Morphological changes in frog mast cells induced by nerve stimulation in vivo

We investigated at both histochemical and ultrastructural levels the effects of unilateral electrical stimulation in vivo of the frog hypoglossal nerve on the mast cells (MCs) within the nerve fascicles and among the axon terminals. The right ventral root of the hypoglossal nerve in different experiments was stimulated respectively for 1, 3, 5, 10 min with over-threshold stimuli (10 Hz; 2 ms duration). The stimulations at 3, 5 and 10 min caused a progressive degranulation and histochemical and ultrastructural changes of the MCs at the stimulated side. The morphological changes consisted of the loss of Alcian Blue secretory content and of a progressive release of safranin+ secretory granules, depending upon duration of stimulation. The ultrastructural study showed that granules are discharged whole into the microenvironment or may release their content through exocytosis. A functional relationship between nerve and MCs is also suggested by the close anatomical association between MCs and pre-terminal axons observed following 10 min of hypoglossal stimulation. No changes in MC morphology occurred after 1 min of electrical stimulation. The results suggest that active cholinergic fibres can modulate MC secretion.