Embryo transfer, a potential risk in disease transmission

The bovine ZP is a 12μm-thick acellular matrix that surrounds the oocyte and the early embryo. It is composed of three major sulfated glycoproteins generally designated as ZP1, ZP2 and ZP3.1 Its thickness is constant up to the blastocyst stage when it becomes progressively thinner until the embryo finally hatches approximately nine days after fertilization. This structure acts as a barrier to pathogens because of its thickness, its acellular nature and the lack of cellular receptors that may attract pathogens.2 Morphologically, it has a sponge-like appearance and is composed of a complex fibrous network containing numerous pores. The pores are channels that granulosa cell foot processes use to communicate with the oocyte prior to ovulation. They are largest at the outer surface and decrease in size towards the inside. After ovulation, the granulosa cells are shed and the structural changes of the ZP tend to obliterate these pores, although not completely.3