Chapter 5
The Integumentary System
157
5
the head against physical trauma, heat loss, and sunlight. (Pity
the bald man.) Eyelashes shield the eyes, and nose hairs filter
large particles like lint and insects from the air we inhale.
Structure of a Hair
Hairs
, or
pili
(pi
9
li), are flexible strands produced by hair folli-
cles and consist largely of dead, keratinized cells. Te
hard kera-
tin
that dominates hairs and nails has two advantages over the
sof keratin
found in typical epidermal cells: (1) It is tougher and
more durable, and (2) its individual cells do not flake off.
Te chief regions of a hair are the
shaf
, the portion in which
keratinization is complete, and the
root
, where keratinization is
still ongoing. Te shaF, which projects from the skin, extends
about halfway down the portion of the hair embedded in the
skin
(Figure 5.5)
. Te root is the remainder of the hair deep
within the follicle. If the shaF is flat and ribbonlike in cross sec-
tion, the hair is kinky; if it is oval, the hair is silky and wavy; if
it is perfectly round, the hair is straight and tends to be coarse.
A hair has three concentric layers of keratinized cells: the
medulla, cortex, and cuticle (±igure 5.5a, b).
Te
medulla
(mĕ-dul
9
ah; “middle”), its central core, consists of
large cells and air spaces. Te medulla, which is the only part
of the hair that contains soF keratin, is absent in fine hairs.
Te
cortex
, a bulky layer surrounding the medulla, consists
of several layers of flattened cells.
Te outermost
cuticle
is formed from a single layer of cells
overlapping one another like shingles on a roof. Tis ar-
rangement helps separate neighboring hairs so the hair does
not mat. (Hair conditioners smooth out the rough surface of
the cuticle and make our hair look shiny.) Te most heavily
keratinized part of the hair, the cuticle provides strength and
helps keep the inner layers tightly compacted.
Because it is subjected to the most abrasion, the cuticle tends
to wear away at the tip of the hair shaF, allowing keratin fibrils
in the cortex and medulla to frizz out, creating “split ends.”
Hair pigment is made by melanocytes at the base of the hair
follicle and transferred to the cortical cells. Various proportions
of melanins of different colors (yellow, rust, brown, and black)
combine to produce hair color from blond to pitch black. Ad-
ditionally, red hair is colored by the iron-containing pigment
trichosiderin
. When melanin production decreases (mediated
by delayed-action genes) and air bubbles replace melanin in the
hair shaF, hair turns gray or white.
Structure of a Hair Follicle
Hair follicles
(
Folli
5
bag) fold down from the epidermal sur-
face into the dermis. In the scalp, they may even extend into the
hypodermis. Te deep end of the follicle, located about 4 mm
(1/6 in.) below the skin surface, expands to form a
hair bulb
(±igure 5.5c, d). A knot of sensory nerve endings called a
hair
follicle receptor
, or
root hair plexus
, wraps around each hair
bulb (see ±igure 5.1). Bending the hair stimulates these endings.
Consequently, our hairs act as sensitive touch receptors.
±eel the tickle as you run your hand over the hairs on your
forearm.
Bronzing
: A bronze, almost metallic appearance of the skin
is a sign of Addison’s disease, in which the adrenal cortex
produces inadequate amounts of its steroid hormones; or a
sign of pituitary gland tumors that inappropriately secrete
melanocyte-stimulating hormone (MSH).
Black-and-blue marks
, or
bruises
: Black-and-blue marks re-
veal where blood escaped from the circulation and clotted
beneath the skin. Such clotted blood masses are called he-
matomas (he
0
mah-to
9
mah; “blood swelling”).
Check Your Understanding
8.
Melanin and carotene are two pigments that contribute to
skin color. What is the third and where is it found?
9.
What is cyanosis and what does it indicate?
10.
Which alteration in skin color may indicate a liver disorder?
For answers, see Appendix H.
Appendages of the Skin
List the parts of a hair follicle and explain the function
of each part. Also describe the functional relationship of
arrector pili muscles to the hair follicles.
Name the regions of a hair and explain the basis of hair
color. Describe the distribution, growth, replacement, and
changing nature of hair during the life span.
Describe the structure of nails.
Compare the structure and locations of sweat and oil
glands. Also compare the composition and functions of
their secretions.
Compare and contrast eccrine and apocrine glands.
Along with the skin itself, the integumentary system includes
several derivatives of the epidermis. Tese
skin appendages
include hair and hair follicles, nails, sweat glands, and seba-
ceous (oil) glands. Each plays a unique role in maintaining body
homeostasis.
A key step in beginning to form any of the skin’s appendages
is formation of an epithelial bud. Tis process is stimulated by
a reduced production of cell adhesion factor (cadherin). Once
the cell-to-cell attractions are broken, the cells can move about
and rearrange themselves, allowing an epithelial bud to form.
Hairs and Hair Follicles
Hair is an important part of our body image—consider, for
example, the spiky hair style of punk rockers and the flowing,
glossy manes of some high-fashion models. Millions of hairs
are distributed over our entire skin surface except our palms,
soles, lips, nipples, and parts of the external genitalia (the head
of the penis, for instance). Although hair helps to keep other
mammals warm, our sparse body hair is far less luxuriant
and useful. Its main function in humans is to sense insects on
the skin before they bite or sting us. Hair on the scalp guards
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