The connective tissue layer surrounding bone is called the periosteum. It has an outer fibrous layer and an inner osteogenesic layer.
They actually divide rarely, which are put in the category of G0 in the cell cycle. in G0, cells are unlikely to divide but still carry out their normal functions. some data suggests that neurons can divide but there is still research going on for this
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Since sea urchins are picky, they are used as indicator organisms in public aquariums to determine if the system is functioning properly. This is because they are very "picky" about water quality. If the water is contaminated, the sea urchins will be the first to show signs of stress, spines laying down or falling off. Do use an aquarium filter and do clean up the day after feeding. Any metal exposed to the seawater will corrode and poison the tank. A dying sea urchin will often spawn out and rot out, causing the others in the tank to spawn and die as welll.
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