We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric proteins moving between the sarcomere and the cytoplasm. We also address in this personal review the recent evidence that indicates that the assembly and dynamics of myofibrils are conserved whether the cells are analyzed in situ or in tissue culture conditions. We suggest that myofibrillogenesis is a fundamentally conserved process, comparable to protein synthesis, mitosis, or cytokinesis, whether examined in situ or in vitro.

Following spinal cord injury, diffusion MRI (DWI) has been shown to detect injury and functionally significant neuroprotection following treatment that otherwise would go undetected with conventional MRI. The underlying histologic correlates to directional apparent diffusion coefficients (ADC) obtained with DWI have not been determined, however, and we address this issue by directly correlating ADC values with corresponding axon morphometry in the normal rat cervical spinal cord. ADC values transverse (perpendicular) and longitudinal (parallel) to axons both correlate with axon counts, however each directional ADC reflects distinct histologic parameters. DWI may therefore be capable of providing specific histologic data regarding the integrity of white matter.

In this chapter, we present the current knowledge on de novo assembly, growth, and dynamics of striated myofibrils, the functional architectural elements developed in skeletal and cardiac muscle. The data were obtained in studies of myofibrils formed in cultures of mouse skeletal and quail myotubes, in the somites of living zebrafish embryos, and in mouse neonatal and quail embryonic cardiac cells. The comparative view obtained revealed that the assembly of striated myofibrils is a three-step process progressing from premyofibrils to nascent myofibrils to mature myofibrils. This process is specified by the addition of new structural proteins, the arrangement of myofibrillar components like actin and myosin filaments with their companions into so-called sarcomeres, and in their precise alignment. Accompanying the formation of mature myofibrils is a decrease in the dynamic behavior of the assembling proteins. Proteins are most dynamic in the premyofibrils during the early phase and least dynamic in mature myofibrils in the final stage of myofibrillogenesis. This is probably due to increased interactions between proteins during the maturation process. The dynamic properties of myofibrillar proteins provide a mechanism for the exchange of older proteins or a change in isoforms to take place without disassembling the structural integrity needed for myofibril function. An important aspect of myofibril assembly is the role of actin-nucleating proteins in the formation, maintenance, and sarcomeric arrangement of the myofibrillar actin filaments. This is a very active field of research. We also report on several actin mutations that result in human muscle diseases.

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NMR q-space imaging is a method designed to obtain information from porous materials where diffusion-diffraction phenomena were observed from which pore size was derived. Recently, the technique has been applied to the study of biological structures as well. Although diffusive diffraction has so far not been observed in multicellular systems, displacement profiles have been used with some success as a means to estimate structure size. However, there have been no quantitative correlations of the retrieved structure sizes with histology. Clearly, the complexity of tissue architecture poses significant challenges to the interpretation of q-space data. In this work, simulations were first performed on a two-compartment model to demonstrate the effects of interference of the diffraction patterns arising from intra and extra-axonal compartments and finite boundary permeability on q-space data. Second, q-space echo attenuation was simulated on the basis of histologic images of various rat spinal cord fiber tracts and the information obtained from the displacement profiles were compared with structural parameters computed from the histologic images. The results show that calculated mean displacements and kurtosis parallel mean axon size and axonal density.