Cephalic flexure formation in the chick embryo.

The cephalic flexure, found in all vertebrate brains, is a ventrally directed bend through the mesencephalon, and a ventral bulging and elongation of the prosencephalon. Most sources say the cephalic flexure is caused by differential growth. We have measured the changing angle of flexure through time and find that flexure occurs between chick embryo stages 10 to 15. We measured, during these stages, the lengths, thicknesses, and volumes of the floor and roof of the mesencephalon and of the prosencephalon. As expected, during flexure the mesencephalic roof elongates much more than the floor. Both roof and floor increase in thickness, and mesencephalic roof volume increases twice as much as floor volume. However, prosencephalon, which does not bend, also has differential growth between roof and floor, but the growth is taken up in complex changes of shape other than flexure. There are sufficient numbers of mitoses in the brain to account for the observed tissue growth, assuming accompanying cell enlargement. We deleted brain parts adjacent to the mesencephalon before flexure and the mesencephalon bent, so migration of cells from or to these adjacent parts to contribute to the differential growth of the mesencephalon is unlikely. We reduced cerebrospinal fluid pressure during flexure by explanting heads to the chorioallantoic membrane, or into New cultures. The mesencephalon of explanted heads bends, but the prosencephalon fails to elongate. We conclude that differential growth may be necessary for mesencephalic flexure in the chick embryo, but other factors that decide the disposition of the products of growth in space must determine the shape.