Asymmetric Endoderm Proliferation Correlates with Morphogenesis of Early Murine Embryos

Two critical morphological changes that take place in murine embryos, axial rotation and closure of the body and gut, occur concurrently at approximately 8.5 days postcoitum. Closure of the midgut region of gut tube occurs as the embryo rolls onto its left side. It has been suggested that the roof of the open gut acts as a hinge as the tube closes around it in a similar manner to the mechanism suggested for neural tube closure. Proliferation patterns support this by presenting a significantly lower roof proliferation index (PI) at 5 axial levels of gut in 8.5 d.p.c. mice. Gut roof cells are closest to notochord which raises the possibility that notochord induces differences in endoderm cells as it does in the basal plate of the neural ectoderm. A recently suggested model for axial rotation states that lateral asymmetries in cell proliferation could push the rotation of the embryo. In this study a right>left asymmetry (R. PI = 50% & L. PI = 43%) was found in the walls of the open midgut region of endoderm. The combination of these data with previous somatopleure data which showed a left>right asymmetry (Anat. Rec. 250, 1998) leads to the conclusion that the opposition of the forces created by the asymmetrical growth could create a torque on the embryo which could help push the embryo through the process of axial rotation.

American Zoologist 38:48A (1999)