Skin and Hair… a shedding point of view
"The largest and heaviest organ of the body"
A hair in and of itself is dead material, but living cells produce it. Hair is made of the protein exokeratin, an extremely strong protein, ensuring skin impermeability. The structure and composition of exokeratin differs from the epidermal keratin.
- Hair Shaft: above the skin surface
- Hair Root: imbedded in the skin surface
- Bulb: the enlarged, hollow portion at the base of the root
- Hair Papilla: projection of dermis into center of the bulb
- Follicle: the skin indentation protecting the root
- Cuticle: the outermost layer of the hair is a single layer of flattened, keratinized cells overlapping like shingles that can be crown-like, petal-like or flat.
- Cortex: inside the cuticle is the thickest and intermediate area. The dead keratinized cells are very densely packed. These cells contain pigment granules, providing the hair color.
- Medulla: a central core of loosely packed cells. Hairs can be hollow, increasing the insulation value of the coat. The total diameter of individual hairs decreases as the number of hairs per follicle increases. The shape of the hair is determined by the shape of the follicle. Straight follicles produce straight hair and curly follicles produce curly hair.
The total diameter of individual hair decreases as the number of hairs per follicle increases. The shape of the hair is determined by the shape of the follicle. Straight follicles produce straight hair and curly follicles produce curly hair. No new hair follicles are formed after birth. Follicle density decreases with age, but follicle size increases!
The hair follicle is a unique composite organ, composed of epithelial and dermal compartments interacting with each other in a surprisingly autonomous way. Each of the follicular compartments is endowed with a specific differentiation and even the shape of the hair shaft is intrinsically programmed from the bulb (Bernard BA. 2003).
- The hair grows from the deepest part of the active hair follicle. Hairs don't grow "through" the skin. They grow from the skin and they're really part of the keratinizing system of the epidermal layer.
- Hair grows in cycles. When it reaches a certain length that is determined by the individual's genetic profile, growing stops. Each cycle consists of four phases: anagen, catagen, telogen and exogen. The duration of each phase and the rate of hair growth determine the length and amount of coat on an animal.
Anagen or Growth Phase
- The anagen phase is the first phase of new hair growth. Pets that do not tend to shed heavily have a longer anagen phase. Pets that continuously shed have shorter anagen phases. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase, an unknown signal causes the follicle to go into the catagen phase.
Catagen or Regressing Phase
- The catagen phase is the transition phase. The catagen phase begins when the cell creation signals to stop. Hair stops growing during this phase as the outer root sheath attaches to the hair.
Telogen or Rest Phase
- Telogen is a rest period between the catagen and anagen phases.
Exogen or Shedding Phase
- The final phase, exogen, is the shedding phase. This phase occurs when the hair falls out and the follicle moves back into the anagen phase. The length of this phase depends on the season.
- At any given moment, some hair follicles are in anagen, some in catagen and some in telogen. The timing and the ratio of the different cycles determine the shedding frequency, the length of the hair and presence or absence of undercoat. Some animals shed poorly because almost all of their follicles are in anagen almost all the time, so their hair just gets longer and longer. Some animals have most of their follicles in telogen and they may be almost completely hairless.
- In dogs: hair growth rate varies between individual dogs and even between the different regions in the same dog. The growth rate is most rapid in the shoulder region, followed by the flank and the forehead regions (Parameswaran Gunuratnam PG, 1983). Each follicle grows independent of the others.
- In domestic short hair cats, the hair growth rate is highest in the summer and lowest in the winter.
- Guard Hairs (primary hairs) form the topcoat, outermost layer of hair that covers most of the skin surface. They are thick, long and stiff. The guard hairs are regularly arranged in broad tracts that follow the contour of the body and give the animal's coat its smooth appearance. Growing one hair per primary follicle, the guard hair provides a waterproof protective top layer covering the undercoat. Guard hairs are most abundant on the back of the animal.
- Under-Hairs form the undercoat (secondary hairs). They are thin, short and soft. They give the coat its softness and provide insulation. Up to 15 secondary hairs can grow from the secondary follicle.
- Tactile Hair (sinus hairs) are very large hairs modified to act as mechanical sensors. Thicker than topcoat or undercoat hair, they have the same structure. However, the base of the tactile hair resides in a blood filled sinus that greatly amplifies the motion of the hair and increases its sensitivity. Merkel cells, specialized sensory cells associated with the tactile hairs, detect the slightest touch. Tactile hairs are concentrated on the sides, top and bottom of the head (whiskers, eyebrows and chin hairs) and are strongly associated with sight and sense of hearing. Tactile hairs shed on a regular basis, but at a much slower rate than other hair types.
References: 1. Linda A.MO .Age, Breed, Sex and Period Effects on Skin Biophysical Parameters for Dogs Fed Canned Dog Food The American Society for Nutritional Sciences J. Nutr. 2002,132:1695S-1697S. 2. Verena K. Affolter, Peter F. Moore Histology features of normal canine and feline skin. Clinics in Dermatology 1994. Volume 12, Issue 4, 491-497. 3. Bernard BA. Hair shape of curly hair. J Am Acad Dermatol. 2003; 48 (6 Suppl): S120-6. 4. Matousek JL, Campbell KL. A comparative review of cutaneous pH. Vet Dermatol 2002; 13: 293-300. 5. Cadieu E, Neff MW, Quignon P, Walsh K, Chase K, Parker HG, Vonholdt BM, Rhue A, Boyko A, Byers A, Wong A, Mosher DS, Elkahloun AG, Spady TC, André C, Lark KG, Cargill M, Bustamante CD, Wayne RK, Ostrander EA.Coat variation in the domestic dog is governed by variants in three genes. Science. 2009, 2; 326(5949): 150-3. 6. William E. Staile. Sensory hair follicles in mammalian skin: The tylotrich follicle. American Journal of Anatomy. 2005, Volume 106 Issue 2, 133-147, 7. H. J. Whiteley. Giant Compound Hair Follicles in the Skin of the Rabbit. Nature 1958,181; 850. 8. Parameswaran Gunuratnam PG., T. Wilkinson. A study of normal hair growth in the dog Journal of Small Animal Practice 1983, Volume 24; Issue 7: 445-453, 9. Wouter H. Hendriks, Michael F. Tarttelina, and Paul J. Moughana. Seasonal Hair Growth in the Adult Domestic Cat (Felis catus). Comparative Biochemistry and Physiology Part A: Physiology 1997, Volume 116, Issue 1; 29-35.
Coat types can be classified according to hair thickness, hair length and hair structure. There are hairless animals, animals with double coats, having both a topcoat and an undercoat and animals with a topcoat but no undercoat. The type and length of hair can vary widely among breeds.
- Based on hair length, cat breeds can be divided into three groups: hairless, short-haired and long-haired. Just like in dogs, hair length in cats is managed by the FGF5 gene. Variation of coat texture is often determined by color of the individual cat.
- Hairless breeds often have little to no hair. Some hairless breeds have noticeable patches of hair on their heads. Great care should be taken to protect and moisturize their skin.
- Hair length is determined by the "long hair gene". Transition from the anagen to telogen phase is delayed in the dominant hair length of cats (C. Drögemüller, 2007). Long-haired cats are classified as having hair 2 inches or longer.
- Single-coated breeds have a topcoat, but lack an undercoat. Single-coated breeds are often referred to as non-shedding breeds because their shedding is so minimal, it is often unnoticed.
- In short-haired cats, the coat lies very close to the body. The appearance depends on the thickness, thinness or absence of the undercoat. Short-haired cats are classified as having hair shorter than 2 inches.
Hair type depends on the breed.
Normal Fur Breeds
- In breeds with normal fur, hair length can run from 7/8 to 1 1/2 inches. Every hair follicle, including topcoat and undercoat, contains roughly 14 hairs.
Rex Hair Breeds
- In rex hair, each follicle may contain up to 50 hairs. The ideal length is about 5/8 inch. The topcoat is shorter and doesn't exceed the length of the undercoat.
Satin Hair Breeds
- Satin hair is the same as normal hair (7/8 to 1 1/2 inches), but the hair shaft is narrower in diameter and transparent. The transparent hair fiber structure gives the hair a very characteristic sheen.
Wool Hair Breeds
- Wool hair breeds have a very long undercoat and fur of 4-5 inches long.
"Shedding is natural; 'non-shedding' is a misconception"
All dogs, cats and rabbits shed to some extent. Hairless animals even shed, but their shedding is just limited to skin shedding.
- Shedding in animals is intimately related to seasonal cycles. In most cases, the cycle of shedding is cued by changes in the amount of daylight. Hair growth and shedding are regulated by fluctuations in the amount of melatonin, the "hormone of darkness", secreted by the pineal gland in response to seasonal sunlight variations.
- Increasing day length stimulates hair growth in the Spring (Baker, 1974).
- Spring shedding is typically heavier because the winter coat is progressively replaced for a lighter, Summer coat.
- Double-coated dogs generally drop their soft undercoats twice a year and lose their topcoat once a year. If they shed all at once, the fur will come out in tufts and is often called "blowing a coat". Other dogs might shed continuously throughout the year or only every 10-12 months. Shedding can take anywhere from three weeks to two months.
- Dogs that live outside usually shed heavily as days lengthen in the Spring, but those that live mostly indoors, often seem to shed at least a little all year.
- In cats, there is no single period of hair shedding. Neighboring follicles are in different phases of the hair cycle at any one time. Domestic cats tend to shed continuously throughout the year, with peaks of activity occurring during Spring and Fall.
- Rabbits shed every 3 months. Every other shed will be a lighter shed and may not be very noticeable. When rabbits shed, it can last up to 6 weeks or more. The shedding period varies greatly from rabbit to rabbit and from breed to breed. Some rabbits have an overall full body shed that only lasts a day or two. Some rabbits start shedding at the head and gradually shed down the back. Others shed tufts of hair at a time. Artificial light or inbreeding disturb the normal genetic response to day length and can modify this cycle.
- The patterns of shedding vary by species, breeds within a species and the location on the body. Like any genetically determined characteristic, shedding can be manipulated through selective breeding.
Factors Influencing Shedding
Different factors influence the hair shedding cycle:
- Shedding seems to be connected to seasonal temperature, but it is actually governed by day length.
- Since shedding is strongly related to changes in the duration and intensity of day light, indoor living animals seem to never stop shedding.
- Inside living animals need to be groomed during these heavier shedding periods to help remove the loose undercoat.
- Outside living animals usually shed heavily as days lengthen in the Spring. Outdoor animals can speed up the shedding process by brushing up against trees and twigs.
- Hair loss due to poor nutrition often involves the whole animal, but may be most obvious over the back and hips where hair follicles have shorter growth cycles and longer inactive periods. As protein is necessary for proper kertanization of the skin, hair of animals deficient in protein will become dry, dull, brittle and will shed easily and may be slow to grow (Mosier, 1978; Watson, 1998; Cline, 2004).
- An improper diet can cause unnatural shedding. B vitamins, especially pantothenic acid, are important for proper hair growth (S. Yu, 2006). Copper is important for hair production and a deficiency will result in a poor hair coat. Zinc can also influence hair growth in dogs (John A Lowe, 1994).
- The breed of dog affects how much the animal will shed.
- Some breeds tend to be low shedding because almost all of their follicles are in the anagen phase almost all the time; their hair continues to grow. Some have most of their follicles in the exogen phase and may be almost completely hairless.
- Dogs with a long hair growth cycle will shed less.
- Dogs with long, fine hair tend to shed less than other breeds. Some breeds shed very little hair. Strictly "non-shedding breeds" do not exist. However, most breeds with single coats are considered "non-shedding" because their shedding is not very noticeable. Short-haired dogs shed the most often and continuously.
- Breeds with a high density of hair follicles have more guard hairs (topcoat) per square inch and less undercoat. They shed less than breeds with a low density of hair follicles but more undercoat associated with each one. These breeds blow their coats with every season change, losing tufts of undercoat at a time.
- Spayed and neutered dogs may have more pronounced undercoats, so shedding can be more noticeable. This is not necessarily true in cats (Baker, 1974).
- In general, no new hair follicles are formed after birth. Initially, a puppy or kitten has simple hair follicles that hold a single hair. The secondary hairs emerge at about the age of 12 weeks. The coat becomes more dense, stiffer and coarser depending on the characteristics of the breed. Puppies or kittens do not actually "lose" their first coat, rather they gain an adult coat. The age of the first shed depends on the season of birth and occurs between 4 and 14 months.
- Blood carries hormones that determine the hair growth phases. Shedding is controlled by hormonal changes that are tied to day length. Some hormones will stimulate hair growth, while others will delay it.
- Female animals tend to shed after a heat cycle and after giving birth. This shedding typically occurs when the litter is five to twelve weeks old. False pregnancy and nervous lactation can cause loss of hair from the chest, belly and sides.
- Hair follicles and skin cells are also strongly influenced by thyroxin. Thyroxin initiates hair growth and increases the rate of new growth. A deficiency in thyroid hormone (hypothyroidism) results in poor hair growth and thinning of the hair, especially over the back.
- Corticosteroids delay hair growth by inhibiting new hair growth resulting in thinning of the hair as well. A lack of growth hormone results in retention of the hair in young animals and lack of hair growth in adults.
- Some animals' hair changes color with the season. This occurrence is another hormonally-controlled aspect of the hair cycle. The activity of melanocytes changes to give hair more or less pigment.
- Allogrooming, when one animal grooms another, is usually experienced after birth, but can go on between adult animals. In cats, allogroomers often groom themselves after grooming a partner (Ruud van den Bos, 1998).
- Allogrooming and autogrooming, self-grooming, offer the pet a feeling of comfort. For some pets, grooming can turn into overgrooming and become a source of excessive hair loss. Some reasons for overgrooming may be boredom, chronical stress, anxiety, nervousness and fear (Bonne Beerda, 1999).
- Undergrooming may signal disease or illness in the pet.
- Fighting or dominance behaviors between animals can cause skin irritations or lesions. The stress of fighting may also increase shedding.
- Anytime an animal is stressed, the skin and coat suffer. All stressors, noise, boredom, fear, sudden changes to environment, inconsistencies in food or water supply, a lack of visual contact, pain and anxiety accentuate hair loss. During the initial phase of stress, such as a visit to the vet or groomer, stress hormones are released into the bloodstream inducing shedding for a couple of days. In cases of daily stress, shedding becomes even more problematic.
- Intensity and speed of shedding varies with different species, breeds and individuals, but stressful conditions typically cause hair to shed first on the body and rear hips.
- The health and beauty of the skin and coat will reflect the overall health and well being of the animal.
- Medications and health issues can have an impact on the skin and coat. Several oral, topical and injected medications cause hair loss. Steroids induce follicle shrinking and cause the hair to fall out. In some breeds, the active or growing phases of the hair cycle is prolonged.
- Skin infections and excessive scratching or biting can cause localized hair loss. Excessive numbers of bacteria in the hair follicle (bacterial pyoderma) may cause circular areas of hair loss, called hot spots. This is very common in dogs with allergies.
- Parasites can induce hair loss. Localization of hair loss suggests which parasite is involved. Fleas attack the rear hips. Lice often attacks the back and rear legs. Mites focus on eyes, ears, mouth and elbows. Ringworms cause hair loss that is usually patchy and characterized by relatively round bald patches with rather distinct edges.
- Bathing too frequently can dry out the hair coat and cause excessive shedding. Attention should be paid to the products used, as skin pH differs between species. Choice of shampoo should be strictly related to skin pH.
- Some dogs may have a pause in hair growth after clipping. After several months of a lack in re-growth of hair, this resolves spontaneously.
References: 1. Baker, K. P. (1974) Hair growth and replacement in the cat. Br. Vet. J.130: 327-335. 2. Wouter H. Hendriks , Michael F. Tarttelina and Paul J. Moughana .Seasonal Hair Growth in the Adult Domestic Cat (Felis catus) Comparative Biochemistry and Physiology Part A: Physiology Volume 116, Issue 1, 1998: 29-35 3. S. Yu , K. J. Wedekind , C. A. Kirk and R. F. Nachreiner .Primary hair growth in dogs depends on dietary selenium concentrations.. The FASEB Journal, 2006;16:A992-A993 4. John A. Lowe, Julian Wiseman and D. J. A. Cole.. Zinc Source Influences Zinc Retention In Hair And Hair Growth In The Dog. J. Nutr. 124: 2575S-2576S, 1994. 5. Ruud van den Bos .The function of allogrooming in domestic cats (Felis silvestris catus); a study in a group of cats living in confinement.Journal of Ethology, Volume 16, N° 1,1998 6. Bonne Beerda, Matthijs B. H. Schilder, JAN A. R. A. M. Van Hooff, Hans W. De Vriesand Jan A. Mol. Chronic Stress in Dogs Subjected to Social and Spatial Restriction.. Behavioral Responses . Physiology & Behavior,1999; Volume 66, Issue 2 :233-242