Deletion of Mthfd1l does not affect apoptosis during early cranial neural tube closure (7–13 somite stages), nor at the time of completion of neural tube closure (18–24 somite stages). Straight line in whole embryos indicates the level of sections. Sections were stained for caspase 3 and DAPI for nuclei. In the merged images, caspase 3 staining is red and DAPI staining is blue. Contrast was increased here to enhance image clarity for publication. (a, b) Representative sections from Mthfd1l+/+ (wild-type) and Mthfd1lz/z (nullizygous) embryos at 7–13 somite stages. (c, d) Representative sections from Mthfd1l+/+ (wild-type) and Mthfd1lz/z (nullizygous) embryos at 18–24 somite stages. (e, f) The apoptotic index (number of cleaved caspase 3-positive cells per section) was determined at the level of developing hindbrain (to the level of first branchial arch). n = three wild-type and four nullizygous embryos (e); n = five wild-type and four nullizygous embryos (f). Three to four sections were averaged per embryo. p value was calculated by Mann–Whitney U test