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SCPP Gene Evolution and the Dental Mineralization Continuum
K. Kawasaki and
K.M. Weiss*
Department of Anthropology, Pennsylvania State University, 409 Carpenter Building, University Park, PA 16802, USA
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Figure 1. Vertebrate phylogeny and the evolution of skeletal mineralization. Many extinct vertebrates (dashed lines) split from our ancestral lineage after the modern jawless vertebrates (lampreys and hagfish), but before cartilaginous fish (Chondrichthyes). The most ancient mineralized tissue has been found in conodonts. Subsequent jawless vertebrates evolved a dermal skeleton. The tooth arose before the divergence of cartilaginous fish. Two WGDs are thought to have taken place, first, in the stem jawless vertebrates (WGD1) and, second, in the stem jawed-vertebrates (WGD2). SPARCL1 arose from SPARC through the WGD2. SCPP genes originated from SPARCL1, but have been found only in teleosts and tetrapods to date. The initial SCPP gene arose before the divergence of ray-finned fish and lobe-finned fish. The vertebrate phylogeny is based on Donoghue et al.(2006).
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Figure 2. SCPP gene clusters in the human (A), chicken (B), frog (C), fugu (D), and zebrafish (E) genomes. Location and transcriptional orientation of SPARCL1 (open box), P/Q-rich SCPP (closed box), acidic SCPP (shaded box), and ARHGAP6 (dashed-line box) genes are schematically illustrated. (A,C) AMEL is located within ARHGAP6 on human chromosome X and in the frog genome. (A) Two large SCPP gene clusters, enamel/milk/saliva and dentin/bone classes, are separated by 17 megabases (Mb). (D) Linkage of SPP1 and the other SCPP genes is unknown. (E) SPP1 and the other SCPP genes reside on different chromosomes. SCPP2-SCPP5 and SPARCL1-SCPP1 clusters are separated by 10 Mb by an intrachromosomal re-arrangement. These results are based on the following sequence assemblies: hg18 (human), galGal3 (chicken), xenTrop2 (frog), fr2 (fugu), and danRer5 (zebrafish).
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Figure 3. Tooth formation. (A) Tooth with enamel. Through the reciprocal interactions between dental epithelial cells and the underlying ectomesenchymal cells, odontoblasts (Od) develop from the ectomesenchymal cells. As odontoblasts retreat centripetally (shaded arrow), these cells deposit the ECM proteins for dentin between the odontoblast layer and the dental epithelial cell layer. Subsequently, ameloblasts (Am), now fully differentiated from the dental epithelial cells, lay down the ECM proteins for enamel on the mineralizing dentin surface. Enamel thus grows centrifugally away from the pulp (closed arrow). As enamel grows to its final thickness, enamel structural proteins are degraded and resorbed by ameloblasts, and further deposition of calcium and phosphates facilitates enamel maturation. (B) Tooth with enameloid. The histogenesis of teleost teeth is considerably different from that of tetrapod teeth covered with enamel. Initially, the ECM proteins for enameloid, not dentin, are secreted by both the inner dental epithelial (IDE) cells and odontoblasts and deposited below the basal lamina abutting the IDE cells (closed arrow). While the activity of the IDE cells diminishes, odontoblasts continue to retreat centripetally (shaded arrow) and deposit ECM proteins for dentin. Major ECM proteins secreted by ameloblasts, odontoblasts, and the IDE cells are shown at the bottom. Note that the deposition of AMTN and ODAM is known only at the enamel maturation stage.
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Journal of Dental Research, Vol. 87, No. 6,
520-531 (2008)
DOI: 10.1177/154405910808700608
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