A light and electron microscopic investigation of the neurosecretory bag cells of Aplysia.

The two bilateral clusters of neurosecretory bag cells of Aplysia were studied with both light and electron microscopy. Autoradiography revealed that the bag cells rapidly accumulate 3H-labelled amino acids and that after 1-2 h, heavy concentrations of silver grains appear over Golgi complexes and in the proximal axons. Intrasomatic injections of CoCl2 or lucifer yellow showed clear branch points and numerous varicosities along individual axons. Many of the bag cells are multipolar. Electron-microscopic observations confirmed that individual fibres branch and showed that the varicosities are packed with dense-cored vesicles similar in size (180 nm diameter) and electron density to those found in the somata. The axons of several cells are usually associated into bundles that travel (within the connective tissue sheath) either rostrally up the pleurovisceral connective or toward the contralateral bag cell cluster. Bundled in groups of tens to hundreds, a total of many thousands of axons fill the sheath around each cell cluster and around the proximal 2-5 mm of the pleurovisceral connective; the number of axon bundles in the sheath decreases rapidly with distance from the cluster. Individual axons reaching the outer edges of bundles from neurosecretory endings near blood sinuses in the sheath, creating an extensive neurohemal release area. Dense-cored vesicles are packed into the endings, often in very close apposition to the plasma membrane. Possible release profiles (omega-shaped) and smaller clear vesicles (85 nm diameter) were observed in the axon endings. A number of axons also enter and travel among the conventional (non-neurosecretory) axons in the core of the pleurovisceral connective nerve. These 'core' bag cell axons project for several millimetres beyond the terminations of the bundled axons of the sheath. The findings support the hypothesis proposed in physiological studies that the distribution and branching of the axonal tree are the basis for the extracellularly recorded wave forms and of the potentiation of electrical signals during bag-cell activity. Additional evidence indicates that exocytosis is the means by which bag-cell hormone is released during afterdischarges.