Morris VB (2016), Analysis of coelom development in the sea urchi...

ECB-ART-44518

Biol Open 2016 Feb 18;53:348-58. doi: 10.1242/bio.015925.

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Analysis of coelom development in the sea urchin Holopneustes purpurescens yielding a deuterostome body plan.

Morris VB .

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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. 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. 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. 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 plan fits frog and chick development and the echinoderm fossil record, and predicts genes involved in coelomogenesis as the source of deuterostome macroevolution.

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Species referenced: Echinodermata
Genes referenced: LOC100887844 LOC115923516

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	Fig. 1. Larva of H. purpurescens at 29 h. Left-side sagittal view showing the oral coelom, archenteron and blastopore and the anterior, posterior, oral and aboral adult orientation (refer to text). Scale bar: 100 µm.
	Fig. 2. Early development of the coeloms and the enteric channel. (A,D,G,J) Sections of larvae at 27 h and 29 h, uncoloured. (B,E,H,K) The same sections coloured with tissues of the hydrocoele (hy) green, coelomic mesoderm (cm) yellow, enteric channel (ec) red and archenteron (ar) blue. (C,F,I,L) Schematics of the sections with the tissues coloured similarly. A-I, sagittal views; J-L, oral view. ab, aboral; ant, anterior; bl, blastopore; ep, hydrocoele epithelium; le, left; or, oral; pos, posterior; ri, right. Scale bar: 50 µm.
	Fig. 3. Origin of the hydrocoele and the coelomic mesoderm from the archenteron in oral view. 33 h larva progressing from aboral to oral. (A,C,E) Uncoloured sections. (B,D,F). The same sections coloured with tissues of the hydrocoele (hy) green, coelomic mesoderm (cm) yellow, enteric channel (ec) red and archenteron (ar) blue. ant, anterior; le, left; pos, posterior; ri, right. Scale bar: 50 µm.
	Fig. 4. Development of the hydrocoele and the origins of the primary podia. Oral view of a 34 h larva progressing from aboral to oral. (A-F) Uncoloured sections showing the origin of the hydrocoele at the head of the archenteron and the different origins of the C, D and E podia and the A and B podia. (G-L) The same sections with the C,D,E epithelia and podia coloured magenta and the A and B epithelia and podia coloured green. ant, anterior; ec, enteric channel; he, hydrocoele epithelium; hl, hydrocoele lumen; ie, inner epithelium; le, left; pos, posterior; ri, right; sp, space between inner epithelia. The podia are labelled A,B,C,D,E. Scale bar: 50 µm. The Z stack from which this figure was constructed can be accessed at http://hdl.handle.net/2123/14231 at Sydney eScholarship Repository, The University of Sydney.
	Fig. 5. Later development of the coeloms. Oral view of a 39 h larva progressing from aboral to oral. (A,B) Epithelia of the C, D and E podia connect with the left anterior archenteron wall; epithelia of the A and B podia connect with archenteron tissue lying centrally; red arrows mark the boundaries between the AB epithelia and the CDE epithelia; coelomic mesoderm (cm) forms from both sides of the archenteron; the enteric channel (ec) is between the AB epithelia and the coelomic mesoderm forming on the right side of the archenteron. (C-F) The hydrocoele develops lobes (A, B, C, D, E) that will separate into the five primary podia. ant, anterior; aw, archenteron wall; le, left; pos, posterior; ri, right. Scale bar: 50 µm.
	Fig. 6. Composition of the hydrocoele. Sections progress along a Z axis slanted from aboral to oral and from posterior to anterior. (A-D) Aboral sections showing the source of the AB epithelia from a stem-like connexion (A, arrow) to the archenteron (ar); arrowhead in A points to the enteric channel; arrow in D points to the CDE lumen. (E-H) Tissue (F, arrow) spreads across the hydrocoele lumen. (I-L) The tissue spreads towards the outer epithelia of the podia. (M-P) AB podial tissue separates from CDE podial tissue along a line marked by red arrows; outer podial epithelia round in, separating the podia. (A-P) Coelomic mesoderm (cm) spreads orally about the hydrocoele. A, B, C, D, E label the five primary podia; pos, posterior; ri, right. 40 h larva. Scale bar: 50 µm.
	Fig. 7. Track of the enteric channel. Oral view, sections progressing from aboral to oral. White arrows mark the boundary between the A and E epithelia throughout. (A-D) Enteric channel (ec, red arrow) starts at the base of a stem-like connexion of the hydrocoele (hy) to the archenteron (ar). (E-H) A and E epithelia grow over the archenteron; A and B epithelia become distinct. (I-L) Lobes develop from the A and E epithelia. (M-P) An opening develops between the epithelial boundaries of the A and E lobes (green arrow). (Q-T) The opening widens into the blastocoele (bl). (A-P) Coelomic mesoderm (cm) wraps about the archenteron, hydrocoele and hydrocoele lobes. 40 h larva. hl, hydrocoele lumen. Scale bar 50 µm. The Z stack from which this figure was constructed can be accessed at http://hdl.handle.net/2123/14231 at Sydney eScholarship Repository, The University of Sydney.
	Fig. 8. The deuterostome body plan. The plan is constructed from core structures of early coelom development in the larva of H. purpurescens and represents the form of the plan in an echinoderm. Red labels represent the plan in a vertebrate. ar, archenteron; cm, coelomic mesodem; ec, enteric channel; hy, hydrocoele; no, notochord; pm, paraxial mesoderm. Left-side sagittal view.
	Fig. 9. Early coelom development in the larva of Heliocidaris erythrogramma. (A,C) Uncoloured sections of 26 h larvae in sagittal views; A is slightly younger than C. (B,D) The same sections coloured with the putative anlagen of the hydrocoele (hy) green, coelomic mesoderm (cm) yellow, enteric channel (ec) red and archenteron (ar) blue. Arrowhead marks the notch between the hydrocoele and coelomic mesoderm anlagen. ab, aboral; or, oral. Scale bar: 50 µm.

References [+] :

David, How Hox genes can shed light on the place of echinoderms among the deuterostomes. 2014, Pubmed, Echinobase