Overview of the Neurocytology of Ganglia and Identification of Putative Serotonin- and Dopamine-Secreting Neurons in the Bivalve Peppery Furrow Shell (Scrobicularia plana)

Abstract

A novel histological and ultrastructural study of the three ganglia types of Scrobicularia plana was made. Adults of both sexes and immature specimens were collected at Ria Formosa lagoon, Portugal. For light microscopy, animals fixed in formalin were processed for various staining, including serotonin and dopamine immunohistochemistry. For transmission electron microscopy, dissected ganglia were fixed in glutaraldehyde and routinely processed. Every ganglion reveals an outer cortex and inner medulla, with the cortex displaying large and small neurons. Their sizes decrease as they are located closer to the medulla. Almost all are unipolar, and only a few are bipolar. Glial cells appear around the neurons and amidst the axons. They usually have smaller sizes and greater nucleus-cytoplasm ratios. The glial morphologies (fusiform, roundish, and triangular) may correspond to glia subtypes and/or functional statuses. Ultrastructural differences from earlier studies were noted, mainly in the type of inclusions and vacuoles. Serotonergic and dopaminergic cell bodies and neurites exist in all ganglia. There is an identical expression pattern in adults (both sexes) and in immature animals. Nevertheless, the extent/intensity of serotonin positivity in visceral ganglia supports that mature animals have a stronger expression, which may be connected the role of that amine in gonad maturation and spawning.