A developmental gradient of dendritic loss in the avian cochlear nucleus occurring independently of primary afferents

Cochlear nerve axons and their target neurons in nucleus magnocellularis (NM) of the chicken undergo extensive parallel structural transformations during development. Between embryonic days 12 and 17 (E12–E17), each immature highly branched axon condenses into a mature calyxlike ending applied to a single NM neuron. Simultaneously, NM neurons are transformed from multipolar cells with many long dendrites into spherical unipolar neurons with only an axon. We tested the hypothesis that cochlear nerve input is necessary for the transformation of NM cells by surgically destroying one otocyst on E3, thereby preventing formation of the nerve. Nucleus magnocellularis neurons from embryos at E11–E12, E13–14, and E17–18 were stained by horseradish peroxidase injected into their axons or by a Golgi‐Hortega method. In camera lucida drawings, the number of dendrites on each cell was counted and the cell's position along the posteriorto‐anterior and lateral‐to‐medial axes of the nucleus quantified. At E11–12, neurons throughout NM on both the deafferented and normally innervated sides of the brain have about ten dendrites. At E13–14, there is a steep spatial gradient in dendritic number bilaterally; cells at anteromedial positions have about two dendrites, while cells in posterolateral positions have an average of nine dendrites. By E17–18, only 14% of the neurons on either side have a dendrite, and these cells are evenly distributed throughout the nucleus. We conclude that cochlear nerve axons are not required for normal spatio‐temporal gradients of dendritic loss, even though the absence of these axons causes severe atrophic changes in NM. Thus, it appears that the characteristic forms of the afferent axon and target neuron in this system must develop in one of two ways. Either NM cells control the structural differentiation of their cochlear nerve afferents, or the forms of axon terminals and their target cells develop independently. These alternative, hypotheses can be evaluated experimentally.