Worm Breeder's Gazette 11(1): 59

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

How do Distal Tip Cells Migrate? (or Cortical Flow Fever)

Ed Hedgecock and David Hall

Figure 1

Migrating distal tip cells are unusual in that the nucleus is 
positioned at the leading rather than the trailing edge of the cell (
Figs. 1, 2).  The leading edge itself is round rather than flat or 
ruffling.  The trailing edge ensheaths some 3-8 distalmost germ cells 
and the trailing lip forms a constriction around the column of germ 
cells.  Despite such novel features, distal tip cell movement may be 
governed by the same cortical flow mechanism recently proposed for 
fibroblast and amoeboid locomotion (Bray and White, Science 239, 883 (
1988)).  In particular, a gradient of cortical contraction, lowest at 
the leading edge, could cause preferential cytoplasmic extension at 
the leading edge.  Like cytoplasm, an untethered nucleus could be 
squeezed into this relaxed region.  A prediction is that actin 
microfilaments producing cortical tension should be more abundant near 
the trailing edge of the cell and perhaps circumferentially aligned.  
In fact, there appears to be a concentration of circumferentially 
aligned microtubules in this region.
How might cell direction be regulated? It is widely believed that 
receptors in the cell membrane couple the cortical cytoskeleton to 
external ligand gradients that guide the cell.  If these receptors are 
diffusible in the cell membrane and functionless until they bind a 
ligand, then the front of receptor-ligand complexes that forms when 
the cell extends over new substratum would tend along ligand isolines (
Fig.  3).  If these complexes orient new microfilaments, cells will 
turn up-gradient and the filaments will align circumferentially during 
straight segments of the trajectory.
The role of the germ cells in distal tip cell migrations is somewhat 
complicated.  They are not strictly required for hermaphrodite distal 
tip cell movements.  Indeed, the male linker cell, which probably 
moves by the same mechanism, does not ensheath germ cells.  Even so, 
the hermaphrodite distal tip cells appear to migrate faster when 
leading a column of germ cells.  The germ cells may be serving two 
roles: (1) filling the cell concavity with an incompressible fluid on 
which to squeeze, and (2) pushing the distal tip cell from behind like 
a battering ram.  The first role would be unnecessary in cells lacking 
a concavity.  Passive movement of the distal tip cells caused by 
proliferation of the germ cells could be significant in late larvae or 
If germ cells are not required for migration, it is not obvious why 
the hermaphrodite distal tip cells trouble to ensheath the distalmost 
cells.  As the male distal tip cells apparently do not ensheath germ 
cells, ensheathment is not needed to regulate germ cell proliferation 
per se.  We speculate that the ensheathment helps 'set' the 
cylindrical geometry of the trailing cell column.  Once established, 
the germ cells maintain this geometry despite extensive proliferation. 
Cells in the somatic primordium could serve this shaping function in 
the male.
[See Figure 1]

Figure 1