PMC

Points of closure in the mouse embryo and phenotypes of failure of closure of the various points along the neuraxis. (a) Schematic figure illustrating the multiple points of closure of the neural tube, directions of closure, and the different locations of neuropores in the developing embryo. (1), site of closure (1) which occurs at the level of somite 3 in the 6-7-somite embryo. Closure (1) is the initiation event of neurulation. Closure then progresses caudally and is completed by closure of the posterior neuropore (PNP) at the 29-30-somite stage of development; (2), second closure site at around the 10-somite stage; (3), closure (3) site which begins soon after closure (2). Arrows depict spreading of neural tube closure to neighbouring regions with completion of anterior neuropore closure soon after initiation of closure (3) and closure of the hindbrain neuropore at the 18–20-somite stage. (b) Phenotype resulting from failure of closure (1): craniorachischisis; (c) phenotype resulting from failure of closure (2): exencephaly; (d) phenotype of failure of the caudal wave of spinal closure, leading to an enlarged PNP and later development of spina bifida. (A), posterior neuropore; (B), branchial arches; (C), developing heart; (D), hindbrain; (E), midbrain; (F), forebrain; ANP: anterior neuropore; HNP: hindbrain neuropore.

Loss of both, LRP4 and LRP5, leads to impaired cranial neural tube closure. (A–C) At E9.0 and E9.5 (somite stages 20–25) all Lrp4 ^−/− single mutant mouse embryos (B) (n = 5), all Lrp5 ^−/− mutants (C) (n = 3) and all wild-type littermate controls (A) (n = 5) displayed a closed anterior neuropore (ANP). (D) In contrast, 88% of all Lrp4 ^−/− ; Lrp5 ^−/− double mutant embryos (8 out of 9) had an open anterior neuropore (frontal view of whole embryo heads, dotted yellow lines and arrowhead indicate the open ANP, scale bar: 500 µm); (D′) Schematic indicating the open anterior neuropore at E9.5 in black. (E–G) At E10.5 wild-type controls (E) (n = 3), Lrp4 ^−/− embryos (F) (n = 4), and Lrp5 ^−/− embryos (G) (n = 3) displayed normal cross morphology of the forebrain and the ANP was closed (frontal view of whole embryo heads). (H) 80% of the Lrp4 ^−/− ; Lrp5 ^−/− double mutant embryos showed an open anterior neuropore (dotted line and arrowhead) at E10.5 (4 out of 5 embryos). Scale bar: 500 μm. (H′) Schematic indicating the open anterior neuropore at E10.5 in black.

(a) Dorsal view of the E8.5 embryo, (b) partially rotated dorsal, (c) top-down view of the embryo showing the open cranial neuropore, and (d) bottom-up view of the embryo showing the open caudal neuropore.

Stages of mouse cranial neurulation, as seen diagrammatically in whole embryos at (A) E8.5 and (B) E9 (with extra-embryonic membranes removed). At E8.5 (five-somite stage) the neural folds are approaching one other at the site of Closure 1 (arrow in A), whereas the cranial neural folds are still wide apart. Ten hours later, at E9 (ten-somite stage), Closure 1 has occurred and neural fold fusion has also been initiated at Closures 2 and 3 (arrows in B). These closure initiation events define two neuropores where cranial closure will later be completed: the anterior or rostral neuropore (ANP) and the hindbrain neuropore (HNP). A third neuropore (the posterior neuropore, PNP) is present in the spinal region. Somites are shown in yellow.