I agree that those “smudges” just above the villi could be anything.

MTCO2 only immunostains mitochondria in the cells of the teratoma ( human).

This ab is also negative, in my hands under identical conditions, for monkey, pig, marmoset. The epitopes must be human -specific?

Eg:   the mouse skeletal muscle is negative in the lower image.

MTCO1 immunolabels mouse and human mitochondria ( as well as many other species, under identical conditions).