Vertebrate embryo somitogenesis is the earliest morphological manifestation of the characteristic patterned structure of the adult axial skeleton. Pairs of somites flanking the neural tube are formed periodically during early development, and the molecular mechanisms in temporal control of this early patterning event have been thoroughly studied. The discovery of a molecular Embryo Clock (EC) underlying the periodicity of somite formation shed light on the importance of gene expression dynamics for pattern formation. The EC is now known to be present in all vertebrate organisms studied and this mechanism was also described in limb development and stem cell differentiation. An outstanding question, however, remains unanswered: what sets the different EC paces observed in different organisms and tissues? This review aims to summarize the available knowledge regarding the pace of the EC, its regulation and experimental manipulation and to expose new questions that might help shed light on what is still to unveil.

Gene expression regulation during embryo development is under strict regulation to ensure proper gene expression in both time and space. The involvement of microRNAs (miRNA) in early vertebrate development is documented and inactivation of different proteins involved in miRNA synthesis results in severe malformations or even arrests vertebrate embryo development. However, there is very limited information on when and in what tissues the genes encoding these proteins are expressed. Herein, we report a detailed characterization of the expression patterns of DROSHA, DGCR8, XPO5 and DICER1 in the developing chick embryo, from HH1 (when the egg is laid) to HH25 (5-days incubation), using whole mount in situ hybridization and cross-section analysis. We found that these genes are co-expressed in multiple tissues, mostly after stage HH4. Before early gastrulation DICER1 expression was never detected, suggesting the operation of a Dicer-independent pathway for miRNA synthesis. Our results support an important role for miRNAs in vertebrate embryo development and provide the necessary framework to unveil additional roles for these RNA processing proteins in development.

MicroRNAs (miRNAs) are small non-coding RNA molecules, with sizes ranging from 18 to 25 nucleotides, which are key players in gene expression regulation. These molecules play an important role in fine-tuning early vertebrate embryo development. However, there are scarce publicly available miRNA datasets from non-mammal embryos, such as the chicken (Gallus gallus), which is a classical model system to study vertebrate embryogenesis. Here, we performed microRNA-sequencing to characterize the early stages of trunk and limb development in the chick embryo. For this, we profiled three chick embryonic tissues, namely, Undetermined Presomitic Mesoderm (PSM_U), Determined Presomitic Mesoderm (PSM_D) and Forelimb Distal Cyclic Domain (DCD). We identified 926 known miRNAs, and 1,141 novel candidate miRNAs, which nearly duplicates the number of Gallus gallus entries in the miRBase database. These data will greatly benefit the avian research community, particularly by highlighting new miRNAs potentially involved in the regulation of early vertebrate embryo development, that can be prioritized for further experimental testing.