Published online by Cambridge University Press: 24 December 2019
Biologists and physiologists began to micromanipulate cells during the last century, using a variety of manipulator systems to dissect or record from cells. The earliest attempt to inject sperm was recorded in 1914, when G.I. Kite injected sperm cells into starfish oocytes, but with inconclusive results (Lillie, 1914). Experiments in which sperm were injected into eggs around the mid-1960s were primarily designed to investigate the early events of fertilization, i.e. the role of membrane fusion, activation of the oocyte and the formation of the pronuclei. Two series of early experiments by independent groups demonstrated major species differences. Hiramoto showed that microinjection of spermatozoa into unfertilized sea urchin oocytes did not induce activation of the oocyte or condensation of the sperm nucleus (Hiramoto, 1962), whereas others demonstrated the opposite in frog oocytes. Ryuzo Yanagimachi and his group later demonstrated that isolated hamster nuclei could develop into pronuclei after microinjection into homologous eggs, and a similar result was obtained after injecting freeze-dried human spermatozoa into a hamster egg (reviewed by Yanagimachi, 2005). These experiments indicated that membrane fusion events can be bypassed during activation of mammalian oocytes, without compromising the initiation of development. The experiments not only provided information on the mechanism of fertilization, but also led to a new technique in clinical embryology.