POSTTESTICULAR DEVELOPMENT OF SPERMATOZOA OF THE TAMMAR WALLABY (MACROPUS-EUGENII)

Tammar wallaby spermatozoa undergo maturation during transit through t he epididymis. This maturation differs from that seen in eutherian mam mals because in addition to biochemical and functional maturation ther e are also major changes in morphology, in particular formation of the condensed acrosome and reorientation of the sperm head and tail. Of s permatozoa released from the testes, 83 % had a large immature acrosom e. By the time spermatozoa reached the proximal cauda epididymis 100 % of sperm had condensed acrosomes. Similarly 86 % of testicular sperma tozoa had immature thumb tack or T shape head-tail orientation while o nly 2 % retained this immature morphology in the corpus epididymis. Th is maturation is very similar to that reported for the common brush ta il possum. Trichosurus vulpecula. However, morphological maturation oc curred earlier in epididymal transit in the tammar wallaby. By the tim e spermatozoa had reached the proximal cauda epididymis no spermatozoa had an immature acrosome and thumbtack orientation. Associated with a crosomal maturation was an increase in acrosomal thiols and the format ion of disulphides which presumably account for the unusual stability of the wallaby sperm acrosome. The development of motility and progres sive motility of tammar wallaby spermatozoa is similar to that of othe r marsupials and eutherian mammals. Spermatozoa are immotile in the te stes and the percentage of motile spermatozoa and the strength of thei r motility increases during epididymal transit. During passage through the caput and corpus epididymis, spermatozoa first became weakly moti le in the proximal caput and then increasingly progressively motile th rough the corpus epididymis. Tammar wallaby spermatozoa collected from the proximal cauda epididymis had motility not different from ejacula ted spermatozoa. Ultrastructural studies indicated that acrosomal cond ensation involved a complex infolding of the immature acrosome. At spe rmiation the acrosome of tammar wallaby spermatozoa was a relatively l arge flat or concave disc which projected laterally and anteriorly bey ond the limits of the nucleus, During transit of the epididymal caput and proximal corpus the lateral projections folded inwards to form a c up like structure the sides of which eventually met and fused. The cav ity produced by this fusion was lost as the acrosome condensed to its mature form as a small button-like structure contained within the depr ession on the anterior end of the nucleus. During this process the dor sal surface of the immature acrosome and its outer acrosomal membrane and overlying plasma membrane were engulfed into the acrosomal matrix. This means that the dorsal surface of the acrosomal region of the tes ticular tammar wallaby sperm head is a transient structure. The dorsal acrosomal surface of the mature spermatozoon appears ultrastructurall y to be the relocated ventral surface of the acrosomal projections whi ch previously extended out beyond the acrosomal depression on the dors al surface of the nucleus of the immature spermatozoon.