Early embryonic coats: morphology, function, practical applications. An overview.

Abstract

Mammalian egg and embryo coats are primarily represented by the zona pellucida which, however, undergoes biochemical and structural changes during fertilization and embryo development. It serves several functions, from ovulation until shortly before implantation. Initially the zona pellucida induces sperm-oocyte interaction, acrosome reaction and prevents polyspermy. Later, it prevents disaggregation of the noncompacted blastomeres and the premature attachment to the oviductal and endometrial surface. Additionally, it protects the embryo against toxins and xenobiotics, as well as bacteria, viruses and phagocytes. As the embryo is covered by the zona pellucida until immediately before implantation, all signals of embryo-maternal signalling have to pass the zona and are detectable within it. Logically we may define the zona pellucida as a mailbox of the embryo-maternal signalling, especial for investigating these messages. Oviductal, uterine and embryonic proteins are incorporated into the zona pellucida as embryonic development goes on and change the zona's morphological and biochemical properties. Whether a protein is able to penetrate the zona, whether it accumulates within the zona or whether it is rejected by the zona depends on its biochemical properties. Three specific proteins have been detected within the embryonic coats (IGFBP3, HB-EGF, P19). New insights into the physiology of the zona pellucida might present new achievements in the in vitro culture of embryos, and present new aspects as to how to prevent zona hardening. Furthermore, knowledge of the zona proteins enables the development of immunocontraceptive vaccines. Consequently, the zona pellucida is not only significant with regard to fertilization, but also during early embryonic development. Investigations of the zona pellucida will yield new insights into early embryo-maternal signalling which in turn may lead to improvements in classic IVF and new approaches to in vitro culture.