Chicken calcium metabolism in an egg-laying period is extraordinary when compared with all other classes of vertebrates, since the egg-laying hens produce up to three hundred eggs with hard-eggshell per a year. The eggshell consists of 5.7 g calcium carbonate (CaCO3) containing about 2.3 g of net calcium. About 6075% of the calcium in the eggshell is derived from dietary sources and the remaining of 2540% from skeletal stores, called medullary bone. Medullary bone is specifically developed in marrow cavities of long bones and plays an important role as a calcium reservoir for eggshell formation. There are osteoblasts, osteoclasts on medullary bone surface, as well as osteocytes embedded in the matrix. In the domestic hens, medullary bone formation and resorption occur alternately during the 24-h egg-laying cycle. These cyclic changes in medullary bone metabolism depend on the site of an egg in the oviduct. Namely, when an egg is in the infundibulum, magnum and or isthmus of the oviduct, osteoblasts actively form medullary bone. On the other hand, when an egg enters into the shell gland and begins to be calcified, osteoclasts resorb medullary bone and mobilise calcium for eggshell formation. It has been reported that osteocytes play a key role for bone formation and resorption. However, the function of medullary bone osteocytes is still unknown. We have recently observed that carbonic anhydrase II (CAII) is localised in medullary bone osteocytes. CAII is a known proton generator which lyses calcified matrix, and is expressed in osteoclasts. In contrast to medullary bone osteocytes, cortical bone osteocytes do not contain carbonic anhydrase. These results suggest that osteocytes may also resorb medullary bone for eggshell formation.
Disclosure: The authors declared no competing interests.