Mechanisms of twinning II.: Laterality and intercellular bonding in monozygotic twinning

OBJECTIVE: To utilize observations on laterality and handedness to gain insight into the initiating mechanism of monozygotic human quadruplets. STUDY DESIGN: In a study of thumbprints and handedness in sets of monozygotic quadruplets, homolateral and heterolateral comparisons were used to distinguish mirror-image from symmetric divisions. RESULTS: Data were consistent with a mechanism involving a mirror-image duplication in the first division and a subsequent symmetric separation of the daughters. In addition, these data failed to substantiate a genetic linkage between dermatoglyphics and handedness. CONCLUSION: Reported and reviewed data tend to support the analogy of monozygotic polyembryony in armadillos and humans. The plausible roles of delayed implantation and intercellular bonding are explored, especially as they relate to ambient calcium concentration. Factors tending to enhance the likelihood of monozygotic twinning include calcium channel blockers, lactation, ovulation induction and prolonged in vitro fertilization incubation.