Tyrone L. Daulton

Mineral Distributions at the Developing Tendon Enthesis

Tendon attaches to bone across a functionally graded interface, “the enthesis”. A gradient of mineral content is believed to play an important role for dissipation of stress concentrations at mature fibrocartilaginous interfaces. Surgical repair of injured tendon to bone often fails, suggesting that the enthesis does not regenerate in a healing setting. Understanding the development and the micro/nano-meter structure of this unique interface may provide novel insights for the improvement of repair strategies. This study monitored the development of transitional tissue at the murine supraspinatus tendon enthesis, which begins postnatally and is completed by postnatal day 28. The micrometer-scale distribution of mineral across the developing enthesis was studied by X-ray micro-computed tomography and Raman microprobe spectroscopy. Analyzed regions were identified and further studied by histomorphometry. The nanometer-scale distribution of mineral and collagen fibrils at the developing interface was studied using transmission electron microscopy (TEM). A zone (∼20 µm) exhibiting a gradient in mineral relative to collagen was detected at the leading edge of the hard-soft tissue interface as early as postnatal day 7. Nanocharacterization by TEM suggested that this mineral gradient arose from intrinsic surface roughness on the scale of tens of nanometers at the mineralized front. Microcomputed tomography measurements indicated increases in bone mineral density with time. Raman spectroscopy measurements revealed that the mineral-to-collagen ratio on the mineralized side of the interface was constant throughout postnatal development. An increase in the carbonate concentration of the apatite mineral phase over time suggested possible matrix remodeling during postnatal development. Comparison of Raman-based observations of localized mineral content with histomorphological features indicated that development of the graded mineralized interface is linked to endochondral bone formation near the tendon insertion. These conserved and time-varying aspects of interface composition may have important implications for the growth and mechanical stability of the tendon-to-bone attachment throughout development.

The nanometre-scale physiology of bone: steric modelling and scanning transmission electron microscopy of collagen–mineral structure

The nanometre-scale structure of collagen and bioapatite within bone establishes bones physical properties, including strength and toughness. However, the nanostructural organization within bone is not well known and is debated. Widely accepted models hypothesize that apatite mineral (‘bioapatite’) is present predominantly inside collagen fibrils: in ‘gap channels’ between abutting collagen molecules, and in ‘intermolecular spaces’ between adjacent collagen molecules. However, recent studies report evidence of substantial extrafibrillar bioapatite, challenging this hypothesis. We studied the nanostructure of bioapatite and collagen in mouse bones by scanning transmission electron microscopy (STEM) using electron energy loss spectroscopy and high-angle annular dark-field imaging. Additionally, we developed a steric model to estimate the packing density of bioapatite within gap channels. Our steric model and STEM results constrain the fraction of total bioapatite in bone that is distributed within fibrils at less than or equal to 0.42 inside gap channels and less than or equal to 0.28 inside intermolecular overlap regions. Therefore, a significant fraction of bones bioapatite (greater than or equal to 0.3) must be external to the fibrils. Furthermore, we observe extrafibrillar bioapatite between non-mineralized collagen fibrils, suggesting that initial bioapatite nucleation and growth are not confined to the gap channels as hypothesized in some models. These results have important implications for the mechanics of partially mineralized and developing tissues.

No evidence of nanodiamonds in Younger–Dryas sediments to support an impact event

The causes of the late Pleistocene megafaunal extinctions in North America, disappearance of Clovis paleoindian lithic technology, and abrupt Younger–Dryas (YD) climate reversal of the last deglacial warming in the Northern Hemisphere remain an enigma. A controversial hypothesis proposes that one or more cometary airbursts/impacts barraged North America ≈12,900 cal yr B.P. and caused these events. Most evidence supporting this hypothesis has been discredited except for reports of nanodiamonds (including the rare hexagonal polytype) in Bølling–Ållerod-YD-boundary sediments. The hexagonal polytype of diamond, lonsdaleite, is of particular interest because it is often associated with shock pressures related to impacts where it has been found to occur naturally. Unfortunately, previous reports of YD-boundary nanodiamonds have left many unanswered questions regarding the nature and occurrence of the nanodiamonds. Therefore, we examined carbon-rich materials isolated from sediments dated 15,818 cal yr B.P. to present (including the Bølling–Ållerod-YD boundary). No nanodiamonds were found in our study. Instead, graphene- and graphene/graphane-oxide aggregates are ubiquitous in all specimens examined. We demonstrate that previous studies misidentified graphene/graphane-oxide aggregates as hexagonal diamond and likely misidentified graphene as cubic diamond. Our results cast doubt upon one of the last widely discussed pieces of evidence supporting the YD impact hypothesis.

Radiation-induced diamond formation in uranium-rich carbonaceous materials

Nanometer-sized diamonds were identified by transmission electron microscopy in a uranium-rich, coal-like carbonaceous assemblage of Precambrian age. This observation, together with estimates of formation efficiencies, supports the hypothesis that diamond can form in carbonaceous material irradiated by the radioactive decay products of uranium. The results also suggest that the formation of carbonados cannot be sufficiently explained by a radiation mechanism alone.

Arguments and Evidence Against a Younger Dryas Impact Event

We present arguments and evidence against the hypothesis that a large impact or airburst caused a significant abrupt climate change, extinction event, and termination of the Clovis culture at 12.9 ka. It should be noted that there is not one single Younger Dryas (YD) impact hypothesis but several that conflict with one another regarding many significant details. Fragmentation and explosion mechanisms proposed for some of the versions do not conserve energy or momentum, no physics-based model has been presented to support the various concepts, and existing physical models contradict them. In addition, the a priori odds of the impact of a >4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are infinitesimal, about one in 10 15 . There are three broad classes of counterarguments. First, evidence for an impact is lacking. No impact craters of the appropriate size and age are known, and no unambiguously shocked material or other features diagnostic of impact have been found in YD sediments. Second, the climatological, paleontological, and archeological events that the YD impact proponents are attempting to explain are not unique, are arguably misinterpreted by the proponents, have large chronological uncertainties, are not necessarily coupled, and do not require an impact. Third, we believe that proponents have misinterpreted some of the evidence used to argue for an impact, and several independent researchers have been unable to reproduce reported results. This is compounded by the observation of contamination in a purported YD sample with modern carbon.

Suspect cubic diamond “impact” proxy and a suspect lonsdaleite identification

The presence of nanometer-sized diamonds in purported Younger Dryas (YD) boundary-dated sediments, carbon spherules, and Greenland ice was cited as evidence of a YD impact event (1). Although cubic and hexagonal (lonsdaleite) diamond have been found in shocked metamorphosed meteorites and are associated with terrestrial impact structures, cubic diamonds are well known to occur in terrestrial deposits that have no associations with impact processes. For example, submicron and smaller cubic diamond crystals have been found recently in carbonaceous spherules isolated in upper soils from various German and Belgian sites (2). Lacking links to impact structures, these diamonds are evidently not products of impact …

Incomplete Bayesian model rejects contradictory radiocarbon data for being contradictory

Kennett et al. (1) apply a Bayesian chronological model in an effort to support the hypothesis of Firestone et al. (2) that “a major cosmic episode of multiple airbursts/impacts occurred at 12,800 ± 300 [B.P.].” Bayesian modeling is a powerful tool because it is intended to incorporate and account for all available evidence. However, Kennett et al. (1) do not include radiocarbon data by Boslough et al. (3) and others in their new analysis because they found it contradictory, undermining their own objectives. Moreover, Kennett et al. (1) dismiss issues raised by the key data they omitted for being contradictory rather than incorporating it their Bayesian model.

The Nano-Physiology of Mineralized Tissues

The nanostructures of bone and partially mineralized tissues determine the toughness (Buehler, 2007) and stiffness (Genin et al., submitted) of these tissues. In the attachment of tendon to bone, tissue compositions and possibly nanostructures vary spatially in concert with microscopic and macroscopic variations in tissue shape, presumably to improve load transfer from tendon to bone (Thomopoulos et al., 2006). We hypothesize that undesirable stress concentrations resulting from a failure to recreate the details of this spatial grading following surgical healing may underlie the low levels of success of surgeries to repair tendon-to-bone attachments such as the rotator cuff (Galatz, 2001). Therefore, a detailed understanding of the gradients in composition and structure of the natural tendon-to-bone attachment as well as an understanding of the mechanisms of their development are critical for our efforts to synthesize surgical grafts that augment tendon-to-bone healing. As a first step towards understanding the nanometer-scale details of the tendon-to-bone attachment, we studied the nanostructure of bone using steric modeling and scanning transmission electron microscopy (STEM).© 2009 ASME

The multiscale structural and mechanical effects of mouse supraspinatus muscle unloading on the mature enthesis

The musculoskeletal system is sensitive to its loading environment; this is of particular concern under conditions such as disuse, paralysis, and extended-duration space flight. Although structural and mechanical changes to tendon and bone following paralysis and disuse are well understood, there is a pressing need to understand how this unloading affects the bone-tendon interface (enthesis); the location most prone to tears and injury. We therefore elucidated these effects of unloading in the entheses of adult mice shoulders that were paralyzed for 21 days by treatment with botulinum toxin A. Unloading significantly increased the extent of mechanical failure and was associated with structural changes across hierarchical scales. At the millimeter scale, unloading caused bone loss. At the micrometer scale, unloading decreased bioapatite crystal size and crystallographic alignment in the enthesis. At the nanometer scale, unloading induced compositional changes that stiffened the bioapatite/collagen composite tissue. Mathematical modeling and mechanical testing indicated that these factors combined to increase local elevations of stress while decreasing the ability of the tissue to absorb energy prior to failure, thereby increasing injury risk. These first observations of the multiscale effects of unloading on the adult enthesis provide new insight into the hierarchical features of structure and composition that endow the enthesis with increased resistance to failure. STATEMENT OF SIGNIFICANCE: The musculoskeletal system is sensitive to its loading environment; this is of particular concern under conditions such as disuse, paralysis, and extended-duration space flight. Although changes to tendon and bone following paralysis are understood, there is a pressing need to clarify how unloading affects the bone-tendon interface (enthesis), which is the location most prone to tears and injury. We elucidated the effects of enthesis unloading in adult mice shoulders showing, for the first time, that unloading significantly increased the risk and extent of mechanical failure and was associated with structural changes across hierarchical scales. These observations provide new insight into the hierarchical features of structure and composition that endow the enthesis with resilience. This knowledge can be used to develop more targeted treatments to improve mobility and function.

Did Nanodiamonds Rain from the Sky as Woolly Mammoths Fell in their Tracks Across North America 12,900 Years Ago?

1. Institute for Materials Science and Engineering, Washington University, St. Louis, MO, USA. 2. Department of Physics, Laboratory for Space Sciences, Washington University, St. Louis, MO, USA. 3. Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, UK. 4. Department of Geography, University of Portsmouth, Portsmouth, UK. 5. Department of Earth and Planetary Sciences, University of California Davis, Davis CA, USA. 6. School of Earth Sciences and Environmental Sustainability, Northern Arizona U., Flagstaff, AZ, USA.