Summary
The elongated spermatid nuclei of monotremes exhibit a circumferentially arranged spiral pattern of chromatin condensation, and ultimately form helical filiform sperm heads up to 50 μm long and either circular or slightly oval in transverse section. The acrosome is formed by the collapse of the proacrosomal vacuole onto the rostral surface of the elongating nucleus. However, genesis of acrosomal material occurs in the absence of a prominent proacrosomal granule. The flagellum becomes inserted into the distal extremity of the nucleus, the most proximal mitochondria of the midpiece directly abutting the nuclear membrane, so that a prominent neck region is absent. The axoneme is simple and, in the midpiece, small dense peripheral fibres are closely applied to the outer surface of each of the nine microtubule doublets. The cortical fibrous sheath of the principal piece is an anastomosing spiral that lacks lateral columnar elements.
The spermatozoal cytoplasmic droplet undergoes migration and is lost during epididymal passage.
Monotreme spermatozoa exhibit a montage of features, some of these being also found in marsupials and some in sauropsidan vertebrates, as well as a number of their own distinctive features. It is concluded that monotreme spermatozoa also have a close affinity with the unspecialised spermatozoa of some eutherian mammals.
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We wish to thank Dr. Mervyn E. Griffiths for providing the Zaglossus material. Permits to collect these native mammals were provided by the National Parks and Wildlife Services, and State Fisheries Services of New South Wales and Queensland. Portion of this work was supported by grants from the Australian Research Grants Committee. The provision of skilled technical assistance and access to electron microscope facilities were kindly provided by Dr. C.D. Shorey, formerly Acting Director of the University of Sydney Electron Microscope Unit, and by Mr. John Hardy, Senior Lecturer-in-Charge of the University of Queensland Electron Microscope Centre. Members of the technical staff of the Departments of Anatomy and Veterinary Anatomy are also acknowledged for their assistance
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Carrick, F.N., Hughes, R.L. Aspects of the structure and development of monotreme spermatozoa and their relevance to the evolution of mammalian sperm morphology. Cell Tissue Res. 222, 127–141 (1982). https://doi.org/10.1007/BF00218293
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DOI: https://doi.org/10.1007/BF00218293