight. It
sheaths the body in protective insulation from
scalp to sole, coating every stretch and fold
between. The skin’s three layers—epidermis, der-
mis, and subcutaneous layer—form the interface
between the body’s internal and external environ-
ments. The endless exchange of information is so
wearing that the skin completely replaces its out-
ermost layer, the epidermis (about 36,000 square
inches of surface area), every three to four weeks.
What looks remarkably the same from day to day
is, in reality, always changing.
In the beginning Attesting to the skin’s impor-
tance for survival and function, the skin and
the BRAINare the first two distinctive organs to
emerge during embryonic development. The cells
of each arise from the primitive neural crest, also
called the neuroectoderm. By three weeks of ges-
tational age the neural crest differentiates. The
cells that migrate inward become NERVE cells,
forming the brain and SPINAL CORD. The cells that
migrate outward become the two major cell types
of cells that form the skin: keratinocytes and
melanocytes.
By seven weeks gestational age the skin devel-
ops hair follicles that, six weeks later, begin to
cover the head with hair. At 20 weeks gestational
age that hair coat, called lanugo, has spread to
cover the entire body. Some babies, especially
those born prematurely, still sport this coat at
birth, which is often disconcerting to anxious par-
ents but quickly falls away. The formation and
function of the sebaceous glands parallels that of
the hair follicles. As hair begins to sprout, the
sebaceous glands secrete a thick, ointmentlike
precursor to sebum, called vernix, that covers the
skin’s surface. Vernix establishes a waterproof bar-
rier that protects the skin as the FETUSfloats in
AMNIOTIC FLUID. Also by 20 weeks the skin contains
SWEAT GLANDS, eyelashes and eyebrows, fingernails
and toenails, and the unique surface ridges on the
fingertips that will become fingerprints.THE INTEGUMENTARY SYSTEM
The integumentary system encloses the body, protecting it from, as well as allowing its interactions with, the external
environment. Physician specialists who treat conditions of the SKIN, HAIR, and NAILSare dermatologists. This section,
“The Integumentary System,” presents an overview of the structures and functions of the integumentary system, a dis-
cussion of dermatological health and disorders, and entries about the health conditions that can affect the skin, hair,
and nails.
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