Bending of the neural plate during mouse spinal neurulation is independentof actin microfilaments

To examine the role of actin microfilaments in mouse spinal neurulation, we stained cryosections of E8.5-10.5 CBA/Ca embryos with FITC-phalloidin, Mic rofilaments are present in the apical region of all cells throughout the ne uroepithelium, irrespective of whether they are involved in bending of the neural plate. Disruption of the microfilaments with cytochalasin D inhibite d closure of the cranial neural folds in cultured embryos, even at the lowe st concentrations tested, and prevented the initiation of spinal neurulatio n (Closure 1) at higher concentrations. In contrast, closure of the posteri or neuropore was resistant to cytochalasin D at the highest concentrations tested, Phalloidin staining and transmission electron microscopy confirmed that cytochalasin D is effective in disassembling microfilaments in spinal neuroepithelial cells. We conclude that spinal neural tube closure does not require microfilament function, in contrast to cranial neurulation which i s strongly microfilament-dependent. Histological examination of cytochalasi n D-treated embryos revealed that bending at hinge points, both in the midl ine (MHP) and dorsolaterally (DLHPs), continues in the absence of microfila ments, whereas the rigidity of non-bending regions of the neural plate is l ost. This suggests that spinal neurulation can continue in the presence of cytochalasin D largely as a result of intrinsic bending of the neural plate at hinge points. Cytochalasin D treatment is a useful tool for revealing t he localisation of hinge points in the neural plate, Analysis of treated em bryos demonstrates a transition, along the spinal axis, from closure solely involving midline bending, at high levels of the spinal axis, to closure s olely involving dorsolateral bending, low in the spinal region. These findi ngs support the idea of mechanistic heterogeneity in mouse neurulation, alo ng the body axis, and demonstrate that contraction of actin microfilaments is not obligatory for epithelial bending during embryonic morphogenesis. (C ) 1999 Wiley-Liss, Inc.