Biology Reference
In-Depth Information
Somitic migration
Myogenic precursor cells migrate from the lateral part of the somites into the
forelimb where they form muscles of the limb (Chevallier et al., 1977; Christ
et al., 1977; Ordahl and Le Douarin, 1992; Williams and Ordahl, 1994). We
prepared slice cultures from chick embryos and followed the dynamics of the
migrating cells. Somitic cells, which were labelled with DiI, did not migrate
along a highly constrained pathway. Rapid bursts of directed migration were
followed by periods in which the cells wandered away from the restricted
pathway. Unlike cultured cells, the somitic cells extended a single, long,
persistent protrusion in the direction of migration. The protrusions moved
laterally, but once stable attachments formed the cells moved forward. Some
cells extended and retracted protrusions, often for long periods of time,
without much accompanying movement suggesting that the formation of
stable adhesions is not tightly coupled to the formation of a protrusion
(Knight et al., 2000). The exaggerated, persistent protrusions were not
observed in cells that moved out of the slice. Instead, the cells extended
random protrusions and migration was no longer directional indicating the
influence of the surrounding environment on the defined migration.
The small GTPase Rac is a key regulator of protrusive activity in cultured
cells (Ridley and Hall, 1992). We expressed wild-type Rac, dominant-negative
Rac (N17Rac) and constitutively-active Rac (L61Rac) as GFP fusion proteins
in the slices to determine whether this molecule contributed to the generation
of the large protrusions in the embryo slices. The exaggerated, persistent
protrusions are not observed in somitic cells expressing dominant negative
Rac, and migration was inhibited. In cells expressing constitutively-active
Rac, many small protrusions formed and retracted frequently, and migration
was random. Thus, Rac regulates the polarization that characterizes the
unusual protrusive activity observed in the somitic cells.
Migration of neuronal precursors in the rostral migratory stream
In rodents, olfactory interneuron precursors are generated in the sub-
ventricular zone (SVZ) of the forebrain in the embryo and throughout the
adult life (Garcia-Verdugo et al., 1998; Luskin, 1993). The neuronal
precursors in the anterior part of the SVZ then migrate to their targets in
the centre of the olfactory bulb and differentiate into mature interneurons.
This restricted pathway composed of migrating neural precursor cells is
known as the rostral migratory stream (RMS). Although the mechanisms that
regulate migration along this pathway are poorly understood, studies from
knockout mice suggest that the polysialic acid chains (PSA) on N-CAM
molecules are involved in this process (Ono et al., 1994; Tomasiewicz et al.,
