Aurélie‘s paper, together with Saima, Stephanie and Seb, and our lovely collaborators from the Evolutionary Biomechanics group at Imperial College and Prof James Iatridis from the Icahn School of Medicine at Mount Sinai, has been published in European Cells & Materials (eCM).
In previous papers by Aurélie, Rebecca and others, we demonstrated that muscle loading is needed for normal development of the spine, including spinal curvature, vertebral shape and vertebral segmentation in the chick embryo. However, the chick embryo does not have the same type of discs as humans (or mammals in general) as it lacks a nucleus pulposus. Therefore, we needed to switch to the mouse to look at the influence of muscles on development of the discs. Our usual mouse line of choice (splotch delayed) wasn’t suitable due to the fact that spine development and vertebral segmentation is known to be abnormal in this line. Therefore we used the “mdg” or muscular dysgenesis line (with grateful thanks to Prof Eli Zelzer, Weizmann Institute) in which skeletal muscles form but do not contract.
In this paper, we investigated how muscle forces affect (1) notochord involution and vertebral segmentation, and (2) intravertebral disc (IVD) development including the mechanical properties and morphology, as well as collagen fibre alignment in the annulus fibrosus. We looked at three different stages of development; Theiler Stage (TS)22 when notochord involution starts, at TS24 when involution is complete, and at TS27 when the IVD is formed. Vertebral and IVD development were characterised using histology, immunofluorescence, and indentation testing. We found that notochord involution and vertebral segmentation occurred independently of muscle contractions between TS22 and TS24. However, in the absence of muscle contractions, we found vertebral fusion in the cervical region at TS27, along with (i) a displacement of the nucleus pulposus towards the dorsal side, (ii) a disruption of the structural arrangement of collagen in the annulus fibrosus, and (iii) an increase in viscous behaviour of the annulus fibrosus. Therefore, mechanical loading due to muscle contractions are important for the later stages of disc development, particularly for annulus fibrosus formation. We believe our results suggest a need for mechanical loading in the creation of fibre-reinforced tissue engineering replacement IVDs as a therapy for IVD degeneration.
The research described in this paper was funded by the Leverhulme Trust and by an ERC Starting Grant.