Occurrence and formation of cytoskeletal proteins in mammalian spermatozoa.

Mammalian spermatozoa are composed of specialized cytoskeletal elements, which appear to have no structural or protein counterparts in somatic cells. Most evident are the outer dense fibres (ODF) and fibrous sheath (FS) of the sperm tail and the perinuclear theca (PT) of the sperm head. The purpose of this study is to review our results on the occurrence and assembly of proteins making up these three elements during spermatogenesis. Our approach was to raise antibodies against the prominent proteins of these elements and to immunolocalize them on testicular sections prepared for histological and ultrastructural analyses. We found that all of the cytoskeletal proteins considered were expressed exclusively during the haploid phase of development and that the proteins of each element had similar if not identical patterns of expression. The PT proteins were synthesized in the first half of spermiogenesis and were associated with acrosome formation, while the ODF and FS proteins were synthesized in the second half of spermiogenesis. The ODF proteins assembled in a proximal-distal direction along the length of the axoneme, while the FS proteins assembled in the opposite direction; both assemblies eventually meeting and overlapping within the periaxonemal cytoplasmic compartment. During assembly the ODF proteins appeared to be temporarily stored in granulated bodies of the cytoplasmic lobe, while the FS proteins were randomly distributed throughout the cytoplasm. In the case of the PT, there appeared to be an interdependence between PT assembly and acrosome formation. The developmental protein distribution patterns observed for each of the elements suggest unique cellular targeting mechanisms adapted by the spermatid to regulate the assemblies of the respective cytoskeletal proteins.