Fluorescent bone viewed through toenails of living animals: a method to observe bone regrowth.

We have developed a new method to observe bone and to document growth in living animals. The technique involves injecting calcein, a fluorescent calcium deposition marker, waiting approximately 4 hr for it to clear the vascular system, and observing bone directly through the toenails of lightly anesthetized living animals. Bone regrowth can be monitored in situ by amputating the digit through the nail plate, waiting the desired number of days, and injecting a second fluorescent label, alizarin red. Bone that has regrown since the amputation appears as a red area distal to the green calcein label on toes of lightly anesthetized animals when viewed under FITC fluorescence. This method has been used to demonstrate blocked bone synthesis and to quantitate significant differences in bone growth in control and experimental toes of individual animals. Advantages of this method include its simplicity, the use of fewer animals to collect sequential data, and increased reliability of repeated microscopic measurements using the same animal.