MUSCLE-CELL DEATH DURING THE DEVELOPMENT OF HEAD AND NECK MUSCLES IN THE CHICK-EMBRYO

Degenerating myofibers have been reported in the embryos and neonates of a number of birds and mammals, but neither the pervasiveness of the phenomenon nor the spatiotemporal patterns of degeneration has been e xamined in detail. Using transmission electron microscopy, we determin ed the patterns of muscle cell death in the chick biventer cervicis, a head extensor muscle. Cell death is most pronounced at incubation day s 10 through 15, and occurs throughout the muscle. This is the period during which many myofiber clusters segregate into individual fibers, each with a separate basal lamina, and secondary myofibers become dema rcated. Cells of largest diameter, presumably the primary myofibers, a re preferentially affected. Degenerating cells exhibit a cohort of cyt ological features consistent with apoptosis, including the presence of dense, darkly-staining, hypercontracted myofibrils, misshapen nuclei with irregular chromatin condensations along the nuclear envelope, and scores of cytoplasmic vesicles and vacuoles. In cross section some la rge diameter muscle cells are characterized by sparse, flocculent cyto plasm that is devoid of myofibrils and organelles. Some show disintegr ating cell membranes. In longitudinal section 200-300 mu m long region s of hypercontracted myofibrils alternate with areas devoid of fibrils ; this arrangement suggests that the myofibrils break into segments th at are in register along one part of a muscle fiber and entirely absen t from the adjacent length of fiber. We have observed similar patterns of muscle cell degeneration in the complexus, splenius cervicis, depr essor mandibulae, and branchiomandibularis muscles. By day 18 of incub ation most signs of degeneration are absent and by hatching (day 21) t he muscle fibers all appear healthy. Many of these cytological changes in embryonic head muscle cells are characteristic of programmed cell death. We hypothesize that large-scale death of myocytes is a normal p art of avian myogenesis and an important mechanism for affecting the t ransformation from embryonic to hatching muscle patterning. (C) 1995 W iley-Liss, Inc.