Craniofacial, caudal, and visceral anomalies associated with mutant sirenomelic mice

Craniofacial anomalies were correlated with mutant murine sirenomelia. Ninety-eight newborn sirens from heterozygous matings were examined and analyzed. In the 96 sirens that had intact craniofacial structures, micrognathia was seen in 39% of the sirens, microstomia in 34%, macroglossia in 26%, and cleft palate in 21%. Even when not cleft, the siren palates were narrower and more highly arched than those of nonsiren littermates. The frequency of abnormal craniofacial development was greater in those sirens that were more severely affected caudally. Even though some earlier studies had indicated a preponderance of males, 46 of the 95 sirens with intact pelvic viscera were females. Fifty-three percent of the sirens were monopodal, 35% were apodal, and 11% were dipodal. A penile-like projection on the genital tubercle occurred on 15 apodal sirens and four monopodal sirens; all but three of these sirens were males. Bladder agenesis was seen in 100% of the sirens, anal atresia in 80%, and bilateral renal agenesis in 43%. No siren was found with bilaterally normal kidneys. The srn gene responsible for sirenomelia might either directly affect the embryo at both the caudal and cranial regions or indirectly affect the embryo by producing lateral mechanical compression at both these sites. The srn gene was earlier characterized as autosomal-recessive; our data confirm this. Sirenomelia was found in only 11% of the newborns from crosses of carrier mice in the colony. Analyses of uterine contents at days 12-14 suggest that the srn gene is fully penetrant, but often lethal, during the fetal period.