Adnexal Units (including diseases) Flashcards
Structure and function of the hair follicle, sebaceous gland, and sweat glands with important associated diseases.
What are the functions of hair?
- Thermoregulation (dependent on length, thickness, density, medulation, piloerection, and gloss)
- Physical protection (UV, chemical, physical, microbial)
- Social interactions (scent, piloerection, color)
- Camouflage
- Stem cells for wound healing
- Dispersing sweat/sebum
Most hair follicles develop before birth, which do not?
- All hair in marsupials
- Secondary follicles in dogs
Which tissues in the hair follicle are ectodermally derived?
- All epithelial components
- Sebaceous glands
- Apocrine glands
Which tissues in the hair follicle are mesodermally derived?
- Hair papilla
- Connective tissue sheath
Which tissues in the hair follicle are neural crest derived?
Melanocytes
Describe where hair first appears on the embryo.
Vibrisse on the chin, eyebrow, and upper lip for first then hair begins to form all over the head and then moves down
What are lanugo hairs?
- First hairs formed in utero
- Non-medulated
- Typically shed in humans before birth
What genes are likely responsible for non-random placement of hair?
- Homeobox genes
- Mice with Engrailed knockout get hair on paw pads
How many stages are there in hair follicle development?
8
(7 in anagen, 8 in catagen)
What happens to the genes and signaling molecules that are involved in the development of the hair follicle when the individual becomes an adult?
Most of the become involved in hair cycling.
What happens first in the development of the hair follicle (stage 1 aka hair placode stage)?
- An epithelial placode/hair placode/hair germ begins to form in the epidermis (goes on to form the entire epithelial portion) due to the influence of Wnt/β-catenin, EDA/EDAR, BMP, noggin, and follistatin
- Then the mesenchymal/dermal condensate forms underneath (goes on to form the dermal papilla) under the influence of Wnt, PDGF, and Shh
What happens second in the development of the hair follicle (stage 2 aka hair peg stage)
Epithelial placode proliferates, invades the dermis, and surrounds the dermal condensate largely mediated by Shh
(later stages will go on to differentiate into inner and outer root sheath and form bulges)
The outer root sheath of the hair peg goes on to form 3 bulges. What does the deepest bulge form?
The arrector pili attachment
The outer root sheath of the hair peg goes on to form 3 bulges. What does the middle bulge form?
Sebaceous gland
The outer root sheath of the hair peg goes on to form 3 bulges. What does the most superficial bulge form?
Sweat gland
What does the fibroblast growth factor 5 (FGF-5) gene do during hair follicle development?
- Responsible for hair length
- Is a catagen inducer in the mature follicle (so abnormalities lead to longer hair)
What does the keratin-71 gene do during hair follicle development?
Is responsible for hair curl
What does the R-spondin 2 gene dictate about hair?
Hair size and texture (and distribution)
What does the Wnt/β-catenin pathway do during hair development?
-“master switch”
- required for hair follicle induction and initial placode development
- originates from epidermal cells
- determines differentiation into hair follicle keratinocytes
- plays an important role in regulating expression of hair shaft keratin genes as nearly all of them have Lef-1 binding sites in their promoter region
- β-catenin is the downstream mediator of Wnt signaling
- once activated β-catenin translocates to the nucleus and interacts with the LEF/TCF family of genes (Lef-1) which then impact downstream genes including homeobox genes
- these are normally inactive (except for inducing anagen) in the adult epidermis but constant activation results in pilomatricomas and trichofolliculomas
What does EDA/EDAR do during hair development?
- Part of another major pathway that stimulates early follicle development/promotes follicle development
- Is essential for the development of multiple ectodermal tissues (hair, teeth, glands) so mutations in this result in ectodermal dysplasias
- Is a downstream mediator of Wnt signaling
- Mice overexpressing this have fused follicles due to lack of proper spacing
What does BMP (bone morphogenetic protein) do during hair development?
- Works in contrast to EDA/EDAR by inhibiting hair follicle/placode formation
- Is essential for correct distribution of follicles and differentiation of inner root sheath/hair shaft
- Induces catagen in the mature follicle
- Its antagonist is Noggin
What does noggin do during hair development?
- Antagonizes BMP
- Promotes placode fate by increasing Lef-1 expression and works to induce anagen in the mature follicle
What does TGF-α/EGF do during hair development?
Induce catagen in the mature follicle
What does the Notch pathway do during hair development?
- Comes from the hair bulb and outer root sheath
- Not that important for embryonic development
- Important for post-natal development leading to differentiation into hair and the the development of the shat and inner root sheath
- Also appears to control the phenotype of keratinocytes as they leave the bulb matrix
What does the Shh (sonic hedgehog) pathway do during hair development?
- Secreted in the follicular placode
- Plays a major role in epithelial-mesenchymal signaling
- Is essential for dermal placode maturation
- Is responsible for the elongation of the hair germ
- Aids in inducing anagen in the mature follicle
What does the Patched-1 (Ptc1) pathway do during hair development?
- Is the receptor for Shh
- Expressed in the germ cells and dermal papillae
What hairs are medullated vs not?
- Most in companion animals and terminal hairs in humans (on scalp, eyebrows, lashes, and elsewhere after puberty)
- Non-medullated hairs form the wool of sheep, fiber of angora goats, most Sphynx hairs, and vellus hairs on the face/arms/body of people
How do primary and secondary hairs differ?
- Primary hairs aka outer/guard hairs are larger in diameter; bulbs deeper in the dermis; have arrector pili, sebaceous glands, and sweat glands
- Secondary hairs aka undercoat/down hairs are more superficial and only have sebaceous glands
- Both are medullated
What is the difference between simple and compound hairs?
- Simple hair follicles only contain 1 hair, compound contain multiple
- Compound hairs develop post-natally (12-28 weeks in dogs)
How do cat and dog hair follicles differ?
Cats have a higher density of hairs with more compound hairs than dogs
- Dogs: compound with 1 large primary and 2 smaller secondaries from 1 pore
- Cats: single primary follicle with 2-5 compound hair follicles around them that contain 3 primary hairs and 6 to 12 secondary hairs
What is acromelanism?
In siamese, Himalayan, Balinse, and Birman cats there is a temperature dependent enzyme that convers melanin precursors to melanin so higher temps hair is light and lower temp hair is dark
Describe sheep hair follicles?
- Wool follicles in clusters with 3 primary follicles and 6x more secondary follicles than primary
- Higher in wool breeds
Where is the mutation in curly coat/sphynx/Devon rexes?
Keratin 71
What are keratin 71 mutations associated with?
Wavy/curly/wooly phenotype in dogs, cats, and humans
What are keratin 25 mutations associated with?
Wavy/curly/wooly phenotype in horses, humans, and mice
What are keratin 27 mutations associated with?
Wavy/curly/wooly phenotype in cattle and mice
How do hair follicle keratins organize differently than other keratin intermediate filaments?
- In the hair follicle, keratins organize such that they are stretched out longitudinally (“macrofibrils”) then twist
- Are crosslinked with keratin associated proteins (KAPs) in addition to disulfide bonds to make it stronger
Which keratin promotes anagen in the hair follicle and how?
K17, attenuates TNF-a-induced apoptosis in matrix keratinocytes
What stimuli are involved in directing hair follicle stem cells into sebaceous cells?
BLIMP1
What molecules are involved in maintenance of stem cell quiescence in the hair follicle?
- NFATc1
- Bmp6
What is the primary mediator of hair follicle elongation and dermal papilla development?
Shh
What is the hair bulb?
Thickening of the proximal end of the hair follicle
Contains rapidly proliferating, undifferentiated matrix cells, melanocytes, and ORS cells
What is the follicular bulge?
Insertion site of the arrector pili muscle
Contains hair follicle stem cells
Not specifically present in dogs
What are club hairs?
Fully keratinized hair present during telogen and catagen
What is the dermal papilla?
Mesodermal signaling center of the hair follicle
Contains fibroblasts
Continuous with dermal connective tissue
Aaro-Perkins corpuscle below; hair matrix above
What is the hair shaft?
The hair, composed of terminally differentiated hair follicle keratinocytes
Central medulla, outer cortex, melanin granules, and hair cuticle
Where in the hair shaft are melanin granules most important for coloration?
Cortex
What is the inferior segment of the follicle?
Most distal portion AKA suprabulbar region
Extends from the bulge/ arrector pili attachment to the bulb
Cycling portion of the hair follicle (ie- not always present)
What is the infundibulum of the follicle?
Proximal portion of the follicle
Extends from the sebaceous duct to the epidermal surface
What is the inner root sheath?
Rigid tube composed of terminally differentiated hair follicle keratinocytes
Surrounded by ORS
Layers (from inside- out):
- IRS cuticle
- Huxley
- Henle
- companion
What is the isthmus of the follicle?
Middle part
Extends from the bulge/ arrector pili attachment to the sebaceous duct
What is the outer root sheath?
Outermost layer of follicle
Proximally, merges with the basal layer of the epidermis
Distally merges with the hair bulb
What is Adamson’s fringe?
Where hair becomes fully keratinized (upper margin of keratogenous zone)
- where tricholemmal keratinization ends
- dermatophyte invasion of the shaft stops here
Differentiates between regenerative vs non-regenerative portion of follicle
What is the dermal papilla composed of?
fibroblasts, collagen bundles, mucopolysaccharide rich stroma, nerve fibers,
single capillary loop
When is the hair bulb maximal in volume?
Anagen
What are the permanent portions of the hair follicular structure?
infundibulum and isthmus
How many hairs/follicle do chinchillas have?
Up to 60
Where are medullated hairs found on companion animals?
Most hairs in companion animals are medullated
Medullated hairs can be primary (outer or guard hairs) or secondary (undercoat, down hairs)
What do primary follicles have that secondary follicles lack?
Epitrichial sweat gland
+/- arrector pili
(both have sebaceous glands)
What animals have non-medullated hair as adults?
Sphynx cats
Wool of sheep
Fiber of angora goats
Human vellus hairs
What are the two tactile hair types in animals?
Sinus hairs
Tylotrich hairs
What is the fibrous connective tissue sheath aka hyalin membrane of the hair follicle primarily composed of?
Collagen III
Where is the outer root sheath the thickest?
Near the epidermis
Just a single layer of flattened cells near the bulb
What do the cells of the outer root sheath in the inferior segment just above the bulb look like?
large and contain glycogen so that their cytoplasm is clear and vacuolated
What do the cells of the outer root sheath do in the isthmus?
undergo trichilemmal keratinization (without keratohyalin granules)
- occurs where inner root sheath begins to slough
–> so is happening where where ORS is not apposed to the IRS
What is trichilemmal keratinization?
abrupt conversion of the non-keratinized ORS cells into anuclear keratin
Where are keratohyalin granules present in the outer root sheath?
Infundibulum
(keratinization is happening just like in the epidermis)
Where are follicular keratinocyte and melanocyte stem cells found in humans and mice?
ORS at the follicular bulge region
Where are follicular keratinocyte and melanocyte stem cells found in dogs?
stem cells are concentrated near the region of the arrector pili attachment
but are also widely distributed in isthmus
What surrounds the ORS?
The glassy membrane - continuous with the basement membrane
* mineralizes in toy poodles and Bedlington terriers
* can be senile change
Where is the inner root sheath present in the hair follicle?
bulb to the isthmus
When in the hair cycle is the inner root sheath absent?
telogen
Where does the inner root sheath keratinize and disintegrates?
same level where trichilemmal keratinization begins in the ORS
- at the isthmus
What is the inner most layer of the inner root sheath?
Cuticle
Forms “shingled roof pattern”
interacts with the hair shaft cuticle to anchor the hair shaft to the follicle
Why does the inner root sheath mold/shape the hair?
IRS keratinizes/ hardens in advance of the hair shaft
What is the outer most layer of the inner root sheath?
companion layer
slippage plane between the stationary ORS and the upward moving IRS
is tightly bound to the Henle layer
Is the Henle or Huxley layer closer to the outside of the hair follicle?
Henle
Which is the first layer of the inner root sheath to keratinize
Henle
*Huxley will keratinize above the Henle layer
What is a granules are a unique feature of the cells of the Henle and Huxley layers of the inner root sheath?
trichohyalin granules (pink on histopathology)
morphological hallmark of the IRS (and the hair medulla)
IRS equivalent of keratohyalin granules
What is the Line of Auber?
A line across the widest part of papilla
Most of the mitotic activity is below this line
What is the hair shaft epicuticle?
Amorphous external layer derived from cuticular cells as an exocellular secretion or from outer portion of cuticular cell membranes
What it the hair shaft cuticle?
Covers hair, interacts with the IRS cuticle shingles
What is the hair shaft cortex?
bulk of the hair shaft
between the cuticle and the medulla
composed of cornified, spindle shaped cells
provides the mechanical strength
melanin here determines hair shaft color
What is the inner most portion of the hair shaft?
medulla
composed of cuboidal cells with trichohyalin granules at the base and air/glycogen vacuoles distally
pigment may be here but doesn’t have much influence on color
What are two mechanisms proposed in the greying of hair?
depletion of the melanocytic stem cell reservoir with repeated cycling
damage to the pigmentary unit via ROS
- melanin is a ROS scavenger for the highly active matrix keratinocytes
What is the wild-type/classic hair color in dogs?
agouti
produces a light tip, a heavily pigmented body and a yellow or red base
What is the wild-type/ classic hair pattern in cats?
tabby/ agouti
produces a black tip, yellow banding in the body and a blue base
In what type of pattern does hair cycling in animals typically occur?
mosaic pattern
How does hair cycling in rabbits and rodents (except Guinea pigs) occur?
synchronized, orderly waves that starts ventrally between the front legs and progresses dorsally and caudally
What stage of the hair cycle dominate in most animals?
telogen
What is induction of anagen dependent on?
Wnt, noggin and Shh signaling (as well as KGF)
What happens during early anagen?
Onset of growth
Dermal papilla moves into the subcutaneous tissue
- via a remaining fibrous streamer
Dermal papilla becomes enclosed by hair matrix cells
Melanin production in the bulb
Hair matrix cells begin to differentiate into hair shaft and IRS
- Dependent on Wnt signaling and BMP
What happens during late anagen?
Dermal papilla is fully enclose and deep into the subcutaneous tissue
Hair shaft is fully developed and surrounded by IRS
Hair reaches the surface and club hair is lost (exogen)
What is IGF-1 in the context of the hair cycle?
A potent anagen stimulator
What is induction of catagen dependent on?
BMP s
TGF-α
FGF-5
vitamin D receptor activity
What happens during early catagen?
Melanogenesis stops
The bulb narrows and mitotic activity stops
- apoptotic keratinocytes may be visible
Dermal papilla changes shape and moves upward
Trailing connective tissue sheath becomes visible
What happens during late catagen?
Shortened hair follicle
Club hair is formed
Trailing connective tissue sheath eventually shrinks and disappears
What happens during telogen?
Hair follicle and the dermal are in more superficial dermis
Club hair firmly anchored in trichilemmal keratin
There is no IRS or trailing connective tissue sheath
What is exogen?
Shedding
Timing varies
o Early exogen – hairs actively retained
o Late exogen – hairs ready to shed
There may be a link between desquamation and shedding
What is kenogen?
Empty follicle after exogen
o Denotes prolonged telogen state ; not always pathologic
ORS with no hair or luminal cornified debris
Is temperature or light the most important part of the affect of season on hair growth?
Photoperiod
Hypothalamus, hypophysis and pineal gland are stimulated by light to produce hormones which influence hair cycling
Hair growth increases in the summer, decreases in winter, shed fall/spring
What is seasonal alopecia in ferrets?
a very dramatic spring molt
How does melatonin influence the hair cycle?
Increases the production of prolactin –> inhibits hair shaft elongation
- induces catagen in most species
- except it induces anagen in cashmere goats
Downregulates apoptosis and estrogen receptor expression in hair follicles
It is a free radical scavenger and DNA repair inducer (protective in anagen?)
Influences sex hormone concentrations which can then affect hair cycling
How does daylight affect melatonin?
Decreased production with long daylight periods
How does daylight affect catagen?
long daylight = less melatonin = less prolactin to induce catagen
How do thyroid hormones influence the hair cycle?
Thyroxine (T4) stimulates anagen and stem cell differentiation
T3 and T4 increase melanogenesis in follicles
T3 and T4 down regulate apoptosis, prevent catagen
Hypothyroid = less T3/T4 = increased number of follicles in telogen
How do glucocorticoids/ cortisol influence the hair cycle?
Role in hair cycling is not fully understood but suppress anagen
- Dexamethasone can induce catagen in mice
Reduced synthesis and increased degradation of hyaluronans and proteoglycans needed for follicular function
How does progesterone influence the hair cycle?
Poorly defined effects on hair cycling
Binds to glucocorticoid receptor in dog and cross-reacts with the testosterone receptor
How do estrogens influence the hair cycle?
Shorten anagen, promote catagen, lengthen telogen in dogs
▪ Opposite effect in people: stimulates scalp hair growth
- Possibly via anti-androgenic effects or differing receptor expression
Stimulates BMP expression
Decreases size of sebaceous glands and epidermal thickness
How do androgens influence the hair cycle?
Humans: androgens stimulate hair growth everywhere
- except the scalp where they inhibit hair growth
▪ Scalp hairs become miniaturized with a shortened anagen
Testosterone seems to have minimal effects on canine hair follicle growth
How does spaying/ neutering can affect coat quality?
Occurs in ~20% of animals
Woolly coat: increased undercoat, increased curl, dulling of color
The pathomechanism is unknown
Where are Langerhans cells found in the hair follicle?
ORS of the upper portion
- This population can serve as a reserve to repopulate the epidermis
What is the immune status of the lower portion of the hair follicle in a healthy state?
Relative immune privilege
Primarily for the ORS stem cell population and anagen bulb
How does the lower portion of the hair follicle sustain an immune privileged state?
Absence of MHC class I expression
- melanocytes work to downregulate this
Bulge has increased expression of the “no danger” signal, CD200
MHC class II expression is reduced greatly in the bulb and some in the bulge
Extracellular matrix has high concentration of glycosaminoglycans
- thought to impede T-cell trafficking
What immunosuppressive molecules does the hair follicle produce?
TGF-β2 and α-MSH
- impair antigen presenting cell function
- prevent NK/ CD8+ cell attacks
IL-10 and MIF (macrophage migration inhibition factor)
What does sebum consist of?
- Lipid
- Keratohyline granules
- Keratin
- Associated cellular debris
- Inorganic salts
- Anti-microbial proteins
- Antiviral gycoprotein interferon
- Albumin
- Transferrin
- Complement
- Glucocorticoid
- Immunoglobulins
What are the functions of sebum?
- Lubricating
- Antimicrobial
- Limit water loss/are water-repelling
What form of secretion do sebaceous glands have?
Holocrine
In what species are sebaceous glands sparse and inconspicuous?
Pigs
In what species are sebaceous glands numerous and prominent?
Cattle, horses, cats, and dogs
What forms “lather” in horses?
Sebaceous secretions and protein-rich (latherin) sweat
Where are sebaceous glands primarily located in sheep?
Infraorbital, inguinal, and interdigital
Where are sebaceous glands primarily located in goats?
Base-of-horn glands
What do dog and cat sebaceous glands produce more of than human sebaceous glands?
- Sterol esters
- Free cholesterol
- Cholesterol esters
- Wax diesters
What do dog and cat sebaceous glands produce less of than human sebaceous glands?
- Triglycerides
- Monoglycerides
- FFAs
- Monoester waxes
- Squalene
What are the free fatty acids present in sebum and what is their primary role?
- Linoleic
- Myristic
- Oleic
- Palmitic acids
(are antimicrobial)
Which bacteria are present in the hair infundibulum that produce FFAs?
- Lipase producing bacteria
- Propionibacterium and Staph
Eccrine aka atrichial sweat glands produce secretions in what manner?
Merocrine
Where do eccrine aka atrichial sweat glands empty to?
- Directly onto the skin surface
- Openings of ducts are not usually associated with hair follicles
- Ducts penetrate cornified epithelium and have a corkscrew appearance
Which sweat gland type is prominent in humans?
Eccrine
Where are eccrine aka atrichial glands in animals?
- Foot pads of carnivores
- Frogs of horses
- Noses of pigs and cattle
What do apocrine aka epitrichial glands look like and where do they open to?
- Distal end may be coiled into a spherical structure or may have a spiral appearance
- Open into the hair follicle
What are the secretions of apocrine aka epitrichial glands like and what do they contain?
- Viscous and cloudy
- Contain IgA
- May have pheromones
- Electrolytes (Na, K, Cl) that can be antimicrobial
How do you test for anhydrosis in horses?
Inject them with epinephrine
What is the genetic mutation leading to anhydrosis in horses?
Aquaporin 5
What tonicity is equine sweat?
Isotonic to hypertonic due to Na, K, and Cl
Primary follicles are located ___ in the dermis than secondary follicles and are ___ diameter.
1) deeper
2) larger
What is a simple hair follicle?
When only a single hair is present
What is a complex hair follicle?
When two or more hairs exit a pore
Describe the organization of dog hair follicles.
Dogs have compound follicles with a primary hair and smaller secondary hairs that occur in clusters of 3
Describe the organization of feline hair follicles.
Cats have a single simple follicle surrounded by 2-5 compound follicles with a primary hair and 6-12 secondary hairs
Describe the organization of equine and bovine hair follicles.
They only have primary hair follicles
Describe the organization of porcine hair follicles.
They have clusters of 3-4 primary follicles
Describe the organization of ovine hair follicles.
Clusters of 3 primary follicles interspersed among secondary follicles which can be 6 or more (the number of secondary follicles is greater in wool sheep than other breeds)
Where are sinus hairs located?
Around the lips, cheek, chin, and eyes (and around the carpi in cats)
What are sinus hairs?
- Hairs that grow from specialized follicles and have sensory/tactile function
- Have a blood-filled sinus
Why are sinus hairs so sensitive?
There are numerous free nerve endings within the inner dermal sheath which extend to the outer sheath
When does the first hair cycle occur and what happens on subsequent cycles?
The first hair cycle occurs in the fetus and through subsequent cycles the hair increases in diameter and coarseness (puppy hair –> adult hair)
What hair cycle stage predominates in dogs with long hair?
Anagen
What stage of the hair cycle does doxorubicin affect?
Anagen
What appendages do sinus hairs have?
- Skeletal muscle
- Poorly developed sebaceous glands
- No sweat glands
How is the acidic pH of the skin maintained?
- Lactic acid and ammonia in sweat
- Amino acids from sebum
What are the most common pustular and nodular diseases without adnexal destruction?
Superficial bacterial folliculitis (common in dog and rare in cats)
Dermatophytosis
Pemphigus foliaceus
Sterile eosinophilic pustulosis
- rare, idiopathic canine disease
- sterile eosinophilic pustules and peripheral eosinophilia
What are some pustular and nodular diseases with adnexal destruction?
Bacterial furunculosis (including interdigital and post-grooming)
GSD pyoderma
Acral lick dermatitis
Actinic furunculosis
Acne (once there is furunculosis)
Dermatophytic kerion
Demodicosis
Pelodera dermatitis
Eosinophilic furunculosis of the face
What are some mural diseases of the hair follicle?
Inflammation directed to follicular wall, inflammation involving only infundibulum, isthmus or hair bulb
- Alopecia areata
- Mural folliculitis due to demodicosis and dermatophytosis
- Pseudopelade
- Eosinophilic mucinotic mural folliculitis in dogs
- Degenerative mucinotic mural folliculitis in cats
- Granulomatous mural folliculitis in dogs
- Follicular mucinosis
What is telogen effluvium?
Rare, widespread alopecia in response to severe metabolic stress
▪ illness, parturition, surgery, anesthesia, etc
▪ Drugs: doxorubicin
Anagen is terminated and all hairs enter telogen –> shed at same time
▪ Usually 1-3 months after the inciting incident
▪ Face is usually spared
▪ Resolves spontaneously
Histopathology: kenogen but no atrophy or lots of early anagen hairs
Non-scarring
What is anagen effluvium/ defluxion?
Very uncommon
Non-scarring
Anagen phase is interrupted and hair is damaged –> falls out in anagen
▪ Trichogram: hair shaft is damaged, tapered fractures of anagen hairs
Caused by toxins, chemotherapy, other insults
Hair loss occurs within days of the insult
What is congenital hypotrichosis/alopecia?
May be born without normal hair (reduced number of follicles in utero)
May have hair loss within 4 wk to 6 mo of life (hair fails to regenerate/ cycle following the 2nd or 3rd catagen)
- Some forms are X-linked (EDA/EDAR mutations in ppl, dogs, cattle
- Some forms are autosomal dominant (Mexican hairless/Chinese crested)
- Some form are autosomal recessive (Sphynx)
- May be associated with other ectodermal defects
What is aplasia of hair follicle with dental dysplasia?
An ectodermal defect
Congenital defect in more than one ectoderm-derived tissue
Humans: mutations in X-linked Eda gene 1
In dogs: X-linked mutation in Eda so occurs primarily in males
- multifocal alopecia of frontotemporal, sacral, abd, proximal limbs
- no arrector pili muscles, sebaceous glands or apocrine glands
- dental alternations of deciduous and permanent teeth
What causes congenital hypotrichosis in Mexican hairless, Peruvian hairless, and Chinese crested dogs?
Frameshift mutation within the FOXI3
- a form of ectodermal dysplasia
- may also have abnormal teeth
Heterozygous dogs are hairless
Homozygous mutants die during embryogenesis
Variably dysplastic follicles, mostly anagen, have sebaceous/sweat glands
What is the most common form of ectodermal dysplasia in most animals?
anhidrotic/hypohidrotic ectodermal dysplasia (XLED or XLHED) caused by mutations in the X-linked EDA gene
What is congenital hypotrichosis in horses?
A form of hair follicle dysplasia
Otherwise normal blue roan Percheron born with a partial alopecia
- progressed to being a complete, body-wide alopecia by 1 year of age Histologic changes consisted of follicular hypoplasia and hyperkeratosis
- Catagen and telogen hair follicles predominated
- Excess melanin within the lumina of follicles and sebaceous glands
Anecdotal reports of a hereditary hypotrichosis in Arabians
- Hair loss is symmetric, on the face, primarily around the eyes
What is congenital progressive alopecia in cattle?
Hair follicle dysplasia in polled Hereford cattle
- born with wiry hair and alopecia on nose and ears
- alopecia progresses with excessive scaling
- hepatic fibrosis, subtle neuro abnormalities
What is lethal hypotrichosis in cattle?
Hair follicle dysplasia in Holsteins
- autosomal recessive
- generalized alopecia
- calves die within hours of birth
What is viable hypotrichosis in cattle?
A hair follicle dysplasia reported in Guernsey, Jersey, Holstein, Hereford
- autosomal recessive with generalized alopecia
- dysplastic HF do not form hair shafts
What is black hair follicle dysplasia?
Reported in dogs and cattle
Collies, JRTs, Münsterländer dogs, salukis, and a New Zealand huntaway dog
Autosomal recessive
Looks a lot like color dilution alopecia, just affects black hairs
▪ Disorder of melanosome transfer or migration
▪ Other color hair will be normal
Coat changes usually by 4 weeks of age (up to 1 yr)
What causes hypotrichosis in sphynx cats?
Mutation in the KRT71 gene
Autosomal recessive
Abnormal shaft production without a decrease in follicle quantity
- follicles are small, curved and kinked
- infundibular hyperkeratosis and dilatation
- shafts that protrude from follicular ostia do not reach normal length
What is color dilution alopecia?
Reported in dogs and cats
▪ Individuals with blue, fawn or grey/ silver coats
▪ Gradual onset, usually by 3 years of age
▪ Weimaraners have their own mild, rare form
Form macromelanosomes due to irregular melanin transfer –> hair fragility
Likely multifactorial, D-locus encodes other follicular changes
Dorsal truck largely affected
Prone to secondary infections
What is follicular lipidosis?
A form of color-linked follicular dysplasia
Variable degreed of hair loss affecting mahogany points in rottweilers
Present within the first year of life
Lipid accumulation in matrix keratinocytes of anagen bulb
- Ballooning of matrix cells, nuclear pyknosis, absence bulb inflammation
- Primary > secondary hairs
What are the common clinical features of follicular dysplasias (not ectodermal dysplasias) in water dogs and which breeds are predisposed?
Loss of coat following damage to hairs
- esp neck where collar lies
- periocular, flank/saddle region
Poor coat quality, color change, poor regrowth after clip
- Primary (guard hairs) affected
- Onset in puppyhood
- Progressive alopecia (may be cyclical but eventually permanent)
Portuguese/Spanish water dogs, Irish water spaniels, curly coated retrievers, Chesapeake Bay Retrievers
* all are technically described separately but share key features
What is the follicular dysplasia seen in Siberian huskies and malamutes?
Onset at 3-4 months of age
Loss of primary but retention of secondary hairs
Coat color change to reddish brown.
What is the follicular dysplasia seen in Doberman pinchers?
Affects red or black dogs
Onset between 1 and 4 years of age
Flank and saddle region involved initially
- With time, the hair along the dorsum is lost but retained elsewhere
What are the histopathologic findings associated with non-ectodermal hair follicle dysplasias in dogs?
Normal epidermis and dermis
Infundibular dilatation and keratin plugging (hyperkeratosis)
Hair follicle w/ irregular outer contours +/- vacuolar change
Scattered clumps of melanin
Small anagen or telogen (but decreased number in anagen)
Variably distorted/atrophic (absent, fragmented, or thin) hair shafts
What is suspected to play a role in the pathogenesis of follicular dysplasias in Irish water spaniels?
Abnormalities of the steroidogenic pathways
Dietary factors and sexual hormones
- Dietary changes improved coat and skin quality
Thought to be an autosomal dominant trait
What is recurrent flank alopecia?
Repeated well-demarcated serpiginous alopecia w/ hyperpigmentation
- Usually affects bilateral flank > dorsal midline, thorax
Hair loss common in late autumn to early spring and resolves by late spring
- Thought to be due to photoperiod and melatonin
- Melatonin is commonly a treatment
Likely strong intrinsic components as well
Strong breed predilection so may have genetic component
What breeds are predisposed to typical recurrent flank alopecia?
Boxers
Rhodesian ridgebacks
Airedale terriers
- May cross over dorsum in an “H” saddle pattern
French Bulldogs
English bulldogs
Schnauzers
What is the histopathology of recurrent flank alopecia?
Mildly acanthotic epidermis
Often hypermelanotic epidermis (including SC and sebaceous glands)
Infundibula are dilated, elongated, and plugged with orthokeratosis
Lower compound HF are short, atrophic, distorted
- “witches’ feet”
Recurrent flank alopecia
What is atypical recurrent flank alopecia?
More common in hunting dogs (ex: German shorthair pointer)
Facial only has been reported in Dogues de Bordeaux and Cane Corso
No hyperpigmentation
Tends to be more generalized, severe, and permanent than the typical form
Histopathology is similar to “typical”
What is alopecia X?
Affects plush coated breeds
- True alopecia X in Pomeranians
- Alopecia X-like in all other breeds
Usually affects between 1 and 5 years of age
Affects males > females, thought to have a hormonal component
- variable results on a large hormone panel study
- 17-hydroxyprogesterone (17-OHP) is increased
Bilaterally symmetrical hair loss and hyperpigmentation
- spares the head and distal limbs
What are the categories of causes for an increased the percentage of hairs in kenogen?
1) Signals that induce a new anagen phase at the end of the telogen phase are missing/reduced
2) Exogen has occurred prematurely, follicle not ready for new anagen
3) Signals which are maintaining the anagen phase (anagen promotion) are missing
Which hormones or stress related molecules are inducers of premature telogen?
Glucocorticoids
Estrogens
Which hormones or stress related molecules prolong anagen?
TSH
T3/4
Which hormones or stress related molecules are inducers of premature catagen?
Glucocorticoids
Estrogens
Substance P
Nerve growth factor
Which hormones or stress related molecules are inducers of anagen?
TSH
T3/4
Which hormones or stress related molecules are inhibitors of anagen?
Glucocorticoids
Estrogens
Testosterone
Substance P
Nerve growth factor
This dog was born looking like this. What genetic mutation do you suspect?
A form of ectodermal dysplasia (probably FOX13)
White arrows: dysplastic epidermal portion of follicle
Black arrows: mesenchymal cells aggregates
What causes the hairless phenotype of American hairless rat terriers?
Mutation in SGK3
Autosomal recessive
Puppies are born with a sparse, fuzzy coat over their entire body
- gradually lose it from nose back
- spare sinus hairs
- may have dental abnormalities
A Spanish water dog presents with regionalized hair loss on the dorsum, the rump, and parts of the lateral thorax. What is occuring?
Follicular dysplasia in Spanish and Portuguese water dogs
note
- B: shaft and IRS formation is impaired in the suprabulbar region
- C: fragmented hair shaft
- D: Vacuolar changes in the IRS and the matrix cells
Color dilution alopecia
What non-cutaneous abnormalities have been found for Pomeranians with alopecia X?
7-hydroxyprogesterone (17-OHP) is increased
–> thought to be due to abnormal activity of the 21-hydroxylase enzyme
baseline cortisol concentrations correlated with progesterone concentrations
potential abnormal cutaneous steroidogenesis characterized by the dysregulation of genes
- however, relevant gene variants were not identified
suggested that there are an increased number of mitochondrial gene mutations
What are the histopathologic findings of alopecia X?
reduced number of anagen follicles
relatively high number of telogen/kenogen/atrophic follicles
looks similar to endocrinopathies (esp hyperestrogenism)
What is trichorrhexis nodosa?
nodes are formed along the hair shaft
hair is abnormally fragile following trivial injury
can be acquired or congenital
described in golden retrievers and horses
What is congenital trichoptilosis?
a hair shaft abnormality that causes hair to split and fracture due to a defect in the hair shaft’s structure or damage to the cuticle layer
reported in related male golden retrievers
What is medullary trichomalacia?
name proposed for a hair shaft abnormality that was recognized in 6 German shepherd dogs
loss of architecture, swelling, and apparent softening of the medulla, followed by longitudinal (lengthwise) splitting and breakage of the hair shaft
What conditions have been reported in dogs that result in structural defects/breakage of the hair?
Trichorrhexis nodosa
Trichoptilosis
Medullary trichomalacia
Color dilution/black hair follicle dysplasia
Bald thigh syndrome
Bald thigh syndrome
note that hair shafts are fractured and trichilemmal keratin is assembled underneath the hair shaft
What is canine pattern alopecia (pattern baldness)?
Mostly affects breed w/ smooth hair coats
- ex. Dachshund, Boston terrier, Chihuahua
- may have genetic basis
Develops within the 1st year of life
Develop a bilaterally symmetric thin coat on the pinnae/nearby skin, caudal thighs, perineal skin, ventral neck/chest/abdomen
Alopecia thought to be associated with miniaturization of the hair follicles
- may have increased telogen
- similar to humans with female or male pattern baldness
- due to a shorted anagen phase?
How does pituitary dwarfism affect hair coat?
affected dogs lack primary hairs and have prominent secondary hairs
- resulting in a puppy coat
over time, bilateral symmetrical alopecia progresses and the skin becomes hyperpigmented
What is pili torti?
Secondary hairs are flattened and rotated
What is the hair shaft disorder of Abyssinian cats?
Only affects whiskers and primary hairs
Onion-shaped swelling at the tip of the hair
* Fracture more easily, coat appears rough and lusterless
What is spiculosis?
Reported in male Kerry blue terriers
▪ Hard, brittle follicular spicules
What are the histopathologic features typical of hyperestrogenism?
decreased anagen follicles
mainly telogen and kenogen follicles
*like alopecia X
*may also have follicular atrophy
What are the typical clinical features of hair loss associated with an endocrinopathy?
Nonpruritic , bilateral symmetrical hypotrichosis /alopecia
Rest of coat is dull, dry, fails to regrow after clipping
*often have other signs of the endocrinopathy
*“Zebra striped pattern” with hypothyroid in Rhodesians
What are the histopathologic features typical of hyperadrenocorticism?
abundant kenogen follicles
severe follicular, dermal, and epidermal atrophy
- atrophy of sebaceous glands to
comedones
+/- calcinosis cutis
What are the histopathologic features typical of hypothyroidism?
primary hairs in anagen or telogen, normal size
secondary hairs in kenogen or atrophic
epidermal hyperplasia (if concurrent infections are present)
rare dermal mucinosis
What is post-clipping alopecia?
characterized by a lack of hair regrowth after clipping
underlying cause is unclear
occur more often in dogs with a long telogen HC phase
-eg. Siberian huskies and other breeds with a thick undercoat
- maybe it just takes a long time to reach anagen again
may also have to do with vascular perfusion changes
What is traction alopecia?
chronic partial ischemia not associated with an underlying vascular pathology
- prolonged traction causes shearing/compression of small vessels
often on top of head in long-haired dogs
once developed, alopecia is long term and generally permanent
histopathology shows atrophy of adnexal units and dermal fibrosis
What are flame follicles?
exaggerated regressing follicles with large spikes of fused keratin protruding thru ORS to vitreous layer –> result in abnormal catagen phase with overabundant tricholemmal keratin
What is alopecia associated with ischemic dermatopathy?
caused by impaired vascular perfusion
- underlying vasculitis or vasculopathy
~50% of cases associated with vaccines
focal or multifocal alopecia typically at vaccine site or extremities
- may have erosions/ulcers/crusts, scale, erythema, hyperpigmentation
What is histopathology finding associated with ischemic dermatopathy?
Histopathology:
- primary and secondary HFs are severely atrophic and may be absent
–> aka “fading” follicles
- Perifollicular collagen is prominent and homogenized
- +/- lymphoplasmacytic cell poor interface dermatitis
- inflammation of the vessel walls is seen rarely in dogs
–> may have loss of vascular distinction or endothelial cells
Other than alopecia, what can high doses of doxorubicin for a long time cause?
ulcerative erythroderma
What is post traumatic alopecia?
Reported in cats
shearing trauma to elastic skin results in separation from blood supply
looks a lot like ischemic dermatopathy
What is proliferative, lymphocytic, infundibular mural folliculitits with apoptosis and parakeratotic casts?
Reported in Labrador retrievers (5 ever)
- Rare, 6 years old
- VERY THICK adherent hyperkeratotic plaques
What neoplasia are frequently associated with widespread hair loss?
Pancreatic neoplasia/biliary carcinoma: Paraneoplastic alopecia (cats)
Thymoma: feline thymoma exfoliative dermatitis
Cutaneous lymphoma
Occasionally neoplasias treated with doxorubicin
What is pseudopelade?
rare immune mediated skin disease in dog and cat
novel lymphocytic isthmus mural folliculitis
manifests clinically as a visually noninflammatory alopecia
causes permanent hair loss
What is degenerative mucinotic mural folliculitis?
rare, presumptively immunological skin disease of cats
generalized alopecia
striking thickening of facial skin
varying degrees of lethargy
causes mural folliculitis of the isthmus with mixed infiltrate
What is granulomatous mural folliculitis?
rare, presumed immunological syndrome
highly variable alopecia
may be an adverse drug reaction in dogs
associated with methimazole in cats
virus associated in goat (catarrhal fever)
What is follicular mucinosis?
very rare, presumed immunologically mediated, alopecic skin disease of dog and cat
What is alopecia areata?
uncommon autoimmune skin disease
Seen in humans, primates, dogs, cats, cattle, poultry, mice, and rats
causes non-scarring well demarcated alopecia +/- leukotrichia
- commonly on face/head but can affect whole body (universalis)
- may also have nail changes
- horses it is usually wide-spread but may be neck, mane, and tail
- can be pruritic if have secondary infection
What are the typical histopathologic findings associated with alopecia areata?
Lymphocytic bulbitis which may progress to isthmus
- “swarm of bees”
What causes alopecia areata and what is the treatment?
caused by a loss of immune tolerance
- IgG against lower hair follicle
- mostly CD3+ T cells (+/- CD8+)
- may have a genetic component
- may be triggered
Treatment is immunosuppression - maybe calcineurin inhibitor?
- mixed response
- can occur in people in the face of cyclosporine treatment
What breed of horse is predisposed to alopecia areata?
Appaloosas and palominos (color not breed) are predisposed
- can have dystrophic hooves (based on one case)
*there is a genetic predisposition in humans
Alopecia areata
(good differential would be cutaneous epitheliotropic lymphoma since it can preferentially affect hair follicles)
What animals are sebaceous glands not present in?
Whale and porpoises
Where on the body of mammals are sebaceous glands not present?
Paw pad and nasal planum
Describe the tail gland in carnivores?
oval region on cats and dogs
around the 5th -7th coccygeal vertebrae
stiff, coarse hairs from simple follicles
has many large sebaceous glands
Where are reserve cells for sebaceous glands found?
surrounding basement membrane
Where do sebaceous ducts open onto?
infundibulum of the hair follicle
What receptors on the sebocytes causes lipid accumulation from circulation?
LDL receptor
FATP4
What hormones influence sebocytes?
androgens - hypertrophy and hyperplasia
estrogens and glucocorticoids - induce involution
retinoids - reduce sebum production
What do members of the Equidae family have that is unique to their sebum?
Lactones
Which carnivore species has an increased amount of sebum production?
Ferrets
What is sebaceous adenitis?
Uncommon skin disease in the dog and rare in cats, rabbits, horses, and humans
Inflammatory condition resulting in T-cell-mediated, immunologic
destruction of the sebaceous glands
Unknown pathogenesis but may have a genetic component
- autosomal recessive in Akitas and poodles
- might be an abnormality in lipid metabolism or storage or keratinization defect
- may be associated with other diseases like hypothyroidism or Leishmaniasis
What breeds of dogs are predisposed to sebaceous adenitis?
Akitas
Poodles
Havanese
Lhasa apso
Chow chow
Springer spaniel
What are the clinical signs of sebaceous adenitis?
Young to middle age onset
Varying degrees of alopecia, hyperkeratosis, and seborrhea, with follicular casts
Long-coated breeds will present in the early stages with hair color and texture change
Short-coated breeds will present with annular areas of scale and alopecia
Lesions commonly start on head, cervical region and around pinnae
Can extend into the ear canal so otitis externa can occur
Secondary bacterial infections are common
What are the histopathologic signs associated with sebaceous adenitis?
mild to moderate acanthosis
moderate to severe hyperkeratosis with follicular plugging
granulomatous to pyogranulomatous reactions at level of sebaceous gland
- histiocytes, neutrophils, lymphocytes and plasma cells
in chronic stages there will be a loss of sebaceous glands
perifollicular fibrosis can be present
Where do epitrichial sweat gland ducts open onto?
Although they are located deep to sebaceous glands, their ducts empty into the infundibulum above the level of the sebaceous gland
Where are epitrichial glands found in largest number and size in most species?
near mucocutaneous junctions, in the interdigital spaces and over the dorsal neck and rump
Where are epitrichial sweat glands concentrated in horses?
mucocutaneous junctions, in the submandibular region, near the mane and coronet
What happens to the volume of epitrichial sweat glands in thoroughbreds in the summer?
increases
Where are epitrichial sweat glands found in rodents and ferrets?
there are no epitrichial sweat glands in rodents or ferrets
What is the treatment of sebaceous adenitis?
revolves around removing scale and follicular casts from the skin and coat and improving the quality of the coat and hair regrowth
Will likely be lifelong
Topical treatments: include shampoos, humectants, and oil soaks
Oral treatments: include omega fatty acids, systemic retinoids, cyclosporine, vitamin A, tetracycline, and niacinamide
- typically not responsive to glucocorticoids
- cyclosporine has been shown to reverse histopathologic changes
What is the structure of sebaceous glands?
may be simple or branched and are composed of 2-8 lobules
What is the structure of epitrichial sweat glands?
coiled and saccular to tubular
secretory cells are a singular row of flat to columnar epithelial cells
duct is composed of a double layer of cuboidal cells
surrounded by a single layer of fusiform myoepithelial cells in most species
What is aquaporin 5?
a water channel in horse epitrichial glands which facilitates rapid movement of fluid during thermoregulatory sweating
What is unique about the structure of the epitrichial gland in the horse?
the myoepithelial cells form a loose basket-weave around the gland and the gland has a rich blood supply
How are epitrichial glands innervated?
They are not directly innervated but are under the control of adrenergic agonists including epinephrine and norepinephrine
How are epitrichial sweat glands controlled in horses?
autonomic adrenergic nervous control is the main mechanism
humoral control by adrenergic agonists secreted from the adrenal medulla during exercise is also important
also appears to be some autocrine control
What causes epitrichial sweating in horses?
fever, exercise, heat, pain, hypoglycemia, hyperadrenocorticsm, and catecholamine release from excitement, stress or a pheochromocytoma
What happens to the epitrichial glands of horses that are continuously exposed to adrenergic agents?
They become refractory
Which immunoglobulin does epitrichial sweat contain?
IgA
What is anhidrosis?
inability to sweat
may have a sudden onset or may be more gradual
can result in life threatening hyperthermia in horses
more common in hot, humid climates
- move them somewhere cooler and drier
ponies, thoroughbreds/warmbloods, and stock-type horses predisposed
Why does anhidrosis occur in horses?
- high levels of epinephrine –> desensitization, downregulation of cell receptors and aquaporin-5 –> degeneration of secretory cells
- macrolides (esp erythromycin) can play a role for foals with R. equi
What are the clinical signs of anhidrosis in horses?
Patchy to inadequate sweat response, reduced ability to cool down after exercise with persistent hyperthermia and tachypnea
- Acutely, they may collapse and die
If chronic, horses can have a dry coat with scale and partial alopecia of the face/neck
How do you diagnose anhidrosis in horses?
History
Response to intradermal testing with epinephrine, terbutaline, or other β-2 agonists
- increasing doses 1:1,000,000 to 1:1,000
- normally, sweating occurs over the test site within minutes at all dilutions
- in anhidrotic animals, sweating only occurs at the highest concentrations of epinephrine and only after 5+ hours
What is the structure of atrichial sweat glands?
small and tightly coiled and reside in the deep dermis and subcutaneous
tissues
secretory cells are cuboidal to columnar and are found in a single layer, lined by a
singly layer of fusiform myoepithelial cells
duct opens directly on the surface of skin
What is the innervation of atrichial sweat glands?
They have a rich nerve supply and sweating increases due to agitation and excitement
What are the common causes of acquired noncicatrichial alopecia due to altered hair follicle function in horses?
Telogen effluvium
Anagen effluvium
Seasonal alopecia
Hypothyroidism (is very rare)
Color dilution alopecia
Follicular dysplasia
Protein, copper, or iodine deficiency
Chemical toxicosis: selenosis*, arsenic, mercury, mimosine
What are the common causes of acquired noncicatrichial alopecia due to defects in hair shafts in horses?
Trichorrhexis nodosa
Piedra
What are the common causes of acquired noncicatrichial alopecia due to inflammation of the hair follicle in horses?
Bacterial pyoderma
Dermatophytosis
Alopecia areata
Linear alopecia
What is noncicatrichial alopecia?
hair regrowth occurs if causative factors are eliminated or corrected
What is cicatrichial alopecia?
permanent destruction of hair follicles; hair regrowth does not occur
What are the causes of cicatrichial alopecia in horses?
Physical, chemical or thermal injury
Severe furunculosis
Neoplasia
Cutaneous onchocerciasis
What is seasonal alopecia in horses?
Seen in several scenarios
1) Icelandic horses in Austria: develops in November, resolves in May/June
- feeding a better diet prevented the problem
2) Horses from the northern hemisphere: facial alopecia in spring-fall
3) Excessive spring shedding: resolves in 1-3 mo
- esp in curly coated horses on mane and tail
*maybe try melatonin?
What are the common follicular dysplasia syndromes?
Mane and tail dystrophy
- esp Appaloosas but consider alopecia areata if an adult onset
Black and white hair follicle dystrophy
Persistent alopecia of the tail in curly coated horses
Color dilution alopecia
What are the cutaneous manifestation of selenium toxicosis in horses?
hoof abnormalities
progressive loss of the long hairs of the mane, tail, and fetlocks
What is proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts?
rare condition
Labrador retrievers from North America
immune-mediated reaction
- genetic/hormonal predisposing factors
Clinical lesions included multifocal, coalescing, verrucous, crusted papules and plaques with erythematous borders and comedones or follicular casts
What are the histopathologic findings associated with proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts?
Ortho- and more prominent parakeratotic hyperkeratosis involving follicular infundibular epithelium, with cast formation and a papillary epidermal surface
Lymphocytic exocytosis affected all strata of follicular infundibular epithelium and epidermis
Variable numbers of acidophilic shrunken keratinocytes, often bordered by lymphocytes (satellitosis), occupied the more superficial strata of the follicular infundibular epithelium and epidermis
What does immunohistochemistry show with proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts?
intraepithelial CD3+ T lymphocytes and cleaved caspase-3-positive apoptotic keratinocytes
What is the treatment of proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts?
Immunosuppression, primarily cyclosporine
Middle-aged female Labrador
proliferative, lymphocytic, infundibular mural folliculitis and dermatitis with prominent follicular apoptosis and parakeratotic casts
(a) Acanthosis and parakeratosis with hair follicle infundibula distended with parakeratotic keratin, providing a papillary appearance to the epidermis (dog 1). Haematoxylin and eosin. (b) Lymphocytes and acidophilic, shrunken keratinocytes, often closely apposed (boxes), within epithelium (dog 1). Haematoxylin and eosin. (c) Eosinophilic fluid within the parakeratotic stratum corneum at the edge of a follicular ostium and vacuoles (arrows) within the parakeratotic stratum corneum (dog 3). Haematoxylin and eosin. (d) Deep within some sections at or near the entrance of sebaceous ducts to the infundibular lumen, there is a thin cornified layer with retained nuclei and with a few apoptotic cells (dog 3). Haematoxylin and eosin.
Which section of the hair follicle is absent in telogen?
The inferior portion
What are the histopathologic hallmarks of the infundibulum in anagen and telogen?
stratified squamous epithelium
keratohyalin granules
What are the histopathologic hallmarks of the upper portion of the isthmus in anagen and telogen?
stratified squamous epithelium
no granules
What are the histopathologic hallmarks of the upper portion of the isthmus in anagen?
cornified IRS
What are the histopathologic hallmarks of the upper portion of the isthmus in telogen?
trichilemmal keratin anchoring club hair
no IRS
What are the histopathologic hallmarks of the lower portion of the isthmus?
glycogenated cells in ORS in anagen
cornified IRS
What are the histopathologic hallmarks of the suprabulbar portion of the inferior portion?
stratified squamous epithelium
trichohyalin granules
gyclogenated cells in ORS
non-cornified IRS
What are the histopathologic hallmarks of the bulbar portion of the inferior portion?
matrical cells
trichohyalin granules
dermal papilla
The lack of which structure plays a role in why birds are so prone to hyperthermia?
no sweat glands in avian skin
What glands to avians have?
uropygial (or preen) glands
pericloacal glands (which secrete mucus)
glands of the ear canal.
