C h a p t e r 4 5
Structure and Function of the Integumentum
1147
the secretion of apocrine glands is oily. In many animals,
apocrine secretions give rise to distinctive odors that
enable animals to mark their territory and attract a mate.
In humans, apocrine secretions are sterile and odorless
until mixed with the bacteria on the skin surface; they
then produce what is commonly known as “body odor.”
The sebaceous glands are located over the entire skin
surface except for the palms, soles, and sides of the
feet. They are part of what is called the
pilosebaceous
unit
, with their ducts opening into the upper third of
the hair follicle. They secrete
sebum
, a waxlike mixture
of triglycerides, cholesterol, and cellular debris. Sebum
lubricates the hair and skin, prevents undue evapora-
tion of moisture from the stratum corneum during cold
weather, and helps to conserve body heat.
Sebum production is an example of holocrine secre-
tion. The cytoplasm of the glandular epithelial cell
produces and becomes filled with the fatty product.
Eventually, the cell’s plasma membrane ruptures, and
the cell dies. Both the secretory product and the cell frag-
ments, which together constitute sebum, are then dis-
charged from the gland into the hair follicle. New cells
are produced by mitosis of the basal cells in the gland.
Sebum production is under the control of genetic and
hormonal influences. Sebaceous glands are relatively
small and inactive until individuals approach adoles-
cence. The glands then enlarge, stimulated by the rise in
sex hormones. Gland size directly influences the amount
of sebum produced, and the level of androgens influ-
ences gland size. The sebaceous glands are the structures
that become inflamed in acne (see Chapter 46).
Hair and Hair Follicles
Hair is a filamentous, keratinized structure that consists
of the hair follicle, sebaceous gland, hair muscle (arrector
pili), and, in some instances, an apocrine gland (Fig. 45-6).
Most hair follicles are associated with sebaceous glands,
and these structures combine to form the pilosebaceous
unit. The arrector pili muscle, located under the sebaceous
gland, provides a thermoregulatory function by contract-
ing to cause goose bumps, thereby reducing the skin sur-
face area that is available for the dissipation of body heat.
Hair color is attributable to the melanin pigments that
the hair contains. Variations mainly reflect the quantity
and ratio of the black to dark brown pigment eumelanin
and the reddish-brown pigment pheomelanin.
The hair follicle is divided into three parts: the infun-
dibulum, which extends from the surface opening to the
level of the opening for the sebaceous gland; the isth-
mus, which extends from the infundibulum to the level
of the arrector pili muscle; and the expanded inferior
hair bulb, which is indented to conform to the shape
of the dermal papilla occupying it. The dermal papilla
contains a rich supply of nutrients and oxygen for the
cells in the hair follicle.
Growth of the hair is centered in the bulb (i.e., base)
of the hair follicle, and the hair undergoes changes as
it is pushed outward. Hair growth goes through three
cyclic phases: the anagen (growth), catagen (atrophy),
and telogen (resting) phases. Anagen hair has long inner
roots and outer root sheaths, is deeply rooted in the der-
mis, is difficult to detach, and does not come out with
regular brushing.
Anagen
follicles are actively replicating
and therefore especially susceptible to nutritional defi-
ciencies and metabolic insults. The
catagen
phase repre-
sents degeneration of the root structure and migration of
dermal papillae and the follicular unit toward the more
superficial layers of the dermis. Catagen hair usually
represents approximately 1% of all scalp hairs.
Telogen
hair has short, white, club-shaped roots. With forma-
tion of new anagen hair below the root, the developing
follicle will eventually replace the telogen hair, leading
to the shedding of approximately 50 to 100 hairs a day.
Nails
Nails are hardened keratinized plates that protect the fin-
gers and toes and enhance dexterity. The nails grow out
from a curved transverse groove called the
nail groove.
The floor of this groove, called the
nail matrix,
is the ger-
minal region of the nail plate (Fig. 45-7). The underlying
epidermis, attached to the nail plate, is called the
nail bed.
Nail
plate
Nail
bed
Lunula Nail
matrix
Eponychium
Nail groove
FIGURE 45-7.
Parts of a fingernail.
Epidermis
Dermis
Hair shaft
Arrector pili muscle
Sebaceous
gland
Hair
follicle
Dermal
papilla
Keratinized
cells
Dermal
blood
vessels
FIGURE 45-6.
Parts of a hair follicle.
