What We Know and Don’t Know About Canine Hair Growth

By Barbara Bird CMG

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.

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