Process through which human hair grows
The
growth of human hair
occurs everywhere on the body except for the soles of the feet, the palms of the hands, the inside of the mouth, the lips, the backs of the ears, some external
genital
areas, the
navel
, and, apart from
eyelashes
, the
eyelids
.
[1]
Hair is a
stratified squamous
keratinized
epithelium
made of multi-layered flat cells whose rope-like
filaments
provide structure and strength to the
hair shaft
. The protein called
keratin
makes up hair and stimulates hair growth. Hair follows a specific growth cycle with three distinct and concurrent phases:
anagen
,
catagen
, and
telogen
. Each phase has specific characteristics that determine the length of the hair.
The body has different types of hair, including
vellus hair
and
androgenic hair
, each with its own type of cellular construction. This varied construction gives the hair unique characteristics, serving specific purposes, mainly warmth (redundant in modern humans) and physical protection.
[2]
Most humans develop the longest thickest hair on their scalps and (mostly observed in males) faces. This hair will usually grow to several feet before terminating, but many humans develop much longer hair.
Growth cycle
[
edit
]
The three stages of hair growth are the
anagen
,
catagen
, and
telogen
phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The growth rate of hair varies from individual to individual depending on their age, genetic predisposition and a number of environmental factors. It is commonly stated that hair grows about 1 cm per month on average; however reality is more complex, since not all hair grows at once. Scalp hair was reported to grow between 0.6 cm and 3.36 cm per month. The growth rate of scalp hair somewhat depends on age (hair tends to grow more slowly with age), sex, and ethnicity.
[3]
Thicker hair (>60 μm) grows generally faster (11.4 mm per month) than thinner (20?30 μm) hair (7.6 mm per month).
[4]
It was previously thought that Caucasian hair grew faster than Asian hair and that women's grew faster than men's;
[3]
but newer research shows that the sexes do not differ significantly in speed of hair growth
[5]
and that the hair of Chinese people grows faster than that of French Caucasians and West and Central Africans.
[6]
The quantity of hairs depends on hair colour (before graying):
[7]
[8]
an average
blond-haired
person has 150,000 hairs, a
brown-haired
person has 110,000, a
black-haired
person has 100,000, and a
redhead
has 90,000.
[9]
Hair growth stops at death; the illusion of growth after death is due to shrinkage of the skin by drying.
[10]
Anagen phase
[
edit
]
The anagen phase, known as the growth phase, is when the hair grows about 1 cm per month.
[11]
It begins in the
papilla
and can last from three to five years.
[12]
The span at which the hair remains in this stage of growth is determined by genetics. The longer the hair stays in the anagen phase, the longer it will grow. During this phase, cells neighboring the
papilla
in a germinative layer divide to produce new hair fibers,
[13]
and the follicle buries itself into the
dermal
layer of the skin to nourish the strand. About 85%–90% of the hairs on one's head are in the anagen phase at any given time.
Catagen phase
[
edit
]
The catagen phase, or the transitional phase, allows the follicle to renew itself (in a sense). During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests," cutting the hair strand off from its nourishing blood supply. Signals sent out by the body (that only selectively affect 1 percent of all hair of one's body at any given time) determine the end of melanin production in the hair bulb and apoptosis of follicular melanocytes.
[14]
Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward.
Growth inhibitors and disorders
[
edit
]
In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet. Exceptions to this rule can be observed in individuals with hair development abnormalities, which may cause an unusual length of hair growth.
[15]
[
better source needed
]
Chemotherapy
[
edit
]
Most
chemotherapy
drugs work by attacking rapidly dividing cells. Rapid cell replication is one of the hallmarks of cancer; however, hair follicle cells also grow and divide quickly. Consequently, the chemotherapy drugs usually inhibit hair growth.
[16]
The dose and type of medicine will determine the severity of hair loss. Once the course of chemotherapy has ended, new hair growth may begin after three to 10 weeks.
Hair loss
[
edit
]
Alopecia
is a hair loss disease that can occur in anyone at any stage of life.
[17]
[18]
Specifically
alopecia areata
is an
autoimmune disease
that causes hair to spontaneously fall out. It is mainly characterized by bald patches on the scalp or other parts of the body, and can ultimately cause baldness across the entire body. This disease interferes with the hair growth cycle by causing a follicle to prematurely leave the
anagen
, or active growth, phase and enter the resting, or
telogen
, phase. The hair growth in the affected follicles is lessened or stopped completely.
Primary Cicatricial Alopecias (PCAs), commonly referred to as scarring alopecias, are a collective group of inflammatory disorders with distinctive pathological features that include the development of fibrosis, permanent destruction of the pilosebaceous unit of the hair follicle (HF), and irreversible hair loss.
[19]
Traction alopecia
is caused by adding too much strain on the hair on one's head. Tight ponytails and other styles that require added tension to the hair are often what cause this disease. It can also occur on the face in areas where the hair is often styled. Plucking or waxing one's eyebrows frequently, for example, can yield suppressed hair growth in the area.
On the scalp, the hair is usually known to be lost around the hair line, leaving the densest amount of hair at the crown. Small
vellus hair
will often replace the hair that is lost.
Radiation therapy to the head
[
edit
]
Human hair follicles are very sensitive to the effects of
radiation therapy
administered to the head, most commonly used to treat cancerous growths within the brain. Hair shedding may start as soon as two weeks after the first dose of radiation and will continue for a couple of weeks. Hair follicles typically enter the
telogen
phase, and regrowth should commence 2.5 to 3 months after the hair begins to shed. Regrowth may be sparser after treatment.
UV-B
[
edit
]
Ultraviolet light levels of either 20 or 50?mJ?cm
?2
in the
UV-B
range have been shown to inhibit hair growth, reduce hair
melanin
and damage
hair follicles
.
[20]
See also
[
edit
]
References
[
edit
]
- ^
Buffoli, Barbara; Rinaldi, Fabio; Labanca, Mauro; Sorbellini, Elisabetta; Trink, Anna; Guanziroli, Elena; Rezzani, Rita; Rodella, Luigi F. (2014).
"The human hair: from anatomy to physiology"
.
International Journal of Dermatology
.
53
(3): 331?341.
doi
:
10.1111/ijd.12362
.
PMID
24372228
.
S2CID
1310059
.
- ^
Schneider, Marlon R.; Schmidt-Ullrich, Ruth; Paus, Ralf (2009-02-10).
"The hair follicle as a dynamic miniorgan"
.
Current Biology
.
19
(3): R132?142.
doi
:
10.1016/j.cub.2008.12.005
.
ISSN
1879-0445
.
PMID
19211055
.
- ^
a
b
Harkey, M. R. (December 1993). "Anatomy and physiology of hair".
Forensic Science International
. Hair Analysis as a Diagnostic Tool for Drugs of Abuse Investigation.
63
(1): 9?18.
doi
:
10.1016/0379-0738(93)90255-9
.
ISSN
0379-0738
.
PMID
8138238
.
- ^
Van Neste DJ, Rushton DH (2016).
"Gender differences in scalp hair growth rates are maintained but reduced in pattern hair loss compared to controls"
.
Skin Res Technol
.
22
(3): 363?9.
doi
:
10.1111/srt.12274
.
PMID
26526232
.
S2CID
19060270
.
- ^
Joseph Castro (January 27, 2014).
"How Fast Does Hair Grow?"
.
Live Science
. Retrieved
July 3,
2020
.
- ^
Loussouarn, Genevieve; El Rawadi, Charles; Genain, Gilles (2005).
"Diversity of hair growth profiles"
.
International Journal of Dermatology
.
44
(s1): 6?9.
doi
:
10.1111/j.1365-4632.2005.02800.x
.
PMID
16187948
.
S2CID
39103960
.
- ^
"How many hairs are on a human head in total and per square inch?"
.
curlcentric.com
. Retrieved
April 18,
2022
.
- ^
"Number of hairs on human head"
.
harvard.edu
. Archived from
the original
on 14 December 2021
. Retrieved
April 18,
2022
.
- ^
"How much hair does a human have on their head?"
.
mevolife.com
. Retrieved
April 18,
2022
.
- ^
"Do your hair and fingernails grow after death?"
.
bbc.com
. Retrieved
April 18,
2022
.
- ^
Ruszczak, Zbigniew (2012).
"Hair Disorders and Alopecia"
. In Elzouki, Abdelaziz Y.; Harfi, Harb A.; Nazer, Hisham M.; Stapleton, F. Bruder; Oh, William; Whitley, Richard J. (eds.).
Textbook of Clinical Pediatrics
. pp. 1489?508.
doi
:
10.1007/978-3-642-02202-9_146
.
ISBN
978-3-642-02201-2
.
- ^
Braun-Falco, Otto (2000).
Dermatology
(2., completely rev. ed.). Berlin: Springer. p. 1101.
ISBN
9783540594529
.
- ^
Nicolas, Jean-Francois; Sequeira, Ines (15 October 2012).
"Redefining the structure of the hair follicle by 3D clonal analysis"
.
Development
.
139
(20): 3741?3751.
doi
:
10.1242/dev.081091
.
ISSN
0950-1991
.
PMID
22991440
.
- ^
Araujo, Rita; Fernandes, Margarida; Cavaco-Paulo, Artur; Gomes, Andreia (2010). "Biology of Human Hair: Know Your Hair to Control It".
Biofunctionalization of Polymers and their Applications
. Advances in Biochemical Engineering/Biotechnology. Vol. 125. pp. 121?43.
doi
:
10.1007/10_2010_88
.
hdl
:
1822/15299
.
ISBN
978-3-642-21948-1
.
PMID
21072698
.
S2CID
21502785
.
- ^
"Natural Hair Growth"
.
Sunday, 11 April 2021
- ^
Grevelman, E. G.; Breed, W. P. M. (2005-03-01).
"Prevention of chemotherapy-induced hair loss by scalp cooling"
.
Annals of Oncology
.
16
(3): 352?358.
doi
:
10.1093/annonc/mdi088
.
ISSN
0923-7534
.
PMID
15642703
.
- ^
Erjavec SO, Gelfman S, Abdelaziz AR, Lee EY, Monga I, Alkelai A, Ionita-Laza I, Petukhova L, Christiano AM (Feb 2022).
"Whole exome sequencing in Alopecia Areata identifies rare variants in KRT82"
.
Nat Commun
.
13
(1): 800.
doi
:
10.1038/s41467-022-28343-3
.
PMC
8831607
.
PMID
35145093
.
- ^
"Hair loss (alopecia) - NHS Choices"
. 2013-09-27. Archived from
the original
on 2013-09-27
. Retrieved
2021-11-26
.
- ^
Wang EHC, Monga I, Sallee BN, Chen JC, Abdelaziz AR, Perez-Lorenzo R, Bordone LA, Christiano AM (Jul 2022).
"Primary cicatricial alopecias are characterized by dysregulation of shared gene expression pathways"
.
PNAS Nexus
.
1
(3): pgac111.
doi
:
10.1093/pnasnexus/pgac111
.
PMC
9308563
.
PMID
35899069
.
- ^
Lu, Zhongfa; Fischer, Tobias W; Hasse, Sybille; Sugawara, Koji; Kamenisch, York; Krengel, Sven; Funk, Wolfgang; Berneburg, Mark; Paus, Ralf (2009).
"Profiling the Response of Human Hair Follicles to Ultraviolet Radiation"
.
Journal of Investigative Dermatology
.
129
(7): 1790?804.
doi
:
10.1038/jid.2008.418
.
PMID
19158839
.