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Hair
density
 Accurate
quantitative evaluation of hair density can be difficult - certainly
time consuming. In classic studies, hair density was defined by
plucking hairs in a marked unit area of skin - primarily to define
hairs in anagen and telogen, but in passing this also enabled hair
density to be evaluated. These days density is commonly evaluated
by a phototrichogram, a close up photo of individual hair fibers
where the surface area is known and consistent between photos. Phototrichograms
are less accurate than counting hairs one by one, but they are quicker.
The average density of hair on an adult who does not have baldness
is 200-300 hair fibers per square centimeter (cm2). Variability
in this value occurs with hair color, age of the individual, location
you are observing on the scalp, and the presence of any hair loss
diseases. The crown is the region that is most densely packed with
hair follicles. In adults the sides and back of the scalp may have
a hair density as low as 150/cm2. For the crown of the scalp pre-puberty,
the density is 250-400 hairs/cm2.
Hair follicle density is a different thing altogether. Hair follicle
density refers to the density of hair follicle structures actually
in the skin. At any one time there are more hair follicles than
hair fibers as not all hair follicles contain hair fibers due to
the growth cycle of the hair follicle. So the hair follicle density
in the skin is higher than the density of hair fibers. The follicular
density (number of hair follicles/cm2) decreases with age. At birth
the follicle density has been shown in one study to be 1135/cm2,
3-12 months = 795/cm2, third decade 615/cm2, and 435/cm2 by 70-80
years of age.
Hair color can be taken as a rough guide to hair follicle density.
Redheads have about 90,000 scalp hair follicles. Black, 108,000,
while brown and blonde haired people can have up to 140,000. The
number of scalp hairs changes with ethnicity too. Far east Asians
can have as few as 80,000 scalp hair follicles.
The follicular density is very high at birth. All hair follicles
form during embryonic development. As an embryo/newborn is relatively
small so there is less surface area over which the follicles are
spread. With growth the follicles must be spread over a larger unit
area of skin. No new follicles are formed at any stage of life after
birth. Also, some hair follicles will atrophy with time, particularly
if individual has androgenetic alopecia. In addition, the figure
dervied from histology is probably an over estimate of the true
density due to the method of analysis. Hair follicle desnity is
examined by cutting skin samples into thin slices and looking at
them under a microscope. Because the angle of cut through the skin
is very often off the vertical plane of the hair follicles, more
follicles are cut through in any one histology section. Horizontal
sections also have some inherant problems as the figure derived
can change depending on how deep the histology section is cut from
the skin. Hair follicles vary in length. In practice, very small
follicles are irrelevant as they contain very fine hair fibers or
none at all.
Hair
density references
- Barman JM, Astore I, Pecoraro V. The
normal trichogram of the adult. J
Invest Dermatol. 1965;44:233-43.
- Courtois M, Loussouarn G, Hourseau C,
Grollier JF. Ageing and hair cycles.
Br J Dermatol. 1995 Jan;132(1):86-93.
- Rushton DH, de Brouwer B, de Coster W,
van Neste DJ. Comparative evaluation
of scalp hair by phototrichogram and unit area trichogram analysis
within
the same subjects. Acta Derm Venereol. 1993 Apr;73(2):150-3.
- Hayashi S, Miyamoto I, Takeda K. Measurement
of human hair growth by
optical microscopy and image analysis. Br J Dermatol. 1991 Aug;125(2):123-9.
- Uno H. The histopathology of hair loss. In:
current concepts. Upjohn, Scope Publications, Kalamazoo, 1988;
123-146.
- Pecoraro V, Astore I, Barman JM. Growth
rate and hair density of the human axilla. A. Comparative study
of normal males and females and pregnant and post-partum females.
J Invest Dermatol. 1971 May;56(5):362-5.
- Saitoh M, Uzuka M, Sakamoto M. Human
hair cycle. J Invest Dermatol. 1970
Jan;54(1):65-81
- Giacometti L. The anatomy of human scalp. Advances
in Biology of Skin. 1965; 6: 107–120.
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