Anura

Anura View in CoL View at ENA

The long bones of all anuran taxa have a typical tubular architecture, with solid walls void of the wide erosion bays observed in the urodeles (small erosion cavities can be nonetheless present in the perimedullary region). In cross-section, the cortices of anuran long bones are more markedly monorefringent than those of the urodeles ( Fig. 3A View Figure 3 ). Likewise, in longitudinal sections, the birefringence of the bone matrix is particularly strong, with alternation of bright and dark phases when rotating the microscope stage ( Fig. 3C View Figure 3 ). These observations indicate that the fibres of the collagenous matrix have a more strictly longitudinal orientation in anurans. In longitudinal sections, osteocyte lacunae appear flat and parallel to the sagittal axis of the bones ( Fig. 3D View Figure 3 ); an aspect confirming that long bone cortices in anurans are made of well-characterized parallel-fibred bone tissue. Sharp cyclical growth marks, in the form of LAGs or birefringent annuli, occur in most anuran bones ( Fig. 3A View Figure 3 ), as also do bundles of radial or oblique Sharpey’s fibres ( Fig. 3B, C View Figure 3 ). The main differences between anuran taxa consist of some variability in the regularity of the spatial orientation of the collagenous meshwork, as well as in the morphology of osteocyte lacunae, which can be circular ( Fig. 3G View Figure 3 ), multipolar or ellipsoid ( Fig. 3H View Figure 3 ) in cross-section.

Although the canaliculi appear short in longitudinal section ( Fig. 3D View Figure 3 ), the cross-sections of most limb bones reveal the presence of complex canalicular networks in the vicinity of the osteocyte bodies ( Fig. 3E–H View Figure 3 ). The aspect of these networks is somewhat reminiscent of the so-called aspidinocytes, which are thin and elongated unmineralized spaces observed in aspidin, a type of acellular bone. The origin and function of these structures have been subject of debates. ( Hancox, 1972; see also the critical discussion of this concept in Francillon-Vieillot et al., 1990b). The canaliculi have a primarily radial orientation, but tend to converge towards the lumen of vascular canals, when present ( Fig. 3E View Figure 3 ). It is rare to observe a direct contact between these canalicular extensions and the peri-somatic part of osteocyte lacunae. Their presence in the layer of lamellar endosteal bone (centripetally deposited) surrounding the medullary cavity ( Fig. 3F View Figure 3 ), testifies that these structures are not anchoring fibres (e.g. Sharpey’s fibres), with which they could have a superficial resemblance. There is little doubt that they indeed represent a complex network of canaliculi, but with ramifications mainly developing at some distance from osteocyte bodies. This situation could indeed explain why it is difficult to clearly show that they originate from the osteocyte lacunae.

Vascular canals are mostly represented by primary osteons, whose lumen varies from 10 to 40 µm in diameter, depending on the level of the section, and the individual or species considered ( Figs 3H View Figure 3 , 4–6 View Figure 4 View Figure 5 View Figure 6 ). Simple vascular canals are also observable, although they are less frequent and always associated, when present, to primary osteons. In many individuals of diverse taxa, the cortical layers (of periosteal origin) show a pronounced deflection under the vascular canals ( Fig. 4B View Figure 4 ). This feature is evidence of the primarily periosteal origin of intra-cortical vascularization. Besides the primary osteons, some secondary osteons occur in the peri-medullary region of Nanorana vicina and Pipa pipa ( Fig. 4A View Figure 4 ). Vascular canals have a preferential longitudinal orientation; however, oblique or even radial canals can be observed in association with longitudinal ones ( Figs 3H View Figure 3 , 4A, B View Figure 4 , 5 View Figure 5 , 6 View Figure 6 ). The distribution of the vascular canals, regardless of their nature, can be random ( Fig. 4C View Figure 4 ), but this condition is rare; they rather tend to be organized either in radial rows ( Figs 4D View Figure 4 , 5B View Figure 5 , 6A–D View Figure 6 ), or in circumferential layers ( Fig. 4F View Figure 4 ), or in a combination of both. In Rhinella marina , the primary osteons are arranged in concentric layers and tend to have a circumferential orientation ( Fig. 4F View Figure 4 ); this locally gives the bone an apparent laminar organization (but the latter does not correspond, of course, to a fibro-lamellar complex).

STATISTICAL ANALYSES