Shell formation and bone strength in laying hens

Abstract: In Sweden almost 3.8% of all eggs are ruined due to cracked eggshells as shell quality declines with age during the laying period. This is a concern for food safety as well as an economical problem. Parallel with reduced shell quality the bone strength declines to an extent that the animal's welfare is threatened. Laying hens of Lohmann Selected Leghorn (LSL) and Lohmann Brown (LB) were monitored during a complete production period. The first sign of declining shell quality was found in mid-production in both hybrids, and even before, in peak production, decreasing bone strength was detected. An imbalance between estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) in the shell gland and a reduced density of tubular glands are suggested as factors involved in the age-related decline of shell quality. Since the majority of eggshell calcium is absorbed in the duodenum, the dramatic drop in active carbonic anhydrase (CA) seen in duodenum at mid-production, may also contribute. In the shell gland CA is considered a key enzyme in the supply of the carbonate ions needed for shell formation. However, CA in the shell gland did not primarily seem to be involved in declining shell quality, in fact number of capillaries positive for CA activity increased with age. The calcium transporter PMCA was located in shell gland and duodenum but was not altered by age. Boosting laying hens with a low continuous dose of estradiol at the end of the laying period improved shell quality in both hybrids, while a daidzein supplementation (50mg/kg feed) in the feed did not. Bone strength was unaffected by both treatments. CA activity in the shell gland may be regulated by estrogen and daidzein, possibly by acting via ERβ. However, a difference in sensitivity to daidzein and estradiol was found between the hybrids which complicates the interpretation of results and must be taken into a count when comparing studies from other investigations.