The medullary bone (MB) is a temporary anatomical structure found in most extant birds, and some extinct dinosaurs and birds. In female individuals during egg-laying period, bone tissue temporarily grows inside the medullary cavity from of the endosteal surface of bone shafts, and are reabsorbed by the end of the period. It is particularly prevalent in femurs, and can completely fill the medullary cavity during the peak. It co-occurs with capillary-rich red bone marrow, but not yellow bone marrow (adipose tissue). The MB contrast with the more permanent cortical bones. Histologically, it is a highly vascularized, mostly woven, endosteally-derived tissue. When eggshell is formed, the speed at which calcium is absorbed in the bird's intestine is lower than the speed of calcium deposit in the oviduct, thus the bird must use a calcium reservoir within the body. Most of this is supplied by the MB, which is a much labile source of calcium than the cortical bones.
History It was first depicted in 1911 and later described in more detail in 1916 within a monograph on the comparative histology of the femur, in the femurs of yellowhammer and Pelecanus erythrorhynchus. It was forgotten and rediscovered by Kyes and Potter in 1934 in the pigeon femur, who found the basic physiological facts of it, that the occurrence of the MB is a cyclic event, restricted to female individuals, and coinciding with the maturation of the ovarian follicles. It had been most extensively studied in chickens, since egg-laying in hens is economically important.
Distribution Among extant amniotes, the MB is only found in birds. In particular, it is not found in non-avian reptiles such as crocodiles, snakes, and lizards.
In birds In a systematic survey of 38 species of birds by μCT, the MB was found in all except Aethia pusilla and Strix varia, though the authors exclude them from the sample on the justification that they died at the end of the laying cycle, when the medullary bones would have been absorbed already. Most other birds have MB in femur and tibiotarsus. In Lonchura punctulata, MB is present in all bones except feet phalanges. Being small, or being a diving flier, is positively correlated with having more MB and less air in bones. Being large or an efficient flier is positively correlated with having less MB and more air in bones. No correlation between the quantity and skeletal distribution of MB and body-size or clutch size, or whether the specimen lived and died in captivity vs the wild. It does not exist in some species of Calidris. During egg-laying period, adult females' stomachs contain teeth and bones, especially vertebrae of Lemmus trimucronatus. They do not eat freshly dead lemmings, and their stomachs do not contain meat or fur of lemmings. They were hypothesized to eat from the cast pellets of avian predators of lemmings, especially Stercorarius pomarinus and Nyctea scandiaca.
Paleontology The MB has been found in some dinosaurs, including Tyrannosaurus, Allosaurus, the ornithopod Tenontosaurus, and pterosaurs, though the putative MB tissue of Allosaurus may be pathologic, since pathological bone can also develop from the endosteal margin of the bone wall, and can look very similar to MB. It has also been found in some extinct birds, including Avimaia schweitzerae, Confuciusornis, and dodo. The presence of MB allows identifying individuals as females who died during egg-laying period. MB has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity before they were full-grown. However, some hypothesize that the presence of MB in fossil only proves the individual had high calcium turnover rates, and not necessarily an egg-laying female. The line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, but crocodiles, which are dinosaurs' second closest extant relatives after birds, do not have MB. This suggests that the MB may be a feature of Avemetatarsalia, and have first appeared in ornithodires, the Triassic archosaur group from which dinosaurs are thought to have evolved. See also.
Physiology
Histology The MB is a granular or powdery soft, fine network of woven-type trabecular bone, in contrast with cortical bone and cancellous bone, which are both hard. It can be separated from the rest of the bone by scraping or by careful grinding and sieving. The presence of MB in a femur can be checked by putting the femur in front of a bright light. Increasing amount of MB decreases the translucency of the bone. Microstructurally, the mineral particles are smaller in MB than in cortical bones. During eggshell calcification, the mineral content and the size of trabeculae of MB decrease markedly, and the smaller mineral particles are preferentially removed.
Endocrinology The MB is not found in males. It is also not found in female chickens before sexual maturity. The MB forms under the influence of testosterone and estrogen. It can be induced by injecting estradiol dipropionate in female pigeons, and injecting estradiol valerate in normal male Japanese quails, and injecting both estradiol and testosterone propionate in castrated male pigeons (injecting only estradiol is ineffective). Curiously, high-dose estrogen injection also causes femurs of mice to become completely filled with bone tissue. This is unique in mice among mammals.
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