Fig. 1

Extracellular miRNAs participant in embryo-maternal interactions. MicroRNAs (miRNAs) are synthesised in the nucleus as primary miRNA (pri-miRNA) transcripts. Pri-miRNAs are then processed by Drosha to form nucleotide hairpin known as miRNA precursors (pre-miRNA). Later pre-miRNAs are transported by exportin-5 from nucleus into the cytoplasm where they are cleaved by Dicer into small RNA duplex. Eventually, the guide miRNA strand is loaded onto Argonaute (AGO) protein family to form the RNA-induced silencing complex (RISC),leading to either mRNA cleavage or translational repression. MiRNAs can be secreted by cells through multiple forms: associated with proteins (e.g. Argonaute family or AGO, nucleophosmin 1), bound to lipoproteins (High-density lipoprotein), encapsulated into apoptotic bodies or membrane-bound extracellular vesicles (EVs). Uterine- and embryo-derived miRNAs have been reported to be associated with exosomes and AGO1, it remains unclear whether other forms of packaging are applied by endometrial cells and/or embryonic cells (question marks)