—Mini Review— The early history of the TYH medium for in vitro fertilization of mouse ova

The TYH medium was first reported as a medium for in vitro fertilization (IVF) of mouse eggs with epididymal spermatozoa, by Toyoda, Yokoyama and Hosi, in 1971. It was a modified Krebs-Ringer bicarbonate solution, supplemented with glucose, Na-pyruvate, antibiotics and bovine serum albumin. In TYH medium, almost all eggs are fertilized within 1 h, if the spermatozoa are pre-incubated for 2 h before insemination, while the sperm penetration is delayed for approximately 1 h when fresh epididymal spermatozoa are used. These findings showed that sperm capacitation can be induced in a chemically defined medium without requiring the female reproductive tract. Although the medium was not specifically named in the original paper, it was later called “TYH” after the initials of the three authors of the original paper. The IVF method using TYH medium is widely used due to its high reproducibility. In this mini-review, we describe the early efforts to develop the TYH medium and briefly discuss the related areas.