In the arcuate nucleus of the normal rat there are two types of neurons. One of them is characterized by a dense cytoplasmic matrix, dilated endoplasmic reticulum cisternae and well developed Golgi complexes; the other cell type by a paler overall appearance. In both of them granulated vesicles of about 1000 Å in diameter with a core of variable density are present. The frequent association between these vesicles and the Golgi complex suggests that they are formed in relation to this organoid. The presence of lysosomes and multivesicular bodies and their close relationship with granulated vesicles is analyzed.
The neuropil of the nucleus is constituted by axons, dendrites and glial processes. The majority of the axon profiles and nerve endings contain a mixed population of clear and granulated vesicles, the latter having a diameter ranging between 600 to 1300 Å.
The finding of granulated vesicles in arcuate neurons is discussed in relation with the known monoamine content of them and the dopaminergic nature of the fibers that form the tuberoinfundibular tract. It is suggested that these granulated vesicles contain primary monoamines.