The neuronal endomembrane system. II. The multiple forms of the Golgi apparatus cis element

The cis element and its associated tubules and vesicles are ideally positioned to play a major role in the sorting of rough endoplasmic reticulum components destined for processing in the Golgi apparatus. Its position is also ideal for playing a major role in the assembly of the saccules which constitute the Golgi apparatus. The present study was undertaken to critically analyze the normal morphology of this Golgi apparatus component. Seventy- to 2000-nm-thick sections of bullfrog spinal ganglia, fixed by osmium impregnation as well as by conventional protocols, were studied using standard and high voltage electron microscopy. Impregnated cis elements were also reconstructed from 170-nm serial sections. These studies found that adjacent neurons within the given ganglion contain cis elements of widely different morphology. In larger neurons, a different cis element organization was also found in different regions of the same cells. Based on structural comparisons, all of the different cis element forms observed could be systematically assembled into a gradual continuum of morphological variation. This continuum was circular in a manner analogous to chromosomal variations seen in highly mitotic tissues. For the sake of discussion, five distinctly different form categories were established. Some forms contained structures that are described herein for the first time. Most notable among these is the cis tubular network, an extensive system of parallel tubules that closely apposed the regularly perforated cis saccule. Osmiophilic vesicles were found to collect in tight clusters that closely apposed certain forms of the cis element. These findings raise the possibility that the cis element and its associated structures may undergo morphological transformations as part of their normal function.