Fine structural studies of the nervous system and the apical organ in the planula larva of the sea anemone Anthopleura elegantissima

Abstract

The nervous system of the planula larva of Anthopleura elegantissima consists of an apical organ, one type of endodermal receptor cell, two types of ectodermal receptor cells, central neurons and nerve plexus. Both interneural and neuromuscular synapses are found in the nerve plexus. The apical organ is a collection of about 100 long, columnar cells each bearing a long cilium and a collar of about 10 microvilli. The cilia of the apical organ are twisted together to form an apical tuft. The ciliary rootlets of the apical organ cells are extremely long, reaching to the basal processes of the cells adjacent to the mesoglea. All three types of sensory cells are tall and slender in profile and are identified by the presence of one or more of the following features: microtubules, small vesicles, membrane-bound granules and synapses. The interneurons are bipolar cells with somas restricted to the aboral end, adjacent to the apical organ. All synapses observed are polarized or asymmetrical. A diagram including all the elements of the nervous system is presented and the possible functions of the nervous system are discussed in relation to larval behavior.

The cellular basis of photobehavior in the tufted parenchymella larva of demosponges

“…Cilia in the apical tuft of the cnidarian planula of Anthopleura sp. also beat as a unit, although no physical connection between its monociliated cells has been found (Chia and Koss 1979).…”

FGF signalling controls formation of the apical sensory organ in the cnidarianNematostella vectensis

“…Nematostella gastrulation (Kraus and Technau, 2006;Magie et al, 2007) is followed by a free-swimming, ciliated planula larva and gradual metamorphosis into a primary polyp (Hand and Uhlinger, 1992). During the planula stage, a sensory ciliary tuft develops at the aboral pole (Chia and Koss, 1979). Larvae swim with this apical ciliary organ facing forward and also settle on this pole.…”

Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)

“…Many cnidarian taxa may lack either the polyp or the medusa, but only in few cases showing direct embryonic development into the polyp has the planula stage been suppressed. The presence of neurons in the larva has been demonstrated in three major cnidarian taxa, namely, the Anthozoa (Chia and Koss, 1979;Miller and Ball, 2000;Hayward et al, 2001;Marlow et al, 2009), Hydrozoa Thomas, 1980, 1981;Thomas et al, 1987;Plickert et al, 1988;Martin, 1992Martin, , 2000, and Scyphozoa (Korn, 1966;Nakanishi et al, 2008;Yuan et al, 2008). In planulae, the nervous system is located in the ectoderm and comprises epithelial sensory cells and basally located ganglionic cells, whose neurites project in a net overlying the mesoglea (Martin, 1988a,b).…”