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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.

Embryonic assembly of a complex muscle is directed by a single identified cell in the medicinal leech