Temporal relationship between fetal bovine skeletal growth and circulating hormonal levels

Skeletal growth and serum hormonal levels in bovine fetuses were studied cross-sectionally from late first trimester to late third trimester of gestation (corresponding approximately to 75-275 days of age, and a crown-rump (CR) 5-105 cm/in size). Measurements of tibial and femoral lengths showed that bone growth proceeds at a 30% faster rate in fetuses of 50-105 cm CR, coincident with the appearance and exponential growth (y = 0.000207 e 0.12522 x; y, dried weights of ossification in grams and x, CR in cm) of a secondary center of ossification in the epiphysis. During this period there is an increase in the proportion of [3H]-thymidine-labeled nuclei (measured by autoradiography) in the proliferative zone of the epiphyseal growth plate and a progressive hypertrophy of chondrocytes in the epiphysis; in serum there is a rise in alkaline phosphatase activity, a rise in the calcium, and a decrease in the phosphorus concentrations. Cellularity (nuclei/area) and the proportion of [3H]-thymidine labeled nuclei in epiphyseal cartilage decline during the period of 15-105 cm CR, except at 25-45 cm CR when both parameters of chondrocyte growth have transiently stabilized. [35S]-Sulfate and [3H]-proline incorporation (cpm/100 micrograms DNA) in epiphyseal cartilage also decline initially during 10-25 cm CR, then attain a stable level during 25-50 cm CR; subsequently, [35S]-sulfate incorporation gradually increases and [3H]-proline incorporation remains at a constant level. The proportion of [3H]-thymidine-labeled nuclei in the epiphyseal growth plate also declines in early gestation, then becomes stabilized at 20-50 cm CR. The whole growth plate thickness varies during gestation and is maximal during 20-50 cm CR. The proliferative zone attains maximal thickness at 20-50 cm CR while the hypertrophied and degenerative zone has maximal thickness at 40-80 cm CR. Gestational changes of hormone levels were quantitated in fetal serum. Glucocorticoids and thyroxine were measured by radioimmunoassay; somatomedinlike bioactivity was measured as the capacity of a serum sample to stimulate [3H]-thymidine incorporation in chondrocytes compared to that of a control serum. Temporally related to the changes occurring in the skeletal tissues, there is a high serum level of glucocorticoids at 10-20 cm CR when the cartilaginous activities are declining, a peak level of serum somatomedinlike bioactivity at 20-50 cm CR when cartilage growth and metabolism become stabilized, and a rise of thyroxine level after 45 cm CR during which time there is an increasing rate of bone formation.(ABSTRACT TRUNCATED AT 400 WORDS)