Caudata

Canoville, Aurore, Laurin, Michel & De Buffrénil, Vivian, 2018, Quantitative data on bone vascular supply in lissamphibians: comparative and phylogenetic aspects, Zoological Journal of the Linnean Society 182 (1), pp. 107-128 : 114

Caudata

Cross-sections from the humeri and femora show that the stylopod elements of urodeles have a narrow medullary cavity surrounded by a thick and compact cortex ( Fig. 2 ; Appendix). In Marmorerpeton, Salamander A, A. davidianus , Andrias japonicus , Ambystoma andersoni , Necturus maculosus and possibly also Pleurodeles waltl , bone cortex in both the humerus and femur is perforated by vast erosion cavities displaying a typical scalloped contour (evidence for osteoclast activity). These cavities are irregularly distributed within the cortex, at variable distance from the medullary cavity, and their total area can be much larger than that of the medullary cavity ( Fig. 2A ). Signs of secondary reconstruction very seldom occur on their walls; it was observed only, to a limited extent, in the two Andrias species.

The cortices of all long bones in our sample are made of parallel-fibred tissue ( Fig. 2A, B ). In cross-section, this tissue appears roughly birefringent in polarized light; however, in some species, the birefringence is poorly characterized (e.g. N. maculosus and both Andrias species; Fig. 2B, C ). Conversely, birefringence is more pronounced in longitudinal sections ( Fig. 2E ). In most individuals, the rotation of the microscope stage reveals in transversal and longitudinal sections a monorefringent band that moves according to the direction of rotation. These observations suggest that the orientation of collagen fibres in the osseous matrix of urodele long bones is basically longitudinal, but that it also includes a significant spiral component. Osteocyte lacunae appear predominantly circular in transversal sections, with some variability in their morphology and orientation ( Fig. 2F ). In longitudinal section, they are flattened and oriented parallel to the surface of the cortices ( Fig. 2E ). Canaliculi are poorly developed ( Fig. 2E ). The orientation of osteocyte lacunae thus confirms the overall polarization of the bone structure. Sharp cyclical growth marks, in the form of annuli or lines of arrested growth (LAGs), are visible on the cross-sections ( Fig. 2C, D ).

Urodele long bones are generally avascular; only a few vascular canals were observed in the femur of A. japonicus (5 canals, at most, in cross-sections) and A. davidianus (2 canals). These are simple vascular canals with average diameters of 60 µm ( A. japonicus ) and 95 µm ( A. davidianus ) and longitudinal to slightly oblique orientations (compared to the long axis of the bone).

In several taxa (e.g. Andrias , Necturus , etc.), the medullary cavity is still partly or totally obstructed by residues of non-resorbed calcified cartilage at the adult stage ( Fig. 2G ). This feature reflects neoteny (cf. Ricqlès, 1964, 1965), a fairly common phenomenon in urodeles ( Duellman & Trueb, 1986). In the N. maculosus specimen, this state only concerns the femur ( Fig. 2G ), because the medullary cavity of the humerus ( Fig. 2B ) is devoid of calcified cartilage.

Figures

Figure 2. Bone histology of the Caudata. (A) Mid-diaphyseal cross section in Andrias japonicus femur (specimen MNHN. AC.1914.224, transmitted polarized light). The bone cortex is globally (but imperfectly) birefringent.The medullary cavity is small, whereas the cortex is very thick, and displays three broad erosion bays, one of which merges into the medullary cavity; (B) Mid-diaphyseal section in Necturus maculosus humerus (specimen ZFMK 27706). Same remarks as in the femur of A.japonicus; (C–D) Histology of the humeral cortex of Andrias davidianus (specimen ZFMK 8568; in polarized and ordinary transmitted light, respectively). Cyclical growth marks, in the form of lines of arrested growth (arrow heads) are particularly pronounced. Note also the abundance in Sharpey’s fibres bundles throughout the cortical bone; (E) Longitudinal section in A. davidianus femur. The insert is a closer view at osteocyte lacunae; (F) Cross-section in A. japonicus femur (ordinary transmitted light). Growth marks are conspicuous throughout the cortex. The white arrow points at one of the rare simple vascular canals observed in the femur of A. japonicus; G, calcified cartilage filling the medullary cavity in N. maculosus femur. Abbreviations, CC, calcified cartilage; EB, intra-cortical erosion bay; Sf, Sharpey’s fibres.