Biology Reference
In-Depth Information
CHAPTER
8
The Nano
-Machinery of Lo
comotion
The mechanisms that control the extent and direction of cell migration in a developing
embryo do so by selective and local modification of the processes of cell locomotion. It is
therefore sensible, in order to avoid an overload of information when guidance mechanisms
are being discussed, to describe the basic machinery of cell migration first, without reference
to navigation, and to refer back to this foundation in the subsequent chapters on guidance
and navigation.
Of the various types of cell locomotion seen across the phylogenetic tree, such as flagellar
swimming, ciliary gliding and amoeboid crawling, by far the most significant to develop-
mental biology is crawling. The other mechanisms may be faster, but they tend to be used
only extracorporeally for the dispersal of gametes. There are several components to crawling
motion. The first is protrusion of a specialized area of the cell periphery, the lamellipodium,
forwards on to the substrate ('forwards' being defined by the orientation of the cell's
polarity, which will be discussed in more detail later). This is followed by advance of the
main parts of the cell (nucleus, organelles, and so on) into the lamellipodium, and the
drawing up of the trailing edge of the cell. These three phases may occur cyclically, but
are often overlapping or simultaneous and their separation in this chapter is mostly for
convenience of discussion.
PROTRUSION: THE ACTIN-BASED NANO-MACHINERY OF THE
LEADING EDGE
Lamellipodia
The lamellipodium of a cell migrating on a flat substrate is a flat plate of cytoplasm, only
100
e
200 nm thick,
1
the internal structure of which is dominated by protein filaments. In most
cells, these are actin microfilaments, although some cells use other proteins for a broadly
similar function; an example is the crawling sperm of the parasitic nematode
Ascaris suum
,
which uses major sperm protein (MSP) instead.
2
The microfilaments at the leading edge
meet the membrane at an angle of about 40
e
60 degrees.
3
It is the presence of these filaments
that prevents the membrane from collapsing inwards under its own surface tension
(
Figure 8.1
).
Neither the filaments nor the membrane are frozen, but exist in the liquid environment of
a living cell and are therefore subject to constant thermodynamic battering by molecules
