Acrosome formation during sperm transit through the epididymis in two marsupials, the tammar wallaby (Macropus eugenii) and the brushtail possum (Trichosurus vulpecula)

In certain Australian marsupials including the tammar wallaby (Macrapus eug enii) and the brushtail possum (Trichosurus vulpecula), formation of the ac rosome is not completed in the testis but during a complex differentiation process as spermatozoa pass through the epididymis. Using transmission and scanning electron microscopy this paper defined the process of acrosome for mation in the epididymis, providing temporal and spatial information on the striking reorganisation of the acrosomal membranes and matrix and of the o verlying sperm surface involved. On leaving the testis wallaby and possum s permatozoa had elongated 'scoop'-shaped acrosomes projecting from the dorsa l surface of the head. During passage down the epididymis, this structure c ondensed into the compact button-like organelle found on ejaculated spermat ozoa. This condensation was achieved by a complex process of infolding and fusion of the lateral projections of the 'scoop'. In the head of the epidid ymis the rims of the lateral scoop projections became shorter and thickened and folded inwards, to eventually meet midway along the longitudinal axis of the acrosome. As spermatozoa passed through the body of the epididymis t he lateral projections fused together. Evidence of this fusion of the immat ure outer acrosomal membrane is the presence of vesicles within the acrosom al matrix which persist even in ejaculated spermatozoa. When spermatozoa ha ve reached the tail of the epididymis the acrosome condenses into its matur e form, as a small button-like structure contained within the depression on the anterior end of the nucleus. During the infolding process, the membran es associated with the immature acrosome are either engulfed into the acros omal matrix (outer acrosomal membrane), or eliminated from the sperm head a s tubular membrane elements (cytoplasmic membrane). Thus the surface and or ganelles of the testicular sperm head are transient structures in those mar supials with posttesticular acrosome formation and this must be taken into consideration in attempts to dissect the cell and molecular biology of fert ilisation.