In 1959, Afzelius observed the presence of two rows of arms projecting
from each outer doublet microtubule of the so-called 9 + 2 pattern of cilia
and flagella, and suggested a possibility that the outer doublet
microtubules slide with respect to each other with the aid of these arms
during ciliary and flagellar movement. The identification of the arms as an
ATPase, dynein, by Gibbons (1963)strengthened this hypothesis, since the
ATPase-bearing heads of myosin molecules projecting from the thick filaments
pull the thin filaments by cross-bridge formation during muscle contraction.
The first experimental evidence for the sliding mechanism in cilia and
flagella was obtained by examining the tip patterns of molluscan gill cilia
by Satir (1965) who observed constant length of the microtubules during
ciliary bending. Further evidence for the sliding-tubule mechanism was given
by Summers and Gibbons (1971), using trypsin-treated axonemal fragments of
sea urchin spermatozoa. Upon the addition of ATP, the outer doublets
telescoped out from these fragments and the total length reached up to seven
or more times that of the original fragment. Thus, the arms on a certain
doublet microtubule can walk along the adjacent doublet when the doublet
microtubules are disconnected by digestion of the interdoublet links which
connect them with each other, or the radial spokes which connect them with
the central pair-central sheath complex as illustrated in Fig. 1. On the
basis of these pioneer works, the sliding-tubule mechanism has been
established as one of the basic mechanisms for ciliary and flagellar
movement.