Abstract
The distribution of C cells in the thyroid gland was investigated histologically and immunohistochemically in 78 thoroughbred horses (37 males and 41 females) aged from 1 to 29 years and which were free of bone and endocrine diseases. C cells are scarce in both the cephalic and caudal ends of the thyroid gland, but large numbers are present in the central area. In annular sections of the thyroid gland with the maximum surface area, C cells are distributed throughout these sections, but an especially high frequency has been detected in the inner margin. C cells increase in number up until around 20-30 days after birth, but there is a tendency to a decrease in number thereafter with age. The thyroid gland of young animals is formed from a small round follicle, which becomes larger and undergoes flattening of the follicular cells with age. Compared to females in which the number of C cells starts to decrease at about 3 months of age, the rate of decrease is gentle in males and is consistently higher than in females.
