Prenatal prednisone exposure impairs fetal long-bone development in mice by disrupting chondrocyte maturation and osteoblast formation.

Prednisone is commonly used to control autoimmune and inflammatory diseases during pregnancy, but its potential effects on fetal skeletal development remain incompletely characterized. This study investigated the dose- and exposure-period-related effects of prenatal prednisone exposure (PPE) on fetal long-bone development in mice and explored the cellular mechanism involving chondrocyte-to-osteoblast transdifferentiation. Pregnant mice received prednisone at 0.25 or 1.0 mg/kg/day throughout gestation or 1.0 mg/kg/day during defined gestational periods. Fetal femurs were collected on gestational day 18 (GD18) for histomorphometric, histochemical, and immunofluorescence analyses. PPE shortened fetal femurs, reduced the primary ossification center (POC), narrowed the proliferative zone (PZ), and expanded the hypertrophic zone (HZ), with generally greater changes after higher-dose or longer-duration exposure. Quantitative cellular analysis demonstrated fewer proliferative chondrocytes, accumulation of hypertrophic chondrocytes, and reduced osteoblast numbers within the POC. Consistently, PPE decreased SRY-box transcription factor 9 (SOX9) and runt-related transcription factor 2 (RUNX2) protein expression while increasing type X collagen (COL10) expression. The accumulation of hypertrophic chondrocytes, together with insufficient osteoblast formation, indicates that PPE impairs the cellular transition from hypertrophic chondrocytes to osteoblasts. Collectively, impaired chondrocyte-to-osteoblast transdifferentiation represents a cellular mechanism contributing to PPE-induced fetal long-bone dysplasia.