MEDULLARY BONE AND AVIAN CALCIUM REGULATION

Medullary bone forms in egg-laying birds in response to gonadal steroi ds and is the most overtly oestrogen-dependent of all bone types. It a cts as a labile reservoir for the supply of eggshell calcium. Previous studies indicate that feeding calcium- and vitamin-D-deficient diets to chickens results in resorption of cortical rather than medullary bo ne. More recent studies in calcium-stressed quail hens question this h ypothesis and suggest that during the first 2 weeks of dietary calcium depletion the medullary bone is resorbed while cortical bone volume r emains intact. The role of the osteoclast in bone resorption is the fo cus of much research that has recently included studies of medullary b one osteoclasts. The functional morphology of the avian cells, i.e. ch anges from quiescent to active osteoclasts with ruffled borders, refle cts the rapid changes in medullary bone turnover that occur during the egg-laying cycle. Unlike mammalian osteoclasts, those from avian sour ces generally appear refractory to inhibitory factors such as calciton in or raised extracellular calcium concentration. However, medullary b one osteoclasts cultured in vitro for several days recover their abili ty to respond to the latter factor by increasing their levels of free cytosolic Ca2+, reducing tartrate-resistant acid phosphatase secretion and reducing their cell spread area. It is suggested that factors suc h as ambient calcium levels and prostaglandins may form part of a syst em of rapid local control for medullary bone osteoclast activity.