Outer dense fibers are filamentous structures located on the outside of the axoneme in the midpiece and principal piece of the mammalian sperm tail. May help to maintain the passive elastic structures and elastic recoil of the sperm tail.
Constituting the main cytoskeletal structure of spermatid flagella, outer dense fibers (ODFs) add elastic recoil, stiffness and protection against shear forces during sperm movement. Human ODFs consist of approximately 10 major and at least 15 minor proteins. The major proteins of the ODF include Odf1, Odf2, and Odf3, which compose a family of proteins that are preferentially expressed during mammalian spermiogenesis. Odf3 (outer dense fiber protein 3), also known as sperm tail protein SHIPPO 1 and TISP50 (transcript induced in spermiogenesis protein 50), is a 254 amino acid protein that is expressed during the latter part of spermatogenesis in flagella of elongated spermatids and mature sperm. Odf proteins are directed to their exact subcellular location by Spags, which are characterized as chaperone-like Odf-binding molecules. There are two isoforms of Odf3 that are produced as a result of alternative splicing events.