Pigment in Hooves

Cheryl Henderson, a US based barefoot trimmer, has a theory that pigmentation increases as the hoof becomes healthier.  She goes as far as saying that the unpigmented inner zone of the stratum medium is 'the enemy of the hoof' because it is more prone to pathogenic action - being less tubule dense and more moisture rich.

Logically, that means Henderson sees the pigment melanin as conferring additional strength to the hoof.

This seems to run counter to formal scientific studies that have concluded the presence of dark melanin pigment does not confer a greater tensile strength or rigidity to the hoof, ie that a 'white' hoof is no weaker than a dark one.

But, Henderson argues that a so-called 'white' hoof (actually creamy coloured) is just as heavily pigmented as a black hoof but just contains pigments of a different colour, ie there are more melanin pigments in the equine hoof than the two that mainstream science has identified.

Some people, in supporting her theory, argue that, just as sunlight stimulates melanogenisis in the skin, a change in the way the hoof meets the ground might stimulate melanin production and, as this increases, the hoof becomes stronger and healthier.

There is no scientific evidence that melanogenisis is triggered by physical stimulation such as ground reaction forces. There is a substance called magnolignan which prevents the formation of melanin - thereby lightening the skin. If that was deficient in unhealthy hooves, a reversal of that deficiency might increase the amount of pigment being produced but that would simply make the hooves darker on the outside - the layer where the pigment naturally occurs.

Coat and hoof pigmentation and skin colour in horses is determined by the presence, the absence or the relative proportions of two melanin pigments  - eumelanin, which is brown/black but thought to always be black in horses, and phaeomelanin, which is reddish and, in humans, found in high quantities in the lips, nipples, genitals and in red hair. 

Most equine skins are black and it seems likely that this is the evolutionary 'default' setting which some humans seek to alter in their ceaseless search for novelty.

If the skin on the pastern is black, the hoof will be black. A 'white' hoof or white part of a bi-coloured hoof always grows from lightly pigmented or unpigmented skin on the pastern.  A horse may grow a black hoof below a white leg but the underlying skin of that leg will be black. 

True albinism (complete lack of melanin) is rare in horses.  I do not know if the 'pink' skin patches found on some horses are completely lacking in pigment or if they contain high levels of phaeomelanin.

Melanin is created in the basal layer of the epidermis by melanocytes. The enzyme tyrosinase plays an important role in melanin production; it is synthesized inside the melanocytes, matures, and transfers into melanosomes, where it produces melanin by binding with the amino acid tyrosine.

The epidermis of the horse has an average thickness of 0.053mm.

Colour, like beauty, is only skin deep -  actually only a fraction of a mm deep.

But in some hooves, most obvious in dark hooves, the pigmented horn seems to flow into the unpigmented layers, ie it does not form a discrete, uniform outer layer. Henderson sees this as evidence that the unpigmented horn has in some way colonised the pigmented layer, or the pigmented layer has in some way degenerated, making the hoof less resistant to pathogens and lacking tensile strength and abrasion resistance.

She does not consider whether the issue is the reverse, ie whether the way a poorly formed hoof meets the ground results in deformations of the superficial pigmented layer.

As the hoof form improves, is what is seen at ground level a more accurate picture of the anatomically normal distribution of tubular and intertubular horn and pigmented and unpigmented horn  - which had been distorted in a forward running / flared / contracted hoof?

The hoof wall proper (the stratum medium) comprises tubules - which are modified hair - around which is intertubular horn that is formed at right angles to the tubules. The bulk of the melanin in the equine hoof is in the intertubular horn and like the pigment of the skin, it lies in a thin superficial layer.

The tubule cortex comprises dense keratinised cells that surround a hollow medulla that contains cellular debris.  The cortex is lightly pigmented or or unpigmented and therefore does not confer colour to the hoof.

The distribution of tubules relative to intertubular horn reduces towards the bone - an arrangement that  is thought to confer a degree of shock absorption. The zones where different types of horn meet and merge are important and actually little understood.   The interface between hoof horn and the highly vascular inner structures close to the bone is arguably the most sensitive to concussive damage - hence the need for an increasing moisture content to the stratum medium. But, the inner structures are also vulnerable to any pathogens that have been able to breach the protective outer layers of horn so, as in all things, there must be a balance.

Pollitt and others argue that it is the intertubular  horn which confers the greatest tensile strength and rigidity to the hoof as the tubules are 3x more likely to fracture than the intertubular  horn.

But, given the need for the outer layers to be more abrasion and pathogen resistant, and the inner layers to be more flexible to help prevent transmission of ground reaction forces to the sensitive structures,  this seems counter intuitive.

What, to my knowledge, has not been tested is the relative abrasion and pathogen resistance of tubular and intertubular horn.  Nor do scientists typically assess the degree and type of deformities present in the specimen hooves they use.

As usual, more questions than answers.