Maturation of the human oocyte in vitro: Nuclear events during meiosis (an ultrastructural study)

Abstract

AbstractStates of meiotic maturation have been traced in human oocytes cultured in vitro from prophase I to telophase II, using electron microscopy.Two hundred ten oocytes, aspirated at laparoscopy after appropriate hormonal stimulation, were cultured for 1–72 h and examined. Most oocytes were inseminated and the final phases of maturation were studied in fertilized ova.The majority of oocytes had resumed meiosis and had matured to metaphase II. Some were arrested at metaphase I, and a few remained inactive at the germinal vesicle stage. Maturation of oocytes to metaphase II was achieved 3 h after laparoscopy; to telophase II, 3 h after insemination. Maturation of some oocytes to metaphase II may have occurred in the ovary.Immature oocytes had eccentric nuclei containing dense fibrillar nucleoli. Corona cell processes formed complex junctions with the oolemma and a type of interaction analogous to neural transmission was observed. Maturing oocytes showed typical barrel‐shaped spindles, but no centrioles or asters. Chromosomes were attached to microtubules via kinetochores. Midbodies were elaborated in the ooplasm and determined the plane of cleavage of polar bodies. Some aspects of oocyte degeneration, aberrations of oocyte maturation, and consequences of aging are also reported.The significance of cytoplasmic maturation that occurs concurrently with nuclear maturation is discussed in relation to assessment of oocytes for in vitro fertilization.