The structural organisation of sperm head components of the wombat and koala (suborder: Vombatiformes): an enigma amongst marsupials
Breed, WG, Leigh, CM & Ricci, M 2001, Journal of Anatomy, vol. 198, pp. 57-66.
Despite many similarities in sperm head morphology, the organisational structure of chromatin was found to differ between koala and wombat sperm; inferring that these structures are species-specific, a finding not previously described in any study.
Sperm head characteristics were observed for both species using transmission electron microscopy (TEM) with the successive incubation of both koala and wombat sperm in 0.01% and 0.1% Triton X-100, an ionic detergent, necessary to determine respective chromatin stability. Application of Triton X-100 showed that koala sperm incubated at 0.1% were dispersed to some degree and displayed large vacuoles. Comparatively, wombat sperm incubated at the same Triton X-100 concentration did not exhibit chromatin dispersal but did exhibit a granular appearance and significant vacuolation of the large acrosome segment. These results indicate variation of interspecific structure and organisation of chromatin between the wombat and koala.
Marsupials, like the koala, exhibit different acrosomal (an organelle on the anterior region of the sperm head) morphologies to eutherian mammal species and appear to be more stable in comparison. Interestingly, wombat sperm head organisational components exhibit very different sperm head characteristics than that observed in koalas; a phenomenon not investigated between any other marsupial species until this study.
Structural components of the acrosome of the spermatozoon were comparable between each species with new posterior and lateral features described for the first time in this study. The authors highlight the need for future investigations to be oriented around sperm-egg interactive processes following the study’s findings.
Summarised by Lauren Mousley
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