VARIABLE HORMONE RESPONSIVENESS OF OSTEOBLAST POPULATIONS ISOLATED ATDIFFERENT STAGES OF EMBRYOGENESIS AND ITS RELATIONSHIP TO THE OSTEOGENIC LINEAGE

A variable response to 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] hor mone treatment was observed for osteoblast cell populations isolated f rom 12- and 17-day-old embryonic chick calvariae. The younger embryoni c cell population showed 2- and 5-fold inductions of osteocalcin and o steopontin gene expression, respectively, and a 25% inhibition of coll agen gene expression when treated with 1,25-(OH)(2)D-3. In contrast, t hese same genes all displayed approximately 80% inhibition of their ex pression when the older embryonic cell populations were treated with h ormone. The hormone response was related to the appearance of the vita min D-3 receptor (VDR) and the developmental state of the two cell pop ulations by assessing the numbers of cells cells that were immunologic ally labeled for two osteoblast lineage, stage-specific surface makers (alkaline phosphatase and SB-5, an osteocyte marker) and the VDR. Usi ng the sequence of marker presentation, with VDR appearing first, foll owed by alkaline phosphatase and then SB-5, models were tested using l ogistic regression analysis to validate this order of marker presentat ion and establish that the two embryonic ages of the cell populations represent discrete stages of their lineage. This analysis indicated th at 1,25-(OH)(2)D-3 treatment progressed the la-day-old embryo cell pop ulations along their lineage and that the hormone promoted the appeara nce of its own receptor (P < 0.001) However, the appearance of the VDR does not appear to be a determinant in the variable responses of the different embryonic aged cell populations to the hormone. These data q uantitatively establish the unique nature of osteoblast cell populatio ns within their lineage progression for cells isolated from embryos of different ages, such that cell populations isolated from younger embr yos are comprised of primarily presumptive or immature osteoblasts, wh ereas cells isolated from older embryos are comprised of mature osteob lasts. These data also demonstrate that the genomic effects of 1,25-(O H)(2)D-3 are dependent on the developmental stage of the osteoblast li neage, and the stimulatory actions of the hormone are targeted to imma ture osteoblasts, whereas the effect of the hormone on mature osteobla sts is inhibitory.