Mitochondrial transcription factor A (TFAM) is essential for human mtDNA transcription and it is also a key regulator of mtDNA copy number. TFAM expression is tissue specific which determines the mitochondrial number or mass. There are databases regarding this query, All the best!!

This is a very complex and not completely solved question. Probably several factors involving complex nuclear-mitochondrial cross-talk add up their effects to determine the final mitochondrial mass (number is not a very useful concept here due to the dynamic nature of the mitochondrial network) in each tissue or cell type. We have published, as an example, that a factor affecting mtDNA copy number is ROS production by mitochondria.

The number of mitochondria in a tissue can be regulated by new mitochondrial production i.e. mitochondrial biogenesis or by continuous fusion or fission process. Both the biogenesis and fusion/fission are expected to be under the genetic control. It is widely known that nuclear factors are required for the mitochondrial biogenesis, but insights into the tissue-specific genetic mechanisms of fusion/fission are lacking and has become an interesting field to work upon.