to sunlight. UV light stimulates melanin production, which
increases the amount of melanin that is deposited into the
keratinocytes. Melanin gives the skin a tanned appearance
and improves protection of the body against UV radiation. A
tan “fades,” or is lost, when the melanin-containing keratin-
ocytes are shed from the stratum corneum. Despite the
protection provided by melanin, repeated or excessive ex-
posure to UV radiation can lead to skin cancer.
Carotene (KAR-o-ten;
carot
= carrot) is a yellow-
orange pigment that gives egg yolk and carrots their
color. This precursor of vitamin A accumulates in the
stratum corneum, fatty areas of the dermis, and subcu-
taneous layer in response to excessive dietary intake. In
fact, so much carotene may be deposited in the skin after
eating large amounts of carotene-rich foods that the skin
color may actually turn orange, which is especially appar-
ent in light-skinned individuals. Decreasing carotene in-
take eliminates the problem.
CONCEPT CHECK
1.
What
are the main components of the integu-
mentary system?
2.
How
is thick skin different from thin skin?
3.
What
is the main pigment that determines skin
color?
Accessory Structures Provide Protection and
Help Regulate Body Temperature
LEARNING OBJECTIVES
1.
Describe
the structure and function of hair.
2.
Identify
the glands found in skin and describe
what they do.
3.
Describe
the structure and function of nails.
air, glands, and nails are accessory structures
of the integumentary system that develop
from the epidermis of the embryo. Each of
these accessory structures performs impor-
tant functions in the body. For example, hair and nails
protect the body, and sweat glands help regulate body tem-
perature. Let’s take a look at hair first.
Hair Protects the Skin and Other
Structures of the Body
Hairs, or pili (Pl-li), are present on most skin surfaces ex-
cept the palms, palmar surfaces of the fingers, soles, and
plantar surfaces of the toes. In adults, hair usually is most
heavily distributed across the scalp, over the brows of the
eyes, and around the external genitalia. The thickness and
pattern of distribution of hair is largely determined by ge-
netic and hormonal influences. Hair on the head guards
the scalp from injury and the sun’s rays; eyebrows and eye-
lashes protect the eyes from foreign particles; and hair in
the nostrils filters insects and foreign particles to protect
the tissues of the respiratory system.
96
CHAPTER 4
The Integumentary System
Each hair is a thread of fused, dead, keratinized epi-
dermal cells that consists of the following (Figure 4.3):
The shaft is the superficial portion that projects above
the surface of the skin.
The
root
is
the
portion
below
the
surface
that
penetrates into the dermis and sometimes into the
subcutaneous layer.
The hair follicle surrounds the root and is composed
of epidermal cells.
Hair root plexuses are nerve endings that surround
each hair follicle. They are sensitive to touch and are
stimulated if a hair shaft is moved.
The base of each follicle is enlarged into an onion-
shaped structure called the bulb. In the bulb is a nipple-
shaped indentation referred to as the papilla of the hair.
The papilla of the hair contains many blood vessels and
provides nourishment for the growing hair. The bulb also
contains a region of cells called the hair matrix, which pro-
duces new hairs by cell division, thus lengthening the hair.
Associated with each hair are sebaceous (oil) glands
and a bundle of smooth muscle cells called arrector pili
(a-REK-tor;
arrect
= to raise), which extends from the up-
per dermis to the side of the hair follicle. In its normal
position, hair emerges at an angle to the surface of the
skin. Under stress, such as cold or fright, nerves stimu-
late the arrector pili muscles to contract, which pulls the
hair shafts perpendicular to the skin surface. This action
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