The infundibular balance organ in amphioxus larvae and related aspects of cerebral vesicle organization

Serial EM reconstructions were used to examine the organization and constituent cell types of the infundibular region of the cerebral vesicle (c.v.) in a 12.5-day larva of Branchiostoma floridae. The balance organ lies just in front of the infundibular cells and consists of 10 electron-dense cells with long, bulbous cilia, each surrounded by a ring of accessory cells. The ciliary bulb cells have axons that terminate in vesicle-filled swellings that lack identifiable synapses. The accessory cells have short basal processes that are minor contributors to the adjacent neuropile. Based on morphology, we suggest a mechanosensory function for the ciliary bulb cells, possibly related to balance or motion detection. Scattered cells of similar type are found elsewhere in the cerebral vesicle, along with a variety of other neurones with caudally projecting axons and varicosities, but few synapses. Instead, nonsynaptic, paracrine secretion appears to be the predominant mode of transmitter release in the neuropile and ventral tracts of the cerebral vesicle. The closest vertebrate homologue of this part of the amphioxus brain is arguably the limbic core of the caudal diencephalon and mesencephalon, including the homeostatic control centres of the hypothalamus. We postulate that this limbic core is an ancient structure traceable at least as far back in evolution as the common ancestor of amphioxus and vertebrates.