Ultrastructural, fluorescence microscopic and microfluorimetric study of the innervation of the axial complex in the sea urchin, Sphaerechinus granularis (lam.).

In juxtaposition with the contractile epithelia of the axial complex of the sea urchin, Sphaerechinus granularis, several types of nerve fibers with different vesicle populations were determined. Nerve terminals, filled with clear vesicles and dense core vesicles, form synaptoid neuromuscular junctions. Close to the somatocoelic epithelia of the axial and terminal sinus septa, numerous axon profiles form a nerve plexus. Among the epithelial cells covering the plexus, two types of nerve cells can be distinguished which presumably produce neurosecretory and aminergic granules, respectively. Monoamine fluorescence (formaldehyde-induced fluorescence, Falck-Hillarp technique) was analyzed microspectrofluorimetrically. The emission spectrum of the fluorophores occurring in the present material shows a maximum at 475 nm and is characteristic of catecholamines; the excitation maximum at 380 nm after formaldehyde treatment is typical of catecholamines at low pH only. Since the peak ratio (370:320 nm) does not change after HCl-vapor treatment, the fluorophores are likely to be indicative of dopamine.