When muscles are injured, they die off in order to make room for the growth of replacement muscles. However, it turns out that these moribund muscle leave behind small evanescent fibers that have been called “ghost fibers.” Ghost fibers seem to be remnants of the gooey stuff that provides the substratum upon which muscle cells sit. This gooey foundation is called “extracellular matrix” or ECM. The ECM consists of acid sugars called “glycosaminoglycans,” which are given the unfortunate abbreviation of GAGs, proteins to which GAGs are attached called “proteoglycans,” and proteins that glue cells to the ECM, such as fibronectin, laminin, and collagen IV. Cells adhere to the ECM by means of receptors embedded in their cell membranes called integrins.
Dying muscle cells leave collagen fibers in their wake and these collagen fibers constitute these so-called ghost fibers. However, these ghost fibers provide the structure into which new muscle cells are inserted. A new study by research teams at the Carnegie Institution for Science and the National Institute of Child Health and Human Development that was published in the journal Cell Stem Cell has established that ghost fibers guide new muscle cells to grow in place and ultimately heal muscle injury in laboratory mice.
Chen-Ming Fan at the Carnegie Institute of Washington in Baltimore, Maryland and his colleagues, in collaboration with and Jennifer Lippincott-Schwartz and her colleagues from the NIH disabled the hind limb muscles of laboratory mice by means of physical injury (laceration), or the administration of toxins. These insults to the skeletal muscles caused the injured muscle fibers to die and disintegrate. They also confirmed that as the skeletal muscle disappeared, they left networks of collagen ghost fibers in their wake.
Then, this team utilized three-dimensional, time-lapse intravital imaging to directly visualize the process of muscle regeneration in live mice. What they saw was stunning. The extracellular matrix remnants or ghost fibers left by the injured skeletal directed muscle stem/progenitor cell behavior during muscle regeneration. The two-photon imaging and second-harmonic generation microscopy employed by this team enabled them to precisely observe the muscle stem and precursor cells in individual mice orient themselves along the ghost fibers and grow new muscle tissue.
The muscle stem cells were quiescent and did not move in uninjured muscle tissue. Only when muscle cells were injured did the muscle stem cells come to life, move to the site of injury and begin the healing process. Both the cell division of these muscle stem cells and their migration were oriented along the longitudinal axes of the ghost fibers.
If the ghost fibers were artificially reoriented, then the muscle progenitors migrated and divided in different planes and gave rise to disorganized regenerated muscle fibers.
From these results, Fan and his team concluded that “the ghost fiber (1) is a key determinant for patterning muscle stem cell behavior and (2) provides the foundation for proportional regeneration. They concluded that “ghost fibers are autonomous, architectural units necessary for proportional regeneration after tissue injury.” They continued, “This finding reinforces the need to fabricate bioengineered matrices that mimic living tissue matrices for tissue regeneration therapy.”