In the tammar wallaby (Macropus eugenii), post-testicular acrosomal shaping involves a complex infolding
and fusion of the anterior and lateral projections of the scoop-shaped acrosome into a compact button-like
structure occupying the depression on the anterior end of the sperm nucleus. The present study has
generated cytochemical and histological evidence to demonstrate that the occurrence of actin filaments
(F-actin, labelled by Phalloidin-FITC) in the acrosome of tammar wallaby spermatozoa is temporally and
spatially associated with the process of acrosomal shaping in the epididymis, through a pool of monomeric
actin (G-actin, labelled by Rh-DNase I) present in the acrosome throughout all stages of epididymal
maturation. F-actin was not detected in the acrosome of testicular spermatozoa, but was found in the
infolding and condensing acrosome of caput and corpus epididymal spermatozoa. When the spermatozoa
completed acrosome shaping in the cauda epididymidis, F-actin disappeared from the acrosomal area. The
strong correlation between the occurrence of F-actin and the events of acrosomal shaping suggested that the
post-testicular shaping of the acrosome might depend on a precise succession of assembly and disassembly
of F-actin within the acrosome as the spermatozoa transit the epididymis. Thus, actin filaments might play a
significant role in the acrosomal transformation, as they are commonly involved in morphological changes in
somatic cells.