Rheuben M B
Department of Anatomy, College of Veterinary Medicine, Michigan State University, East Lansing 48824-1316.
J Exp Biol. 1992 Jun;167:91-117. doi: 10.1242/jeb.167.1.91.
The ultrastructural changes associated with the early stages of degeneration of the larval mesothoracic muscle fibers of Manduca sexta were examined during the prepupal period and on the first day after ecdysis. Over this 5 day period, the muscle fibers decrease in cross-sectional area but increase in apparent surface area compared to the dimensions of early fifth-instar fibers. Large numbers of electron-dense granules or droplets are formed and extruded from the muscle cytoplasm into the hemolymph; this process may account for some of the decrease in muscle fiber mass and may represent a developmental mechanism for recycling nutrients. As the fibers shrink, the thick basal lamina is thrown into folds. Phagocytic hemocytes (granulocytes) congregate in clusters over the surface of the degenerating fibers and appear to remove specifically the basal lamina. The timely removal of the thick larval basal lamina may be essential for subsequent fusion of myoblasts to the residual larval myofibers. The contractile elements within the degenerating muscle fibers become disorganized but are not dysfunctional at the end of the first 12 h after the pupal ecdysis. Tracheoles withdraw from intimate contact with each muscle fiber in its clefts and T-tubules and associate in groups adjacent to it. Mitochondria appear to be degenerating. These structural changes are concurrent with a previously observed decline in resting potential and suggest that a significant change in the electrical properties of the muscle fibers should be expected as well.
在蛹前期和蜕皮后的第一天，研究了烟草天蛾幼虫中胸肌纤维早期退化阶段的超微结构变化。在这5天期间，与五龄初期纤维的尺寸相比，肌纤维的横截面积减小，但表观表面积增加。大量电子致密颗粒或液滴从肌细胞质中形成并挤出到血淋巴中；这个过程可能是肌纤维质量下降的部分原因，并且可能代表一种回收营养物质的发育机制。随着纤维收缩，厚的基膜形成褶皱。吞噬血细胞（粒细胞）聚集在退化纤维表面的簇中，似乎专门清除基膜。及时清除厚的幼虫基膜对于随后成肌细胞与残留幼虫肌纤维的融合可能至关重要。在蛹蜕皮后的前12小时结束时，退化肌纤维内的收缩元件变得紊乱，但并未功能失调。气管从与每个肌纤维裂隙和T小管的紧密接触中撤出，并在其附近成群聚集。线粒体似乎正在退化。这些结构变化与先前观察到的静息电位下降同时发生，表明肌纤维的电特性也应会发生显著变化。
