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The regionalized vertebral column is a hallmark of mammalian morphology and reflects functional differentiation of the vertebral regions. Mammalian vertebrae are serially homologous and morphologically patterened by Hox expression, but also vary in number and gross morphology across species. The trabecular bone inside vertebral centra is more plastic than gross vertebral bone, and structurally adapts to better withstand forces it experiences during life. However, the functional regionalization of vertebral trabecular bone is poorly examined. Are there trabecular "regions” reflecting the differing functions and in-vivo stress patterns of gross morphological vertebral regions? Or is trabecular morphology homogeneous throughout the spine, suggesting that differences in functional demands are borne exclusively by external characteristics? To address these questions, we collected μCT scans and linear measurements of cervical, thoracic, and lumbar vertebrae in four species of large shrews, including two species of the hero shrew Scutisorex, which has a highly modified vertebral column. We compared linear measurements and trabecular bone characteristics of the cranial and caudal ends of each centrum across species. To detect unique vertebral regions, we executed principal coordinates analysis and segmented regression on three versions of our data set: trabecular bone data only, external measurements only, and the two combined. We found that some regionalization is recovered using only trabecular bone data, but trabecular bone regions do not correspond exactly to gross vertebral regions. This reflects divergence between the functional signals of internal and external vertebral bone morphology, which should be further examined in a kinematic context.
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Deciphering an extreme morphology: bone microarchitecture of the hero shrew backbone (Soricidae: Scutisorex )
Biological structures with extreme morphologies are puzzling because they often lack obvious functions and stymie comparisons to homologous or analogous features with more typical shapes. An example of such an extreme morphotype is the uniquely modified vertebral column of the hero shrew Scutisorex , which features numerous accessory intervertebral articulations and massively expanded transverse processes. The function of these vertebral structures is unknown, and it is difficult to meaningfully compare them to vertebrae from animals with known behavioural patterns and spinal adaptations. Here, we use trabecular bone architecture of vertebral centra and quantitative external vertebral morphology to elucidate the forces that may act on the spine of Scutisorex and that of another large shrew with unmodified vertebrae ( Crocidura goliath ). X-ray micro-computed tomography (µCT) scans of thoracolumbar columns show that Scutisorex thori is structurally intermediate between C. goliath and S. somereni internally and externally, and both Scutisorex species exhibit trabecular bone characteristics indicative of higher in vivo axial compressive loads than C. goliath. Under compressive load, Scutisorex vertebral morphology is adapted to largely restrict bending to the sagittal plane (flexion). Although these findings do not solve the mystery of how Scutisorex uses its byzantine spine in vivo , our work suggests potentially fruitful new avenues of investigation for learning more about the function of this perplexing structure.
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- Award ID(s):
- 1811627
- PAR ID:
- 10191863
- Date Published:
- Journal Name:
- Proceedings of the Royal Society B: Biological Sciences
- Volume:
- 287
- Issue:
- 1926
- ISSN:
- 0962-8452
- Page Range / eLocation ID:
- 20200457
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1098/rspb.2020.0457
