Analysis of microtubule number and length in cytoskeletons of cultured chick sensory neurons.

In order to examine the continuity of individual microtubules in growing neurites, microtubules were observed and counted in whole mounted cytoskeletons of chick embryo sensory neurons. At neurite branch points, the microtubules of the proximal segment are divided between the distal segments without the addition of microtubules to the distal branches. Few microtubules ends were seen in the cytoskeletons, and on occasion, the same number of microtubules was counted at widely separated points along a neurite segment. I conclude that microtubules are long in embryonal neurites, and some may extend the entire length from perikaryon to neurite tip. This may limit where microtubule growth occurs to the perikaryon and the neurite tip, where many microtubule ends are. In addition, the great length of most neuritic microtubules may explain why collateral sprouting is rare along embryonal neurites.