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. Expressed in the cytoplasmic lobe of spermatids. Testis-specific (at protein level). Expression restricted to the germ cell fraction, absent in somatic cell fractions such as Sertoli and Leydig cells. Expression detected in the third week postpartum (23 days) after haploid germ cells developed, expression increased with age. Expressed in the tails of elongated spermatids sticking out toward the tubular lumen, and in cytoplasmic droplets still attached to the spermatid tail membrane. Expressed in the tails of mature sperm, from the connecting piece proximal to the head, along the middle and principal pieces, down to the distal end piece. 'Shippo' is a Japanese word for tail. Belongs to the ODF3 family.