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 determ ine variations that occur in chromatin structure during excystment from res ting cysts. Morphometric and densitometric parameters, 22 Markovian chromat in texture variables, and the presence of chromatin extrusion bodies were e xamined in 1-, 5-, and 10-day-old excysting cells. Marked variations in den sitometric values and Markovian variables were found only in 1-day-old excy sting cells, thus suggesting that the chromatin extrusion process, where no t already accomplished in early encystment, is triggered by excystment. Com parative 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, sh owed three and two classes of histograms, respectively, characterised by di fferently condensed chromatin. Moreover, as excystment progresses, the perc entage of macronuclei with decondensed chromatin that also show an extrusio n body of condensed chromatin increases, thus suggesting that the macronucl eus is somehow renewed by extruding chromatin that is unable to decondense. This event appears as a possible mechanism responsable for the 'rejuvenesc ence' of senescent cell lines.