Intracytoplasmic Sperm Injection in Livestock Animals: Historical Development, Fundamental Steps, and Species-Specific Approaches

Intracytoplasmic sperm injection (ICSI) is an assisted reproductive technique aimed at achieving fertilization by microinjecting a spermatozoon directly into the cytoplasm of an oocyte at the metaphase II (MII) stage. ICSI particularly aims to increase the possibility of fertilization in infertility cases associated with sperm motility and low sperm number. Although the ICSI technique became widely adopted following its successful application in human clinical practice in the early 1990s, pregnancy rates after the procedure are lower in livestock animals (cattle, sheep, goats, horse) due to several species-specific constraints. It has been reported that the low embryo development rates are caused by insufficiencies in post injection oocyte activation, delays in sperm chromatin decondensation, and disruptions in male and female pronucleus synchronization. Since the stages of the ICSI procedure, including oocyte collection and maturation, selection of spermatozoa, the microinjection technique, oocyte activation, and culture, contain species-specific differences, this clearly demonstrates that the procedure must be optimized according to the biological characteristics of each species. This review provides an overview of the historical development of ICSI in livestock animals, outlines the key steps of the procedure, highlights species-specific factors influencing its success, and summarizes current strategies developed to overcome these limitations.