Early Embryonic Coats: Morphology, Function, Practical Applications

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.

Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited

“…around 5 µm in the mouse and around 10 µm in the pig [2,3]; the multiple roles of the ZP have been thoroughly reviewed [4,5]. The site of ZP synthesis is the oocyte during its growth phase, at least in the mouse [6][7][8] and/or the follicular cells in other species [3,9].…”

Evaluation of laser-assisted lentiviral transgenesis in bovine

“…The proteins of the zona pellucida are synthesized by the oocyte and the granulosa cells to form an extra-cellular-matrix (ECM) of concentric layers consisting of crosslinked zona proteins 1 -3 (Herrler and Beier, 2000). Zona protein 2 and 3 form filaments of repeating heterodimers which are cross-linked by dimeric zona protein 1 (Fig 4.) (Green, 1997).…”

“…It also prevents disaggregation of the noncompacted blastomeres and the premature attachment to the oviductal end endometrial surface. It protects the embryo against toxins, bacteria, viruses and phagocytes (Herrler and Beier, 2000). After fertilization, hardening of the ZP occurs, which is the result of the formation of disulfide linkages together with specific proteolysis (Iwamoto et al, 1999).…”

“…Chemical removal or thinning of the ZP is usually performed with Tyrode´s acid or pronase, which exhibit embryos to additional stress, such as changes in pH or osmolarity. Mechanically drilled holes by a needle require high technical skills to achieve holes of reproducible size and shape without hurting the oocyte or embryo (Herrler and Beier, 2000). Various reports in the literature demonstrate the feasibility of laser-assisted zona drilling (Herrler and Beier, 2000).…”

Viability of equine embryos after puncture of the capsule and biopsy for preimplantation genetic diagnosis

“…As the encapsulated blastocyst expands, the zona thins and is shed about Day 7. The equine capsule has similarities in composition and time of formation to intrauterine modifications of the early embryonic coats found in many species, notably the neozona of the rabbit (Denker 2000, Herrler & Beier 2000. However, while the rabbit embryonic coat starts to dissolve within a few days of the uterine entry, at the time of implantation (Day 7, Denker 2000), the equine capsule persists until about Day 21 of gestation (Enders & Liu 1991, Betteridge 2007, Allen 2010, creating a physical barrier between the trophoblast and the uterine epithelium during this time.…”