Paired twisted filaments in human pinealocytes.

Paired twisted filaments (PTF) have been confirmed and described ultrastructurally in the normal human pineal gland for the first time. The PTF showed a peculiar double helical structure, measuring 12 to 25 nm in maximal helix width with a half-periodicity of 30 to 35 nm (periodicity of the constrictions). Each filament was about 10 nm wide. The PTF formed parallel aggregates in the perikaryotic cytoplasm of the pinealocytes. In 8 of 12 autopsied middle-aged to elderly individuals, the PTF were present in a small proportion of pinealocytes. The identity of the PTF remains unclear in terms of their apparent similarity to the paired helical filaments observed previously in aged rats. However, the age distribution of individuals with PTF suggests that the intermediate filaments of human pinealocytes are more susceptible to early deterioration during aging than those of other cerebral neurons.