Relationship of in Vitro Acrosome Reaction to Sperm Function: An Update
Sandro C. Esteves and Sidney Verza Jr.

Understanding the mechanisms by which the acrosome reaction is regulated is central to models of fertilization. This article reviews the relationship of the acrosome reaction detected in vitro to sperm function. Proteolytic enzymes in the acrosome digest through the zona pellucida allowing for sperm-oolemma fusion. When this process is impaired, either by lack of an acrosome or acrosomal dysfunction, fertility can be compromised. Optical microscopy and staining with different fluorescent lectins that bind to acrosomal membranes is the method of choice for acrosomal evaluation. Because acrosomal loss can be a result of sperm death, this test should be used in in conjunction with an assay to monitor sperm viability. Different stimulants, such as phosphodiesterase inhibitors, drugs and toxins have been investigated in their ability to affect the sperm ability to undergo in vitro acrosome reaction. Sperm acrosomes are also sensitive to the freezing-thawing process and strategies have been described to minimize cryodamage. The assessment of the acrosome has been shown to be a stable parameter of sperm function and a valid tool to predict the fertilizing potential of human spermatozoa. The acrosome reaction following ionophore challenge (ARIC) is an in vitro assay with good predictability of the sperm’s fertilizing potential for assisted conception techniques including intrauterine insemination and conventional in vitro fertilization. The AR determination has been also used as an important biomarker in studies involving drugs and toxins. Recently, novel aspects of sperm-oocyte fusion have been depicted in humans involving glycoproteins present in the zona pellucida and the female reproductive tract.