Microtubules and filaments in developing axons and optic stalk cells.

Fibrous structures have been studied in the developing optic nerve of chick embryos. The first ganglion cell axons (3-day embryos) were of moderate size, with both neurofilaments and microtubules. Subsequently (4- and 5-day embryos), very small axons were also present. In thesc embryos and in the 4-day hatched chick, the density of microtubules fell within the same range for all but the very small axons, which tended to have more microtubules per unit area. Filaments similar to those previously thought to represent neurofilaments in other parts of the embryonic nervous system were present in the early optic stalk cells, calling into question the reliability of identifying early nerve cells on the basis of 'neurofilaments'.