Triiodothyronine stimulates maturation of porcine growth-plate cartilage in vitro

We studied the effect of triiodothyronine (T₃) on mammalian growth-plate cartilage in vitro. Growth-plate cartilages from fetal pigs scapulae were incubated for 3 or 7 d in serum-free medium alone or median containing T₃. Alkaline phosphatase activity, a marker of hypertrophied chondrocytes, was increased in T₃ (10 nM)-treated growth-plate cartilage 152 ± 36% above that of cartilage incubated in medium alone after 3 d of incubation, and 324 ± 47% after 7 d of incubation. There was a dose-response increase in alkaline phosphatase activity to T₃ over the range of 0.01-10 nM. The rise in alkaline phosphatase activity was specific for T₃ since growth-plate cartilage alkaline phosphatase activity was not increased by cortisol, insulin, parathyroid hormone, or 5% fetal calf serum. Histological studies of growth-plate cartilage showed that T₃ in a concentration-dependent manner increased the width of the zone of maturation (hypertrophied chondrocytes). Histochemical staining for alkaline phosphatase activity demonstrated that T₃ caused the recruitment of new cells into the zone of maturation. T₃ also stimulated incorporation of L-[³H]leucine into protein and ³⁵SO₄ into proteoglycan in growth-plate cartilage. In contrast, T₃ did not increase alkaline phosphatase activity or radiolabeled precursor incorporation into nongrowth-plate scapular cartilage. These studies demonstrate that T₃ directly stimulates maturation and, to a lesser degree, growth-related processes in fetal mammalian growth-plate cartilage.