Hair Biology
Growth and Pigmentation
Andrea M. Park, MDa,*, Sajjad Khan, MDb, Jeffrey Rawnsley, MDa
KEYWORDS
Hair growth cycle Hair pigmentation Gray hair Hair biology Minoxidil Finasteride
Dutasteride PRP
KEY POINTS
The 3 phases of the hair growth cycle are anagen, catagen, and telogen.
Topical minoxidil opens potassium channels and increases the diameter of existing hairs.
Oral finasteride is a Food and Drug Administration–approved type 2 5a-reductase inhibitor that af-
fects the follicle, size, and amount of hair.
Dutasteride is a type 1 and type 2 5a-reductase inhibitor used off-label to treat hair loss.
INTRODUCTION pigmentation as well as briefly reviewing the
various biologic modifiers most commonly used.
For both men and women, having healthy hair de-
notes health, youth, and vitality. In mammals, hair ANATOMY OF THE HAIR FOLLICLE
serves a protective and evolutionary function.
Although hair in humans may not be important Hair can grow individually, in groups of 2 to 3, or
for skin barrier protection from a biologic perspec- even at times in groups of 4 to 5. These groups
tive, hair and pigment, or lack thereof, can have a are known as follicular units. Each individual hair
significant impact on perceived social relevance shaft in the growth phase is composed of 3 main
as well emotional and psychological health. Ac- concentric regions: the medulla, cortex, and
cording to the American Hair Loss Association, cuticle. The medulla comprises the innermost
more than $3.5 billion are spent each year by layer and is formed from transparent cells and air
both men and women in the United States to treat spaces that vary among different hair types. It is
hair loss. The practice of hair coloring has been often difficult to identify on light microscopy and
documented since 1500 BC, and interest in hair at times may be entirely absent. The cells
restoration has not waned. The development of comprising the medulla contain glycogen-rich
synthetic dyes for hair can be traced to the vacuoles and medullary granules, which contain
1860s with the discovery of reactivity of p-phenyl- citrulline. The middle layer is called the cortex
enediamine with air. Today, even with the advent and is the business center of the hair shaft. The
of follicular unit extraction and various synthetic cortex is what comprises the bulk and lends the
hair pigmentation regimens available to rejuvenate mechanical strength to the hair shaft; it is
scalp hair, there remains a great demand for a comprised of a highly structured protein, keratin,
product that could potentially halt, slow, or even which is organized filaments made up of long, he-
reverse hair senescence. This article reviews the lical strands. The cells keratinize without forming
anatomy and physiology of hair growth and granules through a process known as trichilemmal
facialplastic.theclinics.com
Disclosure: None.
a
Department of Otolaryngology Head and Neck Surgery, Division of Facial Plastic and Reconstructive Surgery,
UCLA, 200 UCLA Medical Plaza, Suite 550, Los Angeles, CA 90095, USA; b Medcare Hospital, Dubai, United
Arab Emirates
* Corresponding author. 221 West Pueblo Street, Santa Barbara, CA 93105.
E-mail address: apstl17@gmail.com
Facial Plast Surg Clin N Am 26 (2018) 415–424
https://doi.org/10.1016/j.fsc.2018.06.003
1064-7406/18/Ó 2018 Elsevier Inc. All rights reserved.
