Latimeria

Meunier, François J., Cupello, Camila, Herbin, Marc, Clément, Gaël & Brito, Paulo M., 2021, The lungs of extinct and extant coelacanths: a morphological and histological review, Cybium 45 (1), pp. 21-30 : 25-27

Latimeria ( Figs 3 , 4, 5)

The vestigial lung of Latimeria is structurally similar to the conducting portion of other air breathing organs ( Brien, 1964), and lacks true alveoli and pneumocytes ( Cupello et al., 2017a). Latimeria lung has the same general histological organization than the oesophagus ( Cupello et al., 2017a: fig. 3B-D). The pulmonary sheaths are, from the inner to the outer, constituted by: a mucosa with ciliated cells and goblet cells (an epithelium), a submucosa composed of collagenous and elastic fibres, blood vessels and fat reserve tissues, and a muscularis mucosa ( Fig. 3A ) ( Cupello et al., 2017a). The long and slender residual cord presents a similar structure: a mucosa with goblet cells and with some ciliated cells, and a fibrous submucosa with some blood vessels ( Cupello et al., 2017a: fig. 3G, H). The lumen of the residual cord is reduced, and the sheaths are thin.

The lung plates of Latimeria are positioned at the surface of the vestigial lung and at least on the anterior part of the residual cord of adult specimens ( Fig. 3A ). Each plate is inserted in a membranous bag ( Figs 3A , 4A), which con- stitutes a periostic-like membrane. The plates are relatively thin with a centre that is strongly enriched in proteoglycans and practically deprived of collagenous fibres (Fig. 4A). This centre looks fragile since it tears on the microtome knife, creating an artefactual lumen ( Figs 3A , 4A). Each plate is constituted of a fibrous matrix with enclosed fibro- cytes (Fig. 4B-D). The fibres are collagenous and organized in superposed layers ( Fig. 5A, B ). The cells are more or less star-shaped, and send cytoplasmic extensions in the thickness of the plate (Figs 4E, F, 5C). They are osteocyte-like cells. Scarce lining cells, constituting a periostic membrane, are seen on the surface of the plates, so as fibres which penetrate in the plate (Fig. 4A-C). Here and there osteoblasts enter the collagenous matrix of the external layer: they are becoming osteocytes (Fig. 4C, F).

The extracellular matrix shows various staining intensi- ties with obvious lines. Some of these lines are concentric and parallel (Fig. 4C, E, G), whereas some others form spherule shapes (Figs 4G, 5D). These chromatic lines are Liesegang lines (= waves of Liesegang; Ørvig, 1951: figs 18, 20, 21) that characterize active spheritic mineralizing processes ( Ørvig, 1951; 1968; Francillon-Vieillot et al., 1990;

Zylberberg et al., 1992). Although Latimeria plates are rudimentary anatomical structures, Liesegang lines can be considered as remnants of an overlapping pattern. Liesegang lines are also observed in the overlapped bony plates of some fossil coelacanths (see below). Since the small plates surrounding the lung in Latimeria are mineralized and formed of collagenous fibres with embedded star-shaped cells, it is considered that these plates are made of bony tissue like the ossified plates of † A. araripensis ( Brito et al., 2010) .

† Axelrodichthys and † Swenzia ( Figs 6 , 7 )

Some sections of fossil lung plates in the Cretaceous coelacanth † Axelrodichthys show median detachment that represents a weak zone ( Fig. 6A, B ), as in the plates of Latimeria . Indeed they show the same artefactual brittleness: a lumen in the centre of the plates ( Fig. 6A, B ). The bony plates in fossil coelacanths were probably enriched in proteoglycans that were linked to the spheritic mineralization, as in the small bony plates of extant coelacanths. The thickness of the plates regularly diminishes towards their periphery. The plates are made of primary cellular pseudo-lamellar bone ( Fig. 6B, C ). Osteocytes are seen in the thickness of the laminae ( Fig. 6C ). They are typically star-shaped with numerous ramified cytoplasmic extensions ( Fig. 6D ) that turn in all directions around the cells and penetrate in the bony matrix ( Figs 6C, D, G , 7F ). Here and there vascular cavities or canals are seen in thick lamellae ( Fig. 6E ). Vascular cavities bordered by secondary bone can be seen in other relatively thick plates (see Brito et al., 2010: fig. 4E, F), suggesting processes of bone remodelling.

In their study on the calcified lung of † Axelrodichthys, Brito et al. (2010 : fig. 4B) have misinterpreted concentric lines on sections of bony plates as epigenization artefacts. In fact, these various lines ( Fig. 6B ), either concentric around a central point or superposed all along the osseous plate, are Liesegang lines that characterized a spheritic (= globular) mineralization process: the mineralized crystals and the organic matrix show a radiating arrangement ( Ørvig, 1951, 1968; Francillon-Vieillot et al., 1990: 519). The mineralization processes in the ossified lung plates of † Axelrodichthys are the same than those described for Latimeria lung plates (Fig. 4G; see also Cupello et al., 2017b). The mineralization front of bony lamellae grows by the progressive fusion of calcified granules ( Fig. 6F, G ). These mineralized granules are clearly obvious on SEM images of † Axelrodichthys ( Fig. 7A, C, D ) and † Swenzia ( Fig. 8A, C, D ) plates. The spheritic mineralization is considered as a plesiomorphic character, at the origin of the inotropic mineralization that is characterized by mineral crystals whose axis are oriented parallel to the collagen fibrils ( Ørvig, 1951, 1968). Spheritic mineralization is common in bones and cartilages ( Ørvig, 1951) of teleosteans ( Benjamin et al., 1992; Huysseune, 2000; Zylberberg and Meunier, 2008; Witten et al., 2010). Such a spheritic mineralization has also been described in the tooth dentine of Latimeria ( Castanet et al., 1975) as well as in its scales at the limit of the basal plate and the external layer ( Meunier and Zylberberg, 1999).

The anatomical position and ultrastructural similarities of the plates in extant and fossil coelacanths provide evidences of their homology, and then between the organs that they surround ( Cupello et al., 2017a, b). The presence of bony tissues around the lung raises an important question: how bone is developed from tissues that belong to the walls of an oesophageal derivative? The lung sheaths have a double origin: endodermic (the mucosa) and mesodermic (the outer layers), as for the air bladder of teleosts (Hoar, 1937 in Pelster, 2004). The mesodermic origin in part of the external sheath of the lung can explain the presence of bony plates in the coelacanths’ lung. Globular mineralization has been described on the anterior wall of the air bladder in several physostomid fishes ( Marshall, 1962; Parmentier et al., 2008). However, this mineralization, the so-called “rockerbone” of the Ophidiiformes, is not a true bone tissue with osteocytes ( Parmentier et al., 2008).

Figures

Figure 3. – A: Latimeria chalumnae. Specimen CCC5 (MNHN- ZA-AC-2012-3); Azocarmin preparation, from Millot et al. (1978). Cross section showing the vestigial lung, surrounded by four plates (arrowheads 1 to 4). Two lumen ramifications (lc) of the lung are seen (see Cupello et al., 2017a). The lumen of the lung is bordered by a monostratified epithelium (mu). mu = mucosa; oe = oesopha- gus; sm = sub-mucosa. B: †Axelrodichthys araripensis from the Cretaceous of Brazil, “Josa Collection” (specimen deposited at the MNHN, Paris). Transverse ground cross section of the calci- fied lung showing the imbricated plates (black arrow-heads) around the lumen (black asterisks) (after Clément, 1999: fig. 6). Scale bars: A = 1 mm; B = 5 mm. Deposited at the Laboratoire de Paléontolo- gie, MNHN, Paris.

Figure 5. – Latimeria chalumnae. Specimen CCC 24 (MNHN- ZA-AC-2012-19). SEM images of the small mineralized plates of the lung. A: Smooth and slightly glossy surface of the plate (white asterisk) with the layers of collagen fibres (white arrows). B: Detail of the collagenous bundles in the successive conjunctive layers. C: An osteocyte (white arrowhead) with its cytoplasmic extensions (white arrows). D: Broken margin of a plate, with the artefactual lumen in the centre of the plate (white asterisk), several collagen- ous layers (white arrow-heads) and a bulk of mineralized spherules (white arrows). Inset: detail of the bulk of mineralized spherules. Scale bars:A, D = 100 μm; B = 20 μm; C, inset D = 25 μm.

Figure 6. – †Axelrodichthys araripensis from the Cretaceous of Brazil (“Josa collection”; specimen deposited at the MNHN, Paris). Sections of bony plates of the lung. A: Section of a bony plate showing the central artefactual fracture (black arrowheads), as on Latimeria plates (see Fig. 4A). B: Detail of a sec- tion of a bony plate showing the arte- factual fracture (black arrowheads) and the Liesegang lines (black arrows) indi- cating a spheritic mineralization proc- ess. C: Section of a bony plate showing the histological organization with regu- lar collagenous layers and osteocytes with canalicles crossing the extracel- lular matrix. D: Horizontal ground sec- tion of a bony plate showing numerous star-shape osteocytes with cytoplasmic processes. Inset: detail of an osteocyte with its ramified canalicles. E: Ground section showing some large vascular cavities (white asterisks). F: Cross- ground section of a bony plate show- ing Liesegang lines (black arrowheads) and some mineralized spherules (black arrows). G: Horizontal ground section of a bony plate showing an osteocyte with its cytoplasmic processes (on the left, black arrow) and some mineral- ized spherules (on the right). Scale bars: A = 100 μm; B, C = 25 μm; D, F = 20 μm; inset D, E, G = 10 μm. Deposited at the Laboratoire de Paléon- tologie, MNHN, Paris.

Figure 7. – A-C: †Axelrodichthys araripensis from the Cretaceous of Brazil (“Josa collection”; specimen deposited at the MNHN, Paris). SEM images of the bony plates of the lung. A: External view of bony plates of the lung. Succession of bony plates are visible on the right (white asterisk). B: Detail of the external surface of bony plates (see white asterisk of Fig. 7A). Each bumpy relief (white arrowheads) corresponds to a spheritic mineralization. C: Detail of a spheritic mineralized bulk. D: Detail of some mineralized spherules (arrowheads). An osteocyte is seen (white asterisk) with one osteocytic canalicle (white arrow). Scale bars:A = 200 μm; B, C = 20 μm; D = 5 μm. Deposited at the Laboratoire de Paléontolo- gie, MNHN, Paris.

Figure 8. – A-D: †Swenzia latimerae from the Jurassic of France (specimen MNHN.F.JRE47). SEM images of the bony plates of the lung. A: The bumpy reliefs on the surface correspond to the spherit- ic mineralization. The white asterisk localizes the detail of Fig. 8D. B: Detail of an osteocyte lacuna (white arrow) with one osteocytic canalicle (black arrow). C: Detail of a bulk of mineralized spher- ules (white arrowheads). D: Detail of some mineralized spherules. Scale bars:A = 50 μm; B, D = 10 μm; C = 20 μm.