Jacques Castanet

Assessing a relationship between bone microstructure and growth rate: a fluorescent labelling study in the king penguin chick (Aptenodytes patagonicus)

SUMMARY Microstructure–function relationships remain poorly understood in primary bone tissues. The relationship between bone growth rate and bone tissue type, although documented in some species by previous works, remains somewhat unclear and controversial. We assessed this relationship in a species with extreme adaptations, the king penguin (Aptenodytes patagonicus). These birds have a peculiar growth, interrupted 3 months after hatching by the austral winter. Before this interruption, chicks undergo extremely rapid statural and ponderal growth. We recorded experimentally (by means of fluorescent labelling) the growth rate of bone tissue in four long bones (humerus, radius, femur and tibiotarsus) of four king penguin chicks during their fastest phase of growth (3–5 weeks after hatching) and identified the associated bone tissue types (`laminar, `longitudinal, `reticular or `radial fibro-lamellar bone tissue). We found the highest bone tissue growth rate known to date, up to 171 μm day–1 (mean 55 μm day–1). There was a highly significant relationship between bone tissue type and growth rate (P<10–6). Highest rates were obtained with the radial microarchitecture of fibro-lamellar bone, where cavities in the woven network are aligned radially. This result supports the heuristic value of a relationship between growth rate and bone primary microstructure. However, we also found that growth rates of bone tissue types vary according to the long bone considered (P<10–5) (e.g. growth rates were 38% lower in the radius than in the other long bones), a result that puts some restriction on the applicability of absolute growth rate values (e.g. to fossil species). The biomechanical disadvantages of accelerated bone growth are discussed in relation to the locomotor behaviour of the chicks during their first month of life.

Phylogenetic Signal in Bone Microstructure of Sauropsids

In spite of the fact that the potential usefulness of bone histology in systematics has been discussed for over one and a half centuries, the presence of a phylogenetic signal in the variation of histological characters has rarely been assessed. A quantitative assessment of phylogenetic signal in bone histological characters could provide a justification for performing optimizations of these traits onto independently generated phylogenetic trees (as has been done in recent years). Here we present an investigation on the quantification of the phylogenetic signal in the following bone histological, microanatomical, and morphological traits in a sample of femora of 35 species of sauropsids: vascular density, vascular orientation, index of Haversian remodeling, cortical thickness, and cross-sectional area (bone size). For this purpose, we use two methods, regressions on distance matrices tested for significance using permutations (a Mantel test) and random tree length distribution. Within sauropsids, these bone microstructural traits have an optimal systematic value in archosaurs. In this taxon, a Mantel test shows that the phylogeny explains 81.8% of the variation of bone size and 86.2% of the variation of cortical thickness. In contrast, a Mantel test suggests that the phylogenetic signal in histological traits is weak: although the phylogeny explains 18.7% of the variation of vascular density in archosaurs, the phylogenetic signal is not significant either for vascular orientation or for the index of Haversian remodeling. However, Mantel tests seem to underestimate the proportion of variance of the dependent character explained by the phylogeny, as suggested by a PVR (phylogenetic eigenvector) analysis. We also deal with some complementary questions. First, we evaluate the functional dependence of bone vascular density on bone size by using phylogenetically independent contrasts. Second, we perform a variation partitioning analysis and show that the phylogenetic signal in bone vascular density is not a by-product of phylogentic signal in bone size. Finally, we analyze the evolution of cortical thickness in diapsids by using an optimization by squared change parsimony and discuss the functional significance of this character in terms of decreased buoyancy in crocodiles and mass saving in birds. These results are placed in the framework of the constructional morphology model, according to which the variation of a character in a clade has a historical (phylogenetic) component, a functional (adaptive) component, and a structural (architectural) component.

Phylogenetic, functional, and structural components of variation in bone growth rate of amniotes

SUMMARY The biological features observed in every living organism are the outcome of three sets of factors: historical (inherited by homology), functional (biological adaptation), and structural (properties inherent to the materials with which organs are constructed, and the morphogenetic rules by which they grow). Integrating them should bring satisfactory causal explanations of empirical data. However, little progress has been accomplished in practice toward this goal, because a methodologically efficient tool was lacking. Here we use a new statistical method of variation partitioning to analyze bone growth in amniotes. (1) Historical component. The variation of bone growth rates contains a significant phylogenetic signal, suggesting that the observed patterns are partly the outcome of shared ancestry. (2) Functional causation. High growth rates, although energy costly, may be adaptive (i.e., they may increase survival rates) in taxa showing short growth periods (e.g., birds). In ectothermic amniotes, low resting metabolic rates may limit the maximum possible growth rates. (3) Structural constraint. Whereas soft tissues grow through a multiplicative process, growth of mineralized tissues is accretionary (additive, i.e., mineralization fronts occur only at free surfaces). Bone growth of many amniotes partially circumvents this constraint: it is achieved not only at the external surface of the bone shaft, but also within cavities included in the bone cortex as it grows centrifugally. Our approach contributes to the unification of historicism, functionalism, and structuralism toward a more integrated evolutionary biology.

Lines of arrested growth in bone and age estimation in a small primate: Microcebus murinus

Abstract In primates, age determination using lines of arrested growth (LAGs) from bones has rarely been attempted, andthe reliability of these structures has never been experimentally validated. In order to test skeletochronology inprimates, LAGs were studied mainly in the long bones of known age Microcebus murinus , a small primate, whosepotential longevity may reach 12 years. LAGs were extensively studied in 43 males and 23 females ranging fromjuveniles to 11-year-old adults. All individuals were born and reared in captivity. Some young individuals wereinjected with fluorescent dyes to quantify bone growth rates. LAGs in the diaphysis of the tibia are well correlatedwith age and this skeletal element appears to be the best for assessing skeletochronology in Microcebus murinus .There is strong evidence that the seasonal cycle of photoperiodicity is more important than age alone in producingLAGs. Key words : age determination, skeletochronology, primates, Microcebus murinus INTRODUCTION

Variation of the Outer Circumferential Layer in the Limb Bones of Birds

Abstract. n The core of the limb bone cortex of mammals and birds is made of rapidly deposited, fibro-lamellar bone tissue (also present in non-avian theropods), which is usually surrounded by an avascular outer circumferential layer (OCL) of slowly deposited parallel-fibered bone. We present the first comparative allometric study of the relative OCL thickness (expressed as a fraction of the diaphyseal radius) in modern birds. Body size explains 79% of the OCL variation in thickness, which is inversely correlated with size, that is, shows negative allometry (slope -0.799). This may explain the apparent absence of OCL in the ratites. Since the OCL is deposited at the end of growth, we propose that its relative thickness probably correlates with the amount of slow, residual growth, which our results suggest to be on the average larger in small birds.

The ‘message’ of bone tissue in paleoherpetology

Abstract Some aspects of the historical development of paleohistology in vertebrate paleontology are briefly reviewed, with emphasis on studies of fossil amphibian and reptile bone tissues, as compared with thoses of extant forms. A consideration of bone tissue diversity and of its likely determinism lead to its value as a direct tool in systematics beeing questioned in both extant and extinct forms. Rather, it is suggested that its variations may be used as a biological recorder of ontogenies sensu lato, offering data on species or populations‐specific life history traits and even hints at their larger scale evolution among extinct lineages.

Relationship between bone growth rate and the thickness of calcified cartilage in the long bones of the Galloanserae (Aves).

The histological features of mineralized tissues can be preserved for hundreds of millions of years, and are therefore important potential sources of information for reconstructing the life history traits of extinct species. Bone growth rates and the duration of the growth period have recently been estimated in fossil archosaurs from periosteal ossification (a mechanism responsible for bone diametral growth). Similarly, data on endochondral ossification (the mechanism responsible for bone longitudinal growth) may also yield information on growth duration and rate among extinct vertebrates, as long as potentially informative structures are preserved. However, in order to carry out palaeobiological estimations of growth rate and/or the duration of growth, it is first necessary to quantify in extant species the relationship between these life history traits and the histological features of endochondral ossification that are potentially preserved in the fossil record. Here we analyse the ontogenetic variation of both bone longitudinal growth rate and the thickness of the calcified cartilage in the femora of two Galloanserae (Aves) and find a significant positive relationship between these variables in both species. We discuss possible factors underlying interspecific differences in this relationship, and conclude that it could be applied with caution to draw palaeobiological inferences.