In vitro manipulation of gametes and embryos for evaluating the effect of chemicals on germ cell function and...

The effect of toxicants on gamete fertilizing capacity in mammals can be measured by direct assessment of fertilization and embryo development. Alternatively, an indirect assessment can be made by measuring physiological and biochemical parameters related to sperm and oocyte function. To increase the sensitivity of fertilization assays in vivo, inseminations should be undertaken with the minimum number of viable spermatozoa for adequate conception rates. Internal controls may be used to reduce animal and experimental variation. Fertilization in vitro allows gamete interactions to be investigated in detail but, for maximum conception, sperm/egg ratios 10(3)-10(4) times greater than at the site of fertilization in vivo are required. Several indirect assessments of sperm function show promise as diagnostic markers of infertility. These include sperm penetration into zona-free hamster oocytes and computerized measurements of sperm velocity. In vitro culture of embryos provides a means of assessing the potential of germ cells to promote normal development, although considerable species variation exists with respect to events such as implantation. It is envisaged that in vitro techniques will become increasingly important for screening and monitoring the effects of toxic agents on reproductive processes.