Metabolic state of human blastocysts measured by fluorescence lifetime imaging microscopy

Mammalian embryos undergo large changes in metabolism over the course of preimplantation development. Embryo metabolism has long been linked to embryo viability, suggesting its potential utility in Assisted Reproductive Technologies (ART) to aid in selecting high quality embryos. However, the metabolism of human embryos remains poorly characterized due to a lack of non-invasive methods to measure their metabolic state. Here, we explore the application of metabolic imaging via fluorescence lifetime imaging microscopy (FLIM) for studying human blastocysts. We use FLIM to measure the autofluorescence of two central coenzymes, NAD(P)H and FAD+, in 215 discarded human blastocysts from 137 patients. We find that FLIM is sensitive enough to detect significant metabolic differences between blastocysts. We show that the metabolic state of human blastocysts changes continually over time, and that variations between blastocyst are partially explained by both the time since fertilization and their developmental stage, but not their morphological grade. We also observe significant metabolic heterogeneity within individual blastocysts, including between the inner cell mass and the trophectoderm, and between the portions of hatching blastocysts within and without the zona pellucida. Taken together, this work reveals novel aspects of the metabolism of human blastocysts and suggests that FLIM is a promising approach to assess embryo viability through non-invasive, quantitative measurements of their metabolism.