FACTORS GOVERNING THE STIMULATION OF EMBRYONIC CHICK CARTILAGE BY SOMATOMEDIN

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Abstract
In the stimulation of embryonic chick pelvic rudiments by somatomedin (Sm) in serum, cartilage weight and duration of cartilage exposure to serum determine the stimulation of cartilage by serum and the relative stimulation by Sm and by other non-Sm serum components, respectively. These factors are critical for the use of this system to measure Sm in serum. Initial experiments revealed that incorporation of sulfate (SO4) by cartilage incubated in buffer fell rapidly after 9 h and reached very low levels by 30 h. Incubation in 40% normal serum (NHS) produced significant stimulation of incorporation of SO4 after 4 h, and maintained at least initial levels of incorporation for 24 h. The greatest percentage stimulation by NHS over buffer was seen with prolonged incubation (44 h). Specificity for Sm (discrimination between NHS and hypophysectomized human serum (HHS)) was greater with shorter incubation times. The potency of HHS was 11, 42 and 92% of the potency of NHS following early, intermediate and late measurement of incorporation of SO4 by cartilage, respectively. The best overall results were obtained with intermediate incubation time and measurement of SO4 incorporation ((35S)-SO4 present for the final 5 h of a 25 h incubation), which allowed good precision (.lambda. = 0.17) while maintaining satisfactory specificity for Sm. Since prolonged incubation with late measurement of SO4 incorporation allowed the greatest percentage stimulation by serum with little differentiation between NHS and HHS, stimulation of incorporation of SO4 under these conditions is apparently due to non-Sm factors present in both NHS and HHS. In addition to incubation time, cartilage stimulation by serum was also determined by cartilage weight. Lighter cartilage (from younger embryos) was associated with higher unstimulated incorporation of SO4 (P < 0.01), lower stimulation by added serum (P < 0.01), and inadequate assay precision (P < 0.05): satisfactory assays were generally obtained with cartilage rudiments weighing more than 0.7 mg (dry weight).

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