Spermiogenesis in birds

Abstract

Current knowledge on avian spermiogenesis, including strengths and weaknesses, has been reviewed. Information on avian spermiogenesis considerably lags behind that in mammals because of the paucity of reports in birds. Spermiogenesis in passerine birds has received even much less attention than in non-passerine birds. Mechanisms underlying morphogenesis of the acrosome and nucleus, and roles of microtubular assemblies are poorly understood. The proximal centriole found in non-passerine birds, but hitherto considered to be absent in passerine birds, has recently been described in spermatids and mature spermatozoa of 2 passeridan species, including the Masked weaver for which new and detailed spermiogenetic information is provided in this review. A great deal more studies on spermiogenesis, and spermatogenesis generally, in various avian species are required to considerably enhance knowledge of this phenomenon, contribute to comparative spermatology, provide a basis for appropriate applied studies, and contribute to understanding of phylogeny in this vast order of vertebrates.

Sperm head shaping in ratites: New insights, yet more questions

“…The multiple sites of nuclear constriction characterizing this process in Phase II spermatids are marked by a concentration of circularly disposed microtubules as previously described for the ostrich (Soley, 1997) and various other non-passerine species (Aire, 2014). The cumulative effect of this process transforms the nucleus into a scalloped, elongated structure that eventually manifests as a long, narrow cylinder.…”

“…Most mammalian spermatids typically only display a LM with its point of attachment at the nuclear ring, a sub-plasmalemmal density situated just beneath the base of the acrosome (Barth and Oko, 1989;Fawcett et al, 1971;Holstein and Roosen-Runge, 1981;Ploën, 1971). In contrast, spermatids of a number of avian orders characteristically display both a CM and a LM (Aire, 2014), although the latter is not associated with a nuclear ring. A transitional stage between the two manchettes has been described in some avian species such as the ostrich (Soley, 1997), rhea (Phillips and Asa, 1989), turkey (Aire, 2003) and African collared dove (Mattei et al, 1972).…”

“…Additional evidence has been presented in the literature supporting the role of the CM in effecting nuclear shaping in birds. As summarized by Aire (2007Aire ( , 2014, the round nucleus of Step 4 spermatids (based on the steps of spermiogenesis described in the turkey (Aire, 2003)) in non-passerine birds is surrounded by small groups of microtubules positioned close to the outer nuclear membrane, particularly around the rostral aspect. During the next stage (Step 5) the nucleus becomes pear-shaped (Aire, 2007) or teardrop shaped (Lovas et al, 2012).…”

“…In birds, decondensation of the nuclear chromatin of early spermatids leads to the karyoplasm taking on the form of homogeneously distributed, finely granular material (Aire 2007(Aire , 2014.…”

“…The pattern of chromatin condensation (Fawcett et al, 1971), as well as the limited role of the forming acrosome (Soley, 1996(Soley, , 1997, have also been proposed as factors determining nuclear shaping in birds. The possible involvement of a PT, or indeed the existence of this structure, has not been reported in birds, although the translucent layer beneath the forming acrosome and the peri-nuclear fibro-granular layer posterior to the acrosome (Aire, 2007(Aire, , 2014Fawcett et al, 1971) described in some species would suggest the existence of a PT in this vertebrate order. The recent identification of a unique, transient cytoplasmic structure that manifests during spermiogenesis around the post-acrosomal region of the sperm nucleus in ratites (Du Plessis and Soley, 2013) provides compelling evidence for the presence of an analogous entity, at least in ratites.…”