Histomorphometric bone properties of sexually immature and mature White Leghorn hens with evaluation of fluorochrome injection on egg production traits.

White Leghorn hens were injected i.p. with fluorochromes beginning at 18, 37, and 43 wk of age. Sections of the right femur were collected in ethanol and formalin approximately 2 wk later. Dynamic and static bone variables were determined in both age groups, and the effects of fluorochrome injection on egg production was assessed in the older hens. Fluorochrome bone labeling was nontoxic. Injection of fluorochrome did not affect BW or feed consumption in either age group or egg production traits in laying hens. Fluorochrome labeling was found in the femoral cortex of sexually immature hens. The mineral apposition rate of the periosteal surface was 3.08 micrometers/day and of the osteonal surface was 2.69 micrometers/day. No uptake of fluorochrome label was observed on the periosteal or osteonal surfaces of cortical bone of laying hens. Sexually mature hens had decreased femoral cortical widths, less percentage cortical bone, and more cortical endosteal surface than sexually immature hens. The diffuse labeling of the medullary bone of laying hens indicated active medullary bone mineralization. The central area of the femur cross-section inside of the cortex was examined. Sexually mature hens had less bone area, more bone perimeter, and more bone perimeter covered by osteoclasts in the lamellar cancellous and medullary bone compared with sexually immature hens. Results indicated that the development of osteopenia in hens with the onset and progression of lay may be related to increased resorptive surface of bone and loss of structural and nonstructural bone.