The Fur of the Mongolian Gerbil & Other Gerbil Species: Hair Function & Anatomy

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Hair Function

The Mongolian gerbil is a prime example of a species that has successfully evolved to succeed in its harsh Steppe environment and one of the main things that helped to do this is its thick coat of fur.  They have a dense covering of fur on their tails, fur that cover their ears and fur covered feet.  Gerbil species that live in the hotter regions of their ranges have evolved slightly differently and tend to have less fur on their tails, have naked ears and the amount of hair on the soles of the feet is largely dependent on how sandy their terrain is; providing traction and also insulating the animal from the heat of the sand.
The primary function of the hair itself is to provide some protection against heat loss.  The fur traps the hair that is close to the skin and effectively thermo-regulates the body by providing an invisible, insulating layer.

There is also distinct appearance to their wild coats and all for a good reason.  So depending on their ranges and habitat they live in the fur can vary from wild agouti to a solid coloured sandy coat with a white belly.  The shade of agouti can also vary tremendously as well, all depending on the surroundings in which they live.  This modification of their hair then helps camouflage the animal and the colour tones in the coat help it to blend in better with its surroundings.  Their white bellies help to thermo-regulate the animal and reflect the light and heat much better than its top coat of fur.  The hair fibre also serves another function and forms a tough barrier which serves to protect the epidermis from minor abrasions and/or from ultra violet light.
We can also see how hair can be specialised to serve a certain function.  We can see how the eyelashes help sweep away dirt, dust and debris and also how their vibrissae (whiskers) with their highly developed nerve network around them provides the gerbil with sensory, tactile information about its environment.

Hair may also be of importance in attracting mates and may be based on fur colour and this may hold true for the Domestic Mongolian gerbil who, when used  in experimental studies, illustrated the social preferences of females when encountering males of the different coat colours. The results of this study showed that the Agouti female preferred visiting the arm of the maze where the Agouti males were located, while the females of the mutant coat colours (Black and Argente) preferred to visit those of non-wild type males. (P. Gray-Allen & R. Wong 1990)

So as you can see by using the Mongolian gerbil as an example, just how important hair is to the survival of mammals. These days and far removed from their wild counterparts, the Mongolian gerbils' coat now comes in a wide variety of colours but also of textures; from the smooth, standard coat to a waved coat, and the waving can range from smooth and shiny through to a rough textured wave; some coats can even appear to have satinised fur and some can have virtually no hair at all.  These variants of the standard coat are due to mutations in hair production and as such they affect how the hair grows.  The mutations in hair production serve no purpose in the evolution of the species and in the wild would certainly be to the detriment to the animal.  However, in captivity it is the perceived beauty of these variants that have been selected by fanciers throughout the ages, not just in gerbils, but in a whole host of domestic species.  It is only by selection and by breeders paying a very close eye on their health that these mutations can succeed and flourish while in domesticity.

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The Hair Follicle

The hair follicle is actually a tiny complex mini-organ and a singular follicle is composed of multiple cell layers which comprise of well over 20 different types of cell populations.  These cell layers, the mesenchymal and the epithelial cell layers, along with the sebaceous gland and the tiny erector pili muscle that attaches to the hair shaft, are the main components of the pilosebaceous unit.  The components of these tiny units can often vary in proportion and depend on which kind of hair on the body that they are maintaining. An example of this type of specialisation of the pilosebaceous unit can be observed in the triplet pattern of the hair follicles of the ventral gland. Here the central primary hair follicle also has secondary hair follicles that bud from either side of the primary follicle, all of which erupt through the same pilosebaceous canal.   It was observed that the hairs that grow in these specialised units are not in the same stages of the hair cycle at the same time.  When this is combined with a very short telogen phase it produces an area of skin in which some hair follicles are always in the anagen phase.  A similar condition can be observed in the costo-vertebral spot of the hamster, where the hairs in this region show long phases of anagen coupled with a brief telogen phase.  Similar to the gerbil, this area is larger in the male than the female, and both the hair and sebaceous glands of this region are stimulated by androgens.

This triplet pattern of hair growth can be observed as early as 5 days in the ventral gland  region of the young gerbil.  The long periods of catagen and telogen phases observed elsewhere on the gerbils’ body isn’t evident in the ventral glands hair follicles and they don't appear to stay in a resting phase for any length of time.  The three hair follicles in the unit vary in their stages of growth and each specialised unit always have at least one growing hair follicle. The ventral gland region is an area of the skin that always has some hair follicles in the anagen phase.(Feldman & Mitchell)

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Hair Growth & Anatomy Guide

Photo Credits: Main photo Waved gerbil pups-courtesy Catherine Jakubczyk; Dermal papilla photos 1&2-courtesy H.H. Swanson 1980

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The Hair Growth Cycle

Hair growth & Moulting primer

As mentioned in the flash guide above, the complete hair follicle cycle lasts for  5 recognised phases, however it is the first three phases of this cycle that represents the period of active growth and rest before the hair follicle eventually falls out.   These phases are cyclic in nature and represent the periods of growth,rest and shedding of the hair.

Anagen Phase

anathumb.jpgThe initial phase of growth is known as the anagen phase, which is the active growth phase of the hair.  As the hair shaft grows, materials originating from cells inside the hair follicle are deposited inside the growing shaft.  This period is metabolically demanding and involves the cells above, around and within the dermal papilla dividing, multiplying and differentiating, as they grow upwards to form the hair shaft.  When the matrix cells, which are located in the dermal papilla differentiate, some become keratinised. When the cells keratinise they are unable to function so are pushed upwards and outwards as they die off to form the cortex and cuticle of the growing hair fibre.

The entire length of the anagen phase is genetically predetermined, but its duration can also vary and be dependent on where there hair follicle is located on the body and also the type and size of the hair.  In the developing (juvenile) gerbils coat the typical length of the anagen phase in the top coat and belly hair is around 15-18 days, however the growth phase of the hair around the ventral sebaceous gland is somewhat different and begins around ten days later, with secondary hair follicles budding and then growing next to the primary follicle sites.  The telogen cycle of these hairs are markedly shorter when compared to the other hair follicles.  In the gerbil, the ventral gland is made up of many groups of modified pilosebaceous glands whose canals are much greater in size than those found around the rest of its belly. These modified pilosebaceous glands fill the entire hypodermis (the lowest of the three layers that make up the skin which is also known as the subcutaneous layer) in the adult gerbil.  The hair around the ventral gland area differs from the coat in general in the fact the area lacks a panniculus carnosus.  It has been hypothesised that this lack of a muscle may be necessary for the proper development and functioning of this specialized region in the gerbil.(Feldman & Mitchell)

The hair in the anagen phase is at its thickest and strongest and when it is at its most deeply pigmented.   This can be clearly observed on a gerbil when it is moulting and the deeper pigmented hairs in the anagen phase meet the less pigmented old coat hairs and the distinct shade differences can be seen between the old and new fur.

Catagen Phase

catathumb.jpgAfter the growth of the anagen phase ceases, the follicle then begins to shut down.  This phase is known as the catagen phase which is characterised by a massive and genetically orchestrated programmed cell death in the bulb shaped, transient portion of the hair follicle causing it to shrink considerably.  Protein and pigment production cease, the dermal papilla shrinks along with the epidermal sheath which is then keratinised into a tiny club root and is subsequently pushed upwards to the surface by surrounding epithelial cells.





Telogen Phase

telothumb.jpgThere is a continued regression of the hair follicle into the telogen phase until it appears about half of its original size and what remains of the dermal papilla cells begin their resting phase.  The epithelial cells of the follicle cease both DNA and RNA synthesis and protein synthesis stops.  Only keratin synthesis continues in the epithelial sac to which the hair fibre is still anchored to. In the juvenile Mongolian gerbil, on average the catagen and telogen phases last around 10-11 days.(Feldman & Mitchell)






Exogen and Kenogen Phase

exothumb.jpgkenothumb.jpgThe exogen phase, as opposed to the quiescent resting period of the telogen phase, is a highly active and tightly controlled process and is when the old hair fibres are shed.  Once the hair fibre begins to shed then this marks the final phase known as the kenogen phase.  This is the interval of the hair cycle where the hair follicle remains empty after the shedding of the telogen hair, before the new anagen hair reappears.






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Coat development

Day 1
At birth, with the exception of several delicate mystacial (above the lip) vibrissae the gerbils’ body is typically hairless and the skin appears pink and wrinkled. By the end of day 1, pigment deposits can often be seen and areas around the nose, eyes and the base of the tail darken.  Pigmentation can also be observed starting around the ankle region of the forefeet and on the soles of the hindfeet.  The superciliary vibrissae ( just above the eye) begin to appear.

Day 2
Dorsal hair eruption begins and the chin and around the lips begin being covered in a light down.

Day 3
Hair growth and pigment deposition continue on the dorsal area but the belly remains pink and hairless.  The ears, which are not yet erect, remain hairless.

Day 4
By the 4th day the back and sides (dorsal and lateral) of the gerbils body are covered with fine, developing hair which are noticeable and thickest around the head region but sparse around the rump area.  The limbs are still essentially hairless, but initial growth of hair in the ventral (stomach) region is now observed.

Day 5
The lateral (side of the body) surfaces of the fore and hind limbs are now scattered with fine hair.  On the ventral region, hair growth is restricted to the chest area and the anal area becomes slightly darker in pigmentation.  The rest of the ventral region is still hairless.

Day 6
Hair appears around the anus, while the tail, genitals, abdomen, eyes and ears (which are now unfolded and erect) remain hairless.  Hair on the rest of the ventral areas begin and now grows rapidly so that by day 9 the entire surface is fully covered with fine hair.  The dorsal area of the tail is now covered in bristle, but ventrally the bristle is restricted to the base of the tail.  A few hairs now fringe the tips of the ears and the edges around the eye socket.  The upper surfaces of the feet are now also furred.

On Agouti coats, researchers noted one or more lines of colour difference which extended laterally which were suggestive of moult lines. Over the stomach area, bristle type hair of differing texture from the surrounding fur begins to grow in the region of the ventral gland.   The ventral gland is present on both sexes.

Day 13
By the end of day 13, the fur on the gerbil is apparent on most surfaces of their bodies and nearly all hair has erupted.  On Agouti, apparent moult lines are noticeable between the shoulder blades and progress towards the tail.   Subsequent development consists of a rapid increase in hair length.

Days 17-20
The fur has now developed to the extent it resembles the adult coat in all but texture, the juvenile fur being much softer.

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Moulting and Moult Lines

As a newborn pup the only hair visible are those of the vibrissae, but the young gerbil’s first coat is very rapid in development and is completed around 18- 21 days.   From then to adulthood the coat undergoes two moult cycles, these being the post juvenile moult and a post sub-adult moult.  In the Mongolian gerbil and other rodents, hair growth patterns are often noticed each time the gerbil moults.  However, the hair follicles don’t go through these growth phases all at the same time and instead they pass through  the coat in a wave like fashion; moving from the nose down to the rump and from their bellies to their backs.  It is for this reason that we often notice odd patterns or lines in their coats. We can observe these hair growth patterns each time a gerbil moults.  They are particularly noticeable around the nose and forehead or along their backs and the markings or lines is the result of the new coat meeting the old coat.

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The Post Juvenile Moult and Post Sub-Adult Moult

The Mongolian gerbil, similar to other rodent species, undergoes two maturational moults before reaching adulthood. These phases are termed the post juvenile moult and the post sub-adult moult.  In one study using Nyanzol A dye, the fur was stained a deep purple that lasted until the hair is replaced and the moulting pattern was noted daily as the dyed hairs were replaced by new growth. Here they noted that in the sample of young gerbils used, the post juvenile moulting process started around 32-38 days.(McManus & Zurich)  However In an earlier study where skin samples were obtained on a daily basis then examined under the microscope, they noted the anagen phase of the hair follicles starting on the 28th day on the dorsal fur and the 27th day on the ventral fur.(Feldman & Mitchell)  As indicated in the first study, and echoed again in the second, new hair growth was noted first around the ventral gland, then around the lips, the eyes and inner margins of the ears.  Soon after this, the hair replacement was observed around the shoulder areas and axillary (armpit) regions.  When the post juvenile moult neared its completion, the moult proceeded towards the rump until only patches of the juvenile fur remained. These remnants of old fur could be observed around the occipital (back of the head), rump, ankle and also around the base of the tail and the tip of the tail.  Most of the gerbils appeared at this stage by the 45th day, but the juvenile fur wasn’t completely shed until days 48-55.(McManus & Zurich)

The post sub-adult moult is similar to the post juvenile moult, but tended to be quite variable and was longer in duration.  In the McManus & Zurich study, hair replacement was first observed from days 58-63 and similar to the post juvenile moult it originated around the ventral gland region, chin, genitalia, eyes, shoulders and the inner margins of the ears.  This correlates closely to Feldman & Mitchell study where they observed that on the ventral regions, the telogen phase was from 44 days to 58 days after birth and a new anagen phase was observed again at approximately 58 days which lasts until the 69th day. On the dorsal regions, the telogen phase extended from the 44th day to the 62nd day after birth.  The sub-adult coat was reported to be completely replaced by days 90-112. (McManus & Zurich) This second moult is quite variable in comparison to the first moulting cycle and takes on average twice as long to complete.  The moulting process is of a cyclic nature throughout the gerbils’ life and each cycle can be quite variable in duration.

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The Skin Cycle

Researchers have shown that it is not only the hair that exhibits a cyclic activity, but the skin also undergoes a cyclic activity in its thickness.  From 3-18 days of life the thickness of the skin increases rapidly, mainly due to fat deposits in the hypodermis. This increase in thickness closely corresponds to the anagen phase of the hair cycle.  During the catagen and telogen phases of the hair cycle, the width of the hypodermis decreased causing a subsequent decrease in skin thickness.   A similar skin cycle phenomenon can be seen in other rodents such as mice and rats.(Feldman & Mitchell)

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Feldman, A. and Mitchell, O. G. (1968), The postnatal development of the pelage and ventral gland of the male gerbil. Journal of Morphology, 125: 303–313.

Growth, Pelage Development and Maturational Molts of the Mongolian Gerbil, Meriones unguiculatus- John J. McManus and William M. Zurich-American Midland Naturalist-Vol. 87, No. 2 (Apr., 1972), pp. 264-271

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