Journal of Neuroscience, Vol 8, 3317-3326, Copyright © 1988 by Society for Neuroscience
Embryonic assembly of a complex muscle is directed by a single identified cell in the medicinal leech
J Jellies and WB Kristan Jr
Department of Biology, University of California at San Diego, La Jolla 92093.
The present study examines the morphological development of a highly
organized muscle layer in the leech Hirudo medicinalis, in an effort to
characterize those factors that are important in directing its assembly.
The tubular body wall of the leech contains 3 major muscle layers that are
anatomically distinct: an inner layer of longitudinal muscle, an outer
layer of circular muscle, and a grid of oblique muscle sandwiched between
them. The oblique muscle layer appears later in development than the other
2 and is preceded by several days by the development of a single, complex
cell (here called the comb, or C-cell) whose shape strongly resembles the
organization of the oblique muscle grid. There is a bilateral, mirror-image
pair of C-cells in each segment. The C-cell has a central, longitudinally
oriented soma and projects about 35 fine, parallel processes both medially
and laterally at approximately 45 degrees to the long axis. Using a
combination of intracellular and antibody labels, it was found that
individual muscle cells align themselves with these processes to form
correctly oriented fascicles during development. Photoablation of the
C-cell at early stages resulted in the complete absence of all oblique
muscle fascicles that would have corresponded to that cell; therefore, this
discrete muscle-associated cell is considered to be an identified "muscle
organizer." Such cellular organizers may direct muscular and neuromuscular
assembly in many species.
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