Sperm freezing and in vitro fertilization in three substrains of C57BL/6 mice

Traditional protocols for sperm recovery, cryopreservation, and in vitro fertilization (IVF) have been considerably less efficient for inbred mouse strains, including C57BL/6, than for hybrid and outbred strains. We report here that 3 changes to published and widely used protocols markedly improved fertilization rates for both fresh and frozen-thawed sperm in 3 substrains of C57BL/6 mice (C57BL/6J, C57BL/6NCrl, and C57BL/6NTac). First, the traditional cyroprotective agent was modified by adding amino acids. Second, preincubation of sperm in a preincubation medium containing methyl-beta-cyclodextrin and polyvinyl alcohol enabled collection of progressively motile sperm for IVF. Third, we evaluated 3 media for IVF: human tubal fluid (HTF), modified Krebs-Ringer bicarbonate medium (TYH), and minimal essential medium (MEM). HTF and TYH were modified by adding minimal essential amino acids. The methodology reported here increased the IVF rate of both fresh and frozen-thawed sperm and enabled efficient isolation of capacitated viable sperm. Fertilization rates greater than 65% and 40% were obtained with the 3 tested substrains when fresh and frozen-thawed sperm, respectively, were used for IVF. Higher fertilization rates were seen with frozen-thawed sperm from C57BL/6NCrl and C57BL/6NTac mice than from C57BL/6J mice. Among all strains, fresh sperm from C57BL/6NTac mice gave the highest fertilization rate. Of 190 two-cell embryos, 63 (33.2%) developed to term after transfer to pseudopregnant recipient mice. The protocol we detail here provides reliable cryopreservation and recovery of live mice in 3 substrains of C57BL/6, making sperm cryopreservation and IVF a viable choice for preservation and distribution of mouse lines.