7 - Biology of Hair Follicles Flashcards
Primary purpose of hair in humans
Influence social interactions
First hairs formed
Lanugo hairs
Lanugo hairs are shed
Between the 32nd and 36th weeks of gestation
Responsible for the nonrandom and symmetrical distribution of hair follicles over the body
Homeobox genes
Responsible for dorsal-ventral patterning
Engrailed (type of homeobox gene)
Thicker hair found in Asians is associated with
Increased activity of Edar
Stage 1 of hair follicle development
Epithelial placodes
Epithelial placodes first appear around
10 weeks of gestation
Drives the growth of the fully formed follicle
Dermal papilla
Necessary for establishing epithelial competence, a state in which the epithelial tissue had the potential to form a hair follicle
WNT/beta-catenin pathway
Y/N: Normally the beta-catenin pathway is active in the adult epidermis
No - inactive
Bone morphogenetic protein (promote/inhibit) follicle formation
Inhibit
BMP antagonist
Noggin
Gene deficient in basal cell nevus syndrome
Patched
Stage 2 of hair follicle development
Bulbous peg or hair bud
Proliferation of the worthless cells and lateral expansion of the follicular placode are regulated by
Shh
Deepest portion of the follicle peg forms a bulbous structure surrounding the dermal papilla and goes on to form the
Matrix of the hair follicle
The matrix of the hair follicle gives rise to (2)
Hair shaft
Inner root sheath
The outer root sheath forms two bulges
Superficial bulge - develops into the sebaceous gland
Deeper bulge - future sure of epithelial stem cells
A third bulge develops superficial to the sebaceous gland bud and gives rise to
Apocrine gland
First hairs formed
Lanugo hairs
Duration of _____ determines the length of hair
Anagen
Shape of the _____is thought to determine the shape of the hair
Inner root sheath
Outer root sheath in the infundibulum vs isthmus
Infundibulum: with granular layer
Isthmus: without granular layer (trichilemmal)
Inner root sheath parts (outermost to innermost) (4)
Companion layer
Henle layer
Huxley layer
Cuticle
Provides a slippage plane between the outer root sheath, which is stationary, and the inner root sheath
Companion layer
Some cells within the Huxley layer protrude through the Henle layer and attach directly to the companion layer
Fluegelzellen or wing cells
Cells are overlapping forming a “shingled roof” appearance
Inner root sheath cuticle
Critical for inner root sheath differentiation and lineage
GATA-3
Forms bulk of the hair shaft and contains melanin
Cortex
Receptor of keratinocyte growth factor, found in the matrix keratinocytes
FGF receptor 2
Differentiation of hair shaft cells is dependent on
Lef-1
BMP receptor type 1a
Core of mesenchymally-derived tissue envelopes by the matrix epithelium
Dermal papilla
Produced by the anagen dermal papilla
Keratinocyte growth factor
Free nerve endings transmit
Pain
Lanceolate nerve endings detect
Acceleration
Merkel cells sense
Pressure
Pilo-Ruffini structures detect
Tension
The perifollicular sheath consists of
Inner basement membrane
Outer connective tissue sheath
Has the longest anagen of 2 years to more than 8 years
Scalp hair
_____ of scalp follicles are in anagen
90-93%
Rest are in telogen
Average number of hairs on the scalp
100,000-150,000
Exogen comprises _____ of the telogen hair follicles
1%
An adult loses _____ hairs per day
50-100
Scalp hair growth rate
0.37-0.44 mm/day
1 cm/month
Bulge cells comprise approximately _____ of the cells in the regenerated epidermis during wound reepithelialization
30
Y/N: Bulge cells do not normally move to the epidermis
Yes
Anagen can be divided into _____ stages
7
There is similarity between _____ and the early events of anagen
Wound healing
The onset of _____ is marked by cessation of the mitotic activity of the matrix cells and by the coordinated apoptosis in the cyclic portion of the hair follicle
Catagen
Y/N: Pigment production my melanocytes ceases after matrix cell proliferation stops.
No - before matrix cell proliferation stops
At the end of catagen, the dermal papilla becomes situated
Below the bulge at the lower portion of the isthmus
The migration of the dermal papilla from the subcutaneous fat to the dermis during catagen is necessary for continued follicle cycling. This is illustrated by the syndrome
Atrichia with papules
Increase in shedding of club hairs within weeks of the precipitating event
Immediate telogen release (too soon to be caused by follicles prematurely entering telogen from anagen
Several months could transpire between hair shedding and regrowth. This “lag period” is normally not present or is very short but often lasts months in patients with
Androgenetic alopecia
Melanocytes in the skin arise from (2)
Neural crest cells
Schwann cell progenitors
Commitment of neural crest cells to the melanocyte lineage is regulated by (2)
Pax3
Microphthalmia transcription factors (Mitf)
Melanoblast migration into the dermis and epidermis are controlled by signaling mechanisms activated through
Endothelin receptor type B (Ednrb)
C-kit receptor
Mutated jn Hirschprung disease and piebaldism, respectively
Maintenance of melanocyte stem cells during hair follicle cycling is controlled by
TGF-beta
Notch
Melanocytes producing pigment are located in the
Hair bulb
Hair follicle melanocytes are maximally proliferative during
Early and midanagen
