Microtubule densities and total numbers in selected axons of the crayfish abdominal nerve cord.

Abstract

The ubiquitous cell organelle, the microtubule, has been assigned a putative role in the phenomenon of axoplasmic transport. In this regard the following question can be posed and subjected to experimental test. Do microtubules extend throughout the length of an axon as single, continuous strands, or are they present in the form of long, but discontinuous lengths? The size of the tubules mandates the use of the electron microscope in an attempt to answer this question. Tubule number and density have been measured in selected single axons of the crayfish abdominal nerve cord in two types of geometry: (1) ‘before’ and ‘after’ an axon splits into two branches; (2) at two macroscopically separated points along unbranched axons which may however undergo large changes in their diameters between the two fiducial points. The results for condition (1) are that a conservation law of tubule number holds across the branch point. For condition (2) the conservation law holds for most of the cases examined, but there are clear violations in the case of several axons. The conclusion is therefore that microtubular continuity is not universally valid. Other results are that microtubule density is uniform across the axonal area, that microtubule density obeys left-right symmetry for corresponding pairs of axons, and that axons with similar functions (e.g. motor axons, interneurons) form groups within which the microtubule densities are similar. A limited number of axons from other species of organisms were measured and the microtubule number and density recorded and included in an overall survey. From this collection, an upper limit to the tubule density is found to be in the neighbourhood of 400 microtubules per μm2.