Product Code Database
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 Lability source of calcium than the cortical bones.
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 Pomarine jaeger and Nyctea scandiaca.
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 taxon relatives after birds, do not have MB. This suggests that the MB may be a feature of Avemetatarsalia, and have first appeared in , the Triassic archosaur group from which dinosaurs are thought to have evolved.
Microstructure, 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.
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 , 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.
Birds inherited skeletal pneumaticity from their ancestors. Pneumaticity in bones behind the cranium results from the progressive invasion of bones by Diverticulum of the Lung air sac system after hatching. As these diverticula invade the medullary cavity of bones, there is a simultaneous relocation and decrease in red marrow content, which prevents the formation of MB later during reproductive cycles. In some cases, air sac diverticula do no completely invade the cavities of pneumatized skeletal elements, so small amounts of bone marrow, and hence MB, can persist.
Within a species, different individuals even of the same age can differ in how much the air sac diverticula invade the cavities of skeletal elements, and thus how much red bone marrow can remain. This could explain intraspecific variation in MB. MB was even present in very small bones, such as the ribs of some small passerine birds like Lonchura punctulata, or the bones of the hummingbird Phaethornis superciliosus, or the tiny os opticus of the House sparrow.
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.
| +MB in pigeons during follicular cycle !Size of follicles (mm) !State of MB | |
| <2 | None |
| >4.5 | Some |
| 10 | Extreme |
During eggshell formation or pregnancy, a female animal requires calcium to form the eggshell or the embryo bones. In most mammals, and the turtles, the rate of calcium consumption and bone reabsorption are both in the range of 1–5 mg Ca/kg body weight/hr. This is true even in cows, though cows are unusual among mammals in that their colostrum is significantly more enriched in calcium than subsequent milk, so bone reabsorption may become insufficient after Birth, resulting in milk fever. In contrast, in the chicken, consumption is ~40 and bone reabsorption is ~25. The MB makes up for this serious shortfall. The medullary bone can be mobilized at a rate of 10–15× faster than cortical bone, and 2× as fast as epiphysis, making it the most labile source of calcium in the bird.
During development, the Ca content of a chicken embryo grows exponentially. A fully developed chicken embryo contains 125 mg of Ca, of which 100 mg comes from the shell. The entire shell contains about 2000 mg of Ca. .
In hens, much of the shell is formed during the night, when generally no Ca is consumed and when the Ca content of the digestive tract is gradually decreasing, thus during this time Ca is mostly coming from bones. If the hen during an egg-laying period is on a high-Ca diet, it can replenish all Ca lost to the previous egg during the interval between two eggs. Otherwise, the hen gradually loses cortical bone tissue, but the amount of MB tissue remains fairly constant.
|
|
