Structural Organization and Growth

Skeletal bone provides the framework supporting the body and the mechanism for locomotion, making its strength and rate of maturation vital to each individual. It has long been recognized that the rate at which bone tissue is formed varies in relationship to its structural organization [1], [2]. Woven bone, characterized by an irregular microstructural organization, is the fastest forming type of tissue, with rates of more than 4 μm per day, but sacrifices strength compared to slower forming tissues [3]. Lamellar tissue, which has a highly organized microstructure in which fiber directions change through successive layers, is the slowest forming type of tissue, with rates of less than 1 μm per day [4] but is the strongest type of bone. Parallel-fibered bone lacks the more complex organization of lamellar bone and forms at rates intermediate between those of woven and lamellar tissues [3].

At a higher level of structural organization, fibrolamellar (plexiform) bone [5]–[7] is a vascular based tissue that increases the diametric rate of bone growth by enlarging the surface areas on which new bone can simultaneously form. In this tissue, trabeculae of rapidly formed woven or parallel-fibered bone are extended outward from the growing surface, leaving unfilled spaces around the vascular canals which are subsequently filled with slower forming but stronger lamellar tissue to form primary osteons [8], [9]. The proportion of primary osteons in fibrolamellar bone also correlates with the tissue growth rate [10]. Fibrolamellar tissue is present in many large dinosaurs and mammals [2], [11], [12], [13].

At a finer level of organization, two microstructural features, random directionality of canaliculi in secondary osteons, resembling that in primary woven bone, and reduced abundance and quality of lamellar tissue have been found to be distinctive features of birds and small late Cretaceous theropods [14]. The reduced quantity and quality of lamellar tissue and the increased resemblance of the tissue in secondary osteons to woven tissue in those taxa suggests accelerated osteogenesis in these ontogenetically later forming bone tissues.

In a morphometric study of human tissue, greater numbers of canaliculi were counted on the walls of osteocyte lacunae in woven than in the more slowly forming lamellar bone [15] and another study of seven taxa (frog, chicken, rabbit, bovine, horse, dog and human) found greater numbers of canaliculi departing from osteocyte lacunae in woven than in the slower forming parallel-fibered bone [16].

A visual comparison of samples of woven and parallel-fibered bone tissues from a diversity of taxa (Fig. 1) suggests that canaliculi in three groups of saurischian dinosaurs, modern birds (A), non-avian theropods (B,C) and sauropodomorphs (D, E), are more densely organized than in non-saurischian taxa (H-N). Because canaliculi contain the cytoplasmic extensions of the cells which produced the bone tissue, a greater density of canaliculi may reflect a capacity for higher rates of osteogenesis. However, variation can be seen both within taxa and within individual samples (Fig. 1). To test the hypothesis that the cytoplasm of bone cells in primary tissue is more densely organized in saurischians than in other tetrapods, we measured in thin-section the perimeters of osteocyte lacunae, the lengths of canaliculi, the number of canalicular branching points and the total canalicular and lacunar perimeters (a correlate of the total cellular surface area) within sampled areas of equivalent size. To make the measurement process uniform, image pixel shading was converted to binary values (providing black canaliculi and lacunae on white backgrounds) using a uniform set of conversion parameters across all images, and measurements were made using an image processing algorithm (see Methods for details).

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larger image TIFF original image Download: Fig 1. Osteocyte lacunae and canaliculi in woven cortical bone. (A) Neornithes (Recent great horned owl, Bubo virginianus) ulna. (B) Ornithomimidae (late Cretaceous theropod) metatarsal. (C) Tyrannosaurus (late Cretaceous theropod) phalanx. (D) Titanosauridae (late Cretaceous sauropod) limb. (E) Adeopapposaurus (early Jurassic basal sauropodomorph) rib. (F) Herrerasaurus (late Triassic basal theropod) neural spine. (G) Dinornis (sub-Recent ratite) tarsometatarsus. (H) Ornithischia (late Cretaceous hadrosaurid) ossified tendon. (I) Phytosauria (late Triassic aquatic archosaur) vertebra. (J) Alligator (Recent crocodylomorph archosaur) tibia. (K) Lacertilia (lizard, Tupinambis) humerus. (L) Mammalia (armadillo, Dasypus) femur. (M) Mammalia (prong-horn, Antilocapra) metatarsal. (N) Amphibia (frog, Rana) humerus. Images are shown at the same scale: 10 μm bar in (A). https://doi.org/10.1371/journal.pone.0119083.g001

The measured samples encompass five mammalian genera, three lizard genera, a phytosaur, a crocodylomorph, five modern bird genera, two Cretaceous theropod dinosaurs, two late Triassic theropods, a late Cretaceous sauropod dinosaur (titanosaurid), a late Jurassic sauropod, an early Jurassic basal sauropodomorph, and two late Cretaceous ornithischian dinosaurs.