NORMAL STRUCTURE AND FUNCTION
OF THE HAIR FOLLICLE
APPARATUS
The pilosebaceous unit is a complex apparatus that comprises a hair shaft and its follicle, sebaceous glands, arrector pili muscle, and, in some regions of the body, apocrine glands. Hair is a complex structure that is made of many different keratin proteins linked by disulfide bonds between neighboring cysteine amino acid molecules. The keratin molecules come in acidic and basic forms. An acidic keratin fiber localizes with a basic keratin fiber and cross-links via disulfide bonds.
The
exact function of hair is unknown, but it is theorized to act as an insulator
for heat retention and has been postulated to be important to attract a mate.
No matter what the function, humans can live a normal life without the presence
of hair with no ill effects.
Hair
comes in a variety of colors. The amount of melanin or pheomelanin in the hair
shaft determines the exact color of the hair. With time, the production of hair
pigment decreases, and the hair becomes dull gray or white. This process is
unpredictable in a given individual, and even those within the same family may
show striking differences in hair color change. As people age, the scalp hair
usually tends to thin. This is considered to be a normal physiological process.
There
are two main types of hair in the adult. Terminal hair is thick hair that is
present on the scalp, axilla, and groin and in the beard region in men. Vellus
hair is the fine, thin, lightly pigmented hair that can be found in most areas
of the body where terminal hair is not present. No hair is present on the lips,
palms, soles, glans, or labia minora. Lanugo hair is present during fetal
development and is predominantly seen in premature infants. This type of hair
is shed in utero and replaced with vellus hair before delivery. Reversion of
vellus hair and terminal hair back to lanugo hair is a sign of anorexia
nervosa. Lanugo hair has a soft, fluffy white appearance.
The
hair cycle is an extremely complex and highly coordinated process. The anagen
phase is the growth phase. The anagen phase of the typical adult scalp hair
lasts approximately 2 years. This growth phase is followed by the catagen
phase, which is a short (2 week) transition period during which the hair
follicle trans-forms from a growing, functioning hair into a club hair. This is
followed by the telogen phase, which lasts approximately 2 months and ends with
shedding of the club hair. Anagen hairs have a floppy, pigmented end that is
easily distinguished from the telogen hair. Telogen hair is termed club hair
because of its depigmented bulb at the proximal end. Catagen hairs are
almost impossible to identify because they appear somewhere in the spectrum
between anagen and telogen hair. The length of the anagen phase is responsible
for the overall length of the hair: The longer the anagen phase, the longer the
hair can grow. This process is preprogrammed and is different for all hair
types on the body. The normal scalp can shed up to 100 hairs per day. The hair
follicle is remarkably capable of regeneration after the hair has entered into
telogen phase. An unknown signal causes the hair follicle stem cells, which are
located in the bulge region, to differentiate and begin producing another hair,
restarting the anagen phase. The bulge region is an area found in approximation
to the inser ion of the arrector pili muscle into the hair follicle.
Histological examination of a cross section of a terminal hair shaft reveals a complex architecture. The hair is made up of various concentric layers. The inner-most layer is the medulla, which is pigmented. The next layer is the cortex, followed by the cuticle, the inner root sheath (Huxley’s and Henley’s layers), and the outer root sheath. The outer root sheath seamlessly blends into the epidermis. The hair follicle undergoes trichohyalin keratinization, which is different from the keratohyalin keratinization of the epidermis.
