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 inner ears of a primitive anuran, Xenopus laevis, and an ostariophysan fish, Danio rerio. A prominent ventromedial focus of cell death was described previously in the developing chicken and mouse otocysts. We hypothesize that this focus of cell death might be associated with a signaling center that directs morphogenesis of the surrounding tissue. Amphibian and fish ear anatomies differ considerably from those of birds and mammals, particularly in the structures derived from the ventral part (pars inferior) of the otic vesicle. We reasoned that these anatomical differences between species might result from a difference in the size, location, or presence of a putative morphogenetic signaling center. Using in situ terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) to detect apoptotic cells, we show that developing Xenopus and zebrafish ears have apoptotic cells in the eighth cranial ganglia, the developing sensory patches, and in various positions in the otocyst wall. However, both species lack the persistent ventromedial hot spot of cell death that is prominently situated between the pars superior and pars inferior in the chicken and mouse otocysts.