PRECISE TISSUE BOUNDARY FORMATION AND THE GROWTH PATTERN OF THE EARLY ANIMALS

Epithelium is one of the basic types of animal tissue which line the surfaces throughout the body. Here we show the cell forms evolutionary trend within the epithelium of the worms during 515ma - 470ma. The period saw the Cambrian explosion and the Ordovician biodiversification. Our study demonstrates the “micro evolution” events at cellular and tissue levels. The linear boundary cells separate each annulation of the worm indicate a precise gene expression pattern (precise in a single cell) have been established some half a billion years ago. Meanwhile, the tension filed showing the ground state of the cell polygons minimize their potential energy to support their structural and functional integrity. However, the asymmetry stress field of the cuticle plates suggests their continuous allometry growth type in spite of the worms would renew its cuticle periodically. Therefore the early worms may employ both molting and accelerate growth simultaneously. The structure mechanical property suggest animals tissue growth is a trade off of the structure robustness and plasticity.