Osteocytic vs. anosteocytic bone

Summary

When comparing mammalian osteocyte-containing bone with anosteocytic (“acellular”) fish bone, it is noteworthy that anosteocytic bone is capable of bone remodeling processes and participation in the organism’s mineral homeostasis - two features that are attributed to osteocytic cells. As there is an evolutionary drift towards anosteocytic skeletons throughout teleost fish, osteocytic routes may either not been necessary anymore or it simply indicates that there are other non-osteocytic pathways. The lack of osteocytes in spite of osteoid produced by osteoblasts and bone matrix deposition in anosteocytic fish presumes that some mechanisms prevent the transformation of osteoblasts to osteocytes. The anosteocytic bone may form osteon-like structures, adapt to changing loading regimes and is subject to osteoclastic bone resorption. Considering the multiple types of osteocytic and anosteocytic bone matrices among teleost fish, the availability of mononucleated osteoclasts capable of bone resorption, high bone mineral density and an stabilizing mineralized notochord sheath, are only some observations that need to be studied further. Of note, there is evidence that intermediate stages between osteocytic and anosteocytic bone are present in some species. In fact, the European eel is one example that presents with effective multinucleated osteoclasts, small and few osteocyte lacunae with unknown network characteristics. Further investigation and imaging of osteocytic and non-osteocytic osseous structures will help to provide novel understanding of these essential characteristics in bone biology.

Zusammenfassung

Vergleicht man den zellulären Knochen von Säugetieren mit azellulärem Knochen in Fischen, so ist es bemerkenswert, dass azellulärer Knochen an Knochenumbaumechanismen sowie auch an der Mineralhomöostase des Organismus beteiligt ist– zwei Charakteristiken, die generell mit Osteozyten in Verbindung gebracht werden. Echte Knochenfische (Teleostei) zeichnen sich durch eine evolutionäre Entwicklung von azellulärem Knochen aus. Das bedeutet, dass Osteozyten in ihrer Funktion entweder nicht mehr notwendig waren oder es tatsächlich noch andere Kommunikationswege im azellulären Knochen gibt. Das Fehlen von Osteozyten trotz einer funktionierenden Osteoid- und Knochenmatrixdeposition durch Osteoblasten lässt annehmen, dass Mechanismen vorhanden sind, die den Übergang von Osteoblasten zu Osteozyten verhindern. Azellulärer Knochen ist in der Lage, osteonartige Strukturen zu formen, sich veränderten Belastungen anzupassen sowie osteoklastäre Knochenresorption zuzulassen. Berücksichtigt man die große Vielfalt von zellulären und azellulären Knochenformen unter den Teleostei, sind das Vorhandensein von resorptionsfähigen mono nukleären Osteoklasten, eine hohe Knochenmineralisation sowie die stabilisierende mineralisierte Chorda dorsalis nur einige Besonderheiten, die weiter erforscht werden sollten. Es gibt Anzeichen, die für evolutionäre Stadien zwischen zellulärem und azellulärem Knochen sprechen. So zeigt der Europäische Aal Anguilla anguilla effektive Knochen resorption durch Osteoklasten und wenige kleine Osteozytenlakunen mit unbekanntem Netzwerkcharakter. Die Untersuchung und Bildgebung von osteozytären und nicht osteozytären Strukturen auf den verschiedenen hierarchischen Knochenebenen werden weiter helfen, diese für die Knochenbiologie zu Grunde liegenden Mechanismen besser zu verstehen.

Keywords

Acellular bone - osteocyte - bone remodeling - teleost fish

Schlüsselwörter

Azellulärer Knochen - Osteozyt - Knochenremodeling - Teleostei

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