Spermiogenesis and spermiation in a monotreme mammal, the platypus, Ornithorhynchus anatinus

Spermatogenesis in the platypus (Ornithorhynchus anatinus) is of considerab le biological interest as the structure of its gametes more closely resembl e that of reptiles and birds than marsupial or eutherian mammals. The ultra structure of 16 steps of spermatid development is described and provides a basis for determining the kinetics of spermatogenesis. Steps 1-3 correspond to the Golgi phase of spermatid development, steps 4-8 correspond to the c ap phase, steps 9-12 are the acrosomal phase, and steps 13-16 are the matur ation phase. Acrosomal development follows the reptilian model and no acros omal granule is formed. Most other features of spermiogenesis are similar t o processes in reptiles and birds. However, some are unique to mammals. For example, a thin, lateral margin of the acrosome of platypus sperm expands over the nucleus as in other mammals, and more than in reptiles and birds. Also, a tubulobulbar complex develops around the spermatid head, a feature which appears to be unique to mammals. Further. during spermiation the resi dual body is released from the caudal end of the nucleus of platypus sperm leaving a cytoplasmic droplet located at the proximal end of the middle pie ce as in marsupial and eutherian mammals. Other features of spermiogenesis in platypus appear to be unique to monotremes. For example. nuclear condens ation involves the formation of a layer of chromatin granules under the nuc leolemma, and development of the fibrous sheath of the principal piece star ts much later in the platypus than in birds or eutherian mammals.