Worm Breeder's Gazette 2(2): 27

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.

Structural Studies of Nematode Body-wall Muscles

J.M. Mackenzie Jr., H.F. Epstein

We have pursued detailed studies of the structure of N2 body-wall 
muscle as a necessary condition for deciphering the abnormalities of 
various sarcomere-defective mutants, particularly many which have more 
subtle defects than unc-54 mutants, and also to understand structural 
changes in body-wall muscle cells during development.  
Using polarized light microscopy, we find that the body wall muscle 
cells in relaxed, anesthetized N2 animals are arranged right to left 
+/-11.9  to the anterior-posterior axis.  The sarcomeres within these 
cells are arranged +/- 5.9  to the same axis.  Under these conditions, 
the sarcomere width normal to the cell is 1.67  and the similarly 
measured A band width is 1.0 .  By both polarized light and electron 
microscopy we find that the orientation of the thick filaments is 
parallel to the N2 anterior-posterior axis.  Thus, from these 
observations, the length of the thick filaments is calculated to be 9.
7 , assuming that all the thick filaments are held in register.  It 
would be difficult to accurately measure the length of such very long 
filaments by electron microscopy of thin sectioned material.  In 
roller mutants such as the dominant SU1006 where the cell orientation 
is significantly different from N2 and variable, all the internal 
relationships and dimensions are preserved.  Similarly in dumpy 
mutants such as E61, although cell lengths are shortened, all other 
features such as orientation and internal dimensions are similar to N2.

Electron microscopy of fixed, dehydrated, embedded and stained 
transverse thin sections of N2 body-wall muscles previously obtained 
have been ambiguous as to whether or not the thick filaments are 
arranged in any regular lattice.  On the basis of new sections and 
freeze-etching work, we suggest that indeed there is an hexagonal 
packing of the thick filaments in the A-bands but that this packing 
may become easily disordered due to contraction or to preparation.  
We have been able to identify a new set of structures closely 
associated with the dense bodies to which the thin filaments attach.  
These are similar to the intermediate or 100 angstrom filaments 
observed in other muscles and non-muscle cells, and they may represent 
a kind of cytoskeleton.  Observation of negatively stained, isolated 
and crosslinked dense bodies confirms these associations.  Bridges 
between isolated thick and thin filaments can also be observed by 
these techniques.
Examples of the application of these methods to more subtle 
sarcomere-defective mutants such as R73, a partial revertant of E569 (
unc-54), and E286, a temperature-sensitive mutant (unc-45).In 
connection with biochemical work in the laboratory concerning the two 
body-wall myosins and changes in their relative concentrations during 
development, studies of sarcomere structure during development and of 
immunochemical localization of myosin have been initiated.  The 
dimensions of A-bands and sarcomeres remain in variant from 24 hours 
to 60 hours after hatching.  However, the number of sarcomeres per 
cell changes from about two to eight during this development.
Specific antibody to myosin using a perioxidase method shows strong 
staining only in the sarcomeres of the body-wall and in the pharynx.  
Studies are in progress on the localization of unc-54 affected myosin. 
This method may be useful in determining the molecular composition of 
abnormal structures in various mutants.