Subcellular redistribution of the mitochondrial PG2 epitope during development from cleavage to primordial germ cell...

Mitochondria are central players in diverse cellular functions and their efficient functioning has dramatic impact on embryonic development. Apparently, proliferation and transmission of well functioning mitochondria to the next generation require ingeniously adapted mechanisms, one of which, the 'mitochondrial bottleneck', is thought to occur early in mammalian development during primordial germ cell (PGC) specification. We used an antibody directed against the mitochondrial PG2 epitope, a reliable marker of primordial and adult female germ cells to monitor mitochondrial differentiation in the early rabbit embryo. Early development shows the PG2 epitope either tightly mitochondria-associated (zygote) or diffusely distributed throughout cytoplasm (cleavage stages). Mitochondrial colocalization of the PG2 epitope is regained in the early blastocyst but expression is then retained by the hypoblast and epiblast only, with the epiblast, although the forerunner of PGCs, showing weak and diffuse labeling only. At gastrulation, hypoblast cells lose PG2 expression but intensive PG2 labeling is found again on all mitochondria in the first PGCs and reveals the number of mitochondria to be in the range of 50 to 100 per PGC at this stage. The results highlight the dynamics of PG2 expression during early development and the usefulness of the epitope for testing the bottleneck theory.