Detailed presentation of structure of hair

1. STRUCTURE OF
HAIR
DR. P ANUSHA DSOUZA
25/07/2020

2. Contents
 Introduction
 Types of hair
 Embryology of hair
 Hair structure
 Physiology of hair
 Functions of hair

3. Introduction
• Hair is a cutaneous appendage typical of mammalian
skin.
• Is a keratinized product of hair follicle
• Although it originally evolved as a protective coat, it
has lost this functional value in humans except
probably over the scalp.
• Thus, man is one of the least hairy mammals, with
most body hair being fine and delicate.

4. • Each head holds an average of 85-140 thousand
individual hairs.
• An individual normally loses 100-150 single hairs
each day.
• Hair grows approximately 15 cm every year.
• Daily hair growth rate averages about 0.4mm on
scalp.
• Each hair can grow for up to 7 years before falling.

5. Distribution
• Except for a few regions like the palmo-plantar skin,
mucocutaneous junctions, and the distal dorsal parts of
digits, hair is present all over the cutaneous surface.
• The hair density is maximum on the cheek and forehead
(about 800 hair/cm2),
• followed by the scalp, other parts of the face, trunk, and
extremities.
• The hair density is maximum at birth and gradually
decreases as the age advances

6. Types of hair
Prenatal – lanugo hair
Postnatal -According to the thickness of the shaft,
hair can be classified into
• Vellus hair (<0.03 mm)
• Terminal hair (>0.06 mm)
• Intermediate hair (0.03–0.06 mm)

7. Lanugo hair
• Lanugo (wool-like)
hairs are those that cover
the fetal skin
• Are shed at the fetal age
of 8–9 months or after
birth.
• They are thin,
unmedullated, and light
colored but longer than
vellus hair.

8. Vellus (fleece) hair
• cover most of the body
surface
• are thin, short, unmedullated,
and unpigmented,

9. Terminal hair
• are dark, thick,
medullated, longer
• and cover the scalp,
beard, moustache,
axilla, and pubis
• After Puberty –
Secondary sexual
terminal hairs develop
from vellus hair in
response to androgens

10. Intermediate hairs
• are terminal hairs whose hair bulbs are situated in
the reticular dermis rather than in the subcutis
• and also are thinner than the classical terminal
hairs.
• These are terminal hairs that are in the process of
miniaturization.

11. TERMINAL HAIR

12. Embryology of hair
• Hair germs first observed in embryos in eyebrow
and scalp region –during 3rd month of gestation
• General development of hair begins -4th month
extends in a cephalocaudal direction
• In earliest stage ,consists of an area of crowding
of deeply basophilic cells (keratinocytes) in basal
layer of epidermis (PREGERM STAGE)

13. • Areas of crowding develop into buds that
protrude into the dermis
• Beneath each bud lies a group of mesenchymal
cells from which dermal papilla is later formed
• Forms hair peg first ,then bulbous hair peg is
formed
• Differentiation occurs in lower and upper
portions of the hair follicle and overlying
epidermis

14. • Differentiation in lower portion forms hair cone
which subsequently forms hair cuticle and two
inner root sheaths
• Hair canal at the level of upper dermis is formed
by premature death of central core cells before
keratinisation
• Intraepidermal portion of hair canal produced by
premature keratinisation and dissolution of
matrix cells

15. • 16th week-develops 2 or 3 bulges
• Lowest of the bulges-develops into attachment
for arector pili muscle
• Middle bulge-sebaceous gland
• Upper bulge –if present ,involutes or develops
into apocrine gland

16. • During bulbous stage ,melanocytes concentrate in
pigment matrix region -4th or 5th month
• By 20 weeks ,lanugo hairs cover almost the whole
cutaneous surface

18. Structure of hair
Root
Shaft

19. Root
The active hair follicle (anagen) can be divided into
• the upper infundibulum (up to the entry of the
sebaceous duct)
• the middle isthmus (between the duct and the
attachment of the arrector pilorum muscle)
• the lower inferior segment

20. • The long narrow stalk
connecting the bulb and the
isthmus is termed as the stem.

21. Inferior segment

22. Infundibulum
• “Infundibulum” in Latin stands for the word
“funnel.”
• The funnel-shaped connection of the follicular
canal with the surface epidermis
• The lining of the infundibulum is similar to the
surface epidermis including the formation of a
basket weave- stratum corneum.

23. Isthmus
• Opening of sebaceous duct  insertion of
arector pili
• Multilayered outer root sheath
• Trichilemmal Keratinization …this disintegrates
at or above the level of sebaceous duct
• The arrector pili muscle loops around the follicle
like a sling

24. Inferior segment
STEM : from attachment of arector pili to
Adamson's fringe ,where keratogenous zone
ends
BULB : Adamson's Fringe to Base of
follicle
• The enlarged lowermost part of the inferior
segment, the bulb, is invaginated at the lower end
by the follicular papilla.

25. Stem/ supra bulbar
• The stem is the part of the follicle above the
keratogenous zone up to the insertion of arrector
pilorum.
• Comprised of 3 layers (out in)
Outer root sheath
Inner root sheath
Hair shaft

26. • The hair shaft and the inner sheath are
keratinized at this level.
• All the layers of the inner sheath appear blended
into one and this inner sheath maintains close
contact with the hair cuticle.

28. Keratogenous zone /Adamson's Fringe
• Upper part of the bulb comprising cells that are
still undergoing keratinization
• Where cornification of inner sheath begins
• Differentiates into six different cell lines --
Formation of inner 6 layers of hair follicle
 Medulla
 Cortex
 Cuticle
 Cuticle of inner root sheath
 Huxley’s layer
 Henle’s layer

30. Hair Bulb
• Deepest part
• Lies in s/c fat
• Is invaginated at
base by dermal
papilla
• Made of epithelial
matrix cells
and melanocytes.

31. Dermal papillae
- derived from
condensation of
mesenchymal cells
- Flask shaped ,
invaginates the base
- made of fibroblast like
cells embedded in
extracellular matrix-
proteins & proteo-
glycan's
- melanocytes are found in between matrixial and
germinative cells

32. DP is responsible for
 induction &
maintenance of
folliculo-epithelial
differentiation
 determination of
follicular type
-Target of androgen action

33. • Papilla has single
feeding capillary
• Whereas inferior
segment of follicle
is enriched by rich
network of
capillaries and
nerves in
perifollicular
adventitial dermis.

34. Follicular germinative cells
• Keratinocytes lining the follicular papilla
• Responsible for steady supply of cells of hair
matrix

35. Germinative Matrix Cells
differentiate in
7 pathways
 Outer root sheath
 Henle’s layer
 Huxley’s layer
 Cuticle of inner sheath
 Cuticle of hair
 Cortex
 medulla

36. The inner root sheath
• Extends from upper half of bulbisthmus
Made of 3 layers (inside out)
Henle’s layer
Huxley’s layer
Inner root sheath cuticle

37. • Huxley’s layer –
 2 – 4 cells thick
 keratinizes above Adamson's fringe
 last to keratinize
• Henle’s layer –
 one cell thick
 first to develop Keratohyaline Granules and to
Keratinize

38. • Some cells within the Huxley’s layer protrude
through the henle’s layer to get attached directly
to companion layer Fluegelzellen / wing cells
• By keratinizing before the shaft, the inner root
sheath layers provide a protective mold for the
developing hair shaft and may, therefore, be
involved in the causation of various shaft defects.

39. The outer root sheath
• Continues downwards
from Epidermisbulb
• Made of cuboidal cells
(have clear vacuolated
cytoplasm due to
presence of glycogen)

40. Cell changes in ORS at different levels
 Infundibulum – Granular layer
 Isthmus – Keratinize in Trichilemmal
fashion
 Suprabulbar – Cells become larger with
abundant glycogen
 Bulb – Single flattened cell layer traceable till
base

41. Hair shaft
Consists of 3 layers
• Medulla
• Cortex
• Cuticle

42. Medulla
• Centre layer of the shaft
• Not always present
• Maybe continuous or broken
• consists of a few layers of rounded cells containing
glycogen vacuoles that turn into tiny air-filled
cavities as the hair grows out.
• May contain melanin.

43. Cortex
• Largest and most important layer of hair shaft
• consists of numerous layers of flattened elongated
cells packed together.

44. • The cell organelles and nuclei disappear
gradually as the cells keratinize in the
keratogenous zone (Adamson’s fringe) in the
lower third of the stem.
• Determines the texture of hair
• All chemical and physical changes take place here
• Melanin is found –permanent color changes take
place in cortex

45. CUTICLE
• Consists of 5-10 layers.
• each 0.5 micron thick, of
flattened cells arranged in
overlapping roof-tile
pattern.

46. • The free downward pointing edges of its outermost
layer interdigitate with the upward pointing cells of
the single-layered cuticle of the inner root sheath
thereby ensuring that the inner root sheath and the
shaft move up together.
• Mature cells - thin scales consisting of compact
cuticular keratin associated with ultra high sulphur
proteins (di sulphide bonds )- Tensile Strength
Functions of cuticle-
• it acts as a barrier to physical and chemical insults
• maintains the integrity of the hair shaft

47. • On transmission electron
microscopy - Three distinct
layers
 The outer A layer – Rich
in cystine
 The exo cuticle - Rich
in cystine
 Inner endo cuticle-
Virtually devoid of sulphur

48. HISTOLOGICAL FEATURES OF HAIR

49. Ultrastructure
• Hair is formed of hard keratin with high sulfur content
that is responsible for its extraordinary tensile strength as
against the soft keratin of the skin and the nails that have
a lower sulfur content.

50. Pilosebaceous unit
• Hair along with sebaceous gland and arector pili

51. Arrector pili muscle
• Attached to the
follicle at the base of
epidermis
• Involuntary muscle
• Contracts due to cold
or fright

52. Sebaceous gland
• Attached to the side of
follicle
• Secretes oil/sebum
• Helps hair stay supple
and shiny
• Sebum forms part of acid
mantle that helps protect
the skin from harmful
bacteria and chemical
attack
Sebaceous gland

53. Perifollicular connective
tissue/Sheath
• A thin layer of connective tissue that envelopes the
hair follicle throughout its length.
• consists of an inner basement membrane called the
hyaline or vitreous (glassy) membrane and an
outer connective tissue sheath.

54. • The basement membrane of the follicle is
continuous with the interfollicular basement
membrane.
• It is most prominent around the outer root sheath
at the bulb in anagen hairs
• When transplanted under the skin, this
perifollicular connective tissue has the
remarkable ability to form a new dermal papilla
and induce new hair follicle formation.

55. CONNECTIVE TISSUE
LAYER
FOLLICLE WALL
CUTICLE
HAIR CORTEX
MEDULLA
INNER ROOT SHEATH
OUTER ROOT SHEATH
GLASSY MEMBRANE
Cross section of hair root
PERIFOLLICULAR SHEATH
CONNECTIVE TISSUE
LAYER

56. HAIR FOLLICLE INNERVATION
• Myelinated sensory nerve fibers run parallel to hair
follicles, surrounding them and forming a network.
• Smaller nerve fibers form an outer circular layer,
which is concentrated around the bulge of terminal
follicles and the bulb of vellus follicles.
• Several different types of nerve endings, including
free nerve endings, lanceolate nerve endings, Merkel
cells, and pilo-Ruffini corpuscles are found associated
with hair follicles.

57. • Each nerve ending detects different forces and
stimuli.
Free nerve endings transmit pain,
lanceolate nerve endings detect acceleration,
Merkel cells sense pressure, and
pilo-Ruffini structures detect tension.
• Perifollicular nerves contain neuromediators and
neuropeptides, such as substance P and calcitonin
gene-related peptide, that influence follicular
keratinocytes and alter hair follicle cycling.

59. PHYSIOLOGY OF HAIR
• Various hormones (androgens, thyroid
hormones, insulin, cortisol, estrogens, prolactin,
and growth hormones) affect hair growth
• They act in concert with numerous growth factors
(IGF-1) and cell receptors (Vitamin D receptor).

60. 60

61. Functions of hair
1)Vital functions
(Lost during evolution - its primary roles were to
serve as insulation and protection from the elements)
on scalp, eyes
• Protection against cold and sunlight
• Camouflage
• Mechanical protection
• Disseminate apocrine odour

62. 2)Accessory functions
• Aesthetic
• Cosmetic
• Screen nasal passages
• Tactile sense
• Minimize intertriginous friction

63. References
1. IADVL textbook of dermatology ;4th edition
2. Textbook of dermatology ;samuel .L.Moschella ;3rd
edition
3. Rook’s textbook of dermatology ;9th edition
4. Lever’s histopathology of skin ;10th edition
5. Fitzpatrick’s Dermatology ; 9th edition
6. Internet references

64. THANK YOU