Seasonal cycles of the interstitial cells in the testis of the horned toad (Phrynosoma solare). Seasonal variations in the number and morphology of the interstitial cells and the volume of the interstitial tissue

A histological study has been made of a series of Phrynosoma testes taken at frequent intervals throughout the year. As it was important to differentiate between the amount of interstitial tissue and the number of cells, it was necessary to take into account the percentage of tissue, total testis volume and the number of cells per unit volume. The results show a definite interstitial‐cell cycle. Volume of interstitial tissue and size of individual cells are greatest during the breeding season, but the interstitial‐cell number is then minimal. The maximal number of interstitial cells occurs after the close of the breeding season. There is, apparently, no reversion of interstitial cells to a connective‐tissue‐cell type. The above changes are correlated with the spermatogenetic cycle and with increases in total tubule length, in tubule diameter, and in testis volume. The time in which important changes in ovarian volume occur and at which ovulation takes place is coincident with the testicular changes. Interstitial cells have been stated to have no endocrine function for the reason that their number is at a minimum during the breeding season. If, however, interstitial tissue is responsible for sex characters and activity, the present work would indicate that it is the volume of tissue, and not the number of cells, that is important.