Medullary bone

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. 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. 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. == 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, Curiously, high-dose estrogen injection also causes femurs of mice to become completely filled with bone tissue. This is unique in mice among mammals. Skeletal distribution Generally, the amount of MB deposited in bird limbs follows a consistent pattern across species. Each limb (wing or leg) is such that the proximal part (closest to the torso) may contain air, followed by a part containing red marrow and MB, followed by a part containing yellow marrow. Once MB ends, the rest of the limb is filled with only yellow marrow. For example, in Cygnus atratus, MB ends in the middle of tibiotarsus, so its tarsometatarsus contains only yellow marrow. In Alisterus chloropterus, MB exists in tarsometatarsus, so its tibiotarsus contains only MB. Other patterns of MB are: • When present in the skull, MB was always found in the pterygoid and/or surangular. • When present in the caudal sacrum, it is also present in the pygostyle. • When present in one cervical vertebra, it is also present in the ones closer to the head. For example, if it is present in the axis, then it is also present in the atlas. • MB is positively correlated with red bone marrow. • Pneumatic bones lack blood vessels, and have no or only small amounts of MB. • If a bone is paired on the left and right side, then the amount of MB present in both are similar. == Calcium metabolism ==

The function of MB is to allow the specialized form of calcium metabolism in birds. Most research on calcium metabolism in birds is in chickens, because of their economic importance. Thus, in this section, we default to describing the calcium metabolism of the chicken. The MB first occurs in a pullet about 10 to 14 days before she comes into lay, at the same time when she starts secreting extra sex hormones and retain extra calcium and phosphate. The MB grows in synchrony with the growth of follicles in the ovary, and is mostly absorbed as the egg leaves the ovary, pass down the oviduct, and have eggshell deposited on it. During the buildup of MB, there is great activity of the osteoblast. During the reabsorption of MB, there is great activity of the osteoclast. The "shell gland" in the oviduct of the egg-laying chicken can secret about 5 g of calcium carbonate within 20 hours. In an egg-laying chicken, there is about 0.5 M of calcium, 98% of which is in the medullary and cortical bone. The cost of laying eggs is 0.05 M/day. "an egg-laying chicken on a low calcium diet has a constant mass of medullary bone after laying 6 eggs, but consistently loses about 5 g of mass in cortical bone per egg. After laying 6 eggs, 40% of cortical mass is lost. == References ==