Anatomical analysis of the developmental effects of aneuploidy in man: the Down syndrome.

Detailed anatomical dissections of five Down syndrome (DS) bodies revealed a unique and consistent "internal phenotype" composed of: 1) variations in muscles, 2) peripheral artery variations, and 3) the presence of dilatations and nerve rootlets associated with the spinal accessory and first cervical nerves. Most of these variations may occur occasionally in the normal population, but their more frequent occurrence in the five DS cases provides further support for Shapiro's (1975) hypothesis that one effect of the extra autosome is to increase the instability of normal developmental processes. Three of the variations: 1) the presence of an extra facial muscle, 2) multiple vertebral arteries, and 3) the presence of dilatations and nerve rootlets associated with the spinal accessory and 1st cervical nerves, may be the result of a failure of regression of otherwise transient embryonic structures. A variation of the midfacial muscles of expression, explained as an arrest in development, has not been described previously and, to date, remains a manifestation unique to the trisomy 21 phenotype. Aside from their use in developmental speculation, the sum of these variations may be useful for corroborating diagnoses of suspected post-mortem cases of the DS without cytogenetic confirmation.