Apical Organs, Epithelial Domains, and the Origin of the Chordate Central Nervous System

SYNOPSIS. The apical organ is a key structural landmark in a wide range of invertebrate larvae, but its homolog in chordates has not been identified. A widely accepted explanation of chordate origins, Garstang's auricularia hypothesis, suggests several possibilities. Structural and biochemical evidence both support the idea that the apical organ, in amphioxus, has been incorporated into the frontal eye complex. Structural and positional similarities between this eye-like structure and paired eyes in vertebrates suggest the two may be homologous. The implication is that the cells of the neural retina in vertebrates may be derivatives of the primitive apical organ. Other implications of Garstang's hypothesis are discussed: that it provides (1) an evolutionary rationale for the restriction of Hoxexpression in ectoderm to neural tissue, (2) grounds for doubting that the CNS in chordates and that of protostome invertebrates can in any way be homologous, and a starting point for exploring (3) whether the eyes of ancestral chordates were single or paired, and (4) the origin of the vertebrate telencephalon.