Structural changes associated with freezing of bovine embryos.

Abstract

The structural changes associated with freezing and thawing were examined in bovine embryos at three developmental stages: Day 5, Day 7, and Day 13 (Day 0 day of estrus). Embryos collected at DayS had 8-16 cells, and contained numerous vesicles and primitive junctional regions between some adjacent blastomeres. After cooling to 4#{176}C, the distribution of organelles within blastomeres as well as the spacial arrangement of blastomeres was disrupted. Day 7 embryos were at the early blastocyst stage and contained an intact ring of trophoblast cells enclosing a disc of embryonic cells. Adjacent trophoblast cells were attached by a region of junctional complexes which were structurally unaffected by freezing. Damage to blastocysts after freezing included loss of the integrity of the trophoblast plasma membrane, leading to collapse of the blastocoele. When some collapsed blastocysts were cultured for 24 h after thawing, a smaller intact ring of trophoblast cells had reformed around the embryonic cells and debris from cryoinjured cells was excluded from the blastocoele. Day 13 embryos contained three morphologically distinct cell types: a Layer of trophectoderm, a disc of embryonic cells, and a continuous layer of endoderm cells surrounding the blastocoelic cavity. After freezing and thawing, the embryonic cells were structurally intact while the trophectoderm had substantial damage to all cell components� In conclusion, cryoinjury in bovine embryos may be selective for one cell type within an embryo, and its extent and nature are dependent on developmental stage.