Structure and Function - hair follicles Flashcards
What effect does substance P have on the hair cycle?
anti-proliferative and pro-apoptotic effects: abrupt cessation of anagen, increased apoptosis in hair follicle resulting in premature catagen, increased perifollicular macrophage clusters, incrreased mast cell degranulation
What effect does ACTH have on the hair cycle?
causes mast cell degranulation, activates hair growth in telogen skin, also induces premature transition to catagen
What is the “permanent” portion of the hair follicle?
isthmus and infundibulum
What is the predominant inflammatory cell associated with alopecia areata?
Cd8+ T cell lymphocytes in the bulb – CD4+ T cells in the peribulbar region
What is the role of KGF (keratinocyte growth factor) in the hair cycle?
promotes anagen; produced by anagen dermal papilla –> important for hair growth and cycling
Describe the changes that take place during early catagen (I-IV)
Dermal papilla moves upward, mitotic activity stops, apoptotic keratinocytes, pigmentation stops, trailing connective tissue sheath visible
What is the mutation in hairless Sphynx cats?
keratin 71 gene – keratin 71 is a type II keratin expressed throughout the IRS in mice and humans –> important for molding the hair shaft
What is immediate telogen release?
shortening of the normal telogen phase with exogen as follicles are stimulated to re-enter anagen (terminates telogen retention)
T/F: The number of hairs is reduced with follicular dysplasia.
False - normal number of hairs but hairs have structural changes –> hairs are fragile and malformed
Clinical presentation of mural mucinotic isthmus folliculitis of Lundehunds?
multifocal, patchy alopecia (may wax and wane); scaling, comedones, follicular casts; mild to severe pruritus; thorax, face and/or distal limbs most often affected
Exogenous factors that lead to initiation of anagen
TSH, T3/T4, day length
What effect do thyroid hormones have on the hair cycle
accelerate follicular activity; T4 upregulates human hair matrix keratinocytes; T3 and T4 down-regulate apoptosis, preventing catagen; T3 and T4 prolong anagen in vitro (via downregulation of TGF-beta –> inhibits anagen); anagen shortened*
T/F: Sphynx cats with the Hr mutation lack whiskers.
False - they have whiskers, but are short and curled
What are the two types of color-linked follicular dysplasias?
color dilution alopecia, black hair follicular dysplasia
What transcription factor is important for follicle induction?
Lef-1
What effect does promoting the wingless (wnt) signlaling pathway have?
new hair follicle induction
Poorly pigmented hairs characteristic of the prenatal period
Lanugo hairs
Describe the changes that take place during late anagen (IIIc-VI)
dermal papilla fully enclosed, DP in SQ, hair reaches surface, loss of club hair
Describe changes associated with Naked Foal Syndrome
Hypotrichosis and mild/moderate ichthyosis, nearly complete alopecia, vibrissae distorted**, die at a young age
Regions of the scalp (humans) that do not experience androgenic balding are higher in what enzyme involved in androgen pathways?
aromatase –> converts testosterone to estradiol in these follicles
Where does the outer root sheath keratinize?
level of isthmus – via trichilemmal keratinization
What effect does BMP-4 (bone morphogenic protein) have on Sonic hedgehog (Shh) and Wingless (Wnt) signaling?
inhibits their signaling –> inhibts hair growth
Histopathologic findings with alopecia areata
lymphocytic inflammation around and within the hair bulb; miniaturization of hair follicles below the sebaceous gland, pigmentary incontinence in the peribulbar region; peribulbar fibrosis (“onion skinning”), eosinophilic hair shafts, infundibular hyperkeratosis
T/F: Nail changes are possible with alopecia areata.
True - as well as hoof abnormalities in horses
What is the major component of the hair shaft?
Keratins (keratin intermediate filaments)
T/F: The hair shaft has no attachment to the infundibulum or isthmus.
True
What breeds are associated with post-natal ectodermal dysplasia? What is phenotype?
Pattern baldness: Portugese Water dog, Curly-coated retriever, Chesapeak Bay Retriever, Bald thigh syndrome in Greyhounds, etc.
Exogenous factors that lead to premature catagen induction
glucocorticoids, estrogen, substance P, NGF, TGF-beta-2
Possible pathomechanisms of post-clipping alopecia
clipping at early telogen stage –> hair regrowth delayed; time of year, geographic region; vascular perfusion changes in response to altered cutaneous temperature
Describe the isthmus of the hair follicle
proximal end of the infundibulum to the interface between the completely cornified IRS and the first non-cornifed cell of the Huxley’s layer (=Adamson’s fringe; approximately the insertion of the arrector pili muscle). Contains the HF stem cells
T/F: Exogen is a passive process where the new hair “pushes out” the old hair.
False - active process, involving proteolysis of intercellular adhesion molecules between the club hair and the surrounding epithelium
What produces the trichilemmal cornification?
outer root sheath
Mode of inheritance of naked foal syndrome? What breed is affected?
autosomal recessive (monogenic = ST14 gene) - -Akhal-Teke horses
What is the role of sonic hedgehog signaling in the hair cycle?
critical for induction of anagen; critical for hair follicle morphogenesis
T/F: Hair cycle arrest disorders cannot be differentiated on histopathology.
True - no features consistently differentiate them – all have increased kenogen follicles (likely that anagen is not being induced)
Mode of inheritance of hairless genotype in Xolo dogs?
autosomal dominant
Color-linked follicular dysplasias have been linked to what gene in some dog breeds?
melanophilin
Describe changes associated with X-linked anhidrotic/hypohidrotic ectodermal dysplasia
alopecia at birth; dentition changes - partial adontia, malformation, malposition; no glands are affected
What is the clinical presentation of proliferatve, lymphocytic, infundibular mural folliculitis with apoptosis and parakeratotic casts (PLIMF)?
only reported in Labrador retrievers, All dogs were female, median age of onset 6 years; Clinical signs: Thick, greyish-brown, adherent, hyperkeratotic plaques; variable alopecia; comedones in early lesions
What are the reported triggers of granulomatous mural folliculitis?
viral (caprine- and ovine-herpesvirus-2 – deer and goat), drugs (methimazole in a cat, various in the dog), or idiopathic
T/F: Intact females can show signs of Cushing’s disease due to activation of glucocorticoid receptors by progesterone.
False - even though progesterone DOES bind to the glucocorticoid receptor, intact females do not show cushingoid signs during their cycle
T/F: Dogs have androgen-responsive follicles.
False - do not grow hair in the presence of testosterone, do not lose hair in response to testosterone
What is chronic telogen defluxion?
increased telogen hair shedding occurring for 6 months or more (diagnosis made after ruling out other causes of hair loss)
T/F: Additional ectodermal defects may exist in combination with follicular dysplasia.
True - in people it is classified as a primary defect in at least two of the following: hair, teeth, nails, sweat glands
What has been shown to cause immediate telogen release?
minoxidil and response to rapid light/dark fluctuations
What effect do corticosteroids have on the hair cycle?
suppress anagen and catagen, including trauma-induced anagen; block keratinocyte growth factor (KGF) and hepatocyte growth factor (HGF); inhibits local IGF-1 production (stimulant of anagen); may also have effects on the estrogen receptors; Anagen shortened –> miniaturization and atrophy
T/F: Hair grows faster in the morning.
True - the circadian clock generates a daily mitotic rhythm –> radiation causes more hair loss when administered in the morning during the mitotic peak
What effect does estrogen have on BMP-4 (bone morphogenic protein)? What effect does this have on the hair cycle?
estrogen activates BMP-4 –> leads to inhibition of hair growth (dowregulates Shh, Wnt1 signaling)
T/F: Sphynx cats with the Hr mutation are hairless.
False - they have short fine hairs protruding from the follicular ostia with reduced diameter; hair is often curved and may have kinks, tufts of short hair on the ears, muzzle, tail, feet, and scrotum, whiskers are short and curled.
Histologic changes of hair cycle arrest disorders
hyperkeratosis, follicular dilation, follicular atrophy, telogen hairs, kenogen hairs ***, epidermal melanosis, epidermal atrophy
Exogenous factors that can inhibit anagen onset
glucocorticoids, estrogen, testosterone, BMP-4, IL-6, stress
Clinical presentation of feline proliferative and necrotizing otitis and dermatitis
Kittens and adult cats (median age = 1.75 years); hyperkeratotic, crusty, proliferative, firmly adherent plaques with or without erosions and ulceration - found on concave aspect of pinna and ear canals; lesions may also affect muzzle and paws; pain or pruritus seen
During what phase of the hair cycle does hair pigmentation occur?
anagen – melanosomes migrate and are transferred into hair matrix
From what embryonic tissue is the dermis formed?
mesoderm (composed of mesenchymal cells)
The coat type of cats is determined by genetic variants of what three genes?
KRT71 (keratin 71) = hairlessness in Sphynx, Curly coat in Devon Rex and Selkirk Rex; LPAR6=Curly coat in Cornish Rex and German Rex; FGF5 (fibroblast growth factor-5) = Long hair
Differences between adult coat and puppy coat
Adult coat develops secondary hairs, increased number of hair shafts, increased diameter of hair shafts, decrease in curvature of hair shaft
Histologic features of color-linked follicular dysplasias?
abnormal melanin clumping within hair cortex, medulla, bulb and periadnexally; hyperkeratosis and follicular keratosis
What is the Huxley layer?
part of the inner root sheath, 2-4 layers thick, keratinizes above Henle at the region known as Adamson fringe (keratogenous zone)
Mode of inheritance of hairless genotype in Sphynx cats?
autosomal recessive
T/F: Estrogens are essential for normal hair growth.
False - OVH does not significantly alter the coat
Clinical presentation of feline degenerative mucinotic mural folliculitis
generalized, progressive alopecia with scaling and crusting; leions first often presented on the face or neck; +/- pruritus; swollen face – especially the muzzle (all cases)
Exogenous factors that can lead to induction of exogen
stress, minoxidil
Clinical signs of alopecia areata
multifocal, well-demarcated patches of alopecia; leukotrichia; face most commonly affected; loss of eyelashes and whiskers possible; nail changes possible - trachyonchyia (sand paper nails with fine longitudinal ridges)
Describe the suprabulbar region of the hair follicle
Presence of a not yet fully cornified IRS characterized by trichohyalin granules in the cytoplasm of the cells
T/F: Follicular dysplasia of Weimaraners is more severe than other color dilution phenotypes.
False - similar to color dilution but less severe
Histologic changes with follicular dysplasia of Weimaraners
increased size and number of melanin aggregates; crater defects of hair shaft; dermal melanophages seen less frequently
Poorly pigmented hairs in people typical of fine hairs on face and arms
Vellus hairs
Exogenous factors that lead to prolonged anagen
TSH, T3/T4, Cyclosporine A, caffeine
Describe features of follicular lipidosis
develops in first few months of life; breed-specific: Rottweilers and a Dachshund dog; affects mahogany-colored hair on feet and face; associated with lipid vacuoles within cells of hair follicle matrix
Describe the changes that take place during telogen
hair follicle an dermal papilla in dermis; club hair firmly anchored; no inner root sheath; no trailing connective tissue sheath
When does exogen happen?
late anagen to early telogen - depends on the hair type and species
What two forms of cutaneous lupus erythematosus show lymphocytic interface and mural folliculitis, affecting the infundibulum and isthmus? What other histopathologic changes can be found?
generalized discoid lupus erythematosus and exfoliative cutaneous lupus erythematosus; hair follicle and sebaceous gland atrophy have been reported as well
The coat type of most dogs is determined by genetic variants of what three genes?
RSPO2 (encoding R-spondin-2), FGF5 (fibroblast growth factor-5), KRT71 (keratin 71)
What is unique about the hair follicles of Llamas?
Simple follicles, but may have new anagen hair and old telogen hair occupying the same follicle
What products can increase insulin-like growth factor 1 locally?
cepharanthine and capsaicin (substance P antagonist)
What are the 3 mechanisms by which a thymoma can predispose animals to autoimmune disease?
1) Escape of thymoma-driven thymocytes into periphery w/o critical selection; 2) increased proliferation of thymocytes with an aberrant TCR expression; 3) Mutation in HLA Class II genes, resulting in a defective education of thymocytes by thymic epithelial cells
What are examples of post-natal neuroectodermal dysplasia?
color dilution alopecia, black hair follicular dysplasia, follicular lipidosis
During what phase of melanogenesis is the defect in color-linked follicular dysplasias?
defect in melanosome migration/transfer (neuroectodermal defect)
Skin lesions with granulomatous mural folliculitis
patchy to diffuse alopecia with variable degree of erythema and scaling
What breeds of horses are over-represented with alopecia areata
Appaloosa, American saddlebred, Peru paso
Mode of inheritance of hairless genotype in Chinese crested dogs?
autosomal dominant
T/F: Non-thymoma-associated exfoliative dermatitis responds well to medical therap.
True - 67% response rate reported - drugs used included glucocorticoids +/- cyclosporine
Compound hair follicles are seen in what species?
dogs, cats, other carnivores, and rabbits**
What are the layers of the hair shaft? Which layer is the bulk of the mass of the hair shaft?
cuticle, cortex, medulla; cortex is the bulk of the fiber mass and accounts for the tensile strength of the hair shaft
When are primary hairs shed? Secondary hairs?
Primary hairs are shed all year round; Secondary hairs are shed seasonally**
Describe the skin and hair of Chinese crested dogs with the Hr mutation
Follicles are simple, deformed, dysplastic; comedones with remnants of dermal papilla at the base of the follicle; normal functioning sebaceous and sweat glands
Histopathologic findings with thymoma-associated exfoliative dermatitis
cell-poor to cell-rich lymphocytic interface dermatitis - and mural folliculitis (infundibulum +/- isthmus), trans-epidermal apoptosis, prominent hyperkeratosis due to sebaceous adenitis or loss of sebaceous glands (Lupus + EM + sebaceous adenitis)
What is the Henle layer?
part of the inner root sheath, one cell layer thick; first to develop trichohyalin granules and first to keratinize
Why can’t the Henle’s layer and Huxley’s layer be distinguished in the isthmus?
both are fully cornified at this point
What disease is induced by loss of the immune privilege of the hair follicle?
alopecia areata
Histopathologic findings in proliferatve, lymphocytic, infundibular mural folliculitis with apoptosis and parakeratotic casts (PLIMF)
lymphocyte infundibular mural folliculitis (duh), hair follicle hyperplasia, parakeratosis, keratinocyte apoptosis with lymphocytic satellitosis
What is kenogen?
empty follicle after exogen (hairless telogen)
What time of year is cyclic flank alopecia most common?
hair loss usually occurs in winter or spring (shorter day-length)
Histologic features of follicular lipidosis
lipid vacuoles within cells of hair follicle matrix
T/F: Progesterone cross-reacts with the testosterone receptor.
True - causes negative feedback to the pituitary, inhibiting further gonadotropin secretion
What breed of cow is over-represented with alopecia areata?
Eringer
Histopathologic findings with feline degenerative mucinotic mural folliculitis
moderate to severe inflammation - inflammatory cells included lymphocytes, macrophages, plasma cells, neutrophils; spread from surrounding dermis to isthmus of hair follicle – often with extension to infundibulum
What effect does estrogen have on the hair cycle?
inhibits hair growth – regulates the anagen-telogen hair follicle transition; decreases size of sebaceous glands and epidermal thickness (mice and rats); prolongs anagen in pregnant women; Shortens the anagen period, promotes catagen, lengthens the duration of telogen***; induction of of apoptosis by upregulation of signaling pathways (BMP4)
What is the role of TGF-beta-1 in the hair cycle?
induces premature catagen
What is congenital alopecia/hypotrichosis?
reduction in number of hair follicles formed OR hair follicles formed at normal density –> fail to regenerate following second or third catagen (additional ectodermal defects may exist)
Simple hair follicles are seen in what species?
omnivores and herbivores: horses, cattle, pigs, rats, mice, etc. (NOT rabbits - they have compound hair follicles)
Follicular lipidosis has been described in what breed(s)?
Rottweilers and a Dachshund dog
Name the layers of the hair follicle from Outside to Inside
Connective tissue sheath (fibrous sheath) –> outer root sheath –> inner root sheath (Henle’s layer –> Huxley’s layer) –> cuticle –> hair shaft cortex –> hair shaft medulla
Histopathologic findings with feline proliferative and necrotizing otitis and dermatitis?
marked epidermal and follicular hyperplasia with parakeratotic hyperkeratosis and lymphocytic, infundibular mural folliculitis and apoptosis
T/F: The inner root sheath is not present in telogen.
True - and the entire inferior portion of the hair follicle has regressed
Signalment of cats affected with feline degenerative mucinotic mural folliculitis
no breed predilection, males over-represented; median age of onset = 7 years
T/F: In a telogen follicle, there is no inner root sheath cornification.
True - all trichilemmal cornification from the outer root sheath
What treatment had the most success for proliferatve, lymphocytic, infundibular mural folliculitis with apoptosis and parakeratotic casts (PLIMF)?
cyclosporine
What effect would activation of the sonic hedgehog signaling gene pathway have?
increase duration of anagen, increase size of hair follicle
Which keratinizes first: Henle layer or Huxley layer?
Henle layer
T/F: Humans have androgen-responsive follicles.
True - androgens are associated with increased hair growth (except eyelashes and scalp hair)
Cells in the bulge area express high levels of what surface glycoprotein, that attenuates the inflammatory response
CD200
Clinical signs of follicular dysplasia are apparent during what phase of the hair cycle?
anagen
What is the role of the wingless (Wnt) signaling pathway in the hair cycle?
determines differentiation of epithelial stem cells into hair follicle keratinocytes
Nonmedullated hair of sheep
Wool
Diseases characterstic by isthmic mural folliculitis
pseudopelade, mucinotic mural folliculitis of lundehunds, degenerative mucinotic mural folliculitis, cutaneous and systemic lupus erythematosus (ECLE and GDLE primarily), granulomatous mural folliculitis
Breeds predisposed to cyclic flank alopecia?
Airdale terriers, Boxer, English bulldog, French bulldog
T/F: Feline degenerative mucinotic mural folliculitis responds well to glucocorticoids.
False - all cases published were euthanized after failure to respond to steroids
Describe skin and hair of Xolo dogs with the Hm mutation
Thick epidermis with melanocytes and prominent rete pegs; comedones; many aborted hair follicle plugs –> cannot produce hairs –> most hair follicles undergo atrophy
Histopathologic changes in cyclic flank alopecia
deformed telogen follicle (witch’s foot)
What is the most important environmental influence on the hair cycle?
day length
What are proposed effects of melatonin on the hair cycle?
may affect hair growth via increased concentrations of IGF-1 or GH; may induce anagen via estrogen receptor pathway; hair follicle itself can synthesize melatonin and the melatonin receptor is expressed in the hair follicle
Which portion of the hair follicle is immune privileged: upper or lower? What changes are present in this site?
lower - lower number of APCs, suppressed MHC II antigen presentation, absence of MHC I in ORS and matrix cells; immunosuppressive mediators (alphaMSH, TGFbeta-1, ACTH); bulb devoid of lymphatics
What effect does prolactin have on the hair cycle of sheep and minks?
stimulates shedding
Exogenous factors that can lead to premature telogen induction
glucocorticoids, estrogen
What are the components of the inner root sheath?
inner root sheath cuticle –> Huxley’s layer –> Henle’s layer –> companion layer
Does BMP-4 (bone morphogenic protein) inhibit or stimulate hair growth?
inhibit
What is the role of EGF in the hair cycle?
promotes catagen
Congenital alopecia/hypotrichosis is described in what species? Mode of inheritance?
some forms are X-linked; described in dogs and cattle
What effect does blocking the wingless (wnt) signaling pathway have?
prevents a new hair cycle
Describe the bulbar region of the hair follicle
Highly proliferative matrix cells (transit amplifying cells or matrix progenitors) intermingled with melanocytes; matrix cells enclose the mesenchymal fibroblasts of the dermal papilla – separated from the matrix cells by a thin basement membrane
T/F: Additional ectodermal defects may exist in combination with congenital alopecia/hypotrichosis.
TRUE - abnormal dentition, nails, lack of eccrine glands
What effect does prolactin have on the hair cycle of Cashmere goats?
induces anagen
Describe pathogenesis of acute telogen defluxion
Anagen is prematurely terminated –> all hairs are in telogen due to inciting factor –> telogen hairs shed in 2-3 months when follicles re-enter anagen
What is the role of bone morphogenic protein signaling in the hair cycle?
maintains hair in telogen; prevents anagen initiation
T/F: The medulla is not present in all hair shafts.
TRUE
Describe the infundibulum of the hair follicle
orifice of the hair follicle to a small area at which trichilemmal cornification is seen (approximately the opening of the sebaceous gland duct into the HF); innermost epithelial cells of the infundibulum have keratohyalin granules
What is the predominant inflammatory cell associated with mural mucinotic isthmus folliculitis of Lundehunds?
Cd3+ T cells
What effect does trauma or wounding have on the hair cycle?
causes anagen induction – due to proinflammatory cytokines, decrease of BMP4, upregulation of wnt (wingless) and shh (sonic hedgehog) signaling; does not work during refractory telogen (no anagen induction); TNF-alpha: accelerates wound healing, stimulates hair regeneration through activation of wnt (wingless) signaling
What effect does spaying have on FSH and LH concentrations?
increased FSH and LH, but LH receptors are decreased
T/F: Puppies lose their puppy coast in the first 12-28 weeks of life.
False: they gain an adult coat – primary hairs develop secondary hairs
What effect does estrogen have on human hair?
stimulates scalp hair growth – possible through anti-androgenic effects, or difference in estrogen receptors in human scalp hairs
What is Adamson’s fringe?
Point at which cornification of the inner root sheath stops (i.e. both the Huxley layer and Henle layer are fully cornified)
What signaling pathways are important in anagen initiation and promotion?
Sonic hedgehog (Shh) and Wingless (wnt) signaling
Histopathologic changes in the skin of Sphynx cats with the Hr mutation
abnormal IRS; small curved hair follicles, primarily in anagen; produce hair shafts that are small and fragile; increased kenogen compared to normal; hair follicle density and follicle group size were not significantly different from normal**
T/F: Melatonin can increase thyroid and adrenocortical activity.
False - has no effect on thyroid or adrenocortical activity
T/F: Spontaneous remission is possible with both alopecia areata and mural mucinotic isthmus folliculitis of Lundehunds
False - reported with AA but not with MMIF of Lundehunds
Signalment of dogs affected by alopecia areata?
GSD and Dachshund over-represented; no sex predilection; median agen of onset = 5 years
What is the role of fibroblast growth factor 5 in the hair cycle?
terminates anagen
T/F: Estrogen antagonists accelerate anagen once initiated by endogenous signals.
True - estrogen itself inhibits anagen, when it is removed, then the inhibition is removed
Diseases characterstic by infundibular mural folliculitis
demodectic mural folliculitis, dermatophytic mural folliculitis, cutaneous and systemic lupus erythematosus, thymoma-associated exfoliative dermatitis, proliferative lymphocytic mural folliculitis with apoptosis, proliferative and necrotizing otitis externa in cats, granulomatous mural follicullitis
Describe the changes that take place during early anagen (I-IIIb)
onset of growth, movement of dermal papilla into SQ, melanin production in bulb, DP becomes enclosed, hair shaft formation, hair reaches 2/3’s of follicle
What effect does noggin have on BMP-4 (bone morphogenic protein)? What effect does this have on the hair cycle?
noggin is an antagonist of BMP-4 –> activity of noggin leads to hair growth as signaling by Shh, Wnt1 and STAT3 are no longer suppressed (BMP-4 suppresses these pathways)
What is the role of insulin-like growth factor 1 in the hair cycle?
prevents the hair follicle from entering catagen; mediates growth promoting actions of GH on hair follicle growth
Describe the pathogenesis of anagen defluxion
anagen phase is interrupted and hair falls out in 7-14 days without entering catagen or telogen => immediate anagen release, normal appearing anagen hair bulb; once trigger is removed, hair regrowth in 3-4 months; dystrophic form caused by chemotherapy, toxins, etc. in which the hair bulb is abnormal
What anchors the hair shaft to the hair follicle in a catagen or telogen follicle?
trichilemmal keratin
T/F: Alopecia X is more common in females than males.
False - occurs equally in female and male dogs regardless of neuter status
What is the role of the vitamin D receptor in the hair cycle?
important for maintenance of keratinocyte stem cells in the bulge region; interacts with the wingless (wnt) signaling pathway; interacts with the sonic hedehog singaling pathway
What is the etiology of feline proliferative and necrotizing otitis and dermatitis?
UNKNOWN!! (IHC negative for herpes, calicivirus, papillomavirus) – aberrant immune response suspected due to response to topical or systemic steroids or calcineurin inhibitors
T/F: Color dilution alopecia is common in Weimaraners due to their dilute color.
FALSE
Disease(s) characteristic by inferior/bulbar mural folliculitis
alopecia areata
Portion of the hair follicle from the arrector pili muscle insertion to the entry of the sebaceous duct
Isthmus
Age of onset of Mural mucinotic isthmus folliculitis of Lundehunds?
1.5 years
In what part of the hair follicle are keratohyalin granules present?
infundibulum
Which portion of the hair follicle has a high density of APCs: upper or lower?
upper – the lower portion is “immune privileged” and has low numbers of APCs
