An electron microscope study of the formation of the zona pellucida in oocytes from Triturus viridescens

The formation and morphology of the zona pellucida around the amphibian oocyte was studied by means of electron microscopy. The onset of formation is indicated by undulation of the basal membrane of the follicle cells and the plasmalemma. Gradually a homogeneous ground substance is deposited between the aforementioned layers. The plasmalemma becomes so undulated as to form microvilli which project up into the continually accumulating ground substance. As maturation proceeds, sites on the basal membrane remain in contact with the plasmalemma and eventually extend down as macrovilli. At the time the zona reaches its maximal development and complexity, it consists of two layers, a more peripheral homogeneous layer and an adjacent thicker and more dense zona radiata. The microvilli, which are 0.1 μ in diameter, project up deeply into the zona radiata. The macrovilli, which are 0.2 μ in diameter, by this time extend down through the entire width of the zona pellucida and into (but not in direct contact with) peripheral cytoplasm. As the oocyte reaches maturity, the microvilli will gradually recede and the ground substance of the zona pellucida will serve to form the fertilization membrane. The problem of the site of synthesis and function of the zona is discussed.