Structural organization of neurosecretory cells terminating in the sinus gland of the terrestrial isopod, Oniscus asellus, revealed by paraldehyde fuchsin and cobalt backfilling

The morphology of the brain – sinus gland neurosecretory system in the terrestrial isopod, Oniscus asellus, is described with paraldehyde fuchsin and with cobalt backfilling of the sinus gland. Paraldehyde fuchsin stained the A, B, and β cells located medially in the protocerebrum and the γ cells located in the optic lobe. Cobalt applied to the sinus gland delineates an axon tract that extends from the sinus gland medially along the posterior surface of the protocerebrum and descends into the protocerebrum at the level of the central protocerebral neuropile. Cobalt backfilled to the B, β, and γ cells but not to the A cells. One cell group located distally to the most distal optic lobe neuropile filled with cobalt, but was not stained with paraldehyde fuchsin. It is argued that the B and β cells together comprise the equivalent of the decapod "X-organ." Varicosities, which may represent additional storage and (or) release sites for neurosecretion, appear in the axon tract over the region of the optic lobe. Extensive dendritic arborizations of the B and β cells occur along the anterior-medial side of the central protocerebral neuropile. Additional arborizations of these cells occur in the contralateral protocerebral lobe, suggesting a pathway for neural coordination of left and right sinus glands. Further observations on changes in the staining properties of the β and γ cells during the moult cycle suggest the involvement of β cells with moulting and the involvement of γ cells with egg development.