In vivo and in vitro Radioautographic Investigation of Amino Acid Incorporation into Male Germ Cells

The influence of the blood-testis barrier on the synthetic capabilities of male germ cells in the mouse was investigated by means of radioautography both in vivo and in vitro. In vivo, after i.v. and intratesticular injection of leucine, lysine, arginine, proline, and methionine, a distinct radioactivity-distribution gradient was apparent in the seminiferous tubules, with labeling decreasing from the periphery toward the lumen. With tritiated leucine, quantitative, radioautographic analysis by electron microscopy revealed that labeling intensity was approximately equivalent in type B spermatogonia and pachytene primary spermatocytes, but was only one-third as heavy in spermatids (steps 4–5). In vitro, radioautographic analysis of the same cell types separated by differential sedimentation velocity resulted in a radioactivity-distribution gradient very similar to that obtained in vivo; labeling decreased in the same proportion in spermatids. In all cases, extratubular incorporation of labels remained high in control Leydig cells. These results show that the blood-testis barrier has no significant effect on amino acid incorporation into male germ cells.