Under the Microscope: Hair and Scalp Anatomy

Hair is much more complex than it looks. In this section, we
describe how hair follicles grow and alter with time and other fac-
tors. Understanding how hair follicles function (and what makes
them stop functioning) can help you maintain them, thereby slow-
ing the hair loss process.
Going beneath the skin
Most people think of hair only in terms of what they see above the
scalp, but hair is actually a rather complex organ that goes
beneath the surface. Anatomically speaking, hair is part of your
skin. But because hair is physically distinct from skin, it’s referred
to as a skin appendage. (Other skin appendages include sweat
glands, fingernails, and toenails.) See Figure 2-1.
Skin is composed of three main layers:
 The epidermis: This outer layer of the skin is less than 1 mil-
limeter thick. It’s composed of dead cells that are in a con-
stant state of sloughing and replacement. As dead cells are
lost, new ones from the growing layer below replace them.
 The dermis: This tough layer of connective tissue is about 2
to 3 millimeters thick on the scalp. This layer gives the skin its
strength and contains both sebaceous (oil) glands and sweat
glands.
Sebaceous glands produce an oily substance, which creates a
plug of wax (sebum) to cover the opening to the growing hair
follicle. As the hair grows upward from the skin surface, some
of the waxy substance is taken up by the hair shaft as a lubri-
cant, giving the hair a waxy sheen.
Sweat glands help control body temperature, particularly
when it’s hot. These glands produce a watery, salty sweat; as
the sweat evaporates, body heat is lost.
 Subcutaneous fat and connective tissue: This layer contains
the larger sensory nerve branches and the blood vessels that
nourish the skin. In the scalp, the lower portions of the hair
follicles (called the bulbs) are found in the upper part of this
fatty layer.
Dissecting a hair follicle
The hair follicle must function properly in order to maintain a
healthy head of hair. A hair follicle is a complex structure that
measures about 4 to 6 millimeters in length. Each follicle produces
one to four hair shafts, each about 0.1 millimeters in width (in
other words, these are really, really tiny structures).
The layers of a hair follicle
Hair follicles have three layers surrounding the strand of hair,
which you can see in Figure 2-2:
 The outer root sheath, or trichelemma: This is the outer
layer, which surrounds the follicle in the dermis and then
blends into the epidermis on the surface of the skin, forming
the pore from which the hair grows.
 The inner root sheath: This middle layer is composed of
three parts, with the cuticle being the innermost portion that
touches the strand of hair. The cuticle of the inner root sheath
interlocks with the hair cuticle (described in “the layers of the
hair shaft” section) to give it rigidity.
 The bulb: This is the lower portion of each hair follicle. It con-
tains the inner matrix cells, which produce bundles (also
called spindles) of hair cells that look like fine wires in an elec-
tric cord. These bundles are actually made up of even smaller
bundles, which literally twist as they’re made. The size of the
bulb and the number of matrix cells determine the width of
the fully grown hair.
The layers of the hair shaft
The hair shaft is composed of three layers.
 The cuticle: This layer forms the surface of the hair and is
what you see as the hair shaft emerges from the follicle.
 The cortex: This middle layer comprises the bulk of the hair
shaft and is what gives hair its strength. It’s composed of an
organic protein called keratin, the same material that com-
prises rhinoceros horns and deer antlers.
 The medullas: This is the center, or core, of the hair shaft,
and it’s only present in terminal hair follicles (a fully grown
hair).
The dermal papillae
At the bottom of the hair follicle, there is a bulbous (bulb) portion
which contains a small collection of specialized cells called the
dermal papillae. Scientists believe these cells are at least partially
responsible for determining. how long the hair will eventually
grow, how thick it will be, and the character of the hair will be.
For many years, scientists thought that hair actually grew from the
dermal papillae. Recent evidence shows that the growth center is
not totally controlled from the dermal papillae. Elements that con-
trol hair growth can be found all the way up to the region of the fol-
licle where the sebaceous glands are attached.
If the dermal papillae is removed (which sometimes happens
during a hair transplant (see Chapter 13), the hair follicle may still
be able to regenerate new hair, although the new hair may not be a
characteristically healthy looking terminal hair. It may be shorter,
thinner, and more kinky or wavy.