Case 170317-4

Four-year-old chicken

There was no record of clinical disease in this layer hen.

This bird was in lay and had a shelled egg in the uterus. There were no significant gross lesions.

Four transverse sections through long bone and surrounding skeletal muscle are examined in which there is abundant medullary trabecular bone that fills 60 to 70% of the medullary cavity.

The medullary trabecular bone, the interior rim of the cortical bone and small pockets within the cortical bone are composed primarily of woven bone.

The woven bone is composed mostly of basophilic to amphophilic, fibrillar osteoid with a roughened, scalloped margin that abuts small to moderate numbers of osteoblasts and osteoclasts.

No special stains.

Normal avian medullary bone.

This is an example of a normal bone in an egg-laying chicken. When sexual maturity is reached, a large surge in estrogen changes the function of osteoblasts to forming medullary bone rather than structural bone. This process occurs only in birds and crocodilians. The medullary bone is woven bone that acts as a labile source of calcium in eggshell formation (60% of the eggshell calcium is derived from dietary intake and the remaining 40% is of osseous origin). The medullary bone has little inherent strength. It is fundamentally weaker for two reasons: The collagen fibrils are very irregularly arranged and it is present in isolated spicules. The activity of osteoclast and osteoblast populations in the medullary bone is synchronized with the egg-laying cycle to meet the demand for calcium.

This case was kindly provided by Dr. Kevin Keel.

Whitehead, CC. Overview of Bone Biology in the Egg-Laying Hen. Poultry Science. 83:193-199.