Remodeling in bone without osteocytes: billfish challenge bone structure-function paradigms

Area of Science:

• Comparative bone biology
• Vertebrate paleontology
• Skeletal tissue adaptation

Background:

• Tetrapod bone remodeling is vital for long-term health, orchestrated by osteocytes.
• Most extant fishes (neoteleosts) lack osteocytes, suggesting limited bone repair.
• High activity and long lifespans in some fish should induce significant bone damage.

Purpose of the Study:

• Investigate bone remodeling in osteocyte-deficient billfish rostral bones.
• Determine if billfish bone exhibits repair mechanisms despite lacking osteocytes.
• Challenge the established role of osteocytes in bone remodeling.

Main Methods:

• Microscopic examination of billfish bone structure.
• Analysis of bone tissue mechanical properties (stiffness, strain tolerance).
• Comparison of billfish bone with mammalian and other fish bone.

Main Results:

• Billfish rostral bone shows intense remodeling with features like secondary osteons, similar to mammals.
• Despite lacking osteocytes, this bone exhibits structural complexity and repair.
• Billfish bone is stiff and withstands high strains, unlike other fish bone.
• Osteons in billfish bone are significantly smaller than in mammals.

Conclusions:

• Fish bone physiology is more complex than previously understood.
• Localized bone repair can occur without osteocytes, suggesting alternative mechanisms.
• The primary role of osteocytes in bone remodeling is challenged.
• Billfish bone demonstrates a unique adaptation for repair and mechanical resilience.