Developmental and evolutionary skeletal biology

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As a developmental biologist, I study how the skeleton of embryos of fish and frogs form. I should really say skeletons because all vertebrates have had two skeletons for almost 500 million years. Most of the human or dinosaur bones you see in a museum are the inside (endo) skeleton, which is based on cartilage, most of which is replaced by bone. Your arm skeleton is part of the endoskeleton. The second skeleton was originally on the outside and so is an exoskeleton. Originally it consisted of bone and dentine (the tissue found in teeth), either as small isolated plates or as plates fused into an external armor. During evolution, some plates sank in as teeth and to form the clavicle (collar bone) and parts of the skull. These two skeletons have distinct developmental and evolutionary histories. Currently, we are investigating skeletal development in frogs and fish. Frogs are fascinating because the tadpole skeleton is based in cartilage but the adult skeleton is based on bone. We want to know how much of the tadpole skull skeleton is carried over to adult frogs. If not carried over then do adult skeletal elements arise from a line of cells distinct from those that form the tadpole cartilaginous skeleton? We tack genes that specific for cartilage or bone onto green fluorescent protein (GFP) and inject the construct into frog eggs. The resulting eggs have green cartilages or bones. If the construct is incorporated into cells that make eggs and sperm, then the skeleton of the next generation will also glow green. By using different gene constructs we can track cells making different skeletal tissues at different times during the frog's lifetime. We know the skeletal tissues in frogs - they are cartilage and bone. The situation is much less clear-cut in fish, in which transitions between cartilage and bone occur. So, our fish study is aimed as identifying the types of skeletal tissues present in fish (Atlantic salmon and zebrafish) and the possible transitions between tissues. Then we can use gene-GFP probes to reveal the different cell populations that make these tissues. As much as 30% of fish in hatcheries never make it to market because of skeletal deformities. Therefore, this research has potential significance for the aquaculture industry and for the economy.