Ultrastructural study of the sinus gland of the crab, Cardisoma carnifex.

The sinus gland of Cardisoma carnifex was examined by light and transmission electron microscopy. This neurohemal organ is composed primarily of enlarged, branching axon terminals with numerous finger-like projections, which act as storage and release sites for neurohormones that are assumed to be contained in membrane-bound, electron-dense neurosecretory granules. Also present in the sinus gland are glial cells with cytoplasmic processes which form elaborate wrappings around neurosecretory terminals, and an acellular fibrillar lining of the branching blood sinus. Six types of neurosecretory terminals are identified on the basis of granule size, granule density, and density of the axoplasmic matrix. Images supporting the abutting the blood sinus lining are found. Large multilamellate bodies appear in terminals depleted of neurosecretory granules and may be involved in the recycling of granule membrane.