Main Model
Anterior : Hair shaft
Epidermal Derivatives: Hair (Pilosebaceous Unit)
Scattered in the epidermis are the hair follicles. During development, the epidermis and dermis interact to develop sweat glands and hair follicles.
A hair follicle primordium (called the hair germ)
forms as a cell aggregate in the basal layer of the
epidermis, induced by signaling molecules derived
from fibroblasts of the dermal mesoderm. As basal
epidermal cell clusters extend into the dermis, dermal fibroblasts form a small nodule (called a dermal
papilla) under the hair germ.
The dermal papilla pushes into the core of the hair
germ, whose cells divide and differentiate to form the
keratinized hair shaft. Melanocytes present in the hair germ produce and transfer melanin into the shaft.
A bulbous swelling (called the follicular bulge) on
the side of the hair germ contains stem cells, clonogenic keratinocytes, that can migrate and regenerate
the hair shaft, the epidermis, and sebaceous gland,
forming pilosebaceous units, in response to morphogenetic signals.
The first adult hair follicle cycle starts once morphogenesis is completed about 18 days after birth,
The first hair in the human embryo is thin, unpigmented, and spaced, and is called lanugo. Lanugo is shed before birth and replaced by short colorless
hair called vellus. Terminal hair replaces vellus, which
remains in the so-called hairless parts of the skin (such
as the forehead of the adult and armpits of infants).
Hair follicles are tubular invaginations of the epidermis responsible for the growth of hair.
Hair follicles are constantly cycling between:
1. Growth (anagen) phase.
2. Regression (catagen) phase.
3. Resting (telogen) phase.
During the first 28 days of the telogen phase,
hair follicles become quiescent because of growth inhibitory signals from the dermis (mainly from bone
morphogenetic proteins). Increased Wnt/beta-catenin
signaling promotes stem cell activation to initiate
the growth of new hair during the transition from
telogen to anagen. Anagen, catagen and telogen will
sequentially continue during the life of the individual.
Each hair follicle consists of two parts:
1. The hair shaft.
2. The hair bulb.
The hair shaft is a filamentous keratinized structure
present almost all over the body surface, except on
the thick skin of the palms and soles, the sides of the
fingers and toes, the nipples, and the glans penis and
the clitoris, among others.
A cross section of the hair shaft of thick hair reveals
three concentric zones containing keratinized cells:
1. The cuticle.
2. The cortex.
3. The medulla (the last is absent in thin hair).
The hair shaft consists of hard keratin.
The hair bulb is the expanded end portion of the
invaginated hair follicle. A vascularized connective
tissue core (dermal papilla) projects into the hair
bulb, in close proximity to matrix cells.
The hair shaft is surrounded by:
1. The external root sheath, a downgrowth of the
epidermis.
2. The internal root sheath, generated by the hair
bulb (the hair matrix cells), is made up of three layers of soft keratin (which from the outside to the
inside are the Henle's layer, the Huxley's layer, and
the cuticle of the inner root sheath, adjacent to the
cuticle of the hair shaft).
The keratinization of the hair and internal root
sheath occurs in a region called the keratogenous
zone, the transition zone between maturing epidermal cells and hard keratin. The external root sheath
is not derived from the hair bulb.
The hair follicle is surrounded by a connective
tissue layer and associated with the arrector pili muscle, a bundle of smooth muscle fibers aligned at
an oblique angle to the connective tissue sheath and
the epidermis. The autonomic nervous system controls the arrector pili
muscle, which contracts during fear, strong emotions,
and cold temperature. The hairs stand up and the attachment site of the muscle bundle at the epidermis
forms a small groove, the so called goose flesh.
The hair follicle is associated with sebaceous glands
with their excretory duct connected to the lumen of
the hair follicle. When the arrector pili muscle contracts and the hair stands up, sebum is forced out of
the sebaceous gland into the lumen of the hair follicle.
The color of the hair depends on the amount and
distribution of melanin in the hair shaft. Few melanosomes are seen in blond hair. Few melanocytes
and melanin are seen in gray hair. Red hair has a
chemically distinct melanin, and melanosomes are
round rather than ellipsoid.
A structure that is not recognized in routine histologic sections of hairs is the peritrichial nerve endings
wrapped around the base of the hair follicle. The nerve
is stimulated by hair movement.