Recovery frequency of the external morphology of the severe lordotic individuals at 4 and 7 dpe (a–c) and valorization of the scoring methodology by means of double staining (d–g) at the end of the recovery period (7 dpe). (b, c) Representative case of the transition of a juvenile from a typical lordotic (s-L) to a recovered (s-L/rec) external morphology respectively. (e–g) Representative cases of the internal bone structure of a severe lordotic (s-L), a light lordotic (l-L) and a recovered (s-L/rec) individual after double staining (ds) respectively. Scale bars equal to 1 mm. n, individual number assessed. Asterisks indicate significant differences (p < 0.05).

Comparison of a normal (N) (a, a′) and a recovered fish (s-L/rec) (b, b′) 4 days post exercise. Stained pictures (a, b) and parasagittal sections (a′, b′) of the vertebral bodies located in the center of haemal lordosis. Bo, bone of the vertebral body endplate. Cf, collagen type I fiber bundles. Dc, dense collagen type I matrix. El, elastin. Ev, extracellular vacuole. Fb, fibroblasts. Nc, vacuolated notochord cells. Ne, notochord epithelium. Ns, notochord sheath. Ob, osteoblasts. Se, septum. St, notochord stan. Scale bars equal to 1 mm (a, b) and 50 μm (a′, b′).

Heat map representing the relative mRNA expression levels of genes differentially expressed (p < 0.01) between normal (N) and recovered (s-L/rec) individuals, involved in bone remodeling and/or muscle regeneration.

Basic experimental design. Juvenile zebrafish with normal external morphology were subjected to a swimming challenge test (SCT) for the induction of haemal lordosis. The severe lordotic (s-L) and normal (N) fish were monitored for a week under almost 0.0 TL s⁻¹. Lordosis recovery (s-L/rec) was examined by means of double staining for bone and cartilage (Al.), histology (Hist.) and gene expression analysis (RNA). ed, exercise days. dpe, days post exercise.