SUBSTRUCTURE OF THE CORTICAL SINGLET MICROTUBULES IN SPERMATOZOA OF<i>MACROSTOMUM</i>(PLATYHELMINTHES, TURBELLARIA) AS REVEALED BY NEGATIVE STAINING

Abstract

1. Cortical singlet microtubules are the only microtubular components of the motile spermatozoa of Macrostomum. When negatively stained with phosphotungstic acid, the microtubules display a helical arrangement of the subunits similar to that described for spermatozoa of other species of platyhelminths. 2. The protofibrillar configuration of subunits of the cortical singlets can also occur under conditions of negative staining. One microtubule was clearly observed to be made up of 12 protofibrils. 3. Analysis of various stages in the transition from the helical to the protofibrillar configuration suggests that there are two steps in the conversion. These are (a) a seemingly random lateral separation of subunits to form protofibrils with a longitudinal periodicity of ca. 80 Å, which is characteristic of the periodicity of the intact helical arrangement, and (b) a subsequent change in spacing of the subunits along the length of the protofibril, from ca. 80 Å to 40-45 Å. These observations support the view that if monomers associate to form dimers, the dimers occur along the length of the protofibril rather than between adjacent protofibrils.