Early embryonic coats: Morphology, function, practical applications - An overview

Mammalian egg and embryo coats are primarily represented by the zona pelluc ida which, however, undergoes biochemical and structural changes during fer tilization and embryo development. It serves several functions, from ovulat ion 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 premat ure attachment to the oviductal and endometrial surface. Additionally, it p rotects the embryo against toxins and xenobiotics, as well as bacteria, vir uses and phagocytes. As the embryo is covered by the zona pellucida until i mmediately before implantation, all signals of embryo-maternal signalling h ave 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 protein s 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 zo na or whether it is rejected by the zona depends on its biochemical propert ies. Three specific proteins have been detected within the embryonic coats (IGFBP3, HBEGF, P19). New insights into the physiology of the zona pellucid a might present new achievements in the in vitro culture of embryos, and pr esent new aspects as to how to prevent zona hardening. Furthermore, knowled ge of the zona proteins enables the development of immunocontraceptive vacc ines. Consequently, the zona pellucida is not only significant with regard to fertilization, but also during early embryonic development. Investigatio ns 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 a pproaches to in vitro culture. copyright (C) 2000 S. Karger AG, Basel.