Pluripotent Stem Cell Metabolism and Mitochondria: Beyond ATP

Figure 2

The dynamic localisation and morphology of mitochondria through human development and in culture. Mitochondrial morphology and localisation is determined by the developmental stage and metabolic requirements of the cell [133, 134]. Morphologies in the developing embryo range from spherical organelles with dense matrices and few peripheral arched cristae to long filamentous organelles with sparse matrices and many transverse cristae that maximise the surface area for OXPHOS. The mitochondria also localise strongly with the nucleus and other organelles throughout embryo development to provide ATP for growth and likely to maintain a signalling axis with the nucleus. In primordial germ cells (PGCs), both before and during migration to the gonadal ridge, the mitochondria localise strongly with the nucleus (perinuclear), maintaining a large, vacuous morphology, containing only small vesicular cristae and no transverse cristae [136]. The PGC mitochondrial matrix is clear, suggesting a low level of oxidative activity. During migration, mitochondria increase in number and overall mass. Nine weeks postfertilisation, the PGCs begin to differentiate into the oogonia; by 12 weeks, they begin expansion through mitotic divisions; and by 16 weeks, meiosis commences [190]. During the second stage of prophase in meiosis, zygotene (where the chromosomes closely associate), the mitochondria tightly envelop the nucleus. During the diplotene stage of prophase, when the chromosomes separate, the mitochondria and most other organelles localise to one side of the nucleus forming Balbiani’s vitelline body [191, 192]. It is at this point that the human oocyte arrests until hormonal stimulation up to 50 years later [193]. Upon hormonal activation, the oocyte progresses through folliculogenesis. The primary oocyte contains many spherical mitochondria with very dense matrices and few peripheral arched cristae [135]. Notably, these mitochondria are dispersed throughout the cytoplasm and form complexes with the smooth endoplasmic reticulum (SER) and vesicles [136]. These complexes gradually dissipate throughout ovulation and fertilisation. At the 2 pronuclei (2PN) stage, the mitochondria cluster around the 2PN and the initial fission/fusion events take place giving rise to “dumbbell”-shaped mitochondria although the prevailing morphology is still spherical. During the initial cleavage events, elongated mitochondria begin to emerge approximately 2-3 times the length of the spherical mitochondria with well-developed transverse cristae. During the morula and early blastocyst stages, the ratio of elongated to spherical mitochondria increases, such that by the late blastocyst stage in vivo, there is an approximately even mix in both the inner cell mass (ICM) and trophectoderm cells [135, 136, 139]. This mix of mitochondrial morphologies is also observed in the mouse ICM and trophectoderm cells [141]. Notably, in the blastocyst, the mitochondrial matrix becomes clear while the perinuclear localisation and arching cristae phenotype is retained [135, 139]. In vitro hESC mitochondria are similarly perinuclear with few arching cristae and have clear matrices, although their morphology is almost exclusively spherical with a notable absence of the in vivo elongated mitochondria [30, 142]. After seven days of spontaneous differentiation, hESC take on the mixed mitochondrial population [142]. Somatic cell mitochondria are dispersed throughout the cytoplasm and are often highly elongated, reticulated, and bulbous. Their matrices are dense and their cristae are developed and transverse [30], likely a reflection of the more oxidative nature of somatic cell metabolism. N, nucleus (purple); cytoplasm (blue); electron dense mitochondrial matrix (red); electron sparse mitochondrial matrix (pink).