Structure and innervation of a crustacean neurosecretory cell

Abstract

The crayfish medulla terminalis X organ contains type 1 neurosecretory somata which are distinguishable by size, location, and ultrastructure and which have homologues in the eyestalks of other decapods. Neurosecretory (NS) vesicles (140 nm diameter) in these somata are structurally similar to those in the X organ–sinus gland (Xo-sg) tract and the neurohemal sinus gland. Cobalt iontophoresis into type 1 somata shows that their axons traverse the underlying neuropil of the medulla terminalis as the Xo-sg tract before emerging to the surface of the ganglion and terminating in the sinus gland. The proximal third of these type 1 axons branches extensively in the ganglionic neuropil; only in the neuropil adjacent to Xo-sg tract does electron microscopy reveal many fine postsynaptic neurites containing NS vesicles. Serial thin sections show continuity between these neurites and the Xo-sg tract. Structural evidence suggests that this fine arborization is dendritic and represents the site of informational input from the central nervous system to the Xo-sg complex. This study supports previous electrophysiological recordings within Xo somata which indicate synaptic input to these neuroendocrine cells.