Developmental pathways of periodontal tissue regeneration
Authors: Ripamonti Ugo 
Published: September 2018

Nothing is known on the impact of developmental divergence on periodontal tissue regeneration in vertebrate animals. Molecularly, the induction of tooth morphogenesis is highly conserved deploying across animal phyla a constant and reproducible set of gene pathways, which result in morphogenesis of multiple odontode forms and shapes. Genetic mutations positively affect animal speciation via evolving biting and masticatory forces as well as dietary habits selectively imprinted in animal phyla during evolutionary speciation. The geometry of the attachment apparatus of a tooth is important for the interpretation of the induction of cementogenesis with de novo Sharpey's fibres as in thecodonty, ie, a tripartite attachment of alveolar bone, periodontal ligament and cementum. This review addresses the tooth implantation in different animal clades from the fibrous attachment of the Elasmobranch Carcharinus obscurus dusky shark, reviewing the evolution and functional significance of cementum with functionally inserted Sharpey's fibres. In sharks there is a continuous tooth replacement mechanistically supported by the continuously erupting dental lamina. We show that the arching of the continuously erupting dental lamina, a critical step for the selachians’ tooth differentiation, is prominently characterized by transforming growth factor‐β3 (TGF‐β3) expression not only within the dental lamina but also in cellular condensations in the mesenchymal tissues of the erupting tooth. Such findings indicate the pleiotropic multifaceted activity of a highly conserved mammalian gene across genera, masterminding tooth morphogenesis in both selachians and mammals as well as periodontal tissue induction in the non‐human primate Papio ursinus. In P. ursinus, the induction of cementogenesis entails the expression of TGF‐β3 and osteocalcin with fine‐tuning and regulation of bone morphogenetic proteins BMP‐2 and BMP‐7, and upregulation of TGF‐β3. TGF‐β3 autoinduction and upregulation during the induction of cementogenesis and osteogenesis in P. ursinus provide novel insights into the induction of cementogenesis. It is hypothesized that the evolutionary expression and upregulation of the TGF‐β3 gene may provide the mechanistic insights into the induction of extensive cementogenesis as seen in stem mammals and the induction of trabecular‐like cementum formation in mosasaurs’ tooth attachment. Aspidin, the precursor of cementum, was reported to appear 310‐330 million years ago (Ma) in Odontostraci armoured fish. Studies showed that the differentiation of cementum with inserted Sharpey's fibres is also present in lower amniotes such as Diatectomorpha or Diadectidae, the first herbivorous tetrapods, 323 Ma. In mosasaurs, 168‐165 Ma, there is the induction of extensive trabeculation of cementum though nothing is known on the phylogenetic temporo‐spatial evolution of cementum before Diadectidae and stem mammals. The large trabeculations of cementum as seen in the attachment of extinct mosasaurs invocates a pleiotropic capacity of cemental growth previously unknown. The appearance of cementum facing a vascularized and innervated periodontal ligament space with Sharpey's fibres inserting on to mineralized cementum provides a multiform pleiotropic masticatory apparatus adapted to multiple biting and lacerating forces as well as finely tuned and controlled forces beyond mastication and deglutition. The remarkable cementogenesis as seen in stem mammals but particularly in mosasaurs with cemental trabeculations across the ligament space invocates the developmental capacity of cementum. The large cemental trabeculations as seen in mosasaurs and the cemental growth in stem mammals, together with regenerating scenarios in P. ursinus with large seams of cellular cementum and cementoid populated by contiguous cementoblasts indicate the continuous molecular cross‐talk between cementum, newly formed cementoid matrix, cementoblasts and extracellular matrix soluble molecular signals. This molecular cross‐talk may control the biomolecular homeostasis of both cementum and periodontal ligament, including angiogenesis. A further molecular scenario is invocated by the tight and exquisite anatomical relationships between the cementoid surfaces and the newly formed capillaries. The primitiveness of the craniate masticatory mineralized craniofacial apparatus has been controlled by several yet ancestral common genes not lastly the TGF‐β3 gene. The TGF‐β3 might have been responsible for the induction of cementogenesis not only in extant P. ursinus but also in Diatectomorpha and mosasaurs, thus providing continuous evolutionary mechanisms for the induction of tissue morphogenesis across animal phyla for almost a billion years of evolution, epitomizing Nature's parsimony in controlling tissue induction and morphogenesis. TGF‐β receptor II regulates osterix expression via Smad‐dependent pathways indicating that TGF‐β signalling acts as an upstream regulator of osterix during cementoblast differentiation. The presence of morphogenetic signals within the cemental matrix capable of inducing bone formation needs now to be assigned: bone induction initiated by extracted and partially purified cemental matrices may be the result of a slow release of embryonic remnants of osteogenic signals required and deployed during cementogenesis. The cementum may thus rule the periodontal ligament space homeostasis, remodelling and repair by releasing sequestered morphogenetic signals that were deployed during embryogenesis.