Transitioning from egg to embryo: Triggers and mechanisms of egg activation

Abstract

The transition from mature oocyte to developing embryo requires a coordinated series of events, collectively known as egg activation. Egg activation includes changes to egg coverings to prevent polyspermy, release of oocyte meiotic arrest, generation of haploid female and male pronuclei, changes in maternal mRNAs and protein populations, and cytoskeletal rearrangements. In many animals, egg activation is triggered by fertilization, which increases intracellular calcium within the oocyte and thereby regulates molecular events of egg activation. In other animals, fertilization-independent external signals, including mechanical stimulation of eggs and/or changes in ionic milieu, trigger activation. Recent studies have clarified the upstream portion of pathways leading to eggshell changes and cell cycle resumption and have identified activation-induced changes in maternal mRNA and protein profiles that can identify molecular players in the downstream events of egg activation. We review signals that trigger activation and how they link to subsequent molecular events of egg activation. Developmental Dynamics 237:527-544, 2008.

Phospholipase C-zeta deficiency as a cause for repetitive oocyte fertilization failure during ovarian stimulation for in vitro fertilization with ICSI: a case report

“…This A23187-enhanced Ca 2+ transport within the oocyte is mediated through a single, large Ca 2+ rise rather than the smaller, naturally repeating Ca 2+ oscillations [10,18]. Although previous studies have shown the benefit of using Ca 2+ ionophore with ICSI to enhance Ca 2+ -induced activation of mature oocytes fertilized with 2 Immunofluorescence analysis of PLC-ζ located in the male sperm.…”

“…As a crucial sperm protein governing Ca 2+ -dependent oocyte activation, PLC-ζ released into the oocyte during sperm-oocyte membrane fusion [2] acts on phosphatidylinositol 4,5-biphosphate (PIP 2 ) [18] to induce its hydrolysis forming the second messengers inositol 1,4,5 triphosphate (IP 3 ) and diacylglycerol (DAG) [18]. Subsequent IP 3 binding to its own receptor on the endoplasmic reticulum (ER) [19] opens Ca 2+ channels, which release Ca 2+ from the ER into the cytoplasm of the oocyte [19].…”

“…Subsequent IP 3 binding to its own receptor on the endoplasmic reticulum (ER) [19] opens Ca 2+ channels, which release Ca 2+ from the ER into the cytoplasm of the oocyte [19]. The resulting Ca 2+ oscillations within the oocyte [18,19] disseminate from the cortical region throughout the ooplasm [20,21] at 10-to 20-min intervals [22], triggering oocyte activation [23] to initiate oocyte fertilization and preimplantation embryogenesis [8,14,17].…”

Fertilization and the oocyte-to-embryo transition in C. elegans

“…The oocytes are subsequently ovulated and reach a second arrest point (metaphase of meiosis II in mammals). The developed oocytes are now referred to as eggs and are competent for fertilization (13,14). In mammals, sperm interact with the egg on multiple levels (16); here we will focus only on the interactions between the sperm and egg plasma membrane.…”

“…Fertilization in C. elegans shares many characteristics with mammalian fertilization, including the need for sperm activation in the female reproductive tract, internal fertilization and multiple egg activation events including a membrane block to polyspermy, cortical granule (CG) exocytosis, meiotic resumption, and activation and degradation of selected maternal mRNAs and proteins (Table 1) (1,8,13). In this review, we will discuss C. elegans as a model system for fertilization and the oocyte-to-embryo transition.…”