Monozygotic Does Not Always Mean Identical: An Investigation of Anatomic Variations in Identical Twins

Abstract

Identical (monozygotic) twins develop from a single zygote. As such, it is often assumed that monozygotic twins have identical genotype and phenotype; however, epigenetic and environmental influences may play a significant role in development of monozygotic twins. Anatomic variations are a result of genetic and environmental influences and are often attributed to developmental factors. It is currently unknown whether identical twins with the same genotype develop anatomic variations compared to each other and the general population; therefore, the purpose of this investigation is to document and compare neurovascular anatomic variations in monozygotic twin cadaveric donors. Monozygotic twin (XY) cadaveric donors were dissected using two techniques. An en mass autopsy protocol was utilized to dissect the viscera of the neck, thorax, abdomen, and pelvis, while a regional approach was employed to dissect the upper and lower limbs. Neurovascular anatomic variations were macroscopically visualized, and morphometric data recorded and compared between the donors. Fifteen total neurovascular variations (14 paired structures; one vein, four nerves, ten arteries) were noted in the twin donors (Table 1). Among variations, ten were identified in both donors, while two variations were only found in twin A and three found only in twin B. All ten variations in both donors were paired structures. Six of the ten paired structures demonstrated bilateral variation in both twins, one bilateral variation in twin A (unilateral in twin B), two bilateral variations in twin B (unilateral in twin A), and one unilateral variation in both twins. Of the five variations found in only one donor, four were paired structures, but the variation was only noted unilaterally. Neurovascular variations are not consistent between monozygotic twins. While the mechanism of twinning in these donors is unknown, this study is the first of its kind to demonstrate unique anatomic variations in identical twins. To our knowledge this is the first study to examine gross anatomical neurovascular variations among monozygotic twin cadaveric donors. This study provides a glimpse into the prevalence and inconsistency of anatomical variations in identical twins, which sheds new light on the environmental influences on anatomical variation. Future studies in organ morphologies and histology will further elucidate variations and potentially different pathologies between monozygotic twins.