A conserved regulation of cell expansion underlies notochord mechanics, spine morphogenesis, and endochondral bone lengthening

Cell size is a key contributor to tissue morphogenesis ¹ . As a notable example, growth plate hypertrophic chondrocytes use cellular biogenesis and disproportionate fluid uptake to expand 10-20 times in size to drive lengthening of endochondral bone ^2,3 . Similarly, notochordal cells expand to one of the largest cell types in the developing embryo to drive axial extension ^4–6 . In zebrafish, the notochord vacuolated cells undergo vacuole fusion to form a single large, fluid-filled vacuole that fills the cytoplasmic space and contributes to vacuolated cell expansion ⁷ . When this process goes awry, the notochord lacks sufficient hydrostatic pressure to support vertebral bone deposition resulting in adult spines with misshapen vertebral bones and scoliosis ⁸ . However, it remains unclear whether endochondral bone and the notochord share common genetic and cellular mechanisms for regulating cell and tissue expansion. Here, we demonstrate that the 5’-inositol phosphatase gene, inppl1a , regulates notochord expansion, spine morphogenesis, and endochondral bone lengthening in zebrafish. Furthermore, we show that inppl1a regulates notochord expansion independent of vacuole fusion, thereby genetically decoupling these processes. We demonstrate that inppl1a -dependent notochord expansion is essential to establish normal mechanical properties of the notochord to facilitate the development of a straight spine. Finally, we find that inppl1a is also important for endochondral bone lengthening in fish, as has been shown in the human INPPL1 -related endochondral bone disorder, Opsismodysplasia ⁹ . Overall, this work reveals a conserved mechanism of cell size regulation that influences disparate tissues critical for skeletal development and short-stature disorders.