Sperm migration in the female genital tract controls sperm selection and, therefore, reproductive success as male gametes are conditioned for fertilization while their number is dramatically reduced. Mechanisms underlying sperm migration are mostly unknown, since in vivo investigations are mostly unfeasible for ethical or practical reasons. By presenting a spatio-temporal model of the mammalian female genital tract combined with agent-based description of sperm motion and interaction as well as parameterizing it with bovine data, we offer an alternative possibility for studying sperm migration in silico. The model incorporates genital tract geometry as well as biophysical principles of sperm motion observed in vitro such as positive rheotaxis and thigmotaxis. This model for sperm migration from vagina to oviducts was successfully tested against in vivo data from literature. We found that physical sperm characteristics such as velocity and directional stability as well as sperm-fluid interactions and wall alignment are critical for success, i.e. sperms reaching the oviducts. Therefore, we propose that these identified sperm parameters should be considered in detail for conditioning sperm in artificial selection procedures since the natural processes are normally bypassed in reproductive in vitro technologies. The tremendous impact of mucus flow to support sperm accumulation in the oviduct highlights the importance of a species-specific optimum time window for artificial insemination regarding ovulation. Predictions from our extendable in silico experimental system will improve assisted reproduction in humans, endangered species, and livestock.

Since the discovery of Förster resonance energy transfer (FRET) in the 1940s, its implementation to measure inter-and intramolecular distances has reached wide-spread use in the biological sciences. Because the technique requires labeling of a donor and acceptor site with two different spectroscopically active molecules, a prominent difficulty with biological samples has been how to label with specificity and, therefore, unambigouisly detect and assign the signal contribution from each site. Here we show that transition metal ion FRET (tmFRET) experiments using plasma membrane sheets prepared from deroofed cells can be used as an orthogonal labeling scheme with an unambiguous assignment of the FRET pair signal. As a FRET donor, we used octadecyl rhodamine B (R18), which incorporated into the plasma membrane of deroofed cultured cells. To measure distances on the length scale relevant to membrane proteins, we used the transition metal ion Co 2þ as an acceptor, which, together with R18, gives a R 0 value of 14.9 Å . Although the quenching by Co 2þ of rhodamine B in solution is of low affinity (>10 mM), 100 mM Co 2þ caused appreciable quenching of R18 incorporated into plasma membrane sheets. By first introducing a synthetic fatty acid with a metal-chelating head group, appreciable quenching was observed with less than 1 mM Co 2þ . Comparison of tmFRET experimental measurements to expected theoretical values convincingly demonstrate that these experiments provide information about the distance of closest approach between donor and acceptor (Fung and Stryer, 1978). These experiments provide proof-of-priniciple for measurements of biologically relevant distances in native membranes. We anticipate that FRET experiments on plasma membrane sheets will offer a unique opportunity to explore protein-lipid interactions and discuss several possible future directions for this technique.

Leibniz Institute for Zoo and Wildlife Research silico experimental system will improve assisted reproduction in humans, endangered species, and livestock. fertilization | mathematical modelling | sperm migration | ABM Correspondence: edda.klipp@rz.hu-berlin.de, mueller@izw-berlin.de Bibliography 1. Patrick Abbot and Antonis Rokas. Mammalian pregnancy. Current Biology, 27(4):