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Mesdames et Messieurs,Chers collègues,Chers amis, Avant toute chose, je voudrais remercier chaleureusement mes collaborateurs et amis Jorge Cubo et Michel Laurin pour l’organisation du colloque international « Perspectives sur l’évolution des vertébrés, sujets et problèmes » qui a duré trois jours et s’achève aujourd’hui par cette leçon de clôture de ma chaire. Je voudrais y associer toutes celles et tous ceux qui se sont dévoués pour son bon déroulement, au sein de notre laboratoire à Jussie...

International audience; Aepyornis, a giant subfossil ratite from Madagascar, shows a well-preserved bone histology. Hindlimb bones exhibit an extensive histodiversity; the cortex is initially made of fibrolamellar, well-vascularized primary bone that modulates locally into plexiform or laminar patterns. Lines of arrested growth are generally weakly expressed. Haversian reconstruction can be complete. Perimedullar endosteal deposition is variable but can be extensive. The complex causality (phylogenetic, systematic, ontogenetic and functional… factors) involved in the production of the observed data is discussed.; Aepyornis, ratite géant subfossile de Madagascar, montre une histologie osseuse bien préservée. Les os longs des pattes présentent une forte diversité histologique ; l’os primaire des corticales est initialement du type général fibrolamellaire, fortement vascularisé selon des patrons plexiformes ou laminaires. Les lignes d’arrêt de croissance sont généralement peu exprimées. Le remaniement haversien peut être complet. Le dépôt endostéal périmédullaire est variable, mais peut être très important. La causalité complexe (facteurs phylogénétiques, systématiques, ontogénétiques, fonctionnels…) pouvant rendre compte des structures observées est abordée.

International audience; Research on the bone histology of extant and extinct animals has a long scientific history and an accurate description of microstructural tissues is the cornerstone of the field. Ideally, terminology needs to convey as much information as possible about the structural, developmental, and functional aspects of bone tissues corresponding to the up-to-date knowledge of the time. However, current terms are not always consistent with new observations and advances in the field of bone biology. We provide a brief overview of some ambiguities and their origins, and suggest a new approach of bone tissue classification and description that is congruent with our current understanding of bone as a living tissue, emphasizing its developmental aspects. This approach requires the introduction of a new term, namely ‘woven-parallel complex’, for describing a broad range of complex bone tissue types, including intramembranous and endochondral bones, and different types of primary as well as secondary bone tissues. We reconsider the classical concept of fibrolamellar complex, which we place within the new developmental approach. Finally, using non-archosaurian archosauromorphs as an example group, we demonstrate how the new approach can be utilized in an evolutionary context. The present study demonstrates the relevance and constant evolution of technical terminology along with the advances of the field of science.

International audience; To date, little is known about the structureof the cells and the fibrillar matrix of the globuli ossei,globular structures showing histochemical properties of anosseous tissue, sometimes found in the resorption front ofthe hypertrophied cartilage in many tetrapods, and easilyobserved in the long bones of the Urodele Pleurodeleswaltl. Here, we present the results obtained from theappendicular long bones of metamorphosed juveniles andsubadults using histological and histochemical methodsand transmission electron microscopy. The distal part ofthe cone-shaped cartilage contains a heterogeneous cellpopulation composed of the typical “light” hypertrophicchondrocytes and scarce “dark” hypertrophic chondrocytes.The “dark” chondrocytes display ultrastructural character-istics suggesting that they probably undergo degenerationthrough chondroptosis. However, in the hypertrophic, cal-cified cartilage close to the erosion front by the marrow,several noninvaded chondrocytic lacunae retained cellsthat do not show any morphological characteristics ofdegeneration and that cannot be identified as regularchondrocytes or osteocytes. These modified chondrocytesthat have lost their regular morphology, appear to beactive in the terminal cartilage and synthesize collagenfibrils of a peculiar diameter intermediate between theType I collagen found in bone and the Type II collagencharacteristic of cartilage. It is suggested that the localoccurrence of globuli ossei is linked to a low rate of longitu-dinal growth as is the case in the long bones of postmeta-morphic urodeles.

International audience; The presence of a phylogenetic signal in the variation of osteohistological features has been recently debated in the literature. Previous studies have found a significant signal for some features, but these results were obtained on a small amount of characters and a reduced sample. Here we perform a comprehensive study in which we quantify the phylogenetic signal on 62 osteohistological features in an exhaustive sample of palaeognathous birds. We used four different estimators to measure phylogenetic signal – Pagel's λ, Abouheif's Cmean, Blomberg's K, and Diniz-Filho's phylogenetic eigenvector regressions PVR – and four topologies taken from the literature. Bone size and bone vascular density exhibit a strong phylogenetic signal, whereas all but four of the remaining features measured at the histological level – cellular size in caudal and medial transects of femora, and proportion of oblique vascular canals in rostral and caudal transects of tibiotarsi – exhibit a weaker signal. We also found that the impact of the topologies used in the analyses is very low compared with that of sample size. We conclude that the analysis of a comprehensive sample is crucial to obtain reliable quantifications of the phylogenetic signal.

Biological functions, at all integrative levels of biology, are readily understood as the justification (or explanation) of the actual occurrence of the structures involved into their execution. This brings back directly to Aristotelian " final causes " and it is why modern epistemologists have struggled to offer concepts of function rid of any finalism. The two current answers, known as the etiological and systemic concepts of function respectively echo Mayr's distinction between historical and functional biology (1961). Those two aspects of biology convey either the historical (phylogenetic) or functional (mechanistic) modes of explanations for every actual structuro-functional occurrences. Each aspect is set within distinct epistemological (experimental/nomological vs. comparative/historical) frameworks, dealing with the analysis of proximate and ultimate causes, respectively . A third point of view on causal explanation of form/function relationships was reintroduced in modern science by Seilacher (1970) and Gould, (2002) namely a structuralist (or constructional) input which mainly deals with the constraints (eg. topological) imposed by the materials organisms are built from. Accordingly, three general aspects of causation, historical, functional and structural, may be involved together in the origin of any form/function relationship in biology. Integrating them should bring satisfactory causal explanations of empirical data. However, little progress has been accomplished in practice towards this goal, because a methodologically efficient tool was lacking. We have applied a new statistical method which allows partitioning the variation of a given biological trait among the three causal factors evoked above to analyze growth function in bone. 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. Resting metabolic rate may determine the maximum possible bone growth rate. 3/ Structural constraint. While soft tissues grow through a multiplicative process, growth of mineralised tissues is accretionary (additive, i.e. it occurs only at free surfaces). Bone growth of many amniotes partially circumvents this topological constraint by new bone being laid down not only at the external surface of the bone shaft, but also within cavities included in the bone cortex as it grows centrifugally. The three causal components interfere a great deal: i.e. the functional causation is in part phylogenetically structured. Accordingly, it becomes possible now to decipher quantitatively the complex causality of the structure/function conundrum.

Dans le présent ouvrage, l'enquête a été limitée aux usages biologiques et technologiques du concept de fonction. Elle est en outre de part en part marquée par les débats philosophiques vigoureux, mais très pauvrement connus en langue française, qui se sont développés à l'échelle internationale dans la seconde partie du XXe siècle sur le sens même du mot " fonction ", sur les attributions fonctionnelles et sur l'explication fonctionnelle.

International audience

Omniprésente dans l'ensemble des sciences de la vie, la catégorie de fonction a les allures d'un scandale épistémologique : attribuer une fonction à une structure ou à un processus biologique, c'est en effet suggérer qu'on l'explique par ses effets. Cet ouvrage, sans précédent en langue française, examine les débats philosophiques contemporains sur les fonctions depuis une trentaine d'années, et propose de nouvelles voies d'analyse. Il confronte ces débats à l'usage de la notion de fonction dans un large spectre de disciplines biologiques, psychologiques et médicales. Il soulève aussi la question de savoir si cette notion, aussi ancienne dans l'histoire des techniques que dans celle des sciences de la vie, y a le même sens.