Development and Differentiation of the Anuran Auditory Brainstem during Metamorphosis: An Acetylcholinesterase Histochemical Study

Abstract

The time course of cell differentiation and the presence of histochemically defined areas in brainstem auditory nuclei were examined in developing bullfrogs, Rana catesbeiana, using cresyl violet staining and acetylcholinesterase (AChE) histochemistry. In the medulla, the dorsolateral nucleus (DLN) can be seen as a distinct structure in its adult location only at Gosner stage 40 and beyond. The majority of cells in the DLN are not fully differentiated until late metamorphic climax (stages 45–46) and early postmetamorphosis. The more ventral vestibular nucleus differentiates earlier (stage 37) than the DLN. Adult-like organization of auditory nuclei in the torus semicircularis (TS) of the midbrain cannot be reliably discerned until metamorphic climax stages. Cellular masses in the brainstem reveal AChE from the earliest stage examined (stage 27) but the intensity of staining differs among cell groups. Staining intensity in the DLN is at a peak in recently metamorphosed froglets. The time course of cell differentiation in the DLN precedes slightly or is coincident with the increased, transient presence of AChE. Staining of the superior olive stabilizes at a moderate level in early postmetamorphic stages. Ventral regions of the principal nucleus in the TS stain more intensely than dorsal regions beginning at stage 40. This dorsal-ventral gradient in staining persists in adult stages. There is a transient decline in staining of the laminar nucleus in metamorphic climax stages. Staining intensity in the magnocellular nucleus 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 are an important developmental window for major differentiation and maturational events in the auditory brainstem.

Dynamic visualization of the developing nervous system of the bullfrog, Rana catesbeiana

“…At stage 37, the dorso-medial TS begins growing again, pushing upward until stage 40. At stage 40, the ventral TS and underlying tegmental nuclei expand dorsally, and expanding the left and right TS both medially and dorsally, closing off the connection between the optic ventricle and the aqueduct of Sylvius, as previously described (Kumaresan et al, 1998). The morph video demonstrates a probable mechanism underlying this separation, as it becomes clear that there is substantial increase in density and overall growth in a ventro-dorsal direction of the TS and underlying tegmental regions between stages 40 and 43.…”

“…Anatomical boundaries and terminology (Fig. 1) are consistent with those described for developing (Senn, 1972;Jacoby and Rubinson, 1983;Fritzsch et al, 1988;Kumaresan et al, 1998;Horowitz et al, 2007) and adult (Opdam et al, 1976;Wilczynski and Endepols, 2006) ranid brains.…”

“…These comparisons suggest that major anatomical changes in the visual midbrain occur during a more limited span of larval development than those in the auditory midbrain. Although the emergence of laminations in the tadpole optic tectum (Senn, 1972) and TS (Kumaresan et al, 1998) have been described previously, the morph video allows direct comparison of the different time courses of maturation of these two nuclei.…”

Metamorphic Changes in GABA Immunoreactivity in the Brainstem of the Bullfrog, <i>Rana catesbeiana</i>

“…These ongoing changes, along with the lack of functional data, make it difficult to draw precise boundaries of nuclei in the tadpole brainstem that might be analogous to those in the adult. In this study, we follow the definitions, based on both tract-tracing and cytoarchitecture, proposed in previous studies of tadpole brain organization [Uray and Gona, 1977;Jacoby and Rubinson, 1983;Fritzsch et al, 1984;Boatright-Horowitz and Simmons, 1997;Kumaresan et al, 1998;Boatright-Horowitz et al, 1999;Straka et al, 2001]. Figure 1 shows a series of schematic diagrams (left side of each individual brain section) of the brainstem obtained from bullfrogs at various developmental stages and at approximately the same plane of section.…”

“…Physiological response properties of multi-unit clusters in the midbrain torus semicircularis (TS) vary considerably in sensitivity and sharpness of tuning over larval stages [Boatright-Horowitz and Simmons, 1997;Boatright-Horowitz et al, 1999]. The DLN and the TS both show transient, increased expression of acetylcholinesterase (AChE) in metamorphic climax and early postmetamorphic stages [Kumaresan et al, 1998]. These data all suggest that metamorphosis is a period of substantial neuronal plasticity in the auditory nervous system.…”