The aim of this short review is to emphasize the richness of the comparative histological studies on both fossil and extant Osteichthyes. Some selected examples in both Sarcopterygii (excluding tetrapods) and Actinopterygii show how it is possible to improve our knowledge on bone biology of extinct species but also to obtain new data on their palaeobiology or on their paleobiogeography. After a brief survey of the organization of bony tissues in osteichthyes, we review some examples of skeletal peculiarities in the following extinct and extant taxa: the histological structure of polypterid scales that suggests a hypothesis on the possible age and the biogeographical history of this basal actinopterygian taxon; the ossified lung of the fossil coelacanthids, with a discussion on its potential function; the histological organization of the sarcopterygian derived elasmoid scales (of
Le but de cette courte revue est de mettre en valeur la richesse des études histologiques comparatives de l’os des Ostéichthyens fossiles et actuels. Quelques exemples choisis chez les Sarcoptérygiens (excluant les tétrapodes) et les Actinoptérygiens, montrent qu’il est possible d’enrichir significativement les connaissances sur l’histologie osseuse des espèces disparues, mais aussi d’obtenir de nouvelles données de paléobiologie ou de paléobiogéograhie pour ces espèces. Après un rappel de l’organisation générale des tissus osseux chez les Ostéichthyens, nous présentons quelques exemples appropriés de tissus squelettiques chez des taxons fossiles et actuels suivants : une étude de la structure des écailles des Polyptères, qui permet de poser une hypothèse sur l’âge et l’histoire biogéographique de ce taxon basal d’Actinoptérygiens ; les parois ossifiées du poumon des coelacanthes fossiles, avec une discussion sur le rôle possible de cet organe ; la structure histologique des écailles élasmoïdes dérivées chez les Sarcoptérygiens (
It has long been technically difficult to analyse mineralised structures at the histological level, i.e., the cellular and extracellular structures of bony tissues and associated calcified tissues in Osteichthyes (Actinopterygii and Sarcopterygii, part of which, Tetrapoda, is not covered here), like in tetrapods. The first technical difficulty was to remove the mineral component to be able to use the classical paraffin microscopy. Studying bone after removal of mineral, which is the essential component of bone, appears as an aberration. Even if techniques for sectioning undecalcified material (adapted from petrographic techniques) are relatively old (
As osteichthyan bones have mineral components, the skeleton can fossilize, allowing morphological and histological studies of extinct taxa (
In the present study, I want to give some results acquired from palaeohistological studies but with reference to extant ostechthyan material. Effectively, going back and forth between the fossil material and the extant species is a necessity to improve the interpretation of the observations on the histological organization of extinct species. It is also required to gain an evolutionary perspective on bone biology and, maybe, more generally on osteichthyan palaeobiology, considering that mineralised bony tissues record the influences of biological and/or external events that have accompanied the animals during their lives.
To understand the nature and the function of the skeleton in extant as well as in extinct Osteichthyes, including the Teleostei that include more than half of the extant vertebrate species, it is necessary to give some fundamental explanations about bony tissues and their derivatives.
The osteichthyan bony tissue is a connective tissue enriched with type I collagenous fibres that mineralise. The mineral component of bone is a calcium phosphate that crystallizes as hydroxyapatite.
The cells that synthesize the bone substance are the osteoblasts; when they are embedded in the bone matrix, they become osteocytes that reside in an osteocytic lacuna (
Let us take a look at the extant Holostei. Indeed, these osteichthyans have a cellular bone with well-differentiated osteocytes; but they also show special cells, the cells of Williamson that rest at the surface of the bone and send a cytoplasmic extension housed in a specific canalicule into the osseous tissue (
The collagen is deposited by the osteoblasts as thin microfibrils that are packed to form fibres that are clearly recognizable with a transmission electron microscope, even in some very well preserved fossil material ( an unordered intermingled network; successive strata in which the fibres have the same orientation and are parallel to each other; successive strata whose orientation of the fibres differs between two successive strata with an angle of about 90° (
These three specific types of arrangement are respectively called woven-fibered bone (
The mineral component is generally about 60% of the dry weight of the bone tissue in Osteichthyes (
The progression mode of mineralization in isopedin of teleostean scales (but also in some other taxa) is noteworthy as it involves Mandl's corpuscles (
The bony tissue of Osteichthyes is a metabolically active tissue that must receive nutrients. Osteocytes take on this trophic role at the cellular level. However, when the bone is thick, especially as in cortical bone, bony tissue is, in a way, “supplied” by blood vessels that bring metabolites into the most internal areas of bone (
Osteichtyan bony tissue can occasionally show very thin linear chromophilic structures (about 1 μm thick) (
Generally, in one individual, bony tissue differs from one bone to another or, even, between several areas of the same bone. It can also change all along the life of the animal, as in tetrapods. Bony tissue characteristics can also vary from one species to the next. Because of these numerous bony-tissue characteristics, histologists have constructed typological classifications of bone. Subsequently, they have realized that these typologies may have a functional significance; therefore, it is now possible to establish a functional classification of bony tissue essentially based on the components of bone and on the modalities of bone vascularization (
The ganoid scale of Polypterids is composed of the three layers typical for scales of Palaeonisciforms: a superficial layer of ganoine covering a layer of lacunar and vascular dentine, and a deeper layer, the basal plate, made of bony tissue (
The extant osteichthyan diversity is illustrated by the teleosts, which have colonized a multitude of available biotopes. Looking back into geological times, osteichthyan diversity is essentially represented by the Sarcopterygii and the actinopterygian predecessors of teleosteans. Palaeohistological studies can tackle interesting biological topics such as the so-called “calcified lung” (
In living individuals, the pulmonary walls were reinforced by ossified plates, probably separated by connective tissue; therefore, each plate may have moved independently from the others. This calcified organ present in
Within the actinopterygian clade, the so-called ganoid scales are thick juxtaposed rhomboid scales observed in “basal” taxa. These scales have evolved into imbricated thin and flexible elasmoid scales in various more recent taxa (
The bony tissues in the Osteichthyes have been subjected to few evolutionary trends across geological times (
However, three main trends can be mentioned when we examine the bony skeleton of Osteichthyes over geological times, i.e. from the Palaeozoic to the present. The first one is obvious at the morpho-anatomical level: it is the progressive reduction of the skeleton, especially the dermal skeleton. This reduction of the dermal skeleton is obvious when looking at the Palaeozoic armoured Heterostraci, Osteostraci and Placodermi compared to the extant Teleostei (
Acellularisation of bone is a heterochronic process that has appeared several times in the history of agnathans and gnathostomes. The aspidine of heterostracans, an extinct agnathan group from the Palaeozoic, is a true acellular bony tissue (
The progressive reduction of mineralization had developed in the dermal skeleton and is especially noticeable in the scales (
Another example of the difficulty of classifying skeletal tissues within Osteichthyes is provided by chondroid bone (not reviewed in the present paper;
Cartilages, bones and teeth have long been considered as inert in Osteichthyes. In fact, this is not true, as shown by the extensive remodelling process observed for example in the vertebrae of Tuna (
From a physiological point of view, the osteichthyan skeleton is under the control of various factors: mineral (Ca, P) supply, hormones, vitamins… (