DISCUSSION: WHAT WE KNOW AND DON’T KNOW
ABOUT CANINE HAIR GROWTH
We know that hair goes through four stages:
During anagen or growth
phase the hair is attached to the dermal papilla, the source of growth. This stage has different durations,
depending upon breed and coat type. In studying mouse hair, scientists have
identified eight sub-stages of anagen.
The catagen phase is characterized by the detachment of the hair
from the matrix that created it.
The inner sheath closes around the bottom of the hair shaft forming a club hair. This hair bulb is often visible to the naked eye. An interesting feature of the catagen
phase is the phenomenon of apoptosis or cell death. This is virtually cellular suicide, and is part of the
detachment process. As the hair
shaft detaches from the growth matrix and dermal papilla, it moves upward to a
final resting place closer to the surface of the skin. The hair then enters the
telogen
or resting phase, where neither growth nor apoptosis occurs. The final stage is exogen, or shedding,
which occurs when the hair exits the follicle and the matrix begins to prepare
for new growth to occur.
Although the exact mechanism for signaling
new hair growth, from telogen to anagen and transition from anagen to catagen
stages has not been scientifically unlocked, scientists now know that the hair
follicle contains stem cells which migrate from a location part way up the hair
follicle, called the “bulge”, to the lower region where they engage the dermal
papilla and form the hair matrix that generates a new
hair shaft. Another
interesting fact about hair follicle stem cells is that they migrate not only
up and down the hair follicle and dermis, but onto the surface of the skin as
well. It is thought that these
traveling stem cells participate in wound healing of the skin.
The exact mechanism of communication and signaling that is
required for the dermal papilla to grow a new hair shaft has not been identified;
scientists have identified several factors in this process. The good news is that there is
considerable interest in unlocking this mystery, as it has implications for
other organ regeneration.
DNA programs hair growth. In undercoated breeds of dogs, such as
Husky, Malamute, Chow Chow, Pomeranian, the secondary hairs that form the soft
undercoat have a rapid often seasonal growth and shedding pattern, whereas the
primary hairs are slow growing and have an extended telogen resting phase.
We know that hair (Northern breeds) sheds
(exogen/telopsis phase) partly in response to changes in environmental light
and temperature. But we do not
know the nature of the signaling mechanism. And we do not know what signals a new hair to grow. Dogs with Post-Clipping Alopecia, or
Hair Cycle Disorder, sometimes seem to go through an extended kenogen or empty
follicle stage, as well as an extended telogen or resting phase. Does the hair coat itself play some
role in the transmission of signals to the hair follicle to begin a new anagen
(growth) phase? When we clip the
coat short, do we somehow risk short-circuiting the growth cycle signaling
system? This question cannot be
answered until scientists come up with more information regarding the growth
signal.
Although groomers have noted instances
where a clipped coat grows back with a very different texture, with either the
undercoat or guard hairs being wiry or even kinky, there is no literature
identifying or discussing this phenomenon. Nor does there seem to have been any
scientific study of the alterations that happen following clipping of the
harsh-coated terrier. It is
possible that the study of re-growth of clipped terrier coat might shed some light on the changes in Nordic
breed coats.
One thing that appears with terrier coats
is that clipping alters the ratio of guard hairs to primary hairs, part of
which is responsible for the dilution of color and loss of texture in coats
that are clipped. This may also happen
when clipping coats of Nordic breeds, especially after repeated clipping, or
clipping again before the coat has been thoroughly replaced. When we clip a Siberian Husky, Chow
Chow or Malamute on an once-or-twice-a year basis, we may be interrupting the
re-establishment of the normal hair cycling. These coats are not genetically programmed to completely
replace themselves year after year.
When we take an altered coat and clip it again, do we further throw it
out of balance? Just asking!
There is no scientific indication that
simply removing older telogen hairs does anything to promote hair growth. The groomers’ notion that the old hair
somehow blocks the hair follicle and prevents new hair from sprouting is not
held up by the findings on hair growth.
Departure of old hair is not a signal factor for initiation of anagen
phase or new hair. In fact the
previous notion that a new growing hair somehow pushes up the old hair and
forces it to shed has been disputed by more recent findings that new hair grows
parallel to old and is independent.
Also, the current understanding of shedding as a distinct phase, exogen,
that has its own signal mechanism contradicts this old theory.
Here is a rather simple summary of factors
(perhaps not ALL) that influence hair growth, from the Pet Place.com:
Hormones. Some
will stimulate hair growth while others will delay it.
· Androgens. They cause courser hair with lengthened resting phase
of hair follicles.
· Progesterone. This produces courser hair with lengthened resting
phase of hair follicles and decreased growth rate.
· Estrogen. This results in fine sparse hairs and lengthened resting
phase of hair follicles.
· Thyroxine. This initiates hair growth and increases rate of new
growth. A deficiency in thyroid hormone
· (hypothyroidism) usually results in poor hair growth and
thinning of the hair coat.
Corticosteroids. These drugs retard hair growth by inhibiting new hair growth,
thus alopecia or thinning of hair occurs as a consequence of this type of
therapy.
Growth hormone. The lack of growth hormone results in retention of the
juvenile coat or alopecia in the adult.
Insulin. This
hormone is responsible for normal growth, although diabetics may have alopecia.
Nutrition. Poor
nutrition can result in a loss of hair.
- Protein. Cystine and methionine are requirements for hair growth. Protein-calorie malnutrition is characterized by dry, brittle and sparse hairs.
- B vitamins. These vitamins, especially pantothenic acid, (for copper utilization) are important for proper hair growth.
- Copper. This is important for hair production and a deficiency will result in a poor hair coat.
Some breeds, like the
chow-chow, may have an arrest in the hair growth after clipping. This resolves
spontaneously after several months of a lack of hair re-growth.
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bbird says:
Post Clipping
Alopecia
The
term for poor regrowth of coat following a clip down is post clipping
alopecia. The term was adopted by the veterinary community and is
applied to cases where surgical patches or intravenous patches fail to grow
back properly as well as cases of failure of the coat to regrow following
grooming.
Poor
regrowth, Temporary. The normal
regrowth of an undercoated, Northern breed coat is 8-24 months. The secondary, soft hairs will be
replaced first, sometimes leaving the dog with an unkempt fuzzy look for
months. The stiff, primary or
guard hairs are much slower to fill in.
The situation most often corrects itself when given enough time. Since the top coat, primary hairs are
on a different growth cycle than the undercoat, eventually the guard hairs will
fill in and be present in the original proportion to secondary hairs. Yes, this can take up to two
years!
Poor
regrowth, Permanent. Unfortunately, there are instances when double coated,
especially Nordic, breeds are clipped short and never grow back well. It happens. It is impossible to differentiate a poor and slow regrowth
from a coat that will never grow back.
Groomers tend to want to attribute this sad situation to pre-existing
health conditions, such as undiagnosed low thyroid. Certainly this can happen,
REFERENCES:
Daitsch, Vicki, PhD, Understanding Coat
Funk, www.malamutehealth.org,
2004
Categorizes Post Clipping Alopecia under
Alopecia X.
Jankovic, Slobodan M.; & Jankovic, Snezana
V.(1998). The control of hair growth. Dermatology Online Journal, 4(1).
Retrieved from: https://escholarship.org/uc/item/6gz420mw
Journal of Investigative
Dermatology (2002) 119, 639–644;
doi:10.1046/j.1523-1747.2002.01842.x
Exogen, Shedding Phase of the Hair Growth Cycle: Characterization of a
Mouse Model,Yoram Milner*, James Sudnik, Mario Filippi, Menas Kizoulis,
Michael Kashgarian† and Kurt Stenn
Dr. Rosanna
Marsalla, Lack of Hair Growth in Dogs, PetPlace.com
