Abstract
The distribution of thyroid gland C cells in 171 Thoroughbred racehorses (117 males, including 7 geldings and 47 mares) with a fracture was compared with that in 87 Thoroughbred racehorses without a fracture. No significant difference was found between the number of C cells in the fractured and non-fractured groups. But comparison of male and female fractured groups revealed a tendency to an increase in the number of C cells in 2-year, 4-year and 6-year male racehorses. The number of C cells at the time of fractures of the third metacarpal bone, third metatarsal bone, proximal sesamoid and first phalanx of the fore and hind limbs exceeded the number for fractures of the scapula, pelvis, humerus, femur, radius, tibia, corpus and tarsus, thereby pointing to differences between the bones in calcitonin sensitivity. A positive correlation was found between the frequency of racing starts and the number of C cells in 2-year, 3-year and 4-year-old horses with a bone fracture. No such correlation was found in Thoroughbred racehorses aged 5 years or more, suggesting that horses which do not show an increase in the number of C cells at a young age might have high tolerance of frequent participation in races.
