Biology of Hair and Nails Flashcards
What embryologic layer is hair derived from?
Ectoderm
What embryologic layer are the dermal papillae derived from?
Mesoderm
What is the follicular dermal papillae?
Specialized fibroblast population that is required for hair development. They are mesenchymal cells which protrude into hair bulb
What is the isthmus of the hair follicle?
Midportion from opening of sebaceous gland to insertion of arrector pili muscle - bulge is here (follicular epithelial stem cells) - lined by outer root sheath (ORS) but no inner root sheath (IRS)
What important group of cells is located in the bulge of the follicle?
Follicular epithelial stem cells
What sheaths are present on the isthmus of the hair follicle?
Outer root sheath is present but not the inner root sheath.
What is the infundibulum of the hair follicle?
It sans from the opening of the sebaceous gland to the follicular opening.
What is the inferior segment of the hair follicle?
This is the base of the isthmus to the hair bulb.
What cells make up the inferior segment of the hair follicle?
Matrix cells, envelops the dermal papillae
What sheath linings are present on the inferior segment of the hair follicle?
Lined by inner root sheet and the outer root sheath (non keratinized)
What is the Critical line of Auber?
Below this line, mitotic activity occurs.
Where is the pigment for the hair made?
The pigment produced by melanocytes/melanosomes in the bulb
How far up the follicle, starting at the bulb does the outer root sheath cover?
Extends over the entire length of the hair follicle.
What is the change in keratinization of the outer root sheath as it moves from the bulb to the opening of the follicle?
Trichilemmal keratinization occurs in isthmus with NO keratohyalin granules. Then changes to normal keratinization with keratohyalin granules in the infundibulum.
Where does the inner root sheath extend to?
From the bulb to the bulge area where it disintegrates. The IRS is interlocked w/ the cuticle of the hair shaft.
Does the inner root sheath have keratohyalin granules?
Yes
What are the main signaling molecules involved in hair growth and development?
Wnt and beta-catenin
What diseases results from loss of Wnt signaling?
Ectodermal dysplasia (WNT10A mutation)
What are the differences between pre-puberty and pubertal/post-pubertal hairs?
Pre-puberty: -Lanugo and vellus hairs are both not medullated and not pigment -Terminal on scalp, eyelashes, eyebrows -Vellus on face, trunk, extremities Puberty/post-pubertal Androgens influence some vellus hairs to enlarge → become terminal hairs
What disease process causes terminal hairs to revert to miniaturized vellus-like hairs?
Androgenetic alopecia
What parts of the scalp are more androgen-dependent and which are less androgen-dependent?
Hairs on the frontal scalp are more androgen-dependent, hairs on the occipital scalp are less androgen-dependent.
What molecules are key in involved in starting the first dermal or epithelial message of starting follicular induction?
WNT and Beta-catenin
What molecule is key in forming the epidermal placode?
FGF-20
What signaling molecule is key for inhibiting surrounding epithelial cells during follicular induction?
Dickkopf homolog (DKK)
What molecule is key for the proliferation of follicular epithelium?
SHH
What is the order of the hair follicle cycling?
Anagen, catagen, telogen, kenogen
What is anagen and how long does it last?
The active growth phase lasts 2-6 years
What phase of hair growth determines the length of the hair?
Anagen
What phase are most hairs in?
Anagen (90% of hairs are in this phase)
What is the catagen phase and how long does it last?
A transitional phase, bulb regresses and inner root sheath is lost. Lasts 2-4 weeks
What is telogen and how long does it last?
It is the resting phase, lasts 3 months.
What percent of hair is in the catagen phase?
2% hairs are in this phase
What percent of hairs are in telogen phase?
15% of hairs.
What happens during the catagen phase?
The bulb regresses and the inner root sheath is lost.
What happens in telogen phase?
Proximal hair is club-shaped vs bulb-shaped and The dermal papillae are located higher up in the dermis during telogen
What is the kenogen phase?
Lag period between loss of telogen hair and growth of new anagen hair
What parts of the hair follicle change through follicular cycling and what parts are permanent?
The lower portion (below the isthmus) undergoes growth a regression while the upper region of the follicle including the sebaceous glands, isthmus, and infundibulum remains permanent
If you were to pull out a bunch of hairs where would the bulbs be in relation to depth in the dermis?
If the hairs are removed with the rubber-tipped hemostats, the telogen hairs will have bulbs that are higher up than the other hairs.
At what hair cycle phase are the club hairs released?
Telogen to anagen transition
The epithelial components of the hair follicle come from what cells, with what exception?
Epithelial components come from matrix cells. The exception is the outer root sheath, this comes from the bulge
Where do the cells that make up the outer root sheath come from?
The bulge
What do the dermal papillae do during anagen?
It signals surrounding matrix cells to begin differentiation. Remember that epithelial components come from these except the outer root sheath
What types of cells are made during the catagen phase?
Club hairs are formed, these are cortical and cuticle cells only
Where do the hair germ cells come from?
The bulge
What genes play a key role in telogen to anagen transition?
WNT, SHH, noggin
Where are type I hair keratins expressed more?
In the outer root sheath
What are the type I keratins?
K31-K40
What are the type II keratins?
K81-K86
Where are type II hair keratins expressed?
Hair shaft (a/w hair shaft abnormalities)
How are hair keratins different from epithelial keratins?
Hair keratins have more sulfur in the head and tail domains –> Allows for the formation of tight cross-links w/ associated proteins
What determines if hair is curly or straight?
The shape of the follicle. Round follicle = straight hair, oval follicle = curly hair
What is the significance of healing with the stem cells from the bulge?
Stem cells from bulge and hair germ cells are important in wound healing and repair interfollicular epidermis. (this is why skin comes back w/ split-thickness grafts
What molecules are downregulated on the follicular cells to make it an immune-privileged site?
Downregulated MHC class I, MICA, and Beta2-microglobulin
MHC class II also down-regulated in these areas
What molecules are up-regulated to form the immune-privileged site of the hair follicle?
IL-10, TGF-B1, and alpha-melanocyte-stimulating hormone
How is NK cell action suppressed at the hair follicle?
High expression of killer cell immunoglobulin-like receptors (KIR) and heterodimeric CD94/NKG2A receptors on NK cells.
What is the nail plate?
That is the main part of the nail we see. A convex sheet of onychocytes.
Where does the nail plate come from?
The nail matrix epithelium
Where does the dorsal nail plate (top of the nail) come from?
The proximal nail matrix
Where does the ventral nail plate (the part that touches the nail bed) come from?
Distal nail matrix
What is the pink color of the nail from?
Underlying subungal capillaries
What is the hyponychium?
That is the white part at the end of the nail, the part you cut off
What is the lunula?
It is the most distal portion of the nail matrix. It is the white half-moon part at the most proximal portion of the nail plate you can see.
Where are melanocytes most concentrated in the nail matrix?
In the distal nail matrix
What is the function of the cuticle or eponychium?
Prevents separation of the nail plate and proximal nail fold.
What causes nail pitting?
parakeratosis affecting the proximal nail matrix –> increases granular layer –> nail pitting
Does the nail bed or nail matrix have a granular layer histologically?
No!
What do dermal capillaries in the proximal nail fold look like normally?
Dermal capillaries form regular hairpin loops - nail fold capillary changes (dilated, tortuous, avascular areas) all suggest underlying AI-CTD
How does nail growth change with age?
Slows down with age
What time of day do nails grow faster?
During the day
In what season do nails grow fastest?
Summer
What digits have the fastest and slowest nail growth?
Fastest: Fingers, Middle finger Slowest: Toes, thumb, little finger
What physiologic states cause nails to grow faster?
Pregnancy, hyperthyroidism, arteriovenous shunting
What dermatologic conditions cause nails to grow faster?
Psoriasis, pityriasis rubra pilaris
What dermatologic conditions cause nails to grow slower?
Onychomycosis
Medications that can cause the nails to grow slower?
Itraconazole, Etretinate/acitretin (in psoriasis pts)
Medications that can cause the nails to grow faster?
L-DOPA, Methotrexate, Azathioprine, Etretinate/acitretin (not in psoriasis pts)
