Nonrandom laterality of malformations in paired structures.

Nonrandom mild asymmetric sidedness, a feature of normal morphogenesis, is an even more striking finding among unilateral malformations of paired structures. Each malformation evaluated revealed a significant nonrandom propensity to affect a particular side. Some anomalies were strikingly left-sided, such as postaxial polydactyly and cleft lip, while others were predominantly right-sided, such as fibular aplasia and radial aplasia. Of special interest is the left-sided predilection of hemiatrophy vs. the right-sided predominance of hemihypertrophy. The tendency for certain potentially unapparent anomalies, such as dislocation of the hip and renal agenesis, to be left-sided can be of practical relevance in their detection. These findings imply developmental differences between the two sides for each tissue, allowing one of the sides a greater liability for a particular type of defect in morphogenesis.