1. Structure of Hair follicle
DVL PG FIRST YEAR
• Hair is a miniature,autonomous organ with a
reservoir of pluripotent,self regenerating stem
• The human hair follicle is a component of
pilosebaceous unit, comprises of arector pilli
muscle, the sebaceous gland and the Hair
3. • HF is a complex organ in itself, derived from
composed of more than 20 different cell
populations in total.
4. • Hair follicles grow at a slant, with the major
part of the hair developed from epithelial cells
and only the papilla developing from
mesenchymal cells and fibroblasts.
5. Development of Hair Follicles
• During embryonic development, human HFs
appear at about 8 weeks. HFs are first seen in
the regions of the eyebrows, upper lip, and
chin and the entire body is covered by fourth
• Entire initial hair population completes its
cycle including scalp hair by 22 weeks.
• By the time of birth, in humans, a full
complement of HFs is probably established.
• New HFs are never formed postnatally
6. • Nearing birth, unpigmented, delicate hair
(lanugo) are replaced by terminal hair on the
scalp, eyebrows, and eyelids.
• On the rest of the body, short, fine, and
lightly pigmented hairs called vellus hairs,
replace lanugo hairs
7. • . At puberty, the majority of these vellus hairs
in axillary and pubic regions become terminal
• In males, approximately 90% of vellus body
hair( face,chest,abdomen,arms and legs) and
in females about 35% of body hair is replaced
by terminal hair.
8. Follicular morphogenesis
• Follicular morphogenesis includes three
1. Induction of HF: Here hair placode is formed.
2.Organogenesis of HF: Hair peg is created.
3.Cytodifferentiation of HF: Bulbous hair peg is
10. Stages of hair morphogenesis
There is an aggregation of mesenchyma cells in the superficial
dermis with thickening of the overlying basement membrane.
This is the location of a new HF.
There is a formation of “hair germ” by the elongation of basal
epidermal cells downward and bulging. Meanwhile,
replication of mesenchymal cells occurs, which results in the
formation of the rudimentary dermal papilla
3.Hair peg stage
The epithelial cells grow downward forming a column or hair
peg, which propels the aggregate of mesenchyme
11. 4.Bulbous peg stage
In the hair peg column, three areas of swelling
appear. Uppermost swelling becomes apocrine
gland; Middle swelling develops into sebaceous
gland. The lowermost swelling is the prospective site
of attachment of arrector pili. Arrector pilorum
attaches to the bulge area. Epidermal cells at the
advancing end of the column surrounds a part of
underlying mesenchymal cells, finally resulting in the
12. 5.Primordial hair stage
The basal cells of the column surrounding DP
actively proliferate, which results in an early
matrix formation. This is called the first
primordial hair shaft, which then tends to move
upwards. The central cells in the follicular peg
start showing degenerative changes, the
emerging hair pushes the plug out, and a hair
canal is formed.
14. • The hair follicle is divided histologically into two
regions in relation to the insertion of the arrector pili
• 1. The upper part, which consists of the
infundibulum (from the ostium above to the opening
of the sebaceous duct below) and the isthmus (from
the entry of the sebaceous duct above to the
attachment site of the arrector pili muscle below).
15. • 2. The lower part, which comprises the stem
(from the attachment site of the arrector pili
muscle above to Adamson’s fringe, where the
keratogenous zone ends below) and the hair
bulb (from Adamson’s fringe to the base of
18. During the hair cycle, the upper follicle
remains an innocent bystander, whereas the
lower follicle undergoes repeated episodes of
regression and regeneration.
19. • The hair consists of three concentric layers:
1. The medulla, which is the central axis, consists of
two to three layers of cells containing soft keratin.
2. The cortex, which forms the bulk of the hair,
consists of cells containing hard keratin.
3. The cuticle, which is on the surface, consists of a
single layer of cells containing hard keratin
20. • The hair bulb consists of five major portions
from inside to outward: the dermal hair
papilla; the hair matrix; the hair, consisting of
the medulla, cortex, and hair cuticle; the inner
root sheath, consisting of the inner root
sheath cuticle and Huxley’s and Henle’s layers;
and the outer root sheath
23. • The stem portion consists of the hair, inner
root sheath, and outer root sheath, whereas
the isthmus region has only the hair and the
outer root sheath
(it lacks the inner root sheath).
• The infundibulum is nearly identical to the
epidermis; it is not a component of the root
24. • Hair melanin is formed by melanocytes
situated in the hair bulb; they are also present
in the basal layer of the epidermis in small
numbers, in the outer root sheath of the
lower portion of the follicle.
• The latter cells are DOPA-negative and non-
melanized and act as a melanocyte reservoir
in the skin.
25. • As with the skin, the hair color also depends
on the amount and ratio of eumelanin (black,
high molecular weight, and insoluble) and
pheomelanin (red, low molecular weight, and
soluble) and the number and size of
• Red and blonde hair follicles have spherical
melanosomes; melanosomes in brown and
black hair are ellipsoidal.
26. Hair Cycle
• The hair cycle primarily has three phases:
anagen, catagen, and telogen .
• Anagen Hair Cycle The hair cycle primarily has
three phases: anagen, catagen, and telogen
27. • Anagen is the growing phase of the hair cycle.
It is further subdivided into six substages (I–VI)
the first five are collectively called proanagen,
and the sixth stage, meta-anagen, is defined
by the emergence of the hair shaft above the
• The final length of the hair follicle depends on
this phase.Keratin 6 and Stat 3 are required for
the anagen phase.
28. • At the end of anagen phase, epithelial cell
division declines and ceases, and the follicle
enters a phase of involution known as catagen
(3 weeks). During catagen, the proximal end of
the hair shaft keratinizes to form a club-
shaped structure and the lower end of the
follicle involutes by apoptosis.
• FGF-5, EGF, neurotrophins, and TGF-β are
needed for the catagen phase.
29. • The column of epithelial cells connecting the
bulb of the follicle and the papilla shrinks
further to move the papilla upward close to
the bulb of the follicle.
• The period between completion of follicular
regression and the onset of the next anagen
phase is termed telogen. Here, the club hair
lies within the epithelial sac.
30. • The club hair is eventually shed through an
active process called exogen.
• When a hair in the telogen phase is extruded,
another hair follicle already in its anagen sixth
phase is expected to be there to replace it.
• The 3 month duration of the telogen phase
just permits the new hair to complete its
differentiation to a terminal hair.
31. • In some cases , some follicles remain empty after
the exogen phase; this novel phase of the hair
cycle is called kenogen.
• It has also been observed in prepubertal children
and patients with androgenic alopecia.
• Anagen hairs are long and pigmented with a
hooked root end and have a gelatinous root
sheath, whereas telogen hairs are short and
depigmented with a club-shaped root end and
lack a root sheath
32. • . During the hair cycle on the adult scalp, the
anagen stage lasts from 3–5 years, the catagen
stage about 3 weeks, and the telogen stage
about 3 months.
• At any time, about 85–90% scalp hairs are in
the anagen stage, less than 1% in the catagen
stage, about 13% in the telogen stage, and
about 1% of telogen hair follicles in the
33. • . The hair grows on an average about 0.4 mm
per day.Hair follicular stem cells (HFSCs)
include; the slow-cycling label-retaining cells
located in the bulge, and the primed HFSC’s in
the secondary hair germ of telogen.
• The hair germ is a cluster of cells located
between the bulge and dermal papilla at the
end of the catagen phase.
34. • The primed stem cells of the hair germ
probably originate from the bulge itself.
Towards the end of telogen, these cells
become activated, divide rapidly and initiate
anagen in response to signalling from the
• Later, at the beginning of anagen, the bulge
stem cells become activated and responsible
for the progression of anagen.
35. • The stem cells of the bulge are slow-cycling,
long-lived and multipotent cells which not
only give rise to hair follicles, but can also
regenerate the inter-follicular epidermis after
• The signals from the DP play a major role in
regulation of the hair growth by mesenchyme-
36. • The Wnt/β catenin signalling from the DP is
needed for the initiation and continuation of
the anagen phase, by activating HFSC’s.
• On the other hand, BMP signalling inhibits
• Other factors regulating the hair cycle include
signals from macrophages, adipose tissue,
cutaneous nerves and endocrine signalling
(including androgens, estrogens, prolactin)
37. Applied Aspects
• Various exogenous and endogenous
physiologic factors can modulate the hair
cycle. An example is pregnancy, which is often
accompanied by retention of an increased
number of scalp hairs in the anagen phase.
38. • Three or 4 months after the pregnancy ends, the
normal complement of resting hairs plus those
that have been temporarily retained in the
anagen phase are lost, producing a transient
• Patients on chemotherapy often have hair loss
because the drugs interfere with the mitotic
activity of the hair matrix, leading to the
formation of a thin shaft, which breaks within the