ROLE OF DIFFERENTIAL CELL-PROLIFERATION IN THE TAIL BUD IN ABERRANT MOUSE NEURULATION

In the mouse mutant curly tail, the phenotypes spina bifida and curled tail result from a delay in closure of the posterior neuropore (PNP). At the developmental stage when this delay can first be recognized, t he caudal region of the embryo demonstrates a transiently enhanced cur vature of the body axis which likely inhibits elevation, convergence, and fusion of the neural folds. The enhanced curvature is thought to b e the result of a decreased proliferation in the ventrally located gut endoderm and notochord, together with a normal proliferation of the o verlying neuroepithelium of the PNP. However, the proliferation defect and the enhanced curvature were originally demonstrated at the same d evelopmental stage, while it is expected that reduced proliferation sh ould precede enhanced curvature and delayed PNP closure. The caudal re gion originates from the tail bud and we therefore propose that the en hanced curvature is induced by a disturbed dorso-ventral proliferation pattern in the tail bud. Using flow cytometry, proliferation patterns were determined separately for the dorsal and ventral halves of the t ail bud of curly tail and of control embryos as well as of recombinant embryos having the curly tail phenotype with a genetic background whi ch is matched to the BALB/c control strain. In general, it appeared th at about half of the cell cycle duration in tail bud cells was occupie d by S phase, about 40% by G(0)/G(1) and the rest by G(2)/M. For the c ontrol embryos, no dorso-ventral differences in relative phase duratio n were demonstrated. However, curly tail and recombinant embryos at th e 21-25 somite stage, prior to the onset of enhanced curvature, exhibi ted ventrally a higher proportion of G(0)/G(1) phase cells than dorsal ly, and a complementary relationship for S phase cells. We interpret t hese observations as indicating a prolonged G(1) phase at the ventral side of the tail bud, resulting in a prolongation of the cell cycle an d thus a decreased proliferation. In 26-30 somite stage embryos, prior to the normalization of curvature in curly tail embryos, the dorso-ve ntral proliferation balance was re-established. We conclude that a red uced proliferation in the ventral part of the tail bud of the curly ta il embryo precedes both the onset of enhanced curvature and the previo usly observed reduction in proliferation of the hindgut and notochord, and is a likely candidate for an early event in the pathogenetic sequ ence leading to the curly tail phenotype. (C) 1998 Wiley-Liss, Inc.