Cranial neural crest cells (CNCC) are a stem-like cell population that arises in the anteriormost part of the developing embryo, at the border between the neural plate and the surface ectoderm1. CNCC then undergo an epithelial-to-mesenchymal transition (EMT), delaminate from the neuroepithelium, and migrate dorsoventrally towards various locations in the embryo where they differentiate into a wide variety of cell types2. Studying this cell population is of great interest as it possesses a remarkable plasticity3 and the unique ability to differentiate into both ectodermal and mesenchymal derivatives, such as craniofacial bones and cartilages4. Although CNCC are relatively accessible in the embryo, they are a transient population with a low number of cells, making systemic mechanistic studies difficult to conduct in vivo. CNCC cell lines have been isolated and characterized in the last few years to overcome these limitations. In particular, the O9-1 CNCC cell line is a great tool for studying migratory and post-migratory neural crest development5,6; however, this cell line does not allow the study of the early events prior to migration leading to neural crest induction and specification. In this regard, there have been significant developments in the development of in vitro differentiation protocols to differentiate CNCC in a dish via the use of 3D structures resembling the developing neuroepithelium called neurospheres (NS)7,8- obtained after differentiation of embryonic stem cell (ESC) colonies. These 3D protocols robustly produce high numbers of CNCC, allowing the conduct of biochemical and genomic mechanistic studies9,10. NS are cultured on low attachment plates in N2B27 supplemented medium, together with Fibroblast Growth Factor (FGF) and Epidermal Growth Factor (EGF)10,11 to stimulate cell proliferation. These protocols are carried out in Petri dishes, cultivating numerous NS in the same plate. Within the growing NS, cells aggregate and continue to divide - reaching a diameter of 100-200 µm upon maturity. At maturity (about day 5), NS attach to the substrate and differentiate into CNCC resembling their in vivo counterparts9,12. These CNCC then undergo EMT and delaminate onto the plate surface. Morphological differences can be observed depending on NS size, as larger spheres will appear darker in the core due to lower availability of nutrients and oxygen, leading to cells undergoing apoptosis13. While this type of procedure generates a large number of CNCC at the endpoint of differentiation, it presents several limitations, making the study of the various molecular dynamics occurring during the differentiation process nearly impossible. First, the use of ESC colonies - which vary in size - makes it difficult to control the starting cell number for each experiment. This results in the generation of NS of various shapes and diameters that develop differently by activating specific signaling pathways, leading to altered cell differentiation and, thus not forming a uniform sample at a given time point. Second, culturing multiple NS in the same plate often leads to them fusing together14 and potentially releasing signaling molecules that influence their neighbors' microenvironment and, thus, their development. Altogether, these procedures generate a lot of variability between samples and experiments.
Here, we present a strategy to overcome these difficulties that generate single NS - capable of producing CNCC - by aggregating mouse ESC (mESC) in non-TC treated U-bottomed 96-well plates. Starting from mESC allows studying the specification process and early stages of CNCC development compared to starting from already established neural crest cell lines. This protocol begins with the disaggregation of mESC colonies to obtain a single cell suspension, followed by the seeding of a specific number of mESC in each well of a non-TC treated U-bottomed 96-well plate. The cells are left to aggregate for two days and subsequently moved to a non-TC treated flat-bottomed 96-well plate, in which NS will be able to attach to the plate bottom. By controlling the starting cell number and the microenvironment of each NS during the differentiation process, this protocol reduces sample variability, which increases experimental reproducibility. We believe this will be a convenient platform for designing multiplexed experiments, such as testing the effect of different culture conditions or performing gene perturbation screens.