The axoplasm of the neurons of Periplaneta americana contains numerous neurotubules which are morphologically similar to the microtubules found in non-nervous tissues after sectioning or negative staining. In cross-sections of fixed material such tubules usually appear as electron-dense circles containing a less dense core and surrounded by a non-opaque ‘clear zone’. However, when cockroach ganglia are fixed and incubated in lanthanum hydroxide, the lanthanum is taken up intracellularly by the axoplasm of certain of the neurons and in these is found to stain the entire core of the neurotubules, as well as the clear zone. At least part of the wall of the tubules remains unstained and appears as a ring of non-opaque subunits against an electron-dense, lanthanum-stained background. Since lanthanum staining, under the conditions used here, is sometimes considered to demonstrate the presence of acid muco-polysaccharides, its uptake by the neurotubules may indicate that they contain carbohydrate as well as the protein that is generally considered to form part of the microtubular wall. Alternatively, the lanthanum could indicate the location of other anionic molecules, possibly undergoing extra- or intratubular translocation. The extent to which neurotubules could mediate movements of relatively small water-soluble ions and molecules is considered in relation to their diffusion through the polyanion matrix represented by the core of the tubules.
The lanthanum hydroxide fluoride carbonate system: The preparation of synthetic bastnaesite
