A reexamination of the epithelial sensory cells of amphioxus (Branchiostoma)

Abstract

The rostral epithelium of a newly metamorphosed juvenile of Branchiostoma floridae was examined at the EM level to confirm previous reports on its sensory cells. The majority of the sensory cells are of three types: two type I variants, with simple collars of unbranched microvilli surrounding their cilia, and one kind of type II cell, with an extended collar of repeatedly branched microvilli. The two type I variants differ in the structure and arrangement of the microvilli, basal body and rootlet, and the length of the cilium. Both variants are probably primary sensory cells (i.e. each has its own axon), but the data supporting this conclusion are much better for one variant than for the other. Type II cells are secondary sensory cells, with synaptic terminals borne on short extensions of the cell body. The presence of degenerating type II cells suggests that they may be subject to a regular process of loss and renewal. The results do not resolve the evolutionary issue of how amphioxus sensory cells relate to the epithelial sensory and receptor cells of vertebrates. Being primary, the type I cells resemble the supposed ancestral type more closely than do type II cells. Type II cells may be chemosensory, however, and should not be ruled out a priori as possible homologues of either primary or secondary chemosensory cells in vertebrates.

The oral nerve plexus in amphioxus larvae: function, cell types and phylogenetic significance

“…Gilmour (1996) suggested a sensory role in initiating the cough response for these cells, but further observations are in better accord with there being ciliary e¡ectors, not sensory cells. Neurites from the oral nerve pass beside the bases of some of them, but there is no sign of synaptic specializations as found elsewhere among amphioxus sensory cells by Lacalli & Hou (1999). The function of the oral papilla in feeding is consequently not clear to us at this time.…”

“…The head of amphioxus, although structurally much simpler than the vertebrate head, is of special interest. There is a surprising degree of homology between anterior parts of the amphioxus nerve cord and vertebrate brain (Lacalli et al 1994;Holland 1996), and a number of rostral sensory structures and cell types may also have vertebrate homologues (Stokes & Holland 1995a;Northcutt 1996;Baker & Bronner-Fraser 1997;Lacalli & Hou 1999). However, amphioxus is generally thought to lack placodes and a neural crest, which are the embryonic source for such structures and cell types in vertebrates.…”

The evolutionary history of placodes: a molecular genetic investigation of the larvacean urochordate Oikopleura dioica

“…The presence of ciliated, secondary mechanoreceptors in the oral ectoderm is common to larvaceans, ascidians and cephalochordates (Lacalli and Hou, 1999), however, and a common ancestral organ cannot be excluded, despite differing cell morphologies. Vertebrate Eya1 and Six1 expression is persistent in the stomodeum (Schlosser and Ahrens, 2004), suggesting that the stomodeum of the vertebrate/urochordate common ancestor expressed Eya and Six1/2 and a role for these genes in oral mechanosensory organ development may have been retained in filter-feeding nonvertebrate chordates.…”

BMP and Delta/Notch signaling control the development of amphioxus epidermal sensory neurons: insights into the evolution of the peripheral sensory system

“…In addition to the dorsal CNS, amphioxus also have a PNS comprising several types of sensory neurons (Wicht and Lacalli, 2005). Among these amphioxus sensory neurons, the solitary type I receptor cells are the most abundant population of the epidermal sensory neurons (ESNs) and are scattered along the bodies of developing amphioxus larvae and adults (Holland and Yu, 2002;Lacalli and Hou, 1999) (Fig. 1A,B).…”