Micromanipulation of cells dates from the turn of the last century when biologists and physiologists used a variety of manipulator systems to dissect or record from cells. 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 in the 1960s that microinjection of spermatozoa into unfertilized sea urchin oocytes did not induce activation of the oocyte or condensation of the sperm nucleus, 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 when freeze-dried human spermatozoa were injected into a hamster egg. These experiments indicated that, during activation of mammalian oocytes, membrane fusion events may be bypassed 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.
The first clinical application of microinsemination techniques was partial zona dissection (PZD) developed by Jacques Cohen and colleagues to aid fertilization in human oocytes (Figure 11.1).