Fourteen steps of spermatid development in the tammar wallaby (Macropus
eugenii), from the newly
formed spermatid to the release of the spermatozoon into the lumen of the
seminiferous tubules, were
recognised at the ultrastructural level using transmission and scanning
electron
microscopy. This study
confirmed that although the main events are generally similar, the process
of the
differentiation of the
spermatid in marsupials is notably different and relatively more complex
than that
in most studied eutherian
mammals and birds. For example, the sperm head rotated twice in the late
stage
of spermiogenesis: the
shape of the spermatid changed from a T-shape at step 10 into a streamlined
shape
in step 14, and then
back to T-shape in the testicular spermatozoa. Some unique figures occurring
during the spermiogenesis in
other marsupial species, such as the presence of Sertoli cell spurs, the
nuclear
ring and the subacrosomal
space, were also found in the tammar wallaby. However, an important new
finding
of this study was the
development of the postacrosome complex (PAC), a special structure that
was
first evident as a line of
electron dense material on the nuclear membrane of the step 7 spermatid.
Subsequently it became a
discontinuous line of electron particles, and migrated from the ventral
side of
the nucleus to the area just
behind the posterior end of the acrosome, which was closely located to
the
sperm–egg fusion site proposed
for Monodelphis domestica (Taggart et al. 1993). The PAC and
its possible role in both American and
Australian marsupials requires detailed examination. Distinct immature
features
were discovered in the
wallaby testicular spermatozoa. A scoop shape of the acrosome was found
on the
testicular spermatozoa of
the tammar wallaby, which was completely different to the compact button
shape
of acrosome in ejaculated
spermatozoa. The fibre network found beneath the cytoplasm membrane of
the
midpiece of the ejaculated
sperm also did not occur in the testicular spermatozoa, although the structure
of
the principal piece was
fully formed and had no obvious morphological difference from that of the
epididymal and ejaculated
spermatozoa. The time frame of the formation of morphologically mature
spermatozoa
in the epididymis
of the tammar wallaby needs to be determined by further studies.