No abstract

Cross sections through the middle segment of the anuran rectus abdominis muscle were analyzed morphometrically at nine stages of development, from early larval life through full maturity. The numbers, sizes, and relative distributions of twitch and slow muscle fibers, newly differentiated fibers, degenerating fibers, and satellite cells were determined at each stage. The data indicate that the muscle increases slowly in size and fiber content during early larval life. New fibers appear to form primarily along the medial margin of the muscle. During mid-larval stages, when thyroid hormone levels are rising, new fibers form throughout the medial portion of the muscle. At a slightly later stage, fibers in the lateral region of the muscle begin to degenerate. Structurally normal presynaptic elements are present on both degenerating fibers and the empty basal laminae of fibers that had been removed by phagocytes. Both fiber formation and fiber loss slow during midmetamorphic climax, at the time when thyroid hormone levels reach a peak in anurans and begin to decline. Degenerating fibers appear within the body of the muscle at the end of metamorphosis. By the end of the second postmetamorphic month, neither degenerating nor newly differentiated fibers are present. The muscle continues to grow through adult life primarily by fiber hypertrophy.

A quantitative study was made of the effects of 24 h continuous stimulation on the morphology of the frog neuromuscular junction. The synaptic vesicle concentration in the nerve endings of frog sartorius muscles stimulated in vitro for 24 h at 2 Hz was the same as that in controls stimulated for only 0.3 h at 2 Hz. The control preparations were either freshly dissected or maintained at rest in vitro for 23 h prior to stimulation. Chronically stimulated terminals differed from their controls only in having more cisternae and fewer dense cored vesicles. Varying the lengths of the nerves to both chronically stimulated an in vitro control muscles had little effect on the morphology of the nerve endings. Continuous recording of muscle twitch tension demonstrated that neurotransmission was effective throughout the 24 h period of stimulation. Additional evidence that nerve failure or degeneration was not a factor in the results came from a second set of control and chronically stimulated preparations that were tetanized at 30 Hz for 0.3 h before fixation. Changes attributable to rapid stimulation were evident in 87 to 100% of their nerve terminals. Although the distribution of membrane among various membrane organelles differed from one treatment group to another, the total amount of measurable membrane in the nerve terminals was the same in all of the treatment groups; that is, the total amount of membrane was not altered by maintenance in vitro, chronic stimulation at 2 Hz, rapid stimulation at 30 Hz, reduced nerve length, or any tested combination of these treatments. This conservation of total membrane suggests that membrane exchange between axon and nerve terminal occurs at a relatively slow rate which is unaffected by synaptic activity, and that the local mechanism for recycling synaptic vesicle membrane in frog neuromuscular junctions is more autonomous and durable than has been suspected.

No abstract

No abstract