College of Cat Genetics: Part XX
by Patricia Turner
Study Unit 20
More About Colour
Pigmentary colours are by far the most important to cat breeders and here, too, there are more than one sort. Haemoglobin is a pigment, the respiratory pigment, and this is scarlet when oxygenated and purple when not. Haemoglobin is responsible for the colour of the tongue, tip of nose (in white, red and some tabby, tortoiseshell and piebald cats), the inside of ears (ditto) because it is the blood pigment. In cats haemoglobin does not contribute to the general surface colour because this is normally covered by hair. Colouring in hair is almost entirely dependent on the presence or absence of melanin pigment. Haemoglobin does play a part in the production of colour in the mammalian eye, but even this is mainly determined by intra-ocular melanin.
In cats there are two forms of melanin classified, namely, Eumelanin and Phaeomelanin. The former, which gives brown or black, is a word derived from the Greek (meaning good), while phaeomelanin is derived from the Greek word for dusky.
Melanins occur in the form of minute pigment granules and coat colour in cats largely depends upon the presence of pigment granules giving black or yellow or their absence giving white. Pigment granules are found in the outer layer and the centre of the hair, in the outer layer of the skin, in the iris and other parts of the eye, in the layer of the eye outside the retina and in the retinal cells themselves. They are also found in certain other tissues.
Pigment producing cells are called melanocytes and all those other than cells of the retina migrate in embryonic life from the neural crest. The neural crest itself is the embryonic material giving rise to a wide variety of tissues. The melanocytes of the retina arise from the embryonic optic cup. The process of melanocyte formation from the neural crest and their subsequent migration to the skin surface are complicated and hereditary factors can act upon any one or more than one of them thus affecting the production of pigment within a cell, causing death of the melanocytes, causing inability to produce pigment without also causing the death of the cell, and also delaying or preventing migration to the skin surface.
Coat colour in cats is also affected by the actual structure of the hair itself because defective hair formation may well affect pigmentation because pigment granules may be unable to enter the hair, or even if they can their distribution within it may be abnormal. Changes in the distribution of different kinds of hair in the agouti coat affects its colour because the proportion of hairs with yellow banding depends on the hair type -- no yellow bands being seen at all in guard hairs.
Eumelanin, the brown or black melanin, is an insoluble brown derived from tyrosine, one of the essential amino acids, and is always attached to protein. The tyrosine is apparently converted to dopa which is then oxidised to dopa quinone. After a number of further oxidative changes and polymerization it finally becomes melanin.
Unlike eumelanin, phaeomelanin is soluble in dilute alkali and is also very slightly flourescent under a Woods lamp. The two pigments differ genetically with the genes leading to dilution acting differently on each of them--the dilution gene on eumelanin produces blue or lilac while the dilution genes on phaeomelanin produces cream. An interesting fact is that one hair bulb can produce both eumelanin and phaeomelanin. In cats only two varieties of melanin have been classified although there is another, or possibly more than one other, pigment which is responsible for the production of human red hair, the fur of the red fox and the feathers of the Rhode Island fowl.
(This subject will continue in Study Unit 21.)