Variations in macronuclear chromatin structure and chromatin extrusion in excystment from resting cysts of Colpoda inflata

The macronucleus of Colpoda inflata was studied by image analysis to determine variations that occur in chromatin structure during excystment from resting cysts. Morphometric and densitometric parameters, 22 Markovian chromatin texture variables, and the presence of chromatin extrusion bodies were examined in 1-, 5-, and 10-day-old excysting cells. Marked variations in densitometric values and Markovian variables were found only in 1-day-old excysting cells, thus suggesting that the chromatin extrusion process, where not already accomplished in early encystment, is triggered by excystment. Comparative study of the mean light absorption histograms computed on optical microscopic images of excysting cells, derived from 2- and 25-day-old cysts of a standard culture and from 1-year-old cysts of a senescent culture, showed three and two classes of histograms, respectively, characterised by differently condensed chromatin. Moreover, as excystment progresses, the percentage of macronuclei with decondensed chromatin that also show an extrusion body of condensed chromatin increases, thus suggesting that the macronucleus is somehow renewed by extruding chromatin that is unable to decondense. This event appears as a possible mechanism responsable for the 'rejuvenescence' of senescent cell lines.