Cellular mechanisms in the morphogenesis of the sea urchin larva: The formation of arms

Abstract

On the basis of analyses of time-lapse films, an account is given of the cellular basis of the formation of arms in sea urchin larvae. It is concluded, from a comparison between elongating and non-elongating arms, that the force producing arm elongation is connected with growth of the skeleton. There is, on the other hand, no evidence of autonomous elongation. Growth of the skeleton results in tension in the arm ectoderm, resulting in its extension. The tension is transmitted to the ectoderm by means of a plug of mesenchyme cells attached to the skeleton tip and constantly following its extension. Consequently, when the plug is absent and the skeleton tip projects strongly, no extension occurs. In addition to extension by the skeleton, the pattern of arm formation depends on an apparent invagination and anchoring of the ectoderm in various regions. The future regions of arm formation can be localized at an early stage by the accumulation of pigment cells. It is concluded that the pattern of arm formation is largely determined by the ectoderm.