Role of somite patterning in the formation of Weberian apparatus and pleural rib in zebrafish

Abstract

In the vertebrate body, a metameric structure is present along the anterior–posterior axis. Zebrafish tbx6−/− larvae, in which somite boundaries do not form during embryogenesis, were shown to exhibit abnormal skeletal morphology such as rib, neural arch and hemal arch. In this study, we investigated the role of somite patterning in the formation of anterior vertebrae and ribs in more detail. Using three‐dimensional computed tomography scans, we found that anterior vertebrae including the Weberian apparatus were severely affected in tbx6−/− larvae. In addition, pleural ribs of tbx6 mutants exhibited severe defects in the initial ossification, extension of ossification, and formation of parapophyses. Two‐colour staining revealed that bifurcation of ribs was caused by fusion or branching of ribs in tbx6−/−. The parapophyses in tbx6−/− juvenile fish showed irregular positioning to centra and abnormal attachment to ribs. Furthermore, we found that the ossification of the distal portion of ribs proceeded along myotome boundaries even in irregularly positioned myotome boundaries. These results provide evidence of the contribution of somite patterning to the formation of the Weberian apparatus and rib in zebrafish.

The overlooked first intercostal ligament: Does it help to stabilize the Weberian apparatus?

“…The role of tbx6, a critical gene regulating segmentation, has been examined in zebrafish (Akama et al 2020). In tbx6 null mutants, severe pattern-based defects are found in the axial skeleton, particularly severe in non-Weberian vertebral levels (Akama et al, 2020).…”

“…While the genetics of general body development, somitogenesis, and somite differentiation have been studied in detail in zebrafish, often the studies end before enough skeletal development in the axial skeleton has occurred to determine whether loss of a particular gene affects the ossicle chain or other vertebral-based elements. The role of tbx6 , a critical gene regulating segmentation, has been examined in zebrafish (Akama et al 2020). In tbx6 null mutants, severe pattern-based defects are found in the axial skeleton, particularly severe in non-Weberian vertebral levels (Akama et al, 2020).…”

An atlas of seven zebrafish hox cluster mutants provides insights into sub/neofunctionalization of vertebrate Hox clusters

“…Skeletal structures of zebrafish were observed using X-ray micro-CT scanning as previously described (Akama et al, 2020). Adult zebrafish were fixed with 4% paraformaldehyde in PBS at 4°C overnight and were transferred to 70% ethanol.…”

Vertebral pattern and morphology is determined during embryonic segmentation

“…1,2 In addition, the myotome pattern, derived entirely from paraxial mesoderm cells, guides the growth of spinal processes and ribs. 20,21 The questions raised by the discovery of notochord sheath segmentation prompted us to examine in more detail the connections between embryonic segmentation, myotome borders, and vertebral patterning. We wished to quantify the range of defects across many animals, and test whether specific adult vertebral defects can be linked to specific myotome border defects within the same animal.…”

“…Moreover, the conservation of developmental mechanisms between amniotes and teleosts extends from the genes involved in spine patterning to the cell types responsible for its formation and growth: each vertebra develops from the posterior half of one somite and the anterior half of the next, adjacent somite 1,2 . In addition, the myotome pattern, derived entirely from paraxial mesoderm cells, guides the growth of spinal processes and ribs 20,21 …”