Neurogenesis and apoptosis in the posterior placodal area of embryonic mice

Frabschka A, Washausen S, Knabe W

Abstract

In the posterior placodal area (PPA) of mice, apoptosis contributes to the segregation of otic and epibranchial placodes (OP, EPs)by eliminating rudiments of lateral line placodes (LLPs). Here we investigate the spatial, temporal and functional relationships between placode morphogenesis, neurogenesis, and apoptosis in the PPA. 47 C57BL/6N mouse embryos (E8 to E11.5) were evaluatedusing reconstructions and immunohistochemistry with antibodies against neural stem cell markers (anti-Sex determining region Y-box (Sox) 2 and Sox3) or neuroblast differentiation markers (basic helix-loop-helix transcription factors neurogenin (Ngn) 1 andNgn2). Sox2 and Sox3 are widely expressed in the early PPA. Thereafter, segregation of high-grade thickened placodes either precedes (EP1, OP) or follows (EP2, EP3) gradual restriction of Sox expression to the mature EPs. Exceptions are Sox2-dorsal parts of EP1 and the Sox3-EP3. Ngn1/Ngn2 ratios in EP1 and EP2 constantly remain below 1, whereas they transiently exceed 5 in EP3. "Doomed rudiments" of LLPs express Sox2 and/or Sox3. This explains why LLPs rescued by pharmacological inhibition of apoptosis retain their neurogenic potential (Washausen and Knabe 2018). Regression of EPs is driven by ventral (EP1, EP3) and dorsal apoptotic foci (EP1, EP2, EP3) that eliminate (mainly) Sox2+/Sox3+ placodal remnants. Epibranchial neuroblasts are generated either from Sox2+/Sox3+ (EP1, EP2) or from Sox2+/Sox3-ectoderm (EP3). These facts may contribute to the observed differences regarding the production of Ngn1+ and/or Ngn2+ neuroblasts in different EPs. Whether epibranchial neurogenesis varies individually due todifferences in inductive FGF-signaling (Washausen and Knabe, see accompanying abstract) is discussed.