BONE STRUCTURE AND HISTOLOGY (54)

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Bone is made up of calcium phosphate mineral (in the form of hydroxyapatite) deposited on collagen, a protein formed in long bundles or fibres. In slow growing animals, the collagen fibres are laid down parallel in each layer, producing bone that has a dense packing of mineral crystals all orientated in the same direction. This 'lamellar' bone is in direct contrast to bone formed in rapidly growing animals, where the collagen is laid down in a haphazard way to form an irregular or 'woven' bone.
Woven bone (also termed fibro-lamellar) is typical of mammals and birds that are warm-blooded and hence fast growing, whereas cold-blooded, slow growing reptiles (eg crocodiles) typically have lamellar bone.
Bone from warm-blooded creatures is also typified by a much larger number of vascular (blood vessel) canals and Haversian canals formed in rapidly forming bone. Cold-blooded animals always have far fewer such canals.
Finally, most bone exhibits growth rings. These may be any of a number of types
* Daily - found in teeth of dinosaurs, mammals and crocodiles.
* Seasonal or diet induced. During times of slow growth (eg winter), thinner, denser layers of bone form. Cold-blooded animals are more responsive to the environmental temperature and in warm climates without extremely dry seasons (also slow growth times), tend to show more and better defined growth rings than warm-blooded animals. In mammals these rings are typically much more obvious in teeth than in bone.
* Lines of Arrested Growth (LAGs). These are formed wherever there is a temporary halt to bone deposition. They may be caused by a number of factors and do not necessarily indicate physiological or environmental effects. They are found typically in mammalian jaws and in the long bones of most mammals, where they usually are due to environmental changes. LAGs have been found in many dinosaurs, including Massospondylus, Syntarsus, Brachiosaurus, Hypacrosaurus and Maiasaura, and also in enantiornithine birds. In contrast to mammals, where LAGs are normally found in slow-growing lamellar-zonal bone, in dinosaurs they occur more often in fast-growing fibrolamellar bone. They are assumed to reflect yearly bone deposition, but in dinosaurs there is no direct evidence to confirm this assumption. Growth lines can also form under other influences (unsymmetrical bone remodelling, changes in type of bone deposited etc) and though these are generally distinguishable from true LAGs to the trained eye, they can be a source of confusion. Growth lines vary throughout a dinosaur skeleton, being generally less common in long bones, and may reflect different rates of bone deposition in different parts of the skeleton. Interestingly, although LAGs are found in the femora of virtually all dinosaurs so far examined, one particular growth series of Dryosaurus shows no LAGs, and they are also not found in polar hypsilophodontids from Australia even though other dinosaur types from the same locality do show them.
Thus, although initially the finding of LAGs in dinosaurs was interpreted as evidence for a warm-blooded physiology, today’s consensus is that LAGs do not necessarily correlate with physiology.

Hundreds of slices from bones of all types of dinosaurs have now been examined and all show:
1) Woven bone typical of rapid bone growth
2) Vascular canals equivalent to those of birds
3) Generally poor growth rings but more obvious in teeth
ie in all regards they resemble warm-blooded rather than cold-blooded animals. However, continuing studies have complicated the interpretation of these results. Many small mammals and birds do not show either Haversian canals or fibrolamellar bone, and at least one turtle has been found with a dense Haversian bone. Dinosaurs do appear to differ from other reptiles in their ability to deposit fibrolamellar bone continuously instead of periodically.
In 1993 a comparative study was made of the long bone growth plate of a chicken (bird), dog (mammal) and monitor lizard (cold-blooded reptile) with juvenile Maiasaura bone. In longitudinal section, the line between cartilage and newly formed bone was straight in the dog and monitor, but undulating in the bird and dinosaur. As well as reinforcing the close relationship between dinosaur and bird, the authors conclude that the different bone formation processes that cause the wavy line indicate rapid growth and are consistent with a high metabolic rate .

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