Apoptosis is involved in maintaining the character of the midbrain and the diencephalon roof plate after neural tube closure.

Apoptosis, a major form of programmed cell death, is massively observed in neural plate border and subsequently in the roof plate (RP). While deficiency of apoptosis often results in brain malformations including exencephaly and hydrocephalus, the impact of apoptosis on RP formation and maintenance remains unclear. Here we described that mouse embryos deficient in Apaf1, a gene crucial for the intrinsic apoptotic pathway, in C57BL/6 genetic background exhibited narrow and discontinuous expression of RP marker genes in the midline of the midbrain and the diencephalon. Instead, cells positive for the neuroectodermal gene SOX1 ectopically accumulated in the midline. A lineage-tracing experiment suggests that these ectopic SOX1-positive cells began to accumulate in the midline of apoptosis-deficient embryos after E9.5. These embryos further displayed malformation of the subcommissural organ, which has been discussed in the etiology of hydrocephalus. Thus, the apoptosis machinery prevents ectopic emergence of SOX1-positive cells in the midbrain and the diencephalon RP, and helps in maintaining the character of the RP in the diencephalon and midbrain, thereby ensuring proper brain development.

Local apoptosis modulates early mammalian brain development through the elimination of morphogen-producing cells.

Apoptotic cells are observed in the early developing brain. Apoptosis deficiency is proposed to cause brain overgrowth, but here we show that brain malformations in apoptosis-deficient mutants are due to insufficient brain ventricle expansion as a result of uncompleted cranial neural tube closure. Apoptosis eliminates Fgf8-expressing cells in the anterior neural ridge (ANR), which acts as an organizing center of the forebrain by producing FGF8 morphogen. Deficiency of apoptosis leads to the accumulation of undead and nonproliferative cells in the ventral part of the ANR. The undead cells in apoptosis-deficient mutants express Fgf8 continuously, which perturbs gene expression in the ventral forebrain. Thus, apoptosis within a specific subdomain of the ANR is required for correct temporal elimination of an FGF8-producing region within a limited developmental time window, thereby ensuring proper forebrain development.