GVBD

GVBD stands for germinal vesicle breakdown, a cellular event in oocyte maturation characterized by the disassembly of the oocyte’s germinal vesicle (the nucleus) and the breakdown of the nuclear envelope. GVBD signifies the resumption of meiosis from the prophase I arrest that most immature oocytes experience.

Occurrence and triggers: GVBD occurs in the oocytes of a wide range of animals, including mammals, amphibians, fish, and some invertebrates. In many species it is prompted by hormonal signals that reduce intra-oocyte cyclic AMP levels and alter gap junction communication with surrounding follicle cells. In mammals, the luteinizing hormone (LH) surge leading to a decrease in intra-oocyte cAMP and subsequent signaling events commonly initiates GVBD, followed by progression through meiosis I.

Mechanism: The resumption of meiosis is driven by activation of maturation-promoting factor (MPF), a complex of CDK1 (Cdc2) and cyclin B. MPF activation triggers germinal vesicle breakdown, nuclear envelope breakdown, chromatin condensation, spindle formation, and progression from prophase I toward meiosis I. Upstream regulatory pathways include attenuation of cAMP/PKA signaling and, in some systems, modulation of MAP kinase signaling; the precise cascade can vary among species.

Significance: GVBD is a primary cytological marker of oocyte maturation and is commonly used in reproductive biology to assess developmental competence and the effects of hormonal, environmental, or in vitro conditions on oocyte maturation.

Relation to fertilization: GVBD occurs before the first polar body extrusion and the completion of meiosis I; fertilization typically occurs later, after the oocyte has progressed to metaphase II arrest.