Mechanism of myofibril growth and proliferation in fish muscle

Abstract

The mechanisms of myofibril growth proliferation were investigated in the red and white muscles of fish. In both types of muscle the ratio of lattice filament spacings between the Z disk and M line was found to be greater than that required for perfect transformation of a square into a hexagonal lattice. This mismatch was considered to result in the thin filaments being pulled obliquely instead of at right angles to the Z disk. The angle of pull of the thin filaments was measured in longitudinal sections. The splitting process was found to decrease the degree of pull. Splitting was also observed in transverse sections of the peripheral myofibrils. In both red and white fibres these myofibrils were found to commence splitting when they reached a size of approximately 1–2 mum diameter. Evidence from ultrastructural and autoradiographical studies suggested that growth of the myofibrils within the fibres is centrifugal. The outermost myofibrils appear to be the ones which are being built up and which split. The data indicated that in fish muscle a considerable number of filaments may be added to the daughter regions whilst splitting of the myofibril is still continuing.

Regulation of the evolutionarily conserved muscle myofibrillar matrix by cell type dependent and independent mechanisms

“…In both the leg and TDT muscles, sarcomere branches were more likely to occur near the cell periphery than in the interior (Supplementary Fig. 3c, d ) similar to previous findings in fish muscle 13 .…”

“…Sarcomere branching frequency has been shown to vary according to cell type 11 , 12 , though the mechanisms regulating the degree of connectivity have yet to be determined. Additionally, evidence of sarcomere branching can be seen in muscle images from fish 13 , chickens 14 , 15 , and frogs 16 in addition to mice 12 , 17 and humans 11 , 18 , 19 suggesting that myofibrillar networks may be conserved across vertebrate species. Conversely, insect fibrillar muscles, such as the commonly studied Drosophila indirect flight (IF) muscle, are known to be made up of the textbook individual myofibrils running the entire length of the muscle cell 20 – 22 , implying that a myofibrillar matrix may not occur in invertebrate muscles.…”

A Novel Imaging Method (FIM-ID) Reveals that Myofibrillogenesis Plays a Major Role in the Mechanically Induced Growth of Skeletal Muscle

“…Given these conclusions, one is faced with the question of how an increase in mechanical loading induces myofibrillogenesis. As mentioned several times, one possibility is that the induction of myofibrillogenesis is mediated by hypertrophy and subsequent splitting of the pre-existing myofibrils (i.e., the myofibril splitting model) [21][22][23][24]. However, numerous studies have also described a different type of myofibrillogenesis that involves the de novo formation of myofibrils [60][61][62][63].…”

“…In support of his model, Dr. Goldspink published numerous images of single myofibrils that appeared to split into two smaller daughter myofibrils, as well as quantitative data which demonstrated that longitudinal splits usually occur in myofibrils that are significantly larger than myofibrils that do not contain splits [21-23]. Indeed, the model seemed so convincing that, when discussing how an increase in mechanical loading leads to the growth of muscle fibers, Dr. Goldspink concluded “The number of myofibrils increases ([25]), and this is almost certainly due to the longitudinal splitting of existing myofibrils rather than de novo assembly” [24].…”

“…A longstanding model in the field proposes that the radial growth of muscle fibers can be explained by a process called "myofibril splitting" [21][22][23][24]. The myofibril splitting model was developed by Dr. Goldspink in the 1970s and contends that, during radial growth, myofibrils initially undergo hypertrophy.…”