Some planetary nebulae (PNe) show microstructures of low ionization located in their outer regions, i.e. external to the bright rim defined by the fast vs. slow wind interaction region. Morphological and kinematic analysis of some of these PNe show low ionization knots which clearly share the expansion of the outer shell of the host nebulae (e.g. IC 2553). Sometimes, these knots possess an accentuated degree of symmetry. Given that in situ instabilities are not thought to form symmetrical features, we explore the idea that these symmetric pairs of knots are fossil condensations of the AGB wind being modified by interaction with the expanding ionization front. This very appealing possibility would imply that the mass loss of the pre-PN stages has episodes of highly collimated and symmetric ejections. In other PNe knots of low ionization in the outer shells are not symmetric (e.g. NGC 5882) and could be formed by either simple in situ instabilities and/or fossil condensations.