DEVELOPMENT AND DIFFERENTIATION OF THE ANURAN AUDITORY BRAIN-STEM DURING METAMORPHOSIS - AN ACETYLCHOLINESTERASE HISTOCHEMICAL-STUDY

The time course of cell differentiation and the presence of histochemi cally defined areas in brainstem auditory nuclei were examined in deve loping bullfrogs, Rana catesbeiana, using cresyl violet staining and a cetylcholinesterase (AChE) histochemistry, In the medulla, the dorsola teral nucleus (DLN) can be seen as a distinct structure in its adult l ocation only at Gosner stage 40 and beyond. The majority of cells in t he DLN are not fully differentiated until late metamorphic climax (sta ges 45-46) and early postmetamorphosis. The more ventral vestibular nu cleus differentiates earlier (stage 37) than the DLN, Adult-like organ ization of auditory nuclei in the torus semicircularis (TS) of the mid brain cannot be reliably discerned until metamorphic climax stages, Ce llular masses in the brainstem reveal AChE from the earliest stage exa mined (stage 27) but the intensity of staining differs among cell grou ps. Staining intensity in the DLN is at a peak in recently metamorphos ed froglets. The time course of cell differentiation in the DLN preced es slightly or is coincident with the increased, transient presence of AChE. Staining of the superior olive stabilizes at a moderate level i n early postmetamorphic stages. Ventral regions of the principal nucle us in the TS stain more intensely than dorsal regions beginning at sta ge 40, This dorsal-ventral gradient in staining persists in adult stag es. There is a transient decline in staining of the laminar nucleus in metamorphic climax stages. Staining intensity in the magnocellular nu cleus peaks during stages 40-46 and in early postmetamorphic froglets and then declines in adults, paralleling the pattern seen in the DLN. These data suggest that metamorphic climax and early froglet periods a re an important developmental window for major differentiation and mat urational events in the auditory brainstem.