Worm Breeder's Gazette 11(5): 99

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.

Molt Specifc Changes in the Hypodermal Cytoskeleton and the Origin of Annuli

Bruce W. Draper and James R. Priess

Figure 1

The hypodermal cells of C.  elegans are responsible for transforming 
the round embryo into a long thin worm, apparently by constricting 
circumferentially-oriented bundles of microfilaments at their apical 
surface (Priess and Hirsh, 1986).  These microfilament bundles appear 
to be responsible for both generating and maintaining the shape of the 
worm until the embryonic cuticle has been formed.  We have been 
interested in whether these microfilament bundles are also involved in 
post-embryonic molting periods, when the animal must produce a new 
cuticle and detach from the old.  Since we have found that the 
position of the microfilament bundles across the embryonic surface are 
precisely correlated with where the annuli are seen in the first 
larval cuticle, we predicted that annuli of post embryonic cuticles 
would similarly correspond to microfilament bundles in the hypodermis 
of molting worms.
Synchronized populations of N2 worms were fixed at various stages 
between L3 and adults and stained with Rhodamine-Phalloidin.  We find 
that during each molt, actin microfilament bundles are present and are 
regularly spaced and circumferentially-oriented, very similar to what 
is seen during embryonic morphogenesis (pictured below is L3 molt).  
These bundles largely disappear after molting and do not reappear 
until the following molt.  They are not present in adult worms.  
Visualization of microtubules during the molts reveals that they too 
are regularly spaced and circumferentially-oriented, again similar to 
embryonic morphogenesis.  But unlike the microfilament bundles, 
circumferential microtubules persist during the inter-molt periods and 
are seen in adult worms.
Although the shape of the worm is initially generated by a mechanism 
involving the hypodermal cytoskeleton, larval and adult shape is 
dependent on an intact cuticle and a high internal hydrostatic 
pressure.  This property produces obvious problems for a molting worm: 
how can a worm maintain its shape and internal pressure and at the 
same time reduce its volume in order to shed its old cuticle? We 
propose that nematodes employ the same cytoskeletal machinery used 
during embryonic morphogenesis.  Actin microfilament bundles in the 
hypodermal cells constrict the worm and microtubules distribute this 
pressure evenly along the length of the worm.  The deformation of the 
hypodermal membrane caused by this constriction is once again 
responsible for patterning the cuticle being deposited at this time, 
which accounts for the annuli of larval and adult cuticles.
[See Figure 1]

Figure 1