The sinus gland ofCarcinus maenascontains small numbers of neurosecretory neurones (“intrinsic cells”) as well as glial cells and nerve terminals. Intrinsic cells may be of the same cell type as some extrinsic cells, but are probably multipolar. Exocytosis is a common phenomenon, and may be either “basal” or “interterminal”, and either “simple” or “compound” in character. Elementary neurosecretory granules may apparently fuse within the terminals to form “secondary” granules, and these also release their contents by exocytosis. A distinction is drawn between populations of vesicular inclusions which include elements of diverse size and form, and aggregations of synaptoid vesicles. The mixed populations may be involved in the retrieval of granule membranes. Typical synaptoid complexes are present in a minority of terminals and are characterized by a zonation of vesicular and granular inclusions, the aggregation of vesicles of fairly uniform size and shape adjacent to membrane thickenings, and an affinity of vesicle contents for the ZIO reagent. Granules within one type of fibre differ from typical peptidergic granules, but react negatively to cytoch
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