Intracytoplasmic sperm injection (ICSI) in small ruminants

Small ruminants are an important component of the global production systems of meat and wool, and their reproductive biology is well known. However, the incorporation of assisted reproduction techniques (ART) in the production systems of small ruminants is not as well developed as for other domestic species. Normally, production systems that incorporate ARTs are restricted to artificial insemination or in vivo embryo transfer. Intracytoplasmic sperm injection (ICSI) is one of the ARTs techniques reported for small ruminants and consists of the injection of spermatozoa inside an oocyte, bypassing the natural process of sperm-oocyte interaction. In goats and sheep, there are few live births by ICSI reported, with no reports from other species of small ruminants. Currently, there has not been intensive research about ICSI in small ruminants. However, ICSI has potentially important applications in animal production systems, primarily its use with semen of valued animals, with epididymal sperm, in the fertilization of prepubertal or cryopreserved oocytes. Other applications include more advanced techniques, such as transgenic-ICSI or its combination with spermatogonial transplantation. In this article, we review the "state of the art" of this technique in small ruminants including its historical development, research needs for its improvement and future applications.