Intracellular control of axial shape in non-uniform neurites: a serial electron microscopic analysis of organelles and microtubules in AI and AII retinal amacrine neurites.

Open Access
Publisher Website
Google Scholar
Abstract
AI and AII cat retinal amacrine cells have highly varicose non-uniform, neuritic processes. Processes of both types were reconstructed via a computer system using serial electron micrographs. These reconstructions were analyzed for varicosity volume, surface area and length; neck volume, surface area and length; number of microtubules within the varicosity; number of microtubules within the neck; and volume and surface area of mitochondria and smooth endoplasmic reticulum and large smooth vesicular bodies within the processes. Correlation of these parameters revealed a linear relationship between the number of microtubules in the necks and mean neck cross-sectional area (rS = 0.780, P < 0.001), while microtubule number within the varicosities showed no correlation with varicosity volume (rS = 0.239, P > 0.2). Varicosity volume did, however, correlate strongly with the summed volume of mitochondria and smooth vesicular bodies contained within the varicosity for both cell types examined. The ratio between membranous organelle volume and varicosity volume for AI amacrine processes of 1:6.97 (rS = 0.927), differed from the ratio of 1:1.80 for the AII amacrine processes (rS = 0.987). Similar relationships were observed in other nonvaricose neurites such as optic tract axons. Membranous organelles appear to contribute an additional obligatory volume to the cytosol that can be as much as 7 times the organelles'' direct volume. Apparently, both the cytoskeletal components and the membrane organelles play a direct role in determining neurite shape.