Electron Microscope Observations on in Vztro Cultures of the Isolated Fowl Embryo Otocyst*

In vitro cultures of isolated fowl embryo otocysts were studied with the electron microscope. Hair cells of the developing organ of Corti and crista ampullaris have been examined with particular reference to the structure of the cilia and of the cell membrane. Two types of hair cells could be distinguished on the basis whether or not they possessed a "kinocilium" and "stereocilia," or "stereocilia" only. The cytoplasmic membranes were simple and there were no multiple vesicular layers in any of the hair cells. The supporting elements consisted of supporting cells flanking the hair cells, fibroblasts, and the cartilaginous otic capsule. Both the cochlear and vestibular sensory area showed rich innervation by mainly non-myelinated fibers with partial myelinization in others. There were well developed ganglion cells present. Bare axons penetrated the basement membrane and spread, amongst the supporting cells sheltering them, to the base of the hair cells where they formed bud-shaped nerve endings but, at the stage of development examined, no calyces. These in vitro cultures of the isolated fowl embryo otocyst provided convenient and suitable material for the electron microscope study of the sensory epithelium of the ear and revealed further that the isolated fowl embryo otocyst possesses great powers of self-differentiation also at the ultrastructural level. The ultrastructure of the sensory epithelium of the inner ear has only been studied by a comparatively small number of workers. For example, the organ of Corti of the guinea-pig has been examined with the electron microscope by Engstr6m and Wersiill (1953 a, b, and c). Engstr6m and Sj6strand (1954), Smith and Dempsey (1957), and Spoendlin (1957). The ultrastructure of the macula has been studied by Wersiill, Engstr6m, and Hjor th (1954) and Smith (1956), and that of the cristae ampullares by Wers~ll (1954 and 1956). All these studies have been carried out on guinea pigs, the tissues removed after killing the * Based on a communication presented at the Summer Meeting of the Pathological Society of Great Britain and Ireland in Newcastle-upon-Tyne on July 4, 1958. animal (with or without an anaesthetic) as rapidly as possible so as to avoid any untoward postmortem change. In order to try to overcome this difficulty tissue culture material was used in the present electron microscopical investigations, based on the results of light microscopical studies of in vitro cultures of the isolated fowl embryo otocyst (Friedmann, 1956). Using the watchglass technique first developed by Fell and Robison (1929) for the study of the self-differentiating capacity of avian organ rudiments, Friedmann (1956) has shown that, as first suggested by Fell (1929), the isolated otocyst possesses great powers of self-differentiation leading to the in vitro development of the organ of Corti, the crista, and the macula. I t was thought that further information on the extent of differentiation could best be obtained by means of electron microscopical methods.