Mitochondria provide the cell with energy as well as integration of metabolic pathways for biosyntheses of heme, iron–sulfur clusters, and nucleotides, triggers for cell apoptosis, and reactive oxidative species signaling and includes a circular mitochondrial DNA (a few thousand copies) of about 16.5kb which encodes for some peptides which are required for oxidative phosphorylation, which are transcribed and translated to mt-mRNA, mt-rRNAs and 22 mt-tRNAs, and their own mitoribosomes within the mitochondrial matrix. Many other protein (several hundred) are encoded by nuclear DNA, transcribed, modified in the cytoplasm, then imported into mitochondria. Nucleoids (the mtDNA and associated proteins are ascribed a size (not by me) of something around 100nm) (which means they should be visible in routine transmission electron microscopy.
Looking through many micrographs from my own collection i found several which had unique inclusions within a membrane space (maybe a crista space) which reminded me of the fine texture of DNA in an apoptotic cell which had been digested down into the 2000 to 250bp fragments (laddering of nuclear DNA), actually maybe even finer texture. red dots=approximate size of a cytoplasmic ribosome, bars=approximately 270nm. several micrographs derived from 2 experimental mice, hepatocyte specific knock out of the Gclc gene. 50days old. Mitochondrial changes have been reported (HEPATOLOGY 2007;45:1118-1128) in these animals previously, but intra-cristae inclusions were not reported. These mitochondria have an extreme expansion of the matrix area, pressing cristae to the sides, of the mitochondrion.  Interestingly, there really isn’t a lot of substructure to the matrix (as one might expect some mitoribosomes or mtDNA clusters