Totipotency is an incredibly plastic and transient state during embryonic development which is restricted to the zygote and 2-cell stage in mice. Totipotent embryos are unique in their chromatin architecture, metabolism and transcriptional status. Studying this window of development is important to better understand how normal development at these early stages occurs, and to obtain conceptual and methodological tools to better manipulate the potency of cells in culture. During my PhD I first studied the metabolism of totipotent cells by measuring the mitochondrial membrane potential of 2-cell embryos and comparing it to pluripotent inner cell mass and differentiated trophectoderm of blastocysts. My results show that although these two embryo stages differ in their oxygen consumption and mitochondria matrix shape, there are no major changes in mitochondrial membrane potential. The objective of my PhD was the characterization of the biophysical properties of heterochromatin during the process of chromocenter formation that occurs at the 2-cell stage. I first identified a set of core heterochromatic proteins and revealed their higher potential to phase separate based on an in silico analysis. Using a wide variety of microscopy techniques, I showed that pericentromeric heterochromatin transitions from a liquid state to a more solid or gel like state during the process of chromocenter formation. Overall, my work contributes to a better understanding of the features that characterise totipotency as well as developing state of the art tools to study the biophysical properties of constitutive heterochromatin at these early stages of development.