Head-base bending and disjointed spermatozoa in the emu (Dromaius novaehollandiae): a morphological comparison of two closely related defects.

The accurate assessment of avian sperm abnormalities is hampered by a lack of descriptive data and by the confusing terminology currently in use. Critical appraisal of semen samples from the distal ductus deferens of the emu revealed that two closely related yet separate (distinct) defects previously collectively referred to as "bent sperm" or "crooked-necked sperm" could be identified by light and electron microscopy. Head-base bending typically involved a 180° bend at the base of the nucleus which placed the head and midpiece into close apposition and parallel to each other. No part of the neck or midpiece was involved and bending was restricted exclusively to the base of the nucleus. Incomplete chromatin condensation was always associated with the bend. Disjointed sperm, which superficially resembled "bent" sperm, showed complete separation of the neck from the head-base at the level of the connecting piece. All structural elements of the neck region appeared normal. In both defects the region of contact between the head (nucleus) and the neck/midpiece was enclosed as a unit by the plasmalemma. Both defects were observed to originate in the testis; however, their subsequent expression in the ductus deferens cannot be ruled out. These results confirm that head-base bending of emu sperm represents a head defect, whereas disjointed sperm should be classified as a tail (neck/midpiece) defect.