ATF4^−/− fetal livers are hypoplastic, and the ATF-4 gene is expressed at high levels in wild-type fetal livers.

(A) Gross appearance of livers from wild-type mice (left) and ATF4^−/− littermates (right) at 15.5 dpc. The liver from the homozygous mutant mouse is smaller and paler. (B) The mean total number of cells in the fetal liver of ATF4^−/− mice at 15.5 dpc was significantly lower than that in littermates. The mean number of cells in ATF4^−/− fetal liver at 15.5 dpc was 2.70 × 10⁷ ± 0.29 × 10⁷ (n = 15). The values for ATF4^+/+ (n = 13) and ATF4 ^+/−(n = 19) embryos were 6.31 × 10⁷ ± 0.59 × 10⁷ and 6.15 × 10⁷ ± 0.35 × 10⁷, respectively. Thus, ATF4^−/− embryos had a decrease in cells of 2.3 fold and 2.1 fold, respectively, compared with values in ATF4^+/− and ATF4^+/+ embryos (P < .0001 for both comparisons). There was no significant difference between ATF4^+/+ and ATF4^+/− embryos. Error bars represent SEM. (C) In situ hybridization of a midsagittal section from a 11.5-dpc wild-type embryo with an antisense ATF4 cDNA probe (left, bright-field image; and right, dark-field image). Signal from the hybridization appears bright in the dark-field image. The red arrow indicates the position of the fetal liver. Magnification 10 ×. (D) Higher magnification of results from in situ hybridization for ATF4 in a sagittal section of fetal liver from a wild-type embryo. Magnification 100 ×.