Apoptosis contributes to placode morphogenesis in the posterior placodal area of mice

Abstract

In the embryonic head of vertebrates, neurogenic and non-neurogenic ectodermal placodes arise from the panplacodal primordium. Whether and how growth processes of the ectodermal layer, changes in the transcriptional precursor cell profile, or positional changes among precursor cells contribute to interplacodal boundary formation is subject to intense investigation. We demonstrate that large scale apoptosis in the multiplacodal posterior placodal area (PPA) of C57BL/6 mice assists in the segregation of otic and epibranchial placodes. Complex patterns of interplacodal apoptosis precede and parallel the structural individualization of high-grade thickened placodes, with the fundamental separation between otic and epibranchial precursor cells being seemingly prevalent. Interplacodal apoptosis between the emerging epibranchial placodes, which express Neurogenin2 prior to their complete structural individualization, comes out most strongly between the epibranchial placodes 1 and 2. Apoptosis then moves from interplacodal to intraplacodal positions in dorsal and, with a delay, ventral parts of the epibranchial placodes. Intraplacodal apoptosis appears to exert corrective actions among premigratory neuroblasts, and helps to eliminate the epibranchial placodes. The present findings confirm and extend earlier observations in Tupaia belangeri (Washausen et al. in Dev Biol 278:86-102, 2005), regarded as an intermediate between primates and other eutherian orders. Having now available maps of apoptosis in the PPA of embryonic mice, further investigations into the functions of inter- and intraplacodal apoptosis can be carried out in an experimentally and genetically more accessible mammalian model organism.

Early development of the nervous system of the eutherian <i>Tupaia belangeri</i>

“…It therefore comes as no surprise that many of the works carried out by former students of Hans-Jürg Kuhn still involve 3-D reconstructions (e.g., Schunke and Zeller, 2010;Washausen and Knabe, 2013). However, the need for processing ever bigger data sets, which in our case contain information from large embryonic surfaces as well as from millions of single cells, required a fundamental revision of our pre-existing reconstruction techniques (Knabe and Kuhn, 1996a).…”

“…Other important issues still in need of clarification are the molecular regulation and the very exact functions of large-scale apoptosis first observed during the morphogenesis of placodes in Tupaia belangeri. Fortunately, almost identical apoptotic patterns contribute to the development of epibranchial placodes in mice (Washausen and Knabe, 2013), which facilitates future experimental approaches.…”

Chicken embryos share mammalian patterns of apoptosis in the posterior placodal area

“…; Knabe et al. ; Washausen & Knabe, , , ). However, the nature of its functional contribution has been elusive for many years, and only recently has experimental evidence been provided that, in the PPA of mice, ‘phylogenetic cell death’ (Glücksmann, ) eliminates vestigial lateral line placodes with an astonishingly high developmental potential (Washausen & Knabe, ).…”

“…Later, apoptosis is centred on the mature and regressing epibranchial placodes (Washausen et al. ; Washausen & Knabe, , , ). In T. belangeri , similar patterns of apoptosis accompany the morphogenesis of the trigeminal and lens placodes (Knabe et al.…”

Mechanisms of cranial placode assembly

“…To our knowledge, no experimental data support the contribution of cell fate changes in the secondary coalescence phase. Apoptotic cells were observed at interplacodal boundaries during the separation between otic and epibranchial areas, and later during the resolution of the epibranchial placode into discrete clusters in mammals (Knabe et al, 2009;Washausen and Knabe, 2012;Washausen et al, 2005), but their functional importance in placode separation has not been assessed. Cell shape changes such as a transition from flattened to epithelial morphology (mesenchyme to epithelium transition), or a reduction in placodal cell volume, could both result in overall compaction of placodal areas, but such morphological transitions have not been described so far.…”