Germ cell research: a personal perspective.

My interest in germ cells began when I first witnessed sea urchin fertilization and embryo development during a laboratory class at Hokkaido University, Sapporo, Japan, almost 60 yr ago. Weismann's concept of germ cells that I learned during my undergraduate years became the driving force of my entire research career. During the early years, my associates and I used mainly the golden hamster and the guinea pig as model animals because their spermatozoa had large acrosomes and we could readily follow changes in the acrosomes without killing or staining spermatozoa. We later used the mouse as our model organism because we wanted to produce live offspring with known genetic backgrounds. A summary of the findings we made during those years includes the following: (1) first in vitro sperm capacitation; (2) discovery of sperm hyperactivation; (3) demonstration of the importance of Ca(2+) in sperm acrosome reaction, hyperactivation, sperm-egg fusion, and egg activation; (4) development of the sperm's fusion competence during the acrosome reaction; (5) characterization of sperm-oviduct relationships before and during fertilization; (6) use of zona-free hamster eggs to examine fertilizing ability and chromosomes of human spermatozoa; (7) development and use of intracytoplasmic sperm injection; (8) use of prespermatozoal cells for the production of offspring; (9) sperm preservation by freeze-drying and freezing whole-animal bodies; and (10) mouse cloning by somatic cell nuclear transfer. My current interests and my visions for the future include the following: (1) mass production of mature eggs and spermatozoa in vitro, (2) permanent sperm preservation at ambient temperature, (3) development of safer and more efficient assisted fertilization (reproduction) technologies, (4) development of safe and efficient methods of cloning, (5) production of artificial organs such as an artificial uterus, (6) development of safe and effective male contraceptives, and (7) prevention of cancer through germ cell research.