| The superior olive (SO) is a prominent nucleus of the anuran auditory brainstem and a putative homolog to the medial superior olive of mammals and the laminar nucleus of birds. The study of the morphological and histochemical maturation of the SO is relevant to the understanding of auditory brainstem development in vertebrates because of the extensive plasticity demonstrated by the nuclei of the mammalian superior olivary complex and the similarities between the developmental trajectories of anurans and auditorily precocial mammals (such as humans), including the shift from aquatic to atmospheric sound environments.;We conducted a series of histomorphometric and histochemical studies to quantitate developmental changes in the bullfrog SO and lateral lemniscus (LL) across metamorphosis. The studies of the first block made use of modern stereological tools (optical fractionator, planimetry) geared to minimizing estimation bias and increasing estimation efficiency. The total number of neural cells in the SO nearly linearly increases up to Gosner stage 39. From that stage to the early postmetamorphic period, a quadratic trend with positive curvature best describes changes in the neural population total. The trend break is due to changes in the number of neurons, not glia, and indicates a change in an underlying biological process.;The growth and plasticity associated protein (GAP)-43 has been implicated in axonal outgrowth and synaptic plasticity. The studies of the second block investigated the expression levels of GAP-43 in the developing SO and LL, using immunchistochemistry. The proportion of GAP-43-positive cells in the SO as well as GAP-43 signal in fibers of the LL remain fairly stable over metamorphic development and may slightly decrease postmetamorphically. The continuous need for the activity of the molecule may be related to changes in the size and shape of the animal's head, the expansion of the central nervous system, the remodeling of fiber connections, and differential processes of proliferation, differentiation, and degeneration in auditory and other mechanosensory nuclei. The mild postmetamorphic decline in GAP-43 levels may be due to a decrease in cell proliferation and is consistent with the postnatal reduction in GAP-43 expression in the superior olivary complex and other areas of the mammalian auditory brainstem. |
