Newly Identified Muscle Fiber Structure: Surprising Organization

Longitudinal section illustrating the lattice-like configuration within the sonic muscle in Parophidion vassali. Credit: Marc Thiry/University of Liège

A unique arrangement of muscle fibers has been discovered in Parophidion vassali, a fish that inhabits the Mediterranean Sea and, like many fish, produces sounds using specialized muscles. This unexpected finding made by scientists Eric Parmentier and Marc Thiry at the University of Liège, could change our understanding of muscle contraction. The unique network-like configuration of myofibrils within the muscle fibers could enable rapid contractions while retaining strength. However, further research is needed to fully understand the functional implications of this novel muscle fiber structure.

The history of the description of skeletal muscles can be traced back to the observations made by the Dutch biologist Antoni van Leeuwenhoek in 1712. He reported, using a portable single-lens microscope, the first description of muscle fibers in a whale. William Bowman, an anatomist, provided a more precise description of the muscle in 1840. Subsequent studies have led to increasingly clear descriptions, including the identification of the different molecules that make up the muscle and an understanding of how it works, particularly the model of muscle contraction proposed by biophysicist Andrew Huxley in 1957.

While numerous studies have extended the representation of muscle fiber organization to different taxa over the last 300 years, the general organization of striated muscle fibers has remained perfectly conserved in all groups of vertebrate animals studied to date. However, the proportion of each cellular component can vary from one fiber to another, giving each fiber specific contraction properties. For example, a fiber rich in myofibrils with a poorly developed sarcoplasmic reticulum is found in muscles that develop force during contraction. Conversely, fibers low in myofibrils with an abundance of sarcoplasmic reticulum and numerous mitochondria are present in muscles that develop increased speed of contraction.

Sonic muscles in fish are the fastest muscles as they contract at a frequency of between 100 and 300 Hz, i.e. 100 to 300 contraction/relaxation cycles per second. A recent study has identified a new arrangement of myofibrils within the fibers of a sonic muscle in Parophidion vassali. Instead of being arranged in parallel, the myofibrils form an enormous network within the muscle fiber. This new muscle fiber design could result in a muscle that contracts rapidly while retaining strength. The reduced amount of myofibrils and high volume occupied by the sarcoplasmic reticulum favor fibers that contract rapidly.”

“The network structure of the myofibrils would allow more myosin heads to form cross-bridges with the actin myofilaments, which would increase strength in this fast muscle,” explains Professor Marc Thiry, Director of the Cell and Tissue Biology Laboratory. “In addition, numerous mitochondria are unusually arranged within the Z striae, providing the energy needed to produce long-lasting sounds.”

This new type of organization of striated muscle fibers has never been described in scientific literature and could combine muscular strength and speed. Further studies are needed to understand how it functions and to determine whether there are adaptations in the different molecules involved in these muscles.

Comparison of the organization of “classic” skeletal muscle with parallel myofibrils and sonic muscle with lattice myofibrils in Parophidion vassali. Credit: E.Parmentier/M.Thiry/Université de Liège

Longitudinal section in a classic skeletal muscle. Credit: Marc Thiry/University of Liège

Striated skeletal muscle fibers or cells represent the elementary units of voluntary muscles (muscles that enable movements such as locomotion or posture maintenance) in animals. “Each fiber is characterized by numerous contractile elements, actin, and myosin myofilaments, organized in bundles parallel to the muscle fiber’s long axis, called myofibrils.

In the longitudinal section under the optical microscope, these fibers appear as a succession of light and dark bands located at the same level for each myofibril, giving the appearance of transverse striation to the muscle fiber. A darker line divides the light band in the middle, known as the Z striation. The portion of the myofibril between two Z striae is called a sarcomere and represents the contractile unit of the myofibril. Each myofibril is therefore made up of many sarcomeres placed end to end.

The myofibrils occupy a large cell volume and are surrounded by cisternae of the smooth endoplasmic reticulum (or sarcoplasmic reticulum), which stores the calcium essential for muscle contraction. In addition, mitochondria are located close to the myofibrils; they are the main source of ATP providing energy for muscle contraction.