DNA is transcribed into messenger RNA (mRNA) inside the nucleus of the cell. mRNA is a sequence of codons, which consist of 3 base pairs each. It is transported from the nucleus to the cytoplasm where it may undergo translation into a protein by way of a ribosome. Most codons represent a specific amino acid, but they may also serve as stop signals.

The ribosome reads the codons off of the mRNA and translates that information into a sequence of amino acids. In order to know which amino acid to use for each codon, it uses another kind of RNA, the transfer RNA (tRNA). Each tRNA contains an anticodon and a covalently linked amino acid. This anticodon is the complement of the codon that codes for that specific amino acid, and it is used to recognize the codon and match it to the correct amino acid. Once the ribosome finds a matching tRNA, it takes its amino acid and adds it to the protein.

You can see a picture of this process here:

https://upload.wikimedia.org/wikipedia/commons/0/0f/Peptide_...

Why did I explain all this? Because in the first article, the following is said:

> we confirm and extend prior observations that sperm undergo a dramatic switch in the RNA payload from piRNAs to tRNA fragments (tRFs) upon exiting the testis and entering the epididymis.

So, as sperm passes through the epididymis, it loses these piRNAs and gains tRNA fragments. What are these tRNA fragments? They seem¹ to be incorrectly synthesized tRNA. The tRNAs don't just appear out of nowhere, they're synthesized in the nucleus, just like mRNA. Before they're ready to be used for translation, they must undergo various processes such as aminoacylation. It seems faulty (incorrectly folded?) pre-tRNA molecules are identified and cut up into these tRNA fragments.

The authors of the article have found that this process occurs frequently in the epididymis and that the tRNA fragments are exported to the spermatozoa through vesicles:

> We found that tRNA cleavage occurs robustly in the epididymis and that small vesicles secreted by the epididymal epithelium, known as epididymosomes, carry a very similar population of small RNAs to that gained by sperm during epididymal transit

The epididymis sends tRNA fragments to sperm cells. So what? That's where the second article comes in: the authors compared the development of embryos fertilized by both pre-epididymis and post-epididymis sperm. They found that the pre-epididymis embryos expressed too many regulatory factors during development and were not able to successfully develop.

> We find that caput-derived embryos significantly overexpress ∼50 genes primarily encoding regulatory factors (RNA binding proteins and chromatin-associated factors)

If they overexpressed regulatory factors and failed to develop, it can be inferred that they underexpressed the actual genes and proteins required for development. This apparently causes what they describe as "pleiotropic defects in implantation and post-implantation development".

Meanwhile, the post-epididymis embryos were able to develop normally. It can be inferred that these normal embryos did not express as many regulatory factors, and that the tRNA fragments gained in the epididymis suppressed the expression of such factors.

It is already known in medicine that embryo-associated risk factors for abortion will probably lead to abortion within the first couple of weeks. This seems to be what's happening here: an epigenetic factor intrinsic to the embryo prevented its development past a certain early point. In this case, the female will likely not even know she was pregnant. This seems to be a rather common occurrence. Nevertheless, it's still quite interesting to figure out exactly why this happens.

Since the sperm of all males must pass through the epididymis, this seems to be most relevant for the science assisted reproduction and the procedures involved. It is my understanding that this knowledge can help decide the precise location in the genitourinary tract from which to surgically extract a male's sperm. Also, injection of the epididymal small RNAs into the embryos is apparently sufficient to rescue it from abortion, so this research might form the basis of a prophylactic procedure.

¹ https://www.ncbi.nlm.nih.gov/pubmed/21976287