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Structure of hair


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  • 1. SHUMEZ. H
  • 2.  Hair is a cutaneous appendage typical of mammalian skin. It has no vital function in humans, yet its psychological functions are extremely important.
  • 3. TYPES OF HAIR LANUGO (wool like) Fine, soft, unmedullated, unpigmented VELLUS HAIR ( ≤0.03 mm ) Soft, unmedullated, pigmented TERMINAL HAIR ( ≥0.06 mm ) Coarse, medullated, pigmented
  • 4. DISTRIBUTION Cheek - 830/450/sq. cm Forehead - 765/sq. cm Scalp - 350/sq. cm Chest & limbs - 50 – 100/sq. cm
  • 5. ANATOMYMature hair follicle divided into UPPER SEGMENT Infundibulum Isthmus LOWER SEGMENT Stem Bulb
  • 6. INFUNDIBULUM Extends from Skin surface (Ostium) above to the entry of Sebaceous duct below. Lining is similar to the surface epidermis.
  • 7. ISTHMUS Extends from entry of Sebaceous duct above to attachment of Arrector muscle below. Narrow part of the hair follicle. Hair shaft is fully keratinized. Inner root sheath begins to disintegrate.
  • 8. STEM Longest part of hair follicle. Extends from insertion of Arrector pili muscle above upto the keratogenous zone. Consists of 3 layers : Outer root sheath Inner root sheath Hair shaft
  • 9. BULB Lower most part of hair follicle. Follicular papilla lies in the center of the follicular bulb. The bulk of the follicular bulb is formed of Epithelial matrix ( matrical cells ). Few melanocytes are interspersed.
  • 10. ADAMSON’S FRINGE Upper part of the bulb. Keratogenous zone. Divided into 6 layers: Medulla Cortex Hair cuticle Cuticle of inner sheath Huxley’s layer Henle’s layer
  • 11. FOLLICULAR PAPILLA Varies according to the phase of hair cycle. Composed of specialized fibroblast like cells embedded in extracellular matrix. Often contains a loop of capillary blood vessels. Volume of dermal papilla maybe responsible for controlling size of hair follicle.
  • 12. HAIR SHAFT 3 layers : MEDULLA CORTEX HAIR CUTICLE Medulla is a variable structure. Maybe continuous, interrupted or absent. Contains few layers of rounded cells containing glycogen.
  • 13.  Cortex forms bulk of the hair shaft. Consists of numerous layers of flattened elongated cells packed together. Cell organelles and nuclei disappear as the cells keratinize.
  • 14.  Hair cuticle consists of 5 – 10 layers of flattened cells arranged in overlapping “roof – tile” pattern. The upwards pointing edges of the hair cuticle interlock with the downwards pointing edges of cuticle of inner sheath.
  • 15. INNER ROOT SHEATH 3 layers : Cuticle Huxley’s layer Henle’s layer Henle’ s layer – 2 cells thick, keratinizes first Huxley’s layer – 4-6 cells thick At the Isthmus the IRS disintegrates .
  • 16. OUTER ROOT SHEATH Most peripheral part of hair follicle. Single layered at lower tip of hair bulb and 4-6 layered at the isthmus. Keratinize at the level of Isthmus. Trichilemmal keratinization. Occasionally “companion layer” maybe seen in between IRS and ORS.
  • 17. PERIFOLLICULAR FIBROUS SHEATH Thin layer of connective tissue which envelopes the hair follicle througout its length. PFS is variable: Early Anagen : Thin, containing collagen and fibroblasts Anagen : Thickened and trilayered Catagen : Hyalinization of PFS Telogen : Thinning of PFS
  • 18. ULTRASTRUCTURE Hair is formed of hard keratin with high sulfur content. High sulfur content responsible for its extraordinary tensile strength. Fibrillar component consists of macrofibrils, 7 nm thick arranged in longitudinal lamellated form. S-H linkages of cysteine at the bulb are converted to S-S linkages of cysteine higher up.
  • 19. HAIR FOLLICLE INNERVATION A plexus of longitudinally arranged sensory nerve fibres surrounds the isthmus region. Other nerve endings found are : Free nerve endings Pilo – Ruffini nerve endings Merkel nerve endings
  • 20. HAIR CYCLE Hair follicles undergo a repetitive sequence of growth and rest known as hair cycle. Hair cycle varies between follicles in different regions of the skin in the same species . Hair growth occurs in 3 stages : Anagen Catagen Telogen
  • 21. ANAGEN Period of active hair growth. Duration of this phase responible for final length of the hair. Usually lasts for 2 – 3 years. Duration of Anagen genetically determined. About 85% of all hairs are in this phase at any time.
  • 22.  Onset of mitotic activity in epithelial cells of Dermal papilla. Lower part of follicle elongates downwards along a preformed dermal tract ( stele ). Dermal papilla expands from a tightly packed ball of cells into a flask shaped structure. A network of capillary blood vessels develop around the lengthening follicle.
  • 23.  Epithelial cells in the hair bulb undergo vigorous proliferative activity. The melanocytes become active adding colour to this newly forming hair. Anagen consists of 6 substages. Differences in the length of hair is due to variable duration of the last stage ( VI ).
  • 24. CATAGEN Short transition stage that occurs at the end of the anagen phase. Signals the end of active growth of hair. Usually lasts about 2 – 3 weeks.
  • 25.  At the end of Anagen, epithelial division declines and ceases. Proximal end of the hair shaft keratinizes to form a club shaped structure. Lower part of the follicle involutes by apoptosis. Basement membrane surrounding the follicle becomes thickened to form “glassy membrane”. Base of the follicle along with dermal papilla moves upwards to lie below the level of Arrector muscle attachment.
  • 26. TELOGEN Resting phase of the hair follicle. Usually lasts for about 3 months. About 10 – 15% of all hairs are in this phase at any time.
  • 27.  Quiscient period between completion of follicular regression and onset of next anagen phase. Resting club hair lies within an epithelial sac. Streak of fibrous tissue extends from base of follicle to quiscient papilla beneath – fibrous tract
  • 28. EXOGEN / TELOPTOSIS Active process of shedding of club hair. Defective Exogen gives rise to Trichostasis spinulosa.
  • 29. KENOGEN / LAG PHASE Latent phase in the hair cycle where hair follicle is empty after hair shedding and persists in this stage for a variable duration. Seen in patients with AGA and prepubertal children.
  • 30.  Approx. 100000 follicles are present on the average scalp. About 70 – 100 hairs in telogen are shed daily. Hair on the scalp grows at an average rate of 0.45 mm daily. It grows faster on the crown > vertex > temple.
  • 31. FUNCTIONS OF HAIR Tactile perception Protection of scalp from sunlight and trauma. Protection of eyes from foreign bodies. Screening nasal passages. Reduce friction in intertriginous areas.
  • 32. ANDROGENS Follicles that respond to androgen are grouped as androgen dependent hair. Converts axillary and pubic follicles into terminal follicles. Similar course in beard, moustache, abdomen, chest area in men. Increases the duration of anagen hence thickness of hair shaft. Increases size of hair follicle.
  • 33.  However, Scalp hair respond in a regressive manner to androgens. Pubertal recession of frontal hair in boys and are resposible for AGA. Enzyme 5α reductase is responsible for conversion of weaker androgens into more potent form. Finasteride inhibits 5α reductase type 2 and prevents AGA.
  • 34.  Growth Hormone - synergistic effect on androgen dependent hair. Estrogen - retains club hair in follicle. Prolactin - androgen like effect. Throid hormone - Both hypo and hypersecretion can cause alopecia. Retards the rate of hair growth. Retains club hair in the follicle.
  • 35.  IGF – I : promotes hair growth and proliferation. Insulin - acts through IGF – I and promotes hair growth. Cortisol - enhances proliferative actions of IGF – I Shaving has no effect on the rate of hair growth.
  • 36. DURING PREGNANCY Increase in proportion of follicles in anagen. Reduction in hair density. Post partum - large number of follicles enter into telogen, leading to increased shedding - Postpartum telogen effluvium
  • 37. MOLECULAR CONTROL OF HAIRCYCLING ANAGEN Activation of various mediators - IGF – 1, HGF, SHH, Whn, beta catenin IGF – 1 plays an important role. It promotes hair growth and proliferation. IGF – 1 receptors are located in dermal papilla and ORS. Insulin acts through IGF – 1 and promote hair growth.
  • 38. CATAGEN It is marked by down regulation of IGF – 1 receptors and HGF expression in hair bulb. TGF – β, IL – α1, TNF – α are the mediators of catagen activation. Stress, trauma, dexamethasone, ACTH, es tradiol also initiate catagen phase.
  • 39. TELOGEN Bone morphogenetic protein – 4 plays an important role in suppressing follicular growth and differentiation. BMP – 4 acts on BMPR – IA expressed by keratinocytes. BMP – 4 antagonist Noggin accelerates the onset of anagen.
  • 40. COMMON CAUSES OF HAIRFALL Androgenetic Alopecia Alopecia areata Infection Postpartum Hypothyroidism Hyperthyroidism Stress
  • 41.  Inadequate diet Low serum iron Major surgery / chronic illness Fungal infection of scalp Trichotillomania Oral contraceptives Chemotherapy Other drugs – Vit. A, β blockers, Antidepressants, Anticoagulants, etc.
  • 42. CAUSES OF HYERTRICHOSIS Hypertrichosis lanuginosa Naevoid hypertrichosis Hypo/hyperthyroidism Anorexia nervosa Dermatomyositis Epidermolysis bullosa Porphyria Drugs – Systemic steroid, minoxidil, psoralen, diazoxide, cyclo
  • 43. CAUSES OF HIRSUTISM Polycystic Ovarian Disease Ovarian tumours SAHA syndrome Congenital Adrenal Hyperplasia Hyperprolactinaemia Adrenocortical disease Idiopathic hirsutism Drugs – phenytoin, androgens, OCPs, diazoxide, minoxidil.