[Microfolds in the suprablastoporal zone of the frog gastrula and their relationship to the geometry and mechanics of gastrulation].

Spatial distribution and orientation of microfolds arising during invagination of the outer layer of suprablastoporal zone into the blastopore dorsal lip and changes of the lip shape were studied in Rana ridibunda embryos using statistical analysis of a normal individual variability. Active invagination of the cells into the lip correlated with deviation of the orientation of microfolds from the normal in the points of their intersection with the zone of dorsal lip inflection and their orientation is normalized upon transition of the cells across the inflection zone. Frequency distribution of the angle of microfold deviation from the normal is close to the exponential and, therefore, the angle of deviation is an analog of the potential energy of cells-components of the microfold: the bigger the deviation angle, the higher the potential energy. The minimum potential energy is observed at the normal orientation of microfolds, i.e., when it coincides with the radius of the dorsal lip curvature at the point of intersection with the microfold. The following mechanism of dorsal lip formation has been proposed: equatorial contraction of cells upon their invagination into the dorsal lip causes deviation of cell flux orientation from the normal orientation and the normal orientation is restored through an increase in the local curvature of dorsal lip. When the orientation of cell fluxes is normalized, invagination of cells in the dorsal lip ceases. The wave of normalization overtakes the wave of cell invagination into the dorsal lip at the lip angle length 120 degrees. At this moment, the archenteron roof is mechanically detached from the superficial cells of the suprablastoporal zone and lateral blastopore lips and this determines separation of the presumptive notochord.