library_construction_protocol: E3.5 blastocysts, zona pellucida was removed passing the embryos by mouth pipetting through three consecutive drops of Tyrode's acidic solution pH 2.5 (Sigma T1788). After careful examination under the microscope for disintegration of the pellucida, tyrode's solution was immediately neutralized with a double volume of PBS pH 7.4 (Gibco #10010023), and embryos were placed into 20 μL of dissociation buffer composed of TrypLE™ Express Enzyme (Gibco # 12604013) / Accutase (Gibco # A1110501) in a 1:1 ratio, in a low binding 200 μL microcentrifuge tube. To avoid attachment of the embryos to the pulled crystal capillaries, capillaries were coated with 1% BSA in PBS beforehand. Blastocysts were incubated for 20 minutes in dissociation buffer at 37ºC, pipetted up and down gently 60 times with 20 μL low binding pipette tips, incubated 10 additional minutes at 37ºC and pipetted up and down again gently 40 times to disaggregate the last ICM cell clumps. After dissociation, the resulting cell suspension was neutralized with 40uL of 1% cell culture grade BSA (Sigma # A8806). In the case of 8-cell and morula stage embryos, pellucida was removed as abovementioned and embryos were placed in 20 μL of Accutase (Gibco # A1110501) for 20 min at 37ºC. After incubation, embryos were pipetted up and down 50 times until complete disaggregation and neutralized by the addition of 40 μL of 1% cell culture grade BSA (Sigma # A8806) to avoid the formation of clumps after dissociation.  Then, cell suspensions were immediately placed on ice until processing for scRNA-seq Cell samples were loaded onto a 10x Chromium Single Cell controller chip B (10x Genomics) following manufacturer's intructions (Chromium Single Cell 3'GEM, Library & Gel Bead Kit v3, ref. PN-1000075). Intended targeted cell recovery of ∼1000 - 10000 cells depending on the embryonic stage. Generation of gel beads in emulsion (GEMs), barcoding, GEM-RT clean-up, cDNA amplification and library construction were all performed as recommended by the manufacturer.