Prenatal ossification as an indicator of exposure to toxic agents.

Following exposure to bromodeoxyuridine (BUDR), acetazolamide (ACZM), trypan blue (TRBL), cortisone (CORT), or diphenylhydantoin (DPH), alizarin-stained, cleared fetuses were examined at 18 days postcoitus for unossified cervical vertebral centra; number of ossified caudal vertebrae; number of ribs; and ossification of sternebrae, metatarsals, metacarpals, and phalangeal rows. At all teratogenic doses, in no vehicle-treated groups, and rarely in lower-dose groups, there were significant increases in frequency of unossified cervical centra, the first vertebra (C1) being most often affected, and C7 least often affected. In the high-dose CORT group, there was a significant correlation between unossified C1 and cleft palate. No association between abnormality and reduced ossification of cervical vertebrae was seen in other series examined, nor was there any correlation between litter size and abnormality. With minor complications, the number of ossified caudal vertebrae was significantly reduced after exposure at teratogenic dose levels to all compounds except DPH. Although caudal and cervical ossification were correlated with each other in those series examined, neither was correlated with abnormality. Frequency of 14 ribs was increased in BUDR, ACZM, and TRBL but not CORT or DPH. Other parameters were essentially unaffected. Significantly increased frequency of abnormality, when contrasted with untreated or vehicle-treated groups, was seen at high-dose levels in all but DPH treatments, and mortality was increased in ACZM D9-11, TRBL, and CORT. These studies show that reduced ossification of cervical centra is an excellent indicator of prenatal exposure to noxious substances, and caudal vertebrae appear to be useful as well. Increased frequency of 14 ribs occurred for all strong teratogens utilized if they were administered on day 7 or day 8 postcoitus.