Vertebrate body axis extension involves progressive generation and subsequent differentiation of new cells derived from a caudal stem zone; however, molecular mechanisms that preserve caudal progenitors and coordinate differentiation are poorly understood. FGF maintains caudal progenitors and its attenuation is required for neuronal and mesodermal differentiation and to position segment boundaries. Furthermore, somitic mesoderm promotes neuronal differentiation in part by downregulating Fgf8. Here we identify retinoic acid (RA) as this somitic signal and show that retinoid and FGF pathways have opposing actions. FGF is a general repressor of differentiation, including ventral neural patterning, while RA attenuates Fgf8 in neuroepithelium and paraxial mesoderm, where it controls somite boundary position. RA is further required for neuronal differentiation and expression of key ventral neural patterning genes. Our data demonstrate that FGF and RA pathways are mutually inhibitory and suggest that their opposing actions provide a global mechanism that controls differentiation during axis extension.

Neuromesodermal progenitors (NMps) contribute to both the elongating spinal cord and the adjacent paraxial mesoderm. It has been assumed that these cells arise as a result of patterning of the anterior neural plate. However, as the molecular mechanisms that specify NMps in vivo are uncovered, and as protocols for generating these bipotent cells from mouse and human pluripotent stem cells in vitro are established, the emerging data suggest that this view needs to be revised. Here, we review the characteristics, regulation, in vitro derivation and in vivo induction of NMps. We propose that these cells arise within primitive streak-associated epiblast via a mechanism that is separable from that which establishes neural fate in the anterior epiblast. We thus argue for the existence of two distinct routes for making central nervous system progenitors.

Neural tissue formation is induced by growth factors that activate networks of signal transduction cascades that ultimately lead to the expression of early neural genes, including transcription factors of the SoxB family. Here,we report that fibroblast growth factor (FGF)-induced Erk1/2 (Mapk3 and Mapk1,respectively) mitogen-activated protein kinase (MAPK), but not phosphatidylinositol 3′-OH kinase (PI3K, Pik3r1), signalling is required for neural specification in mouse embryonic stem (ES) cells and in the chick embryo. Further, blocking Erk1/2 inhibits the onset of key SoxB genes in both mouse ES cells (Sox1) and chick embryos (Sox2 and Sox3) and, in both contexts, Erk1/2 signalling is required during only a narrow time window, as neural specification takes place. In the absence of Erk1/2 signalling, differentiation of ES cells stalls following Fgf5 upregulation. Using differentiating ES cells as a model for neural specification, we demonstrate that sustained Erk1/2 activation controls the transition from an Fgf5-positive, primitive ectoderm-like cell state to a neural progenitor cell state without attenuating bone morphogenetic protein (BMP) signalling and we also define the minimum period of Erk1/2 activity required to mediate this key developmental step. Together, these findings identify a conserved, specific and stage-dependent requirement for Erk1/2 signalling downstream of FGF-induced neural specification in higher vertebrates and provide insight into the signalling dynamics governing this process.

The progressive generation of chick and mouse axial tissues – the spinal cord, skeleton and musculature of the body – has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

Construction of the trunk/caudal region of the vertebrate embryo involves a set of distinct molecules and processes whose relationships are just coming into focus. In addition to the subdivision of the embryo into head and trunk domains, this "caudalisation" process requires the establishment and maintenance of a stem zone. This sequentially generates caudal tissues over a long period which then undergo differentiation and patterning in the extending body axis. Here we review recent studies that show that changes in the signalling properties of the paraxial mesoderm act as a switch that controls onset of differentiation and pattern in the spinal cord. These findings identify distinct roles for different caudalising factors; in particular, Fibroblast Growth Factor (FGF) inhibits differentiation in the caudal stem zone, while Retinoic acid (RA) provided rostrally by somitic mesoderm is required for neuronal differentiation and establishment of ventral neural pattern. Furthermore, the mutual opposition of FGF and RA pathways controls not only neural differentiation but also mesoderm segmentation and might also underlie the progressive assignment of rostrocaudal identity by regulating Hox gene availability and activation.

Withdrawal of differentiating cells from proliferative tissue is critical for embryonic development and adult tissue homeostasis; however, the mechanisms that control this cell behavior are poorly understood. Using high-resolution live-cell imaging in chick neural tube, we uncover a form of cell subdivision that abscises apical cell membrane and mediates neuron detachment from the ventricle. This mechanism operates in chick and mouse, is dependent on actin-myosin contraction, and results in loss of apical cell polarity. Apical abscission also dismantles the primary cilium, known to transduce sonic-hedgehog signals, and is required for expression of cell-cycle-exit gene p27/Kip1. We further show that N-cadherin levels, regulated by neuronal-differentiation factor Neurog2, determine cilium disassembly and final abscission. This cell-biological mechanism may mediate such cell transitions in other epithelia in normal and cancerous conditions.

RNA interference (RNAi) provides an effective method to silence gene expression and investigate gene function. However, RNAi tools for the chicken embryo have largely been adapted from vectors designed for mammalian cells. Here we present plasmid and retroviral RNAi vectors specifically designed for optimal gene silencing in chicken cells. The vectors use a chicken U6 promoter to express RNAs modelled on microRNA30, which are embedded within chicken microRNA operon sequences to ensure optimal Drosha and Dicer processing of transcripts. The chicken U6 promoter works significantly better than promoters of mammalian origin and in combination with a microRNA operon expression cassette (MOEC), achieves up to 90% silencing of target genes. By using a MOEC, we show that it is also possible to simultaneously silence two genes with a single vector. The vectors express either RFP or GFP markers, allowing simple in vivo tracking of vector delivery. Using these plasmids, we demonstrate effective silencing of Pax3, Pax6, Nkx2.1, Nkx2.2, Notch1 and Shh in discrete regions of the chicken embryonic nervous system. The efficiency and ease of use of this RNAi system paves the way for large-scale genetic screens in the chicken embryo.

By analyzing cellular and molecular changes in key cell populations in the tailbud during embryogenesis, this work uncovers critical signaling events that determine vertebrate body length.