Mori, S. et al. Nat. Commun. 10, 3802 (2019).

Organoids can serve as a proxy for developmental processes such as organogenesis. While a variety of different tissues have been modelled with organoids, the limb bud has not been one of them. Mori et al. adapted the SFEBq (serum-free floating culture of embryoid body-like aggregates with quick reaggregation) approach. They found that culturing aggregates of mouse embryonic pluripotent stem cells (PSCs) in the presence of Matrigel and BMP4 induces hindlimb bud-like tissue, consisting of an outer epithelial layer with aggregated cells internally. Retinoic acid promotes forelimb bud fate, while retinoic acid antagonists favor a hindlimb bud fate. Moreover, if BMP was provided in a locally restricted manner, the epithelium locally thickened in response, reminiscent of dorsoventral patterning. The developed protocol, therefore, mimics early stages of appendage formation, while further adjustments are needed to enable distal elongation as in the developing mouse embryonic limb.