Ultrastructural study of the rat terminal nerve: organization of neuronal subsets and acetylcholinesterase cytochemistry

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Abstract
The ultrastructural distribution of acetylcholinesterases (AChE) has been used as a cytochemical criterion for the identification of distinct neuronal subsets in the terminal nerve and ganglion of the rat. The terminal nerve bundles spreading on the medial surface of the olfactory bulbs contain three types of axons: AChE-containing axons (0.45 μm mean diameter) are individually ensheathed in Schwann cell processes (Type 1); large varicose AChE-negative fibres occur less frequently and run individually within the outer layer of the terminal nerve (Type 2); AChE-negative axons (0.23 μm mean diameter) resemble olfactory and vomeronasal axons (Type 3). Furthermore, close anatomical relationships between terminal and vomeronasal nerves have been demonstrated. Our ultrastructural and cytochemical data strongly suggest that fibre exchange occurs between both systems. Our results are discussed with reference to previous data and hypotheses concerning the terminal nerve structure and functions.