Figure 1. Overview of mouse early embryonic development and the NBC organogenesis approach.

(A) Key steps in early mouse development from zygote to blastocyst are illustrated: Fertilization involves the fusion of two haploid gametes to form a zygote. The zygote (0.5 dpc) undergoes cleavages to give rise to an eight-cell embryo by 2.5 dpc. Blastomeres then become compacted and polarized, forming a 16-cell embryo (morula), and subsequently the blastocyst. Differentiation of the blastomeres sets up embryonic and extra-embryonic lineages via formation of the inner cell mass (ICM), which gives rise to the embryo, and the trophectoderm, which gives rise to the placenta²⁴. NBC involves injection of ESCs into 3.5 dpc blastocysts. (B) Schematic overview of the NBC organogenesis approach. Blastocyst injection of mouse ESCs into genetically programmed blastocysts restores forebrain structures in resulting chimeras. Schematic modified from⁶. (C) CRISPR/Cas9-based gene targeting enables rapid engineering and testing of complex genotypes, which can then directly be analyzed in vivo. Mouse brain schematics were generated based on a reference image from the Allen Mouse Brain Atlas.