Abstract :
The spermatozoon fibrous sheath contains longitudinal columns and circumferential ribs. It surrounds the axoneme
of the principal piece of the mammalian sperm tail, and may be important in sperm stability and motility. Here we
describe its assembly during spermiogenesis in a marsupial, the brush-tail possum, and compare its structural organization
with that of eutherian mammals, birds and reptiles. Transmission electron microscopy showed that possum
fibrous sheath assembly is a multistep process extending in a distal-to-proximal direction along the axoneme from steps
4 to 14 of spermiogenesis. For the most part, assembly of the longitudinal columns occurs before that of the circumferential
ribs. Immunohistochemical and immunogold labelling showed that fibrous sheath proteins are first present
in the spermatid cytoplasm; at least some of the proteins of the sheath precursors differ from those in the mature
fibrous sheath. That immunoreactivity develops after initiation of chromatin condensation suggests that fibrous sheath
proteins, or their mRNAs, are stored within the spermatid cytoplasmic lobule prior to their assembly along the axoneme.
These findings are similar to those in laboratory rats, and thus suggests that the mode of fibrous sheath assembly
evolved in a common ancestor over 125 million years ago, prior to the divergence of marsupial and eutherian lineages
