A micromanipulation apparatus was used to produce holes in the zonae pellucidae of unfertilized mouse oocytes. A microneedle loaded with acid Tyrode's solution was brought into contact with the zona surface, and positive flow was used in conjunction with mechanical pressure to cause a localized dissolution of the zona. Treated eggs were then fertilized in vitro in comparison with control cells. The zona drilling procedure decreased the sperm count required to achieve fertilization by a factor of approximately 100. The rate of polyspermy in zona-drilled oocytes was not greater than in controls, and oocytes fertilized after drilling, when implanted into pseudopregnant foster females, developed to term at the same rate as controls. The results demonstrate that zona drilling is a safe, effective method of increasing the efficiency of fertilization in vitro and may be useful both in agriculture and medicine for conferring fertility upon males with low sperm counts.
With respect to the male gamete, mammalian fertilization is an inefficient process. Only one sperm of millions initially deposited in the female reproductive tract actually penetrates the egg. The importance of a high sperm count to male fertility is underscored by studies of human oligos ermia in which are associated with infertility (Belsey et al., '80; Yovich and Stanger, '84). The requirement for a high sperm count is in part due to attrition of the sperm population during passage from the cervix to the site of fertilization. Once the oocytes are reached, as few as 100 sperm may remain to carry out the fertilization process (Braden and Austin, '54;