ReviewVariation in osteocytes morphology vs bone type in turtle shell and their exceptional preservation from the Jurassic to the present
Graphical abstract
Highlights
► Three different morphologies of osteocytes are recognized in turtle bone. ► Osteocyte morphology is ruled by bone type in the turtle shell. ► Preservation of osteocytes in fossil bone is independent of geologic time. ► We show preservation of osteocytes in turtles from 150 Ma to Present.
Introduction
Bone is the result of phylogenetic, functional, and structural influences [1] evidenced in the hierarchical levels of bone tissue: macrostructure (cancellous and cortical bone), microstructure (Haversian systems, osteons, and lamellae), and nanostructure (mineral, collagen, and non-collagenous proteins) [2]. Of the three types of cells comprising bone, osteocytes are the most abundant, making up 95% of all cells in bone ([3] and references therein), yet little is known of osteocyte biology and function. Recent studies have begun to elucidate the role of osteocytes in bone formation, bone function, bone maintenance and bone pathology [4], [5], [6], [7], [8], [9], [10], [11], [12], but many questions regarding the fundamental biology of these cells remain. Issues that are still poorly understood include: 1) the potential variation in the morphology of osteocytes in bones with different origins (intramembranous vs endochondral) and their different roles in vertebrate body plans, (e.g., do osteocytes function differently in long bones vs bony flat plates); 2) what temporal limits exist on osteocyte preservation in the bony matrix, and whether preservation is dependent on taxon, bone type, geologic time, depositional environment or other factors; and 3) if these cells persist and can be shown to be endogenous to the organisms, can chemical/molecular analyses of these remnants shed light on the physiology, phylogeny, and/or ecology of extinct organisms across geological time. Turtles are an ideal organism to inform on these issues, because their bony shell is unique among vertebrates, in that their carapace is endochondral in origin, while the plastron has an intramembranous origin [13] (see Graphical abstract). Additionally, their shell consists of bones with three well differentiated bone types or layers [14], and a robust and continuous fossil record extending approximately up to 230 million years (Ma).
Osteocyte lacunae and associated lacunocanalicular network (LCN) have been described in many fossil specimens [15], [16], [17], including non-avian dinosaurs from the Late Cretaceous (80 Ma) of Mongolia. Two different morphs of these structures have been identified: flattened-oblate and stellate [18]. More recently, three dimensional osteocytes and blood vessel morphs have been isolated from the bone matrices of various Mesozoic (dinosaurs) and Cenozoic (mammals-birds) vertebrates [19], [20], but an in-depth examination of turtle bone for such preservation has not been previously conducted.
Here we describe osteocytes from four extant turtle taxa: sea turtle Caretta caretta (Cryptodira, Chelonidae), box turtle Terrapene carolina (Cryptodira, Testudinidae), freshwater turtle Trachemys scripta (Cryptodira, Emydidae), and freshwater side-necked turtle Podocnemis expansa (Pleurodira, Podocnemididae). We then examine the preservation of osteocytes in Cenozoic and Mesozoic fossils closely related to these extant species, and describe morphological variation in osteocytes from each of the three layers of bone that forms the shell—external cortex (EC), cancellous bone (CB), and internal cortex (IC). Finally, we compare these morphologies with osteocytes recovered from long bones (femora and humeri).
Section snippets
Sampling
Complete skeletons of each extant species were donated for research from the Amphibian and Herpetological collections from the North Carolina Museum of Natural Sciences. We studied carapace and plastron elements of C. caretta, Tr. scripta, and P. expansa, as well as the right femur of T. carolina and C. caretta. Specimens used for comparing osteocyte morphology are described in the caption of Fig. 4.
Bone histology
Eleven bone thin sections (Table 1) were taken following the procedure described in Ref. [21],
Osteocyte morphology in turtles
Two morphologically distinct types of osteocytes can be identified in ground sections of extant and extinct turtle bone or in isolated osteocytes after demineralization of each of these morphs specifically associated with variations in bone tissue (Fig. 1, Fig. 2, Fig. 3). These two morphologies are also conserved through ontogeny for each bone type, and for both shell elements (carapace and plastron), as is supported from hatchling, juvenile, and adult specimens of Mongolemys elegans from the
Discussion
Turtle osteocytes retain the typical stellate morphology mentioned by Ref. [22], also reported in non-avian dinosaurs [16] and other vertebrates [23], [24]. One notable difference, however is that FO1 osteocytes (Fig. 1a) are more abundant in the turtle shell than in any other vertebrate, possibly due to the proximity of the internal cortex to the soft body of the turtle. The very flat shape of FO1 osteocytes observed in this layer of lamellar bone may be the result of the density and
Acknowledgments
This work was supported by National Science Foundation grants OISE 0638810, EAR 0642528, and EAR 0824299, Geological Society of America Southeastern section Graduate Student Research Grant 2010 the Smithsonian Institution, the Panama Canal Authority, Mr. Mark Tupper, and SENACYT. Thanks to one anonymous reviewer for helpful comments. For access to samples of modern and fossil bones, thanks to B. Stuart and V. Schneider (North Carolina Museum of Natural Sciences, Raleigh, NC, USA). For support
References (34)
- et al.
Mechanical properties and the hierarchical structure of bone
Med Eng Phys
(1998) - et al.
The organization of the osteocyte network mirrors the extracellular matrix orientation in bone
J Struct Biol
(2011) - et al.
Serial FIB/SEM imaging for quantitative 3D assessment of the osteocyte lacuno-canalicular network
Bone
(2011) - et al.
MT1-MMP modulates the mechanosensitivity of osteocytes
Biochem Biophys Res Commun
(2012) - et al.
Distinguishing between archaeological sheep and goat bones using a single collagen peptide
J Archaeol Sci
(2010) The bigger the C-value, the larger the cell: genome size and red blood cell size in vertebrates
Blood Cells Mol Dis
(2001)- et al.
Phylogenetic, functional, and structural components of variation in bone growth rate of amniotes
Evol Dev
(2008) - et al.
Oxygen tension is an important mediator of the transformation of osteoblasts to osteocytes
J Bone Miner Metab
(2007) The amazing osteocyte
J Bone Miner Res
(2011)- et al.
AFM analysis of the lacunar–canalicular network in demineralized compact bone
J Microsc
(2011)
The biological function of DMP-1 in osteocyte maturation is mediated by its 57-kDa C-terminal fragment
J Bone Miner Res
The three-dimensional morphometry and cell–cell communication of the osteocyte network in chick and mouse embryonic calvaria
Calcif Tissue Int
Mechanical loading stimulates BMP7, but not BMP2, production by osteocytes
Calcif Tissue Int
Hepatocyte growth factor is crucial for development of the carapace in turtles
Evol Dev
A comparatie histological study of fossil and recent bone tissues
Tex J Sci
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