Neuronal survival and peripheral tissue size during human embryogenesis: study of a partial twinning

The relationship between somatic tissue size and development of the nervous system was studied in a partially divided human uniovular twin. In the neck region where tissues were symmetrical in each twin the spinal cords, sensory ganglia and sympathetic ganglia were also normally formed. The bodies remained fused below the necks, but the spinal cords and related neuronal structures had developed separately. Mirror image asymmetry of tissue size existed in relationship to each spinal cord. The asymmetry was reflected in size and neuron content of motor and sensory structures. Stellate ganglia on the sides of the missing arms were also hypoplastic. This abnormal human development reproduces animal experiments in which limb ablation reduces embryogenetic survival of related neurons. It is indicates that human neurons are affected by alterations in tissue bulk during ontogenesis.