| Abstract: | The embryonic brain was dissected in urodele amphibians at the early postneurulation stages. Tangential mechanical tensions were shown to exist in the embryonic brain. The reaction of neuroepithelial cells characterizing the topology of tensions was found by the use of dissections in two interperpendicular directions. The cells were oriented along the acting force in the case of unidirectional tension. In the case of two interbalanced tensions the cells were inclined along the lines of force of greater tension. Three types of tangential tensions were revealed which differ in force, direction, range of action and life time. The life times of tangential tensions were shown to depend on their force and range of action. The strongest tensions were short-lived and, besides, limited in space. Weak tensions were long-lived and spread all over the brain. In all cases of dissections the cells inducing tangential tensions reacted in the same way: by elongation of cell bodies along the normals to the brain layer. It is suggested that the tendency of cell elongation can cause the tangential tension of the layer. It was found that the partial removal of tensions enhances the curvatures of brain layers. The cells reacted to the tension removal in accordance with their position in the layer. If the cells are located in the grooves, they are shortened. If the cells are outside the grooves, they are elongated. It was found that after the tension was removed the nuclei migrated along the cell bodies. The migration of the nuclei depends on the direction of the layer flexure. The nuclei always migrated to the external surface of evagination or to the internal surface of flexure. It is suggested that the tangential tensions stabilize the changes in the brain shape. |
