Tritiated thymidine in the chick embryo inner ear

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Development of the chick (Gallus domesticus) inner ear has been studied, and the maturation of cells that detect sound has been analyzed at the E.M. level [1,2,3]. Other workers showed correspondence between ultrastructural maturation and behavioral responses [4,5]. In mammals [6] hair cells mature after ceasation of mitosis {Fig.l}, in a pattern so that older cells are in the base of the cochlea while younger cells are in the apex [7]. But, electrophysiology indicates that cells at the base do not function first. Chicks are precocious with well developed sensory organs at birth, and their embryonic development follows, on a very short time span, a sequence that resembles that of the human ear. This study was undertaken to standarize tritiated thymidine (TT) because resolution of TT in avian embryos differs significantly from mammals [6]. Embryos were injected with 100 μl of TT, and sacrificed 1 or 2 hours later in order to label only those cells that were actively dividing cells at the time of the injection. Specimens were fixed and processed for autoradiography [6].Actively dividing cells incorporate TT after short exposure, with minimal background. It seems that vestibular sensory epithelia {Fig.2} have more dividing cells than the auditory sensory epithelia {Fig.3}, even though the vestibule develop before the cochlea. The ratio between the number of labeled cells over the length of the sensory epithelia is lower in the auditory basilar papilla (0.098 cell/(μm) than in the vestibular utricle (0.77 cell/μm) and saccule (1.66 cell/ μm). When dividing cells were analyzed in the basilar papilla alone, and their distribution displayed along the length of the cochlea over time, older cells were opposite to the VIIIth nerve fibers that innervate those hair cells. A lateral and a longitudinal gradient has been established and hair cells closer to the nerve in the mid-basal area mature earlier than hair cells at both ends of the cochlea [2]. This finding, if occuring in mammals, may explain why mid-frequency are the first to appear [5]. The first 1/3 of the chick cochlea contains mainly short hair cells and are innervated primarily by efferent nerve fibers, which arrive in the cochlea almost a week after the afferent do. Moreover, tall hair cells extend 2/3 of the cochlear length from apex to mid-base and show mature innervation patterns before the short hair cells do. In the short embryonic cochlea, frequencies may be produced first in the what will later be the mid-region because, early in development, that area contains more mature receptors [1,2,3].