Formation of the outer dense fibers during spermiogenesis in the rat.

The morphogenesis of the outer dense fibers (ODF) in rat spermatids has been studied by electron microscopy, and the synthesis and incorporation of proteins into the ODF during this process have been followed by radioautography using 3H-proline and 3H-cystine as precursors for ODF proteins. In the first phase (step 8-14), nine very fine fibers termed anlagen of the ODF develop in association with the microtubule doublets. These first appear along the most proximal portion of the axoneme in step 8 of spermiogenesis; during steps 9-14 they gradually increased in length in a proximal-to-distal direction, being first observable along the forming midpiece and later along the principal piece as well. In the second phase (steps 15-16), the rudimentary fibers suddenly increase in diameter, with the most dramatic growth occurring in step 16, and assume a close resemblance to the mature ODF. This striking transformation, which appears to result from simultaneous deposition of electron-dense material along the length of anlagen of the ODF, coincides with a period of rapid incorporation of 3H-proline-and 3H-cystine-containing proteins, which become permanent structural components of the ODF. These proteins, which comprise the bulk of the ODF, are synthesized in the cytoplasm of spermatids during the acrosome and early maturation phases. In the final phase (steps 17-19) the fibers continue to enlarge very slowly, assuming their definitive form in step 19 of spermiogenesis. Thus formation of the ODF in the rat is a lengthy multistep procedure, requiring from step 8-19 of spermiogenesis and utilizing proteins synthesized throughout most of this period.