The Survival and Development of Sheep Eggs Following Complete or Partial Removal of the Zona Pellucida

Abstract

Summary. The effect of`Protease' on the zona pellucida of fertilized and unfertilized sheep eggs was examined, and the effects ofthe treatment and of mechanical removal of part of the zona on the subsequent development of fertilized eggs was assessed in vitro and in vivo.The zonae of follicular eggs were digested by`P rotease' within 25 min of exposure. In ovulated eggs, the resistance of the zona to digestion decreased with age. Zonae of eggs collected 2 days after oestrus (Day 2) were extremely resistant whereas nearly all the Day-5 eggs lost their zonae within 10 min. Fertilization had little effect upon the susceptibility of zonae to digestion by`P rotease'.Neither age of egg nor method of treating the zona had any effect on subsequent development of fertilized Day-2 and Day-6 eggs in culture, but there were effects of both factors on the proportion of eggs which developed to normal Day-25 embryos.`P rotease' treatment of Day-2 and Day-3 eggs failed to digest the zona completely and a high proportion of such eggs developed to normal embryos whereas only a few eggs of similar age, in which part of the zona was removed mechanically, developed normally. The zonae of Day-4 to Day-6 eggs were invariably removed by`P rotease' and the method of treatment had no effect on the proportion of eggs which developed to normal embryos. Irrespective of method of treatment, more Day-6 than Day-4 eggs developed to normal embryos. Culture for 2 days following treatment did not increase the survival of Day-6 eggs when they were transferred to recipients. Ovulation and ageing of eggs appear to be associated with changes in the zona pellucida which influence its susceptibility to digestion by proteolytic enzymes. In early cleavage stage eggs, the major r\l=o^\leof the zona may be protection of the inner cell mass from the uterine environment.

Assisted hatching in assisted reproduction: a state of the art

“…Compaction is the formation of the structural junctions between the blastomeres. Once compaction occurs, the zona pellucida is no longer essential [25].…”

Conception Rate After Uterine Transfer of Cow Eggs, in Relation to Synchronization of Oestrus and Age of Eggs

“…Moore & Shelton (1964) found that transfers to the oviduct were more successful than those to the uterus, but Trounson & Moore (1974) found that the site of transfer of Day-3 eggs had no effect upon their subsequent survival. In sheep, most eggs are present in the uterus 66 hr after ovulation (Holst, 1974) In the present study, the conception rate after uterine transfer of cow eggs of different ages has been examined in relation to the degree of synchronization of the donor and recipient.…”

Assisted hatching in assisted reproduction

“…Compaction is the formation of the structural junctions between the blastomeres. Once compaction occurs, the zona pellucida is no longer essential 14 . To ensure the above function, zona hardening naturally occurs, and is evidenced by an increased resistance to dissolution by different chemical agents 10 .…”