Ventromedial focus of cell death is absent during development of Xenopus and zebrafish inner ears

We present the normal patterns of programmed cell death in the developing i nner ears of a primitive anuran, Xenopus laevis, and an ostariophysan fish, Danio rerio. A prominent ventromedial focus of cell death was described pr eviously in the developing chicken and mouse otocysts. We hypothesize that this focus of cell death might be associated with a signaling center that d irects morphogenesis of the surrounding tissue. Amphibian and fish ear anat omies differ considerably from those of birds and mammals, particularly in the structures derived from the ventral part (pars inferior) of the otic ve sicle. We reasoned that these anatomical differences between species might result from a difference in the size, location, or presence of a putative m orphogenetic signaling center. Using in situ terminal deoxynucleotidyl tran sferase (TdT)-mediated dUTP nick-end labeling (TUNEL) to detect apoptotic c ells, we show that developing Xenopus and zebrafish ears have apoptotic cel ls in the eighth cranial ganglia, the developing sensory patches, and in va rious positions in the otocyst wall. However, both species lack the persist ent ventromedial hot spot of cell death that is prominently situated betwee n the pars superior and pars inferior in the chicken and mouse otocysts.