The Questions of Developmental Biology

The means by which sperm are propelled vary according to how the species has adapted
to environmental conditions. In some species (such as the parasitic roundworm Ascaris), the
sperm travel by the amoeboid motion of lamellipodial extensions of the cell membrane. In most
species, however, each sperm is able to travel long distances by whipping its flagellum. Flagella
are complex structures. The major motor portion of the flagellum is called the axoneme. It is
formed by microtubules emanating from the centriole at the base of the sperm nucleus (Figures
7.2 and 7.3). The core of the axoneme consists of two central microtubules surrounded by a row
of nine doublet microtubules. Actually, only one microtubule of each doublet is complete, having
13 protofilaments; the other is C-shaped and has only 11 protofilaments (Figure 7.3B). A threedimensional
model of a complete microtubule is shown in Figure 7.3C. Here we can see the 13
interconnected protofilaments, which are made exclusively of the dimeric protein tubulin.
Although tubulin is the basis for the structure of the flagellum, other proteins are also critical for
flagellar function. The force for sperm propulsion is provided by dynein, a protein that is
attached to the microtubules (Figure 7.3B). Dynein hydrolyzes molecules of ATP and can convert
the released chemical energy into the mechanical energy that propels the sperm.
This energy allows the active sliding of the outer doublet microtubules, causing the flagellum to
bend (Ogawa et al. 1977; Shinyoji et al. 1998). The importance of dynein can be seen in
individuals with the genetic syndrome called the Kartagener triad. These individuals lack dynein
on all their ciliated and flagellated cells, rendering these structures immotile.