Development of the ampullae in the semicircular canals of chick embryos: A scanning and transmission electron microscopy investigation.

Our scanning ultrastructural studies in chick embryos revealed that the early bud (7days embryo) of the septum cruciatum (s.c.) is represented by bulges of both sites of the central portion of the crista, that thus takes the shape of a spindle. During the following stages these buds become small roundish swelling that progressively grow thinner and elongate. In 1-and 4 day chickens, they display lateral depressions and cluster of microvilli that make the septum resemble the cladode of Opuntia vulgaris. From the 8th day, tufts of hairs appear on the crista, including a longer kinocilium and a bundle of stereocilia decreasing in height. In the vertical semicircular canals, the central portion of the crista on the area included between the two protrusions of the s.c. is hairless and clepsydra –shaped. During the early stages studied, transmission electron microscopy observations have shown differentiated sensorial cells of type I and II according to Wersall and the respective cytoneural synapses.The supporting cells, instead, are not yet well differentiated. On the 14th day, some cells show a darker cytoplasmic matrix and evident infolding of the basal and lateral plasmalemma delimiting compartments with numerous mitochondria. These features become more marked during the following days, and after birth dark cells resemble those of adults.Stress is laid on the mismatch between the early differentiation of ampullar receptors and the late one of dark cells, as well as on its possible significance.