Growth and mineralogy in dental plates of the holocephalan Harriotta raleighana (Chondrichthyes): novel dentine and conserved patterning combine to create a unique chondrichthyan dentition

Moya Meredith Smith, Charlie Underwood, Christopher Healy, Zerina Johanson, Tomasz Goral

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25 Citations (Scopus)
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Abstract

The dentition in extant holocephalans (Chondrichthyes) comprises three pairs of continuously growing dental plates, rather than the separate teeth characterizing elasmobranchs. We investigated how different types of dentine in these plates, including hypermineralized dentine, are arranged, and how this is renewed aborally, in adult and juvenile dentitions of Harriotta raleighana (Rhinochimeridae). Individual plates were analysed using x-ray computed tomography (µCT), SEM in back scattered mode with EDX and EDR analysis, and optical microscopy on hard tissue sections.

Results
Harriotta dental plates are made entirely of dentine tissue, mostly as trabecular dentine, bone itself being absent. Hypermineralized dentine forms in restricted ovoid and tritor spaces within trabecular dentine, inside a shell of outer and inner dentine layers. Trabecular dentine is ubiquitous but changes to sclerotic osteodentine near the oral surface by increasing density, remaining less mineralized than the hypermineralized dentine. All structures are renewed aborally, within a vascular dental pulp, a tissue suggested to be a source of stem cells for tissue renewal. Ca density profiles and concentrations of Mg, P and Ca ions reveal extreme differences in the level and type of mineralisation. Early mineralization in ovoids and tritors has very high levels of Mg, then a sudden increase in mineralization to a high total mineral content, whereas there is gradual change in trabecular dentine, remaining at a low level.

Conclusions
Hypermineralized dentine fills the prepatterned ovoid, rod and tritor spaces, early at the aboral surface within the trabecular dentine. Deposition of the hypermineralized dentine is from surfaces that are lined with large specialised odontoblasts, within cell body spaces connecting with extensive, ramifying tubules. Early mineralisation occurs amongst this maze of tubules that penetrate far into the dentine, expanding into a mass of saccules and membranous bodies, dominating in the absence of other organic matrix. This early stage has hydroxyapatite, including significant Mg, initiated as a poorly crystalline phase. In the hypermineralized dentine, crystal formation occurs as clusters of variably shaped crystals, forming from a sudden phase transition. In later hypermineralized dentine, high MgO+CaO+P2O5 suggests that almost pure Mg containing tricalciumphosphate (TCP (ß-Ca3(PO4)2) (Mg whitlockite) is present, with little or no hydroxyapatite. Serial replacement of tritors and ovoids may occur within the dental plate, representing a relic of patterning, as classically found in elasmobranch dentitions.
Original languageEnglish
Article number11
Number of pages30
JournalZoological letters
Volume5
Issue number1
DOIs
Publication statusPublished - 12 Mar 2019

Keywords

  • Dental evolution
  • Dental plates
  • Holocephali
  • Hypermineralized dentine
  • Osteodentine
  • Whitlockite

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