  A living cell uses long tubular appendages for locomotion and sensory
purposes. Hence, assembling and maintaining a protrusion of correct length is
crucial for its survival and overall performance. Usually the protrusions lack
the machinery for the synthesis of building blocks and imports them from the
cell body. What are the unique features of the transport logistics which
facilitate the exchange of these building blocks between the cell and the
protrusion? What kind of `rulers' and `timers' does the cell use for
constructing its appendages of correct length on time? How do the multiple
appendages coordinate and communicate among themselves during different stages
of their existence? How frequently do the fluctuations drive the length of
these dynamic protrusions out of the acceptable bounds? These questions are
addressed from a broad perspective in this review which is organized in three
parts. In part-I the list of all known cell protrusions is followed by a
comprehensive list of the mechanisms of length control of cell protrusions
reported in the literature. We review not only the dynamics of the genesis of
the protrusions, but also their resorption and regrowth as well as regeneration
after amputation. As a case study in part-II, the specific cell protrusion that
has been discussed in detail is eukaryotic flagellum (also known as cilium);
this choice was dictated by the fact that flagellar length control mechanisms
have been studied most extensively over more than half a century in cells with
two or more flagella. Although limited in scope, brief discussions on a few
non-flagellar cell protrusions in part-III of this review is intended to
provide a glimpse of the uncharted territories and challenging frontiers of
research on subcellular length control phenomena that awaits vigorous
investigations.
