Intracytoplasmic sperm injection in the bovine induces abnormal [Ca²⁺]_i responses and oocyte activation

Abstract

Fertilisation by intracytoplasmic sperm injection (ICSI), a technique that bypasses the membrane fusion of the gametes, has been widely used to produce offspring in humans and mice. Success with this technique has lent support to the hypothesis that in mammalian fertilisation, a factor from the sperm, the so-called sperm factor, is responsible for oocyte activation and that the fusion process is not involved in the generation of the hallmark [Ca²⁺]_i signalling seen following fertilisation. However, the success of ICSI has largely eluded large domestic species, such as the bovine, porcine and equine, casting doubt on the current model of oocyte activation at fertilisation in these species. Using Ca²⁺ imagery and a series of treatments to manipulate the chemical structure of the sperm, we have investigated the early events of oocyte activation in response to ICSI in the bovine. Our results demonstrate, for the first time, that following ICSI, the majority of bovine oocytes are unable to mount [Ca²⁺]_i oscillations, although, in few cases, the initiation of [Ca²⁺]_i oscillations can occur in a manner indistinguishable from in vitro fertilisation. We also show that bull sperm possess a full complement of sperm factor. However, either the release and/or activation of the sperm factor are compromised after ICSI, leading to the delivery of a defective Ca²⁺ stimulus, which results in premature termination of embryo development.