Senescence: a new player in the development of neurogenic epibranchial placodes

Washausen S, Hakenes T, Knabe W

Abstract

Senescence denotes a form of cell-cycle arrest that is linked to tumour suppression and aging. Other less well understood roles are related to embryonic development, as illustrated by the expression of key components of the senescence pathway in two major signaling centres (roof plate, apical ectodermal ridge). Here we aim to clarify, whether senescence contributes to the development of neurogenic epibranchial placodes in mice. Histological serial sections from C57BL/6N mouse embryos (E8 to E11.5) were immunoreacted with antibodies against the senescence inducer cyclin-dependent kinase inhibitor p21. Established other features of epibranchial placode development were demonstrated using anti-cleaved caspase-3 (apoptosis), anti-phospho-Histone H3 (proliferation), and anti-neurogenin (neurogenesis) antibodies. Strictly localized p21 immunopositivity was found in the lateral thirds of the pharyngeal pouches and, thus, immediately adjacent to the three paired epibranchial placodes. These are exactly the positions where Begbie et al. (1999) have identified epibranchial signaling centres that, in the chicken, express bone morphogenetic protein 7 and induce epibranchial neurogenesis. Proceeding rostrocaudally, the onset (E8.5 to E9), climax (E9.5 to E10.5) and decline of epibranchial neurogenesis in mice is followed by theonset, climax and decline of endodermal p21 immunopositivity with a delay of about 12 to 24h. Additionally, spatiotemporally regulated patterns of senescent ectodermal cells occur within as well as adjacent to each epibranchial placode. Our findings suggest that senescence contributes to the maintenance and/or to the termination of epibranchial signaling centres as well as to the morphogenesis of epibranchial placodes. Whether senescence and apoptosis cooperate during these tasks is discussed.