Dermatology Book

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

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  1. See Also
    1. Folliculitis
    2. Acne Vulgaris
    3. Alopecia
    4. Hirsutism
  2. Background
    1. Normal scalp has more than 100,000 hairs
    2. Hairs normally lost per day: 50 to 100
  3. Distribution
    1. Vellus hair
      1. Non-pigmented fine "peach fuzz" hair covering body
      2. Found in children and adults
      3. Increased in Hypertrichosis
    2. Terminal hair
      1. Androgen-dependent male-type body hair
      2. Face (Mustache, beard and sideburns)
      3. Body (Chest, areola, linea alba, inner thighs)
      4. Increased in Hirsutism
  4. Physiology
    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
  5. Development
    1. Hair follicles are laid down at 8 weeks gestation
  6. Phases of cyclic hair growth
    1. Anagen: 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
    2. Catagen: Transition or rapid involution phase (2-3%)
    3. Telogen: Resting phase (10-15% of hair cycle)
      1. Mustache hair resting phase lasts 1.5 months
      2. Leg hair resting phase lasts 3 to 6 months
      3. As telogen ends, hair ejected and anagen starts again
  7. 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

Hair Diseases (C0018500)

Definition (MSH)Diseases affecting the orderly growth and persistence of hair.
ConceptsDisease or Syndrome (T047)
MSHD006201
EnglishDISEASE OF HAIR, DISORDER HAIR, Disorder of hair, HAIR DIS, HAIR DISEASE, Hair Diseases, Hair Disorder, Hair Disorders, Hair/hair follicle disease
Spanishtrastorno del pelo
Parent ConceptsHair Diseases (C0018500), Disease of Skin and Appendages (C0037272), Functional disorder (C0277785), skin disorder (C0037274), Hair finding (C0423842), Ambiguous concept (C1274012)
SourcesCOSTAR, CST, MSH, MTH, NCI, NDFRT, SCTSPA, SNOMEDCT
Derived from the NIH UMLS (Unified Medical Language System)


Structure of dermal papilla (C0221931)

ConceptsTissue (T024)
EnglishDermal papilla, Follicular papilla, Structure of dermal papilla
Spanishestructura de la papila dermica, papila dermica
Parent ConceptsDermis (C0011646), Microscopic skin structure (C1283406), Subdivision of papillary layer of dermis (C1182813)
SourcesMTH, SCTSPA, SNOMEDCT, UWDA
Derived from the NIH UMLS (Unified Medical Language System)


Pilo-sebaceous apparatus (C0221945)

ConceptsBody Part, Organ, or Organ Component (T023)
EnglishPilo-sebaceous apparatus, Pilosebaceous unit
Spanishaparato pilosebaceo
Parent ConceptsEntire skin appendage (C1280892), Heterogeneous cluster (C0932491), Skin (C1123023)
SourcesSCTSPA, SNOMEDCT, UWDA
Derived from the NIH UMLS (Unified Medical Language System)


Hair shaft structure (C0221961)

ConceptsBody Part, Organ, or Organ Component (T023)
EnglishHair shaft, Hair shaft structure, Shaft of the Hair
Spanishestructura del tallo del pelo, tallo del pelo
Parent Conceptshair (C0018494), Hair Part (C1512329), Subdivision of hair (C1182697)
SourcesAOD, MTH, NCI, SCTSPA, SNOMEDCT, UWDA
Derived from the NIH UMLS (Unified Medical Language System)


Structure of hair medulla (C0221970)

ConceptsBody Part, Organ, or Organ Component (T023)
EnglishHair medulla, Structure of hair medulla
Spanishestructura de la medula del pelo, medula del pelo
Parent ConceptsHair shaft structure (C0221961)
SourcesMTH, SCTSPA, SNOMEDCT
Derived from the NIH UMLS (Unified Medical Language System)


Hair root (C1182707)

ConceptsBody Part, Organ, or Organ Component (T023)
EnglishHair root, Root of hair
Parent ConceptsHair Part (C1512329), hair (C0018494), Subdivision of hair (C1182697)
SourcesNCI, UWDA
Derived from the NIH UMLS (Unified Medical Language System)


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]
ConceptsOrganism Function (T040)
Englishcatagen
Parent Conceptshair cycle process (C1818620)
SourcesGO
Derived from the NIH UMLS (Unified Medical Language System)


telogen (C1752470)

Definition (GO)The resting phase of hair cycle. [PMID:12230507]
ConceptsOrganism Function (T040)
Englishtelogen
Parent Conceptshair cycle process (C1818620)
SourcesGO
Derived from the NIH UMLS (Unified Medical Language System)


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]
ConceptsOrganism Function (T040)
Englishanagen
Parent Conceptsdevelopmental growth (C1660152), hair cycle process (C1818620)
SourcesGO
Derived from the NIH UMLS (Unified Medical Language System)



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