Studies on zona hardening in rat oocytes that are matured in vitro in a serum‐free medium

Publisher Website
Google Scholar
Abstract
The present study confirms that zonae pellucidae of rat oocytes became resistant to chymotrypsin digestion (zona hardening) after undergoing in vitro maturation (IVM) in a serum‐free medium. However, zona hardening and did not occur when empty zonae without oocytes were cultured in the same IVM conditions, suggesting that oocyte‐derived factor(s) is responsible for zona hardening in oocytes matured in the serum‐free medium. Zona hardening occurred primarily after dictyate oocytes were cultured for 6–8 hours in the IVM medium without serum. Zona hardening could be prevented or alleviated if oocytes were cultured in the IVM medium containing bovine foetal calf serum, a soybean trypsin inhibitor, or β‐mercaptoethanol, and in vitro fertilization rates for such oocytes were normal. Possible mechanisms of the phenomenon of zona hardening in oocytes matured in serum‐free conditions are discussed in relation to its possible relevance to the cortical reaction and the physiological block to polyspermy.