In vitro fertilization in the rat: Observations on living eggs

The purpose of this study was to observe and record some of the key events during in vitro fertilization in the rat. Freshly ovulated eggs were incubated with epididymal spermatozoa at 37°C and removed 4.5 to 7.5 hours later for microscopic examination. The head of the fertilizing spermatozoon penetrated the zona pellucida with its long axis perpendicular to the zona: this orientation was maintained during subsequent incorporation into the vitellus. Sperm motility was drastically reduced soon after sperm-egg fusion. Simultaneously, the flagellum, most of which was still outside the zona, assumed a characteristic curved posture. Time-lapse cinematography demonstrated that the vitellus frequently underwent surface movements during the tail incorporation process, suggesting that its cortex was undergoing significant changes. This study presents the first long-term observations on the fertilization of living rat eggs in vitro.