Fig. 1. Schematic diagram of development of the chick inner ear and its sensory patches. (A) The inner ear of a bird originates by invagination of the otic placode and remodelling of the resulting otic vesicle to form the labyrinth. The mature structure contains seven vestibular sensory patches, involved in perception of gravity and acceleration: three cristae (in the ampullae of the semicircular canals) and four maculae, those of the utricle, the saccule, the lagena, and the macula neglecta (which we neglect). The basilar papilla, an elongated sensory region extending along the cochlear duct, serves auditory function. (B) The patterns of Notch ligand expression in a sensory patch before, during and after hair-cell differentiation. One of the earliest genes to be expressed in prospective sensory patches codes for the Notch ligand Ser1. Within the Ser1 domains, nascent hair cells expressing Delta1 can be detected from E3.5 in vestibular regions, and from E5 in the cochlear duct. Delta1 expression foreshadows the differentiation of hair cells, which become identifiable by morphological and other molecular criteria about 24 hours later. Notch1 itself is expressed in sensory as well as non-sensory regions of the developing otocyst.