Cryopreservation of boar spermatozoa offers an effective means of long-term storage of important genetic material. Many
researchers have investigated how to improve reproductive performance by artificial insemination (AI) using cryopreserved
boar spermatozoa. Recently, we and other groups reported that high conception rates (70–80%) can be achieved by AI
with frozen-thawed boar spermatozoa using a modified temperature program during freezing, or a novel cryopreservation
extender to improve sperm quality (including sperm survivability, motility, membrane status and fertilization ability) after
thawing, or a novel sperm infusion method, deep intra uterine insemination. However, these techniques have not yet been
used for commercial pig production. The variation in sperm freezability among boars or among ejaculations in an identical
boar is one of the main reasons for this problem. In our previous study, it was revealed that some components of seminal
plasma have a negative effect on the freezability of boar sperm. One of these factors is bacteria-released endotoxin
(lipopolysaccharide: LPS). LPS binds to Toll-like receptor-4 (TLR-4) expressed on the sperm surface, resulting in induction of
apoptosis. On the other hand, seminal plasma suppresses cryo-capacitation induced by thawing stress. On the basis of
these findings, we designed a novel protocol of AI using frozen-thawed boar sperm.