Does Birth Trigger Cell Death in the Developing Brain?

Advancing birth by 1 d advances cell death. A, Timeline of experimental procedures. B, C, Cell death was very similar between the on-time and advanced groups when compared by postnatal age (left), but not by postconception age (right) in both the CA1or and NAcc. Asterisks indicate significant differences by Bonferroni post hoc tests: **p < 0.01, ***p < 0.001, ****p < 0.0001. In addition, cell death did not differ between pups with spontaneous advanced birth versus those with advanced birth after RU-486 treatment of the dam (Extended Data Fig. 1-1). D, E, Photomicrographs of AC3-labeled tissue (counterstained with thionin) in representative sections from the on-time and advanced birth groups in the CA1or and NAcc. Arrowheads point to cells shown at higher magnification in the insets. Peak cell death occurred at P1, despite the fact that this was 20 dpc for on-time births and 19 dpc for advanced births; ac, anterior commissure; MS, medial septum; ne, neuroepithelium. Scale bars = 100 μm (main photograph) and 20 μm (insets). F, Advancing birth by a day did not change the magnitude of cell death over the first three postnatal days in the CA1or, but decreased cell death in the NAcc; ***p = 0.0001. G, RU-486 did not alter cell death in fetuses 3 h after treatment of the dam in either the CA1or or NAcc. Means ± SEM are plotted for all figures except for F, which shows estimated marginal means ± SEM.

Delaying birth by a day does not delay cell death. A, Timeline of experimental procedures. B, C, Patterns of cell death in on-time versus delayed birth groups best match when plotted as a function of postconception age (right), rather than postnatal age (left) in both the CA1or and NAcc. Asterisks indicate significant differences on each day by Bonferroni tests: **p < 0.01, ****p < 0.0001. D, E, Photomicrographs of AC3-labeled tissue (counterstained with thionin) in representative sections of mice from the on-time and delayed birth groups in the CA1or and NAcc. Arrowheads point to cells shown at higher magnification in the insets. Peak cell death occurred at 20 dpc, despite the fact that this was P1 for the on-time birth group and P0 for the delayed-birth group; ac, anterior commissure; MS, medial septum; ne, neuroepithelium. Scale bars = 100 μm (main photograph) and 20 μm (insets). F, Delaying birth by a day increased cell death magnitude overall in the CA1or but did not affect the magnitude of cell death in the NAcc; ***p = 0.0007. G, Progesterone treatment of the dams increased progesterone levels in fetuses (as described in Results) but did not affect cell death in either brain region. Means ± SEM are plotted for all figures except for F, which shows estimated marginal means ± SEM.

Cell death declines between P0 and P1 in pups born 2 d late. Cell death in the CA1or (A) and NAcc (B) was higher on P0 than on P1 (p = 0.04 and p = 0.02, respectively) in pups born 2 d late (on 21 dpc) to dams treated with progesterone on days 17 and 18 postconception. This outcome is in contrast to that seen for mice born on-time (where cell death in the CA1 and NAcc is low on P0 and peaks on P1; Figs. 1, 2B,C ) but is expected if a developmental program governs cell death when birth occurs late; N = 3–6 mice per group. Means ± SEM are plotted, *p < 0.05.

Overall brain growth is determined by postconceptional age. FB growth was best predicted by postconception age (right), rather than postnatal age (left) whether birth was advanced (A) or delayed (B). Because mice with spontaneous advanced births differed from mice with advanced birth after RU-486 treatment on this measure, the data for those groups are plotted separately (dotted and hashed lines, respectively), as well as combined (solid lines). However, the comparison between on-time and advanced groups remained significant whether considering spontaneous only, treated only, or all advanced births (p < 0.0001 in each case). Asterisks indicate significant differences on each day by Bonferroni post hoc tests. Brackets indicate main effect of group. **p < 0.01, ***p = 0.001, ****p < 0.0001. Means ± SEM are plotted.