The Otolithic Organ in the Developing Chick Embryo Scanning Electron Microscopic Study on the Utricular Macula

The long and short axes of utricular maculae were measured to calculate growth curves using an ocular micrometer under stereomicroscope. The mean lengths of the long and short axes on the 6th incubation day were 0.06 mm and 0.05 mm, respectively. Thereafter: 1.00 mm and 0.74 mm on the 11th incubation day; 1.34 mm and 1.03 mm on the 15th incubation day; and on hatching 1.51 mm and 1.07 mm, respectively. The surface structure, particularly the otoconia and the sensory hairs of the utricular maculae, on the 4.5th incubation day to the 7th post-hatching day of White Leghorn chicks were observed with a scanning electron microscope (SEM). A number of microvilli and primary cilium on the inner surface of the otocyst were found between the 4.5th and the 5th incubation day. Immature sensory hairs appeared on the 5th incubation day. The polarity of the sensory hairs became evident on and after the 9th incubation day. Mature sensory hairs were seen all over the utricular maculae between the 1st and the 3rd post-hatching day. Mature sensory hairs were mainly seen near the striola, and immature ones peripherally. Then, the various forms in the maturation phase of sensory hairs were seen between the striola and the periphery. These arrangements suggest some process of maturity from the peripheral region to the central striola region. Immature otoconia were first seen on the 6th incubation day. Mature otoconia were recognized on and after the 11th incubation day, and they were observed on the whole surface of the utricular maculae. Global substances on the macula with a rough surface and spongy structure were observed suggesting otoconial precursors transforming themselves into otoconial crystals.