The role of microfilaments in cranial neurulation in rat embryos: effects of short-term exposure to cytochalasin D.

During the late stages of cranial neurulation in mammalian embryos, the neural epithelium becomes concave. A thick subapical band of microfilament bundles, attached to junctions which are both vertical and horizontal in orientation, can be seen by TEM. Prior to this the neural epithelium is first biconvex and then V-shaped in transverse section, microfilament bundles are absent, and the subapical junctions are only vertical in orientation. In order to determine the role of microfilaments in cranial neurulation, rat embryos were exposed to cytochalasin D (0.15 micrograms ml-1) for 1 h at three stages of development: convex neural fold stage, early concave (prior to midline apposition at the forebrain/midbrain junction: 'preapposition') and later concave ('postapposition'). They were subsequently washed and cultured in addition-free medium for 5, 12, 24 or 36 h, then examined alive and by LM, TEM, or SEM. The degree of neural fold collapse varied with the stage of development: at the convex stage there was only slight opening out of the neural groove; early concave (preapposition) neural folds collapsed laterally to a horizontal position; later concave (postapposition) neural folds showed widening of the midbrain/hindbrain neuropore and slight neuroepithelial eversion at the anterior neuropore. Neural epithelium which had been concave prior to cytochalasin D treatment changed in structure so that the cells were broader and shorter; most of the subapical junctions were vertical in orientation, and microfilament bundles were represented either as a mass of amorphous material adjacent to the junctions, or as separated and broken filaments. Re-elevation of neural folds in 'recovery' cultures was accompanied by regeneration of apical microfilament bundles and horizontal junctions. Embryos which had been exposed to cytochalasin D at the convex or later concave stage of cranial neural fold development were able to complete cranial neural tube closure; none of the early-concave-stage embryos achieved apposition at the forebrain/midbrain junction, and all had major cranial neural tube defects. The results suggest that contraction of apical microfilament bundles plays an essential role in elevation of the neural folds and in the generation of concave curvature during the later stages of cranial neurulation. During the convex neural fold stage, microfilaments are important in maintaining neuroepithelial apposition in the neural groove, but are not crucial to maintenance of the convex shape. Successful formation and maintenance of the forebrain/midbrain apposition point at the appropriate time is considered to be essential for subsequent brain tube closure.