The writings are an intermix of what the ancestor was like and what the path of evolution might have been. The morphological homology of the deuterostome phyla was approached in the past using similarities between larval forms and similarities in the arrangement of coeloms, as well as by applying characters common to, or characteristic of the phyla. Then secondly, accepting a hypothesis of duplication, morphological homology between the echinoderm, hemichordate and chordate body plans will be, predictably, in the similarities between the structures of one echinoderm arm or ray and the anterior-posterior axial structures of hemichordates and chordates. Duplications are in agreement with the fossil record of the co-existence of bilateral and three and five-rayed echinoderms during the Cambrian ( Smith et al., 2013). This is so, first because the pentameral body plan can be explained by duplications of the four columns of plates arising from one Jackson's growth zone ( Morris, 2012). It is this unit of four columns of plates and Jackson's growth zone that is key to understanding the pentameral body plan of echinoderms. The columns of four plates grow from a zone at the tips of the arms or rays that is referred to as Jackson's growth zone. Each interambulacral region was thus split into two. (1994) referred back to Jackson who in 1912 described each arm or ray as a set of four columns of plates: the two central columns were ambulacral columns and each outer column was a single interambulacral column. The ambulacra are the radial canals and tube feet and the interambulacra are the plates and tissues between them ( Hyman, 1955). Traditionally, there is a further division into five ambulacra separated by five interambulacra. The body plan of echinoderms is in general pentamerous, that is to say it is made up of five arms or rays and its structure and evolutionary origin are better understood when treated as a pentameral body plan rather than, as it is often described, a radial plan. The plan fits frog and chick development and the echinoderm fossil record, and predicts genes involved in coelomogenesis as the source of deuterostome macroevolution. Applied to the phyla, the medial coelom is the hydrocoele in echinoderms, the notochord in chordates and the proboscis coelom in hemichordates: the lateral coeloms are the coelomic mesoderm in echinoderms, the paraxial mesoderm in chordates and the lateral coeloms in hemichordates. The deuterostome body plan thus has a single axial or medial coelom and a pair of lateral coeloms, all surrounding an enteric channel, the gut channel. The analysis shows an early separation into a medial hydrocoele and lateral coelomic mesoderm with an enteric channel between them before the hydrocoele forms the pentameral plan of five primary podia. This review covers progress toward understanding many of the molecular mechanisms underlying this sequence of morphogenetic events.An analysis of early coelom development in the echinoid Holopneustes purpurescens yields a deuterostome body plan that explains the disparity between the pentameral plan of echinoderms and the bilateral plans of chordates and hemichordates, the three major phyla of the monophyletic deuterostomes. Remarkably, each event seamlessly occurs at the right time to orchestrate formation of the primitive body plan. Multiple cell biological processes conduct each of these movements and in some cases the upstream transcription factors controlling this process have been identified. As part of the anterior remodeling, mesodermal coelomic pouches bud off the lateral sides of the archenteron tip.
The archenteron then remodels to establish the three parts of the gut, and at the anterior end, the gut fuses with the stomodaeum to form the through-gut. At the end of archenteron extension, a second wave of EMT occurs to release immune cells into the blastocoel and primordial germ cells that will home to the coelomic pouches. Shortly thereafter, invagination of the archenteron occurs. The first of these movements is an epithelial-mesenchymal transition (EMT) of skeletogenic mesenchyme cells, then EMT of pigment cell progenitors.
One outcome of that specification is the expression of transcription factors that control each of the many subsequent morphogenetic changes. The cells participating in sea urchin gastrulation are specified early during cleavage. An extraordinarily simple and elegant process of gastrulation is observed in the sea urchin embryo. This process provides the basic embryonic architecture, an inner layer separated from an outer layer, from which all animal forms arise. Gastrulation is arguably the most important evolutionary innovation in the animal kingdom.
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