PIGMENTATION IN GUINEA-PIG HAIR 



Microscopical as Well as Chemical Studies Reveal Many Differences Between the 

 Black and Red Colors Genetic Investigation Reveals but a Single 

 Difference — the Problem is Suggested of Explaining the Many 

 Observed Differences as Results of a Single Funda- 

 mental Physiological Difference 



Harrison R. Hunt, ir<75/ \'irginia University 



AND 



Sewall Wright, Bureau of Animal Industry, Washington, D. C. 



IX RECENT years many intcresling 

 facts have been discovered concern- 

 ing the inheritance of coat color in 

 guinea-pigs. It seems to the writers 

 that the time is now ripe to correlate 

 these facts of heredity with the color, 

 form, and cheinical constitution of the 

 pigments which im])art the color to the 

 different forms of guinea-pig hair. Prog- 

 ress in science is frequently made by 

 attacking a problem from a new angle. 

 Such a correlation as the one just men- 

 tioned should throw some light on the 

 ])hysiology of pigment formation in 

 mammals. 



In all, twenty-eight sam])les of guinea- 

 I^ig hair have been studied so far. These 

 were dehydrated in absolute alcf)hol, 

 cleared in xylol, imbedded in ])araffin, 

 sectioned and mounted in the usual way, 

 and studied, unstained, with the oil 

 immersion. 



The cortical, or peripheral, and the 

 medullary, or central, regions of the 

 guinea-pig's hair present characteristic 

 differences. The medulla contains ex- 

 tensive communicating air sjjaces (see 

 figures), and its substance stains rcadil}^ 

 with eosin. The cortical material lacks 

 this reaction with eosin and air s])aces 

 are entirely absent. The cuticle forms 

 a thin outer covering for the hair 



The color of the hair depends ui)on 

 two forms of pigonent — diffuse and 

 granular. The former lends cf)lor to the 

 hair in much the same way that sub- 

 stances in solution may color a solvent. 

 The adjective "granular" suflicicntly 

 explains the character of the other kind 

 of pigment. 

 178 



The color an 1 the distribution of these 

 pigTiient substances in the medulla and 

 cortex demonstrate some interesting 

 facts in the physiology of pigment for- 

 mation. 



OBSERVATIONS 



Black Hair. — The most noteworthy 

 fact in the case of black hair is that both 

 the medulla and the cortex ha\'e an 

 abundance of black granular pigment. 

 (Fig. 10.) The cortical granules are 

 short cylindrical rods whose diameters 

 are, roughly, half or two-thirds their 

 length. The average length is about 

 0.8 of a micron. Their long axes lie 

 lengthwise of the hair. The medullary 

 granules vary in size and shape, the 

 smallest ones being about 0.2 of a 

 micron in diameter and the largest 

 a])proxiinately 3}/2 micra in their great- 

 est dimension. 



In the same samples of hair, the 

 granules are often more closely aggre- 

 gated in cross-sections of small diameter 

 than in larger sections. The smaller 

 cross-sections were jjrobably cut nearer 

 than the larger ones to the distal ends of 

 the hair. Since the tips of the hairs are 

 usually darker in color than the bases, 

 one would expect to find the condition 

 just described — more granules near the 

 tips. 



Sepia llair. — In sepia hair the 

 granules are black. They vary con- 

 siderably in size and are abundant in 

 both cortex an<l medulla. The cortical 

 granules as in l)lack hair are predomi- 

 nantly short bacillus-like rods, oriented 

 with their long axes parallel to the long 

 axis of the hair. The distribution and 



