Control of somite number/size in zebrafish upon reduction of progenitor cells

Abstract

Somites are transient embryonic structures, formed in a sequential and rhythmic manner from the presomitic mesoderm (PSM) in a process termed somitogenesis. The somites are precursors of the repetitive structures of vertebrates: the vertebral column with their ribs and associated skeletal muscles. Each new pair of somites is generated from the PSM in a sequentially and rhythmic manner: a new pair of somites is formed every 30 minutes in the zebrafish, 2 hours in the mouse and 4-5 hours in humans. The total number of somites is also species specific and varies from ~30 in humans and zebrafish, to ~60 in mouse. Amazingly, embryos where large portions of PSM progenitor cells, at the blastula stage or later stages of development, were physically removed develop into smaller embryos, yet they form the same number of somites (Cooke, 1975). These experiments illustrate the regulative capacity and robustness of somite formation and lead to the clock and wavefront model of somitogenesis (Cooke and Zeeeman, 1975). A plethora of molecular and genetic evidences have emerged to support the clock and wavefront model. However, how the embryos regulate the total somite number has not been so much explored. Therefore, we set out to re-visit the work of Cooke (1975) using new genetic tools available in zebrafish, where it is possible to reduce the size of the mesoderm progenitor population genetically at different time-points of development and access its impact on somite number and size. We made use of heat-shock transgenic lines hsp70:msgn1, hsp70:dkk1:GFP and hsp70:fgfdn:GFP – that allowed us to modulate the mesoderm progenitor population upon a heat-shocked during gastrulation, trunk or tail development by interfering with the levels of Mesogenin1, Wnt and Fgf signalling, respectively. We conclude that Wnt signalling plays a role not only in the regulation of the total somite number but also in regulating somite size.