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ACVD Exam > The Hair Follicle > Flashcards

The Hair Follicle Flashcards

1
Q

What are the functions of hair?

A
  • 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
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2
Q

Most hair follicles develop before birth, which do not?

A
  • All hair in marsupials
  • Secondary follicles in dogs
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3
Q

Which tissues in the hair follicle are ectodermally derived?

A
  • All epithelial components
  • Sebaceous glands
  • Apocrine glands
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4
Q

Which tissues in the hair follicle are mesodermally derived?

A
  • Hair papilla
  • Connective tissue sheath
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5
Q

Which tissues in the hair follicle are neural crest derived?

A

Melanocytes

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6
Q

Describe where hair first appears on the embryo.

A

Vibrisse on the chin, eyebrow, and upper lip for first then hair begins to form all over the head and then moves down

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7
Q

What are lanugo hairs?

A
  • First hairs formed in utero
  • Non-medulated
  • Typically shed in humans before birth
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8
Q

What genes are likely responsible for non-random placement of hair?

A
  • Homeobox genes
  • Mice with Engrailed knockout get hair on paw pads
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9
Q

How many stages are there in hair follicle development?

A

8

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10
Q

What happens to the genes and signaling molecules that are involved in the development of the hair follicle when the individual becomes an adult?

A

Most of the become involved in hair cycling.

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11
Q

What happens first in the development of the hair follicle (stage 1 aka hair placode stage)?

A
  • 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
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12
Q

What happens second in the development of the hair follicle (stage 1 aka hair peg stage)

A

Epithelial placode proliferates, invades the dermal, 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)

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13
Q

The outer root sheath of the hair peg goes on to form 3 bulges. What does the deepest bulge form?

A

The arrector pili attachment

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14
Q

The outer root sheath of the hair peg goes on to form 3 bulges. What does the middle bulge form?

A

Sebaceous gland

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15
Q

The outer root sheath of the hair peg goes on to form 3 bulges. What does the most superficial bulge form?

A

Sweat gland

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16
Q

What does the fibroblast growth factor 5 (FGF-5) gene do during hair follicle development?

A
  • Responsible for hair length
  • Is a catagen inducer in the mature follicle (so abnormalities lead to longer hair)
17
Q

What does the keratin-71 gene do during hair follicle development?

A

Is responsible for hair curl

18
Q

What does the R-spondin 2 gene do dictate about hair?

A

Hair size, texture, and distribution

19
Q

What does the Wnt/β-catenin pathway do during hair development?

A

-“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

20
Q

What does EDA/EDAR do during hair development?

A
  • 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
21
Q

What does BMP (bone morphogenetic protein) do during hair development?

A
  • 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
22
Q

What does noggin do during hair development?

A
  • Antagonizes BMP
  • Promotes placode fate by increasing Lef-1 expression and works to induce anagen in the mature follicle
23
Q

What does TGF-α/EGF do during hair development?

A

Induce catagen in the mature follicle

24
Q

What does the Notch pathway do during hair development?

A
  • 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
25
Q

What does the Shh (sonic hedgehog) pathway do during hair development?

A
  • 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
26
Q

What does the Patched-1 (Ptc1) pathway do during hair development?

A
  • Is the receptor for Shh
  • Expressed in the germ cells and dermal papillae
27
Q

What hairs are medullated vs not?

A
  • 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
28
Q

How do primary and secondary hairs differ?

A
  • 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
29
Q

What is the difference between simple and compound hairs?

A
  • Simple hair follicles only contain 1 hair, compound contain multiple
  • Compound hairs develop post-natally (12-28 weeks in dogs)
30
Q

How do cat and dog hairs differ?

A
  • 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
31
Q

Describe horse hair follicles

A
  • Simple hair follicles
32
Q

Describe pig hair follicles

A
  • Clusters of primary hairs with 3-4 primary per cluster
33
Q

What is acromelanism?

A

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

34
Q

Describe sheep hair follicles?

A
  • Wool follicles in clusters with 3 primary follicles and 6x more secondary follicles than primary
  • Higher in wool breeds

ACVD Exam (21 decks)

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