Hair

Aka: Hair, Pilosebaceous Unit, Pilosebaceous Follicle, Hair Follicle, Hair Growth, Hair Bulb, Hair Root, Hair Shaft, Hair Medulla, Hair Cortex, Hair Cuticle, Dermal Papilla, Anagen, Catagen, Telogen, Arrector Pili Muscle, Hair sheath, Internal Root Sheath, External Root Sheath, Vellus Hair, Terminal Hair, hair Anatomy

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II. Background

  1. Normal scalp has more than 100,000 hairs
  2. Hairs normally lost per day: 50 to 100

III. Distribution

  1. Vellus Hair (from vellus follicles)
    1. Non-pigmented fine "peach fuzz" hair covering body
    2. Found in children and adults
    3. Increased in Hypertrichosis
  2. Terminal Hair (from terminal follicles)
    1. Androgen-dependent male-type body hair
    2. Terminal Hairs are long, thick and pigmented
    3. Face (Mustache, beard and sideburns)
    4. Body (Chest, areola, linea alba, inner thighs)
    5. Increased in Hirsutism

IV. Anatomy: Images

  1. See Skin Anatomy
  2. dermHairFollicleGrayBB944.gif Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)
  3. dermHairFollicleXsGrayBB945.gif Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)

V. Physiology: General

  1. Pilosebaceous Unit (Pilosebaceous Follicle)
    1. Hair Follicle
    2. Sebaceous Gland
    3. Arrector Pili Muscle
  2. Hair Follicle
    1. Hair producing unit based in Hypodermis
    2. Cylinder that ascends through surface epithelium
    3. Hair held within center of follicle's cylinder
    4. Entire follicle encased in connective tissue
  3. Hair Bulb
    1. Located at base of Hair Follicle in Hypodermis
    2. Includes Hair Root which surround Dermal Papilla
  4. Dermal Papilla
    1. Forms center base of Hair Bulb
  5. Hair Root
    1. Epithelial cells actively divide about Dermal Papilla
    2. Hair Root produces 5 cell layers
      1. Hair Shaft forms from inner 3 epithelial layers
      2. Hair sheath forms from outer 2 epithelial layers
  6. Hair Shaft
    1. Inner 3 layers from Hair Root (around Dermal Papilla)
    2. Three layered keratinized structure
      1. Hair Medulla (innermost core)
      2. Hair Cortex (highly keratinized bulk of hair)
      3. Hair Cuticle (hard thin outer later)
  7. Hair sheath
    1. Outer 2 layers from Hair Root (around Hair Shaft)
    2. Two layers
      1. Internal Root Sheath (lightly keratinized)
      2. External Root Sheath
  8. Arrector Pili Muscle
    1. Smooth Muscle attached to Hair Follicle
    2. Contraction erects hair
    3. Innervated by Sympathetic Nervous System
      1. Activated by fear or cold
    4. Minimal function in humans
      1. Results in Goose Bumps
      2. Evolved in furry animals
        1. Increases insulation against heat loss
        2. Animal appears larger

VI. Physiology: Development

  1. Hair Follicles are laid down at 8 weeks gestation
  2. At birth, the scalp is covered by 100,000 terminal follicles
    1. Vellus follicles cover the remainder of the body at birth
  3. With the surge in androgens at Puberty, some vellus follicles transition to terminal follicles
    1. Androgens promote terminal follicle development in the axilla, genitalia and chest
  4. Scalp Hair Follicle diameter changes over time and per gender
    1. Female follicle diameter peaks by age 35 to 45 years, and then decreases with Menopause
    2. Male follicle diameter peaks by mid-20s

VII. Physiology: Phases of cyclic Hair Growth

  1. Follicle cycles are independent of their neighboring hairs (each cycling independently)
  2. Anagen: Active growing phase (85-90% of hair cycle)
    1. Thigh Hair Growth phase lasts 1 to 2 months
    2. Eyebrow and eyelash growth phase lasts 1 to 6 months
    3. Scalp Hair Growth phase lasts 2 to 6 years
      1. Scalp hair grows 0.4 mm/day or 6 inches per year
  3. Catagen: Degeneration, transition or rapid involution phase (2-3%)
  4. Telogen: Resting phase (10-15% of hair cycle)
    1. Hair shedding occurs during this phase
      1. Scalp hairs are shed at 100 hairs/day during Telogen phase
    2. Mustache hair resting phase lasts 1.5 months
    3. Leg hair resting phase lasts 3 to 6 months
    4. As Telogen ends, hair ejected and Anagen starts again

VIII. Pathophysiology

  1. See Folliculitis
  2. See Acne Vulgaris
  3. See Alopecia
  4. See Hirsutism

IX. References

  1. Habif (1996) Clinical Dermatology, Mosby, p. 24
  2. Murphy in Cotran (1989) Robbins Pathology, p. 1277-8
  3. Wheater (1987) Functional Histology, p. 130-2
  4. Dakkak (2024) Am Fam Physician 110(3): 243-50 [PubMed]

Images: Related links to external sites (from Bing)

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Related Studies

Ontology: Hair Diseases (C0018500)

Definition (MEDLINEPLUS)

The average person has 5 million hairs. Hair grows all over your body except on your lips, palms, and the soles of your feet. It takes about a month for healthy hair to grow half an inch. Most hairs grow for up to six years and then fall out. New hairs grow in their place.

Hair helps keep you warm. It also protects your eyes, ears and nose from small particles in the air. Common problem with the hair and scalp include hair loss, infections, and flaking.
Definition (MSH) Diseases affecting the orderly growth and persistence of hair.
Concepts Disease or Syndrome (T047)
MSH D006201
SnomedCT 156407009, 267862002, 279425004
English HAIR DISEASE, Hair Disease, HAIR DIS, HAIR DISORDER NOS, Disorder hair, Hair disorder NOS, Hair Diseases [Disease/Finding], disorders hair, disease hairs, hair disorder, hair disorders, hair disease, disorder hair, diseases hair, hair diseases, Hair Problems, Hair disease, Hair--Diseases, Hair disorder, Disorder of hair, Disorder of hair (disorder), Hair Disorder, Hair Diseases
Italian Patologia dei peli, Patologia dei peli NAS, Malattie dei peli e dei capelli
Dutch haaraandoening NAO, haaraandoening, Haarziekte, Haarziekten, Ziekte, haar-, Ziekten, haar-
French Trouble des cheveux, Anomalie des cheveux SAI, ANOMALIE CAPILLAIRE SAI, Trouble pileux, Maladies des cheveux, Maladies du cheveu, Maladies du système pileux
German Erkrankung des Haars, Haarveraenderungen NNB, HAARVERAENDERUNGEN NNB, Haarveraenderungen, Haarkrankheiten
Portuguese Alteração dos pêlos NE, Alteração do revestimento piloso, DOENCA DO CABELO NE, Anomalia do sistema piloso, Doenças do Cabelo
Spanish Trastorno del pelo NEOM, Enfermedades del Pelo, trastorno del pelo (trastorno), trastorno del pelo, Trastorno del pelo, Enfermedades del Cabello
Japanese 毛髪障害, 毛髪障害ＮＯＳ, ﾓｳﾊﾂｼｮｳｶﾞｲNOS, ﾓｳﾊﾂｼｮｳｶﾞｲ
Swedish Hårsjukdomar
Finnish Hiusten sairaudet
Russian VOLOS BOLEZNI, ВОЛОС БОЛЕЗНИ
Czech Porucha ochlupení, Porucha oclupení NOS, nemoci vlasů, nemoci ochlupení, vlasy - nemoci
Croatian KOSA, BOLESTI
Polish Choroby włosów
Hungarian Haj rendellenesség, Haj betegség, Haj betegség k.m.n.
Norwegian Hårsykdommer
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: Structure of dermal papilla (C0221931)

Definition (NCI) A projection of the dermis into the epidermis.
Concepts Tissue (T024)
SnomedCT 89986003
English Follicular papilla, Dermal Papilla, Dermal papilla, Structure of dermal papilla (body structure), Structure of dermal papilla
Spanish estructura de la papila dérmica (estructura corporal), estructura de la papila dérmica, papila dérmica
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: Pilo-sebaceous apparatus (C0221945)

Concepts Body Part, Organ, or Organ Component (T023)
SnomedCT 33416008
English Pilosebaceous unit, Pilo-sebaceous apparatus, Pilo-sebaceous apparatus (body structure)
Spanish aparato pilosebáceo (estructura corporal), aparato pilosebáceo
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: Hair shaft structure (C0221961)

Definition (NCI_CDISC) The segment of the hair that projects above the skin surface. (NCI)
Definition (NCI) The segment of the hair that projects above the skin surface.
Concepts Body Part, Organ, or Organ Component (T023)
SnomedCT 37111008
English hair shaft, hairs shaft, shaft hair, HAIR SHAFT, Hair shaft, Hair shaft structure (body structure), Hair shaft structure, Shaft of the Hair
Spanish estructura del tallo del pelo (estructura corporal), estructura del tallo del pelo, tallo del pelo
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: Structure of hair medulla (C0221970)

Concepts Body Part, Organ, or Organ Component (T023)
SnomedCT 6359004
English Hair medulla, Structure of hair medulla (body structure), Structure of hair medulla
Spanish estructura de la médula del pelo (estructura corporal), estructura de la médula del pelo, médula del pelo
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: hair root (C1182707)

Definition (NCI) The portion of the hair that is enclosed within the hair follicle.
Definition (NCI_CDISC) The portion of the hair that is enclosed within the hair follicle. (NCI)
Concepts Body Part, Organ, or Organ Component (T023)
English Root of hair, Hair root, hair roots, root hair, hair root, HAIR ROOT, Hair Root
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: catagen (C1752468)

Definition (GO) The regression phase of the hair cycle during which cell proliferation ceases, the hair follicle shortens, and an anchored club hair is produced. [PMID:12535193]
Concepts Organism Function (T040)
English catagen
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: telogen (C1752470)

Definition (GO) The resting phase of hair cycle. [PMID:12230507]
Concepts Organism Function (T040)
English telogen, hair resting phase
Derived from the NIH UMLS (Unified Medical Language System)

Ontology: anagen (C1819703)

Definition (GO) The growth phase of the hair cycle. Lasts, for example, about 3 to 6 years for human scalp hair. [PMID:12230507]
Concepts Organism Function (T040)
English anagen
Derived from the NIH UMLS (Unified Medical Language System)

Related Topics in Anatomy

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