Involvement of programmed cell death in morphogenesis of the vertebrate inner ear.

An outstanding challenge in developmental biology is to reveal the mechanisms underlying the morphogenesis of complex organs. A striking example is the developing inner ear of the vertebrate, which acquires a precise three-dimensional arrangement of its constituent epithelial cells to form three semicircular canals, a central vestibule and a coiled cochlea (in mammals). In generating a semicircular canal, epithelial cells seem to 'disappear' from the center of each canal. This phenomenon has been variously explained as (i) transdifferentiation of epithelium into mesenchyme, (ii) absorption of cells into the expanding canal or (iii) programmed cell death. In this study, an in situ DNA-end labeling technique (the TUNEL protocol) was used to map regions of cell death during inner ear morphogenesis in the chicken embryo from embryonic days 3.5-10. Regions of cell death previously identified in vertebrate ears have been confirmed, including the ventromedial otic vesicle, the base of the endolymphatic duct and the fusion plates of the semicircular canals. New regions of cell death are also described in and around the sensory organs. Reducing normal death using retrovirus-mediated overexpression of human bcl-2 causes abnormalities in ear morphogenesis: hollowing of the center of each canal is either delayed or fails entirely. These data provide new evidence to explain the role of cell death in morphogenesis of the semicircular canals.

The molecular biology of hair cell regeneration in the avian cochlea.

The sensory cells of the ear, the hair cells, are damaged by loud noise or certain types of drugs. In the bird cochlea, new hair cells are produced to replace those that are lost. Regeneration also occurs in the vestibular epithelia of birds, fish, and mammals but does not occur in the mammalian cochlea. In order to further our understanding of the regeneration process in the bird cochlea, we have begun to identify the genes that are involved. However, the small size of this organ has made it difficult to use traditional molecular biology methods to address these problems. Recently, many molecular techniques have been adapted for use with small amounts of tissue. Northern blot analysis, the ribonuclease protection assay, semiquantitative PCR and differential display of mRNA are all techniques that are being used to greatly improve our understanding of hair cell regeneration and may eventually provide the information necessary to induce regeneration in hearing-impaired humans.

Overproduction and translational regulation of rp49 ribosomal protein mRNA in transgenic Drosophila carrying extra copies of the gene.

During Drosophila early development the translation of ribosomal protein mRNAs is regulated specifically and coordinately. In this study we assayed for changes of ribosomal protein rp49 gene expression in flies transformed with extra copies of the gene. RNA blot analysis revealed that the rp49 transcript was overrepresented in most of the transformed fly lines: flies carrying three times more genes than the wild type contained up to seven times more mRNA. The abundance of other ribosomal protein mRNAs was not affected. Despite the large differences in rp49 mRNA content, the proportion of the rp49 mRNA which was associated with polysomes during oogenesis and early embryogenesis did not differ significantly from the wild type, implying that rp49 protein is overproduced in the transgenic flies. The results indicate that the basis for coordinate r-protein gene expression lies in the intrinsic properties of r-protein genes, rather than in a dynamic system that separately modulates the expression of individual genes.