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 i n ethanol and formalin approximately 2 wk later. Dynamic and static bo ne variables were determined in both age groups, and the effects of fl uorochrome injection on egg production was assessed in the older hens. Fluorochrome bone labeling was nontoxic. Injection of fluorochrome di d not affect BW or feed consumption in either age group or egg product ion traits in laying hens. Fluorochrome labeling was found in the femo ral cortex of sexually immature hens. The mineral apposition rate of t he periosteal surface was 3.08 mum/day and of the osteonal surface was 2.69 mum/day. No uptake of fluorochrome label was observed on the per iosteal or osteonal surfaces of cortical bone of laying hens. Sexually mature hens had decreased femoral cortical widths, less percentage co rtical bone, and more cortical endosteal surface than sexually immatur e hens. The diffuse labeling of the medullary bone of laying hens indi cated active medullary bone mineralization. The central area of the fe mur cross-section inside of the cortex was examined. Sexually mature h ens had less bone area, more bone perimeter, and more bone perimeter c overed by osteoclasts in the lamellar cancellous and medullary bone co mpared with sexually immature hens. Results indicated that the develop ment of osteopenia in hens with the onset and progression of lay may b e related to increased resorptive surface of bone and loss of structur al and nonstructural bone.