Abstract
Using morphological, histological, and TEM analyses of the cranium, we provide a detailed description of bone and suture growth in zebrafish. Based on expression patterns and localization, we identified osteoblasts at different degrees of maturation. Our data confirm that, unlike in humans, zebrafish cranial sutures maintain lifelong patency to sustain skull growth. The cranial vault develops in a coordinated manner resulting in a structure that protects the brain. The zebrafish cranial roof parallels that of higher vertebrates and contains five major bones: one pair of frontal bones, one pair of parietal bones, and the supraoccipital bone. Parietal and frontal bones are formed by intramembranous ossification within a layer of mesenchyme positioned between the dermal mesenchyme and meninges surrounding the brain. The supraoccipital bone has an endochondral origin. Cranial bones are separated by connective tissue with a distinctive architecture of osteogenic cells and collagen fibrils. Here we show RNA in situ hybridization for col1a1a, col2a1a, col10a1, bglap/osteocalcin, fgfr1a, fgfr1b, fgfr2, fgfr3, foxq1, twist2, twist3, runx2a, runx2b, sp7/osterix, and spp1/ osteopontin, indicating that the expression of genes involved in suture development in mammals is preserved in zebrafish. We also present methods for examining the cranium and its sutures, which permit the study of the mechanisms involved in suture patency as well as their pathological obliteration. The model we develop has implications for the study of human disorders, including craniosynostosis, which affects 1 in 2,500 live births.
Highlights
The objective of this study is to further understand the zebrafish cranial vault and suture development and their homeostasis in adult animals
The vertebrate head skeleton is a complex structure composed of the neurocranium and viscerocranium, which originate from the neural crest (NC) and mesodermal mesenchyme [3,4,5,6,7]
Our examination indicates that the earliest ossification of the zebrafish cranial vault is observed in the supraoccipital bone at 5.9 mm standard length (SL), as seen though Alizarin red staining
Summary
The objective of this study is to further understand the zebrafish cranial vault and suture development and their homeostasis in adult animals. This topic has only briefly been described in previous work [1, 2]. It is composed of two types of bilaterally developing bones: frontal and parietal. These bones are formed in the process of intramembranous ossification, in which condensing mesenchymal cells directly differentiate into the osteogenic linage. The zebrafish frontal bone has dual origin; its anterior part arises from the NC and its posterior from the mesoderm. Cell lineage-tracing studies in mice suggest that calvarial bones develop independently from the dermal mesenchyme, the term “membrane bones”, as opposed to the term “dermal bones”, has been proposed as being more adequate [4]
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