Localization of serotonin and fmrfamide in the bivalve molluscMytilis edulis at early stages of its development

Expression of serotonin (5-HT) during CNS development of the cephalopod mollusk, Idiosepius notoides

Cephalopods are unique among mollusks in exhibiting an elaborate central nervous system (CNS) and remarkable cognitive abilities. Despite a profound knowledge of the neuroanatomy and neurotransmitter distribution in their adult CNS, little is known about the expression of neurotransmitters during cephalopod development. Here, we identify the first serotonin-immunoreactive (5-HT-ir) neurons during ontogeny and describe the establishment of the 5-HT system in the pygmy squid, Idiosepius notoides. Neurons that are located dorsally to each optic lobe are the first to express 5-HT, albeit only when the lobular neuropils are already quite elaborated. Later, 5-HT is expressed in almost all lobes, with most 5-HT-ir cell somata appearing in the subesophageal mass. Further lobes with numerous 5-HT-ir cell somata are the subvertical and posterior basal lobes and the optic and superior buccal lobes. Hatching squids possess more 5-HT-ir neurons, although the proportions between the individual brain lobes remain the same. The majority of 5-HT-ir cell somata appears to be retained in the adult CNS. The overall distribution of 5-HT-ir elements within the CNS of adult I. notoides resembles that of adult Octopus vulgaris and Sepia officinalis. The superior frontal lobe of all three species possesses few or no 5-HT-ir cell somata, whereas the superior buccal lobe comprises many cell somata. The absence of 5-HT-ir cell somata in the inferior buccal lobes of cephalopods and the buccal ganglia of gastropods may constitute immunochemical evidence of their homology. This integrative work forms the basis for future studies comparing molluscan, lophotrochozoan, ecdysozoan, and vertebrate brains.

Neuronal development in larval polychaete Phyllodoce maculata (Phyllodocidae)

The existing view on neuronal development in polychaetes, as undergoing neurogenesis beginning in the rudiments of central ganglia and then extending peripherally, has been contrasted with the latest findings in molluscs, their sister trochozoan group, which show a peripheral to central mode of neurogenesis. The current study addresses this issue by examining early neuronal development in the polychaete Phyllodoce maculata using immunolabeling against acetylated alpha-tubulin, serotonin, and the FMRFamide. The first nervous cell was detected 20 hours before hatching, at the early trochophore stage. A solitary serotonergic neuron was located at the posterior-dorsal extreme of the larva and issued anterior projecting fibers, which outline the future ventral nerve cords and prototroch nerve. Two more serotonergic dorsal peripheral cells and three peripheral FMRFamidergic cells appeared soon thereafter. The fibers of these early cells formed a scaffolding, which prefigured the future adult nervous system (cerebral ganglion, ventral cords, prototroch and esophageal nerve rings) in prehatched trochophores. Shortly before hatching, the larval nervous system developed, including the apical organ, meridianal nerves in the episphere, and posttrochal nerves that innervate the feeding apparatus. After hatching, the rudiments of the adult nervous system started to develop along the paths already established by the earliest peripheral neurons. Thus, the general strategy of neurogenesis in a representative polychaete trochophore appears to resemble that of molluscs. The first neuronal cells to appear are peripheral in origin, located near the posterior margins of the embryo. Their similar anatomical appearance suggests that they share a similar functional role in trochophore development and behavior.

Neuronal development in larval chiton Ischnochiton hakodadensis (Mollusca: Polyplacophora)

Chitons are the most primitive molluscs and, thus, a matter of considerable interest for understanding both basic principles of molluscan neurogenesis and phylogeny. The development of the nervous system in trochophores of the chiton Ischnochiton hakodadensis from hatching to metamorphosis is described in detail by using confocal laser scanning microscopy and antibodies raised against serotonin, FMRFamide, and acetylated alpha tubulin. The earliest nervous elements detected were peripheral neurons located in the frontal hemisphere of posthatching trochophores and projecting into the apical organ. Among them, two pairs of unique large lateral cells appear to pioneer the pathways of developing adult nervous system. Chitons possess an apical organ that contains the largest number of neurons among all molluscan larvae investigated so far. Besides, many pretrochal neurons are situated outside the apical organ. The prototroch is not innervated by larval neurons. The first neurons of the developing adult central nervous system (CNS) appear later in the cerebral ganglion and pedal cords. None of the neurons of the larval nervous system are retained in the adult CNS. They cease to express their transmitter content and disintegrate after settlement. Although the adult CNS of chitons resembles that of polychaetes, their general scenario of neuronal development resembles that of advanced molluscs and differs from annelids. Thus, our data demonstrate the conservative pattern of molluscan neurogenesis and suggest independent origin of molluscan and annelid trochophores.