Electron Microscopy of Developing Sympathetic Fibres in the Rat Pineal Body The Formation of Granular Vesicles

Publisher Summary The development of the sympathetic innervation of the rat pineal body was studied at the ultrastructural level. Special attention was paid to the granular vesicles, in the hope of obtaining some further insight into their mechanism of formation. There is now considerable evidence that vesicles play an important role in the synthesis, uptake, storage, and release of catecholamines. Small granular vesicles (SGV) could be observed in small amounts, a few hours after birth. Their number increased enormously during the first two weeks, while the number of the large granular vesicles remained within the range observed in the adult from the day of birth. There was also an increase in the proportion of small granular to granular vesicles, as well as in the size of their granules. This fact was taken as evidence that SGV were forming from agranular vesicles (AV) with the appearance at the vesicle membrane of a small granule that progressively increased in size. Some of the large granular vesicles (LGV) appeared to be formed by a similar mechanism. However, most of the LGV originate by pinching off from sacs and tubules of smooth endoplasmic reticulum that had accumulated dense material. In developing adrenergic fibres, a part of the LGV—the granules—and probably the membranes of the SGV are formed locally in peripheral axons and nerve terminals.