A personal tribute to Professor Ryuzo Yanagimachi

Professor Ryuzo Yanagimachi, known to many friends and colleagues as Yana, died on 27 September 2023. Yana was one of the foremost pioneers in the discoveries that led to the present day success of human IVF. I first met him at the Worcester Foundation in 1964 and we remained friends until his passing.

Born near Sapporo on Japan's largest Northern Island – Hokkaido – he completed a BSc in Biology and a PhD in Embryology at Hokkaido University. In his thesis he made significant contributions to fertilisation in fish (sperm chemotaxis) and the sexual organisation of the parasitic barnacle.

He spent his postdoctoral years (1960-1964) with M C Chang at the Worcester Foundation for Biomedical Research in Massachusetts, where he began his formative studies on the process and mechanism of mammalian fertilisation. His research with the golden hamster concentrated on the in vitro conditions required to enable the spermatozoon to penetrate the oocyte (capacitation), the acrosome reaction, sperm egg fusion, the block to polyspermy by the release of the cortical granules from the oocyte and the activation of the oocyte with the formation of the male and female pronuclei.

Unfortunately, the conditions in which he was able to demonstrate fertilisation outside the female reproductive tract did not support development of the activated oocyte beyond the formation of the male and female pronuclei. However, this did not deter him from making a detailed analysis of the early stages of mammalian fertilisation.

On returning to Hokkaido University in 1964, he was unable to obtain an assistant professorship but in 1966 he was offered and accepted a position at the University of Hawaii Medical School, where he spent the rest of his life.

One of his most important discoveries that later became adopted in human IVF was the ability hamster and human sperm to form male pronuclei when directly injected into hamster oocytes. This technique commonly referred to as ICSI (intracytoplasmic sperm injection) was translated to human IVF and is now commonly used to overcome human male infertility problems eg, immotile sperm and low sperm numbers.

In the late 1990s, at the time when Professor Sir Ian Wilmut's team reported on the cloned sheep (Dolly), Yana and his co-workers successfully cloned mice from somatic nuclei producing over 50 cloned mice which were bred over several generations without obvious abnormalities.

Recognition of Yana's contributions to the study of reproduction are many and include
the Marshall Medal from the Society for Reproduction and Fertility (1994), the International Prize for Biology, Japan (1996), a member of the US National Academy of Sciences (2001), selected as an outstanding scientist in the Hall of Honour at the US National Institute of Child Health and Human development (2003) and the Kyoto Prize in Advanced Technology, Japan (2023).

As a person Yana was always affable and welcoming and ready to share his vast knowledge of fertilisation in the animal kingdom. His challenging questions to those he knew and who worked with him were thought provoking leading to constructive discussion. His deep love of all that nature has to offer, which is more than adequately summed up in one of his sayings 'Unlike People, Nature Never Lies'.