['Blastocyst biogenesis occurs over several cell cycles during the preimplantation period comprising the gradual expression and membrane assembly of junctional protein complexes which distinguish the outer epithelial trophectoderm (TE) cells from the inner cell mass (ICM). In the human, TE integrity and the formation of a junctional seal can often be impaired. Embryos likely to result in a successful pregnancy after transfer are mostly selected according to morphological criteria. Recent data suggest that non-invasive measurement of amino acid turnover may be useful to complement such morphological scores. Whether morphological and metabolic criteria can be linked to poor TE differentiation thereby underpinning developmental predictions mechanistically remains unknown.', 'We examined TE intercellular junction formation in human embryos by immunofluorescence and confocal microscopy and correlated this process with morphological criteria and amino acid turnover during late cleavage.', 'Our results show that TE differentiation may be compromised by failure of membrane assembly of specific junction constituents. This abnormality relates more closely to metabolic profiles than morphological criteria.'] Are human embryos developing in vitro susceptible to impaired epithelial junction biogenesis correlating with abnormal metabolic activity?