Abstract
The mechanism of induction of micromelia in6-day-old chick embryo by 6-aminonicotinamide (6-AN) was investigated. Six-day-old chick embryo exposed to 6-AN did not show micromelia when tenfold excess of nicotinamide over 6-AN was co-administered. The ability of nicotinamide to prevent the induction of micromelia was partially offset after 4hr of exposure to 6-AN and completely disappeared after6hr. The length of time necessary for the induction of micromelia was not affected by the concentration of 6-AN. These results indicate that exposure to 6-AN for only a short period of 6 hr is sufficient to commit the limb to micromelia and that cellular components involved in the induction of micromelia alter during this period. During this period, newly synthesized proteoglycan monomers typical of cartilage decreased in average molecular size, and isolated femora did not respond to parathyroid hormone (PTH) but to dibutyryl cyclic AMP to stimulate growth of cartilage in organ culture.
