Linkage of manchette microtubules to the nuclear envelope and observations of the role of the manchette in nuclear shaping during spermiogenesis in rodents

Abstract

Structural features of the mouse and rat manchette and the role of the manchette in shaping the spermatid nucleus were investigated. Rod‐like elements about 10 nm in diameter and 40–70 nm in length were seen linking the innermost microtubules of the manchette and the outer leaflet of the nuclear envelope in step 8 through step 11 rat and mouse spermatids that either had been routinely fixed for electron microscopy or had been isolated and detergent extracted. Rod‐like linkers were also seen joining the nuclear ring to the plasma membrane and nuclear envelope. These linkers may ensure that under normal conditions the manchette remains in a defined position relative to these membranous components. A variety of compounds (taxol, cytoxan, and 5‐fluorouracil) were found to perturb the manchette and to affect nuclear shaping. In addition, sys and azh mutant mice were used to determine the consequences of defective manchette formation. These genetic conditions and chemical treatments either produced manchettes that were not in their normal position (azh, sys, and taxol) and/or caused the manchette to appear abnormal (azh, sys, cytoxan, 5‐fluorouracil, and taxol), and all resulted in a deformation of the step 9–11 spermatid nucleus. In all instances where the manchette was present, either in normal or ectopic locations, the sectioned nuclear envelope was parallel to the long axis of the microtubules of the manchette. In general, areas of the nuclear envelope where the manchette was not present, or where it was expected to be present but was not, were rounded (normal animals, sys, cytoxan). In addition, there are indications using certain compounds (cytoxan and 5‐fluorouracil) as well as in the azh and sys mouse that the manchette may exert pressure to deform the nucleus. It is suggested that the rod‐like linkages of the manchette ensure that the nuclear envelope remains at a constant distance from the manchette microtubules and that this is a major factor acting to impart nuclear shape changes on a region of the head caudal to the acrosome during the early elongation phase of spermiogenesis. The manchette microtubules, which are also known to be linked together, may act as a scaffold to deform this part of the nucleus from its spherical shape, perhaps in concert with forces initiated by other structural elements. Evidence from sys animals indicates that structural elements, such as the acrosomal complex over the anterior head (acrosome‐actin‐nuclear envelope), may affect nuclear shaping over the acrosome‐covered portion of the spermatid head. It is suggested that chromatin condensation primarily reinforces the shape changes already brought about by the manchette and other structural elements acting on the nucleus. After chromatin condensation, spermatid nuclear shape changes are minor.