Exploring placental ultrastructure: A review of electron microscopy techniques and emerging methods for resolving 3D organelle architecture.

Trophoblast cells line the surface of placental villi, facilitating the exchange of nutrients, gases, and wastes between the maternal and fetal circulations. The fusion of cytotrophoblast (CTB) cells into the surrounding multinucleated syncytiotrophoblast (STB), is accompanied by a shift in cellular ultrastructure (subcellular architecture). Mitochondria undergo a remarkable decrease in size and alteration in morphology following trophoblast differentiation, and have thus been the subject of investigations due to their crucial role in producing energy for placental development. Observing this shift in structure has relied on the use of electron microscopy, which has offered insights into underlying mitochondrial functions. Since the initial use of electron microscopy to study villous trophoblasts in the 1950s, novel techniques have emerged that have the capacity to interrogate placental ultrastructure with unprecedented resolution. This review discusses the evolution of electron microscopy techniques to study the placenta over the last 70 years. Moreover, we discuss emerging methods for resolving 3D organelle structure within the placenta, which offer more physiologically pertinent information and context for complex topologies. Further, we discuss advanced methods of cryo-electron tomography (cryo-ET) that present the placental field with an exciting opportunity to determine the complex relationship between mitochondrial architecture and protein structure in the human placenta. By specifically focusing on mitochondrial imaging, we showcase the capacity for volume electron microscopy and cryo-ET to reveal the role of organelle structure in placental development.