Worm Breeder's Gazette 1(1): 15

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.

Post-embryonic Somatic Cell Lineages

J. Sulston, R. Horvitz

After hatching, the number of somatic cells in the hermaphrodite of 
C.  elegans increases from about 530 to about 780.  The precise 
lineages which lead to this increase in cell number have been 
determined by direct observation of developing nematodes.  Young worms 
are mounted on a thin block of agar on a glass slide under a cover 
slip which contains a small amount of E.  coli spread at its center; 
the edges are sealed with silicone grease or Vaseline to prevent 
dehydration.  When so mounted, the young nematode develops normally 
and can be observed in the light microscope.  Under Nomarski 
differential interference contrast optics, which detects refractive 
index variations, nuclei (and often nucleoli) are very distinct and 
can be followed as they migrate, divide, and, in some cases, die.  
Using this technique to observe nuclei in developing larvae, we have 
determined all of the somatic cell lineages which occur after hatching.

All tissues in the nematode undergo some post-embryonic divisions.  
Hypodermal cell divisions occur around the time of the first three 
larval moults; six cells on each lateral side of the newly-hatched 
animal follow the same basic program of divisions.  (Two of these 12 
cells modify this program to produce most of the posterior lateral 
ganglia as well as hypodermal cells).  Two additional hypodermal blast 
cells on each side of the head follow a different program.  Another 
blast cell on each side of the tail produces both hypodermal and nerve 
cells.  Most of the intestinal cells divide once, just before the 
first moult.  Substantial mesodermal development derives from a single 
cell present in the newly hatched larva; a series of divisions during 
the first larval stage produces 14 somatic muscle cells, 2 non-muscle 
cells (probably secretory in function) and 2 myoblasts; these 
myoblasts migrate anteriorly until they reach the position where the 
vulva will form; they divide just before the third moult to produce 16 
sex muscles, which function in egg-laying.  Much of the neuronal 
development occurs in the ventral nerve cord and its associated 
ganglia.  At hatching only 33 neurons are present in these structures, 
but divisions of 12 precursor cells and one neuroblast (followed by a 
number of programmed cell deaths) increase this number to 89.  Each of 
the 12 precursors divides to give a hypodermal cell and a neuroblast.  
All 13 neuroblasts then follow the same asymmetric program of 
divisions.  Each of 5 of the distinct anatomical classes of neurons in 
the adult ventral cord seems to be derived from the equivalent progeny 
cells of the 13 neuroblast lineages (e.g.  the posterior daughter of 
the posterior daughter of each neuroblast becomes a 'Class D' neuron.) 
Three of the 12 new ventral cord hypodermal cells divide further just 
before the third moult to produce the 22-cell vulva.  A number of 
other cell lineages also occur in the hermaphrodite.
In the male, this developmental program is altered.  The number of 
somatic cells in the male increases from about 530 to about 950; the 
extra cells are mostly derived from new lineages which occur in the 
In addition, some of the somatic lineages of the hermaphrodite are 
modified to generate male-specific structures.  For example, the male 
mesodermal lineage produces 14 somatic muscle cells, one non-muscle 
cell, and 41 male specific muscles which control the tail before and 
during copulation.  In the male ventral cord there are fewer 
programmed cell deaths and an extra round of cell divisions.  Two 
posterior ventral cord hypodermal cells divide to form part of the 
male tail, whereas the three related cells which produce the vulva in 
the hermaphrodite do not divide.
We have begun to isolate and characterize mutants which affect these 
cell lineages.  Some of these mutants display general defects in 
postembryonic cell division and, because of their aberrant gonadal 
development, produce sterile animals when homozygous.  Other mutants 
show more specific effects, such as blocks in the ventral hypodermal 
lineage which produces the vulva.  One mutant is generally deficient 
in ectodermal divisions, with both the ventral and lateral hypodermis 
and part of the male tail affected.  Some mutants show cell divisions 
which do not normally occur in wild-type individuals; in one, for 
example, extra posterior lateral neurons are generated.  Two genes 
affect the migrations of the ventral cord precursor cell nuclei which 
normally precede their divisions.
Using both mutants and the laser system developed by John White to 
disrupt the normal cell lineages, we hope to learn more about the 
regulation of these developmental processes.