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Figure 1 General overview of the intramembranous and endochondral composition of the zebrafish and human skeletons. (A) Human adult skeleton. In the head, intramembranous bones such as those of the calvaria (the top portion of the neurocranium) dominate the human skull in surface area, while endochondral bones mostly occupy the cranial base. All of the bones that compose the trunk and appendicular skeletons are endochondral, except for portions of the clavicle and scapula. (B) Zebrafish adult skeleton. The zebrafish skull, trunk and appendage skeletons are composed of both intramembranous and endochondral bones. The zebrafish skull is composed of 134 bones, 78 of which are endochondral. The zebrafish trunk skeleton is composed of intramembranous vertebrae and ribs. The zebrafish appendage skeleton is composed mostly of endochondral bones, while the fin ray exoskeleton is completely intramembranous.