Compliance in the neck structures of the guinea pig spermatozoon, as indicated by rapid freezing and electron microscopy

Electron microscopy has been used to investigate whether the transversely striated columns of the connecting piece in the neck region of guinea pig spermatozoa, undergo lengthening and shortening as a result of the forces generated during motility. Motile spermatozoa were subjected to near-instantaneous rapid freezing, followed by freeze-substitution fixation and epoxy embedment. Thin sections passing longitudinally through the striated columns revealed that the periodicity was indeed variable. The repeat period, taken to have an unstressed width of 60 nm, could be found extended to 75 nm in some specimens, and reduced to 54 nm in others. The estimates of the coefficients of variation were 6.6% for the width of the 'dense' band and 33.5% for the 'pale' band. The 'pale' band in the extended state showed longitudinal striae. Such variations in length, which - it is suggested - are physiological, and passively induced, would have functional implications for the flagellum - for both bend initiation and bend growth. Also, hypothetically, any mechanism that could increase the degree of compliance in these columns, such as perhaps phosphorylation of the constituent proteins, could permit the flagellum to develop the exaggerated bend angles and asymmetries of the 'hyperactivated' state.