Fig. 8

Summary schematic of the early neurogenetic events of the Pacific oyster, Crassostrea gigas. Serotonin-immunoreactive (5-HT-ir) elements are blue; FMRFa-ir elements are violet; and VAChT-immunoreactive (VAChT-ir) elements are green. The apical pole is always upward, and the ventral side is on the right. a At the early trochophore stage, only 5-HT-ir cells are present in the apical region of the episphere. Apical neurons (an) have a flask shape and bear cilia. Paired dorsal (dn) and ventral neurons (vn) located in the hyposphere region express FMRFa-ir. The dorsal cells have a ciliated sensory dendrite and an axon extending towards the ventral cells (arrowheads); VAChT-ir elements are absent. b By the late trochophore/early veliger stage, the axons of the dorsal cells pass a group of ventral cells to reach the caudal region of the larvae. VAChT-ir neurons appear posteriorly (pn) and send axons anteriorly along the FMRFa-ir processes of the dorsal cells. Thus, the paired ventral nerve cords are established by early FMRFa-ir and VAChT-ir nerve elements. FMRFa-ir and VAChT-ir flask-shaped cells are incorporated into the apical organ (AO), and AO neurons are located between the ventral cords but have no connections with them. c In the D-hinge veliger stage, solitary FMRFa-ir neurons (pn) appear posteriorly near the posterior VAChT-ir cells. Axons of apical, ventral, and posterior neurons grow along the ventral cords. Two commissures (asterisks) connect the left and right ventral cords (vnc). The AO is located between the two cerebral ganglia forming the AO/CG-complex. 5-HT-ir, VAChT-ir, and FMRFa-ir neurites (arrowheads, n) extend from the AO/CG neuropil. d In the umbo stage, the pedal (PG) and visceral ganglia (VG) adjoin the longitudinal ventral cords. No specific cerebro-pedal cords can be separated within the bands of the ventral cords. The CPG, PG, and posterior group contain FMRFa-ir, 5-HT-ir, and VAChT-ir neurons, while the VG only contains FMRFa-ir and 5-HT-ir cells. Both ventral cords and commissures contain FMRFa-ir, 5-HT-ir, and VAChT-ir neurites. 5-HT-ir and VAChT-ir processes extending from the CG richly innervate the ciliated velum. 5-HT-ir and FMRFa-ir neurites innervate the velum anterior edge. Abbreviations: a – anus, an – apical neuron, CG – cerebral ganglia, dn – dorsal neurons, m – mouth, n – peripheral neurites, PG – pedal ganglia, pmo – presumptive mouth opening, pt. – prototroch, VG – visceral ganglia, vn – ventral neurons, vnc – ventral nerve cords