Reference: Biol. Bull. 197: 123-131. (October 1999) Development of the Larval Serotonergic Nervous System in the Sea Star Patiriella regularis as Revealed by Confocal Imaging FRANCIS CHEE* AND MARIA BYRNE Department of Anatomy and Histology, University of Sydney F13, NSW 2006, Australia Abstract. Development of the nervous system in the lar-vae of the sea star Patiriella regularis was reconstructed in three dimensions. The optical sectioning and image process-ing capabilities of the confocal microscope made it possible to identify the precise location and timing of development of serotonergic cells in relation to subsequent development of larval features. Similarities between this system and the serotonergic systems in larvae of other echinoderms were explored. Neuronal-like immunoreactive cells and pro-cesses first appeared in late gastrulae as a collection of cells scattered across the animal pole. These cells subsequently gave rise to basal axons positioned along the basal lamina. Immunopositive cells located in the stomodaeal region marked the beginnings of formation of the adoral ciliated band. Cells were also present in the mid-dorsal epithelium. Advanced bipinnaria had pyramidal immunoreactive cells within the adoral band and ovoid immunoreactive cells within the preoral and postoral ciliated bands. Processes originating from neurons in the transverse region of the preoral ciliated band extended into the buccal cavity, sug-gesting that these cells have a sensory role in feeding. An anterior ganglion formed in the late bipinnaria. innervating the preoral and postoral ciliated bands. This connection has not previously been described. It thus appears that the ciliated bands in the bipinnaria larvae of P. regularis com-municate via serotonergic nerve tracts. Introduction Serotonin (5-hydroxytryptamine. 5-HT) is a ubiquitous monoamine and functions as a neurotransmitter in the adult Received 2 November 1998: accepted 28 June 1999. * E-mail:
[email protected] Abbreviations: ADNP. adoral nerve plexus; FSW. filtered seawater; PBS. phosphate buffered saline; AG, anterior ganglion; 5-HT. serotonin. nervous systems of a large range of animal groups (Collier, 1958). In echinoderms, serotonin was first isolated in the gonads of adult asteroids (Welsh and Moorehead, 1960), and its cellular location has been documented in studies of the larval nervous system of echinoids, asteroids, and ho-lothuroids (Bisgrove and Burke, 1986: Burke et ai, 1986; Bisgrove and Burke, 1987: Nakajima, 1988; Bisgrove and Raff, 1989: Nakajima et ai, 1993; Moss et al., 1994; Chee and Byrne, 1997). These studies indicate that serotonin functions in a neuronal capacity in these larvae. The feeding larvae of echinoderms typically have a well-developed se-rotonergic nervous system that innervates the ciliated bands and is suggested to play a sensory role in feeding and metamorphosis. The similarities between the serotonergic systems in the larvae of several echinoderm classes have been taken to suggest that these systems are homologous (Burke et al., 1986). In general, the increasing complexity of the serotonergic nervous system in the feeding larvae of echinoderms parallels the development of the ciliated bands (Burke, 1983). During sea star development the number of serotonergic immunoreactive cells increases from a few cells at the sastrula stage to a complex nervous system in competent larvae prior to metamorphosis (Nakajima, 1988). In this study we investigate the larval nervous system of the sea star Patiriella regularis. This species has planktotrophic development through bipinnaria and brachiolaria feeding stages (Byrne and Barker. 1991). Following from previous observations (Chee and Byrne, 1997), we document the expression of serotonin in P. regularis from the first ap-pearance of neurons in gastrulae through the formation of a complex three-dimensional nervous system. Assisted by confocal microscopy, we were able to reconstruct develop-ment of the serotonergic-like nervous system in three di-mensions with respect to morphogenetic change. Although 123
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