In situ appearance of the cold-stable microtubules in the growing axons of the tectal plate of mouse investigated immunocytochemically after polyethyleneglycol (PEG) embedding.

Tubulin immunostaining of semi-thin sections after polyethylene glycol embedding was used in the tectal plate of the embryonic mouse at 10 days postmating to analyze the effects of cold treatment on the microtubules of the different cell types seen at this stage. Three sets of microtubules are observed. In the radially oriented bipolar columnar cells, dense bundles of microtubules are present in the ventricular processes between the cell nucleus and the ventricular surface. In the mitotic cells, located just at the surface of the ventricle, microtubules are among condensed chromosomal figures. In the apical region, the intermediate zone, tangentially oriented axonal profiles contain dense bundles of microtubules among tangentially oriented young neurons. Cold treatment does not modify the organization of the cells. However, it depolymerizes whole cytoplasmic and mitotic microtubules of the bipolar cells and a large number of microtubules in the growing axons. In the axonal profiles, the cold-stable fraction of microtubules displays the appearance of short fragments. Some of these are regularly organized, suggesting that they could be the remnants of the same individual microtubule. These fragments are approximately 1 micron long and seem to represent nearly 10% of the total microtubules in the axons. These cold-stable fragments might fulfill a function in the axon analogous to the microtubule organizing centers in the perikaryon and their presence can explain some properties of the growing axons suggested by previous studies on the guidance of neurites and growth cones as well as on the growth of isolated axons.