While it is tempting to interpret the intratesticular Cd gradient in shark testis as due to developmental programming of genes encoding the number of Cd-binding molecules per cell, there are alternative explanations. One possibility is that it reflects stage-related differences in the number, ratio, and size of germ cells vs. Sertoli cells in each region; see Fig. 1). Another explanation is that Cd accumulation decreases with development of the blood-testis barrier. In shark testis, tight junctions form between adjacent Sertoli cells when spermatocysts are in late sper-matogonial or early spermatocyte stages of development, and exclusion of dextran-rhodamine (10 000 Mr) from the germinal compartment is observed in isolated M and PoM, but not PrM, spermatocysts. buy birth control online
Higher levels of Cd in immature compared to mature rat testes, and increased Cd accumulation after Cd pretreatment, have been attributed to the status of the blood-testis barrier. Nonetheless, the barrier per se cannot account for the Cd gradient in shark testis. Although Cd exposure in vivo and in vitro perturbs the blood-testis barrier in sharks, as measured by dextran-rhodamine exclusion in PoM cysts, it does not alter the pattern of Cd accumulation.