The morphology and development of normal and abnormal spermatozoa in the emu, Dromaius novaehollandiae

Abstract

The morphological features and development of normal and abnormal sperm were studied in the emu, Dromaius novaehollandiae, using both light and electron microscopy. Detailed descriptions of normal as well as abnormal forms were documented. Where possible, the origin and development of the defects were examined and interpreted. Based on these observations, a system for the morphological classification of ratite sperm abnormalities was proposed. Emu sperm typically revealed the basic filiform morphology which is common in other ratites and non-passerine birds in general. At the ultrastructural level, emu sperm displayed a conical acrosome covering the tip of the nucleus and a long distal centriole running the entire length of the midpiece. Mitochondria of the pars spiralis were more numerous than in other ratites, but no intermitochondrial cement was present in the mitochondrial sheath. Although sharing basic ultrastructural features, a distinct difference between sperm of the emu and other ratites was the complete absence of a perforatorium and endonuclear canal. Another distinctive feature revealed by all forms of microscopy was the presence of a cytoplasmic appendage located at the base of the nucleus and which appeared to represent the point of cytoplasmic release during spermiation. Based on the examination of a total of 15 semen samples collected during the middle of the breeding season from the distal deferent ducts of commercially slaughtered adult emus, a total of 14% morphologically abnormal cells were identified. Head defects were represented by bent, microcephalic, macrocephalic or round heads. Acephalic sperm were also present in all samples. Zones of incomplete chromatin condensation and retained cytoplasmic droplets appeared to be implicated in head bending, while giant heads were often associated with multiple tails. In acephalic sperm the tail was complete, but the head was absent, often being replaced by a small spherical structure. Tail defects were subdivided into defects of the neck/midpiece and those of the principal piece. In the neck/midpiece region two anomalies were noted, namely abaxial tail implantation and disjointed sperm. Abaxial tail implantation involved the eccentric positioning of the centriolar complex relative to the head base while disjointed sperm were characterized by the complete separation of the head and midpiece in the neck region, but within the confines of the plasmalemma. Defects observed in the principal piece were subdivided into short, coiled and multiple tails. Cytoplasmic droplets were classified as a separate defect. A small percentage of sperm displayed multiple abnormalities. Defective alignment and/or migration of the centriolar complex (diplosome) was implicated in a number of the defects observed, including head-base bending, disjointed sperm, acephalic sperm and abaxial sperm. Spermiogenesis in the emu broadly paralleled the development of spermatids reported in other nonpasserine birds. However, a previously undescribed morphological feature of avian sperm development was observed. During the circular manchette stage of spermatid development, a unique transient structure appeared. It was associated with the outer nuclear membrane and formed regular finger-like projections into the cytoplasm. This structure was present during both the circular and longitudinal manchette stages of development and was particularly obvious towards the apical aspect of the nucleus where it abutted the cone-shaped acrosome. There were no obvious connections between the structure and the microtubules of the manchette. During the final stages of spermatid development the structure abruptly disappeared. Despite employing immunogold labelling techniques and electron tomography, the nature and function of this structure remained unresolved. Descriptions of a similar structure in various lizard species and the Caiman crocodile, although restricted in its location to the nuclear membrane immediately beneath the acrosome, would appear to reinforce the phylogenetic link previously identified between birds and crocodiles.