[Polyploidy occurring during in-vitro fertilization of human oocytes: frequency and possible causes]

Characterization of calcium oscillation patterns in caprine oocytes induced by IVF or an activation technique used in nuclear transfer

Routine activation of nuclear transfer (NT) eggs involves the application of a single intracellular calcium [Ca2+]i rise, stimulated by an electrical pulse, as opposed to [Ca2+]i oscillations, which is the natural mode of sperm-induced activation at fertilization in all mammalian species tested to date. It has yet to be shown that caprine oocytes exhibit an increase in calcium at fertilization in a manner similar to other mammals. The objective of the present study was to evaluate and characterize the ([Ca2+]i) oscillation patterns of caprine metaphase II (MII) oocytes during IVF and during an activation techniques used in nuclear transfer. Additionally, the effect of cytochalasin B (cyto B) in the NT process was evaluated for its impact on [Ca2+]i oscillations and subsequent embryo development. Mature in vitro and in vivo derived caprine oocytes were activated by 5 microM ionomycin, an electrical pulse(s), or IVF. The intracellular Ca2+ response was determined using the [Ca2+]i indicator Fura-2 dextran (Fura-2D). Ova treated with ionomycin or stimulated by an electrical pulse exhibited a single [Ca2+]i rise, whereas IVF-derived oocytes showed oscillations. IVF [Ca2+]i showed some variation, with 62% of in vitro matured oocytes exhibiting oscillations, whereas 8% of in vivo matured oocytes exhibited oscillations demonstrating a correlation between [Ca2+]i responses and maturation technique. Knowing the [Ca2+]i profile of activated eggs, one may be able to optimize the activation methodology used in a production nuclear transfer setting which could potentially improve development to term for NT embryos.

Triploidy after in vitro fertilization: cytogenetic analysis of human zygotes and embryos

Tripronuclear zygotes obtained from a clinical IVF program were studied cytogenetically. Successful analysis was possible of 42 specimens at the zygote stage and 21 embryos after the first or second cleavage division. In the majority of zygotes (88%) the expected triploidy was confirmed, whereas only 14% of embryos had solely triploid cells. Therefore it is concluded that after tripolar cleavage division, many different types of mosaicism may originate from irregular chromosome distributions. Since the findings in individual blastomeres in embryos resulting from multipronuclear zygotes do not reflect the genetic content of the whole embryo, these embryos are less suitable in a model system for preimplantion diagnosis. The distribution of the sex chromosomal types (XXX, XXY, and XYY) confirmed theoretical expectations. Since in abortion material or in liveborn triploidy cases, the XYY karyotype is hardly ever observed, this indicates that most likely the 69,XYY karyotype has a very high embryonic mortality.

Electro-rotation of mouse oocytes: single-cell measurements of zona-intact and zona-free cells and of the isolated zona pellucida

Passive electrical properties of oocytes and of zonae pellucidae, and the mechanical coupling between them, can be elucidated by means of rotating-field-induced rotation. In low-conductivity media (25-100 microS/cm) rotation of mouse oocytes (with or without their zonae) requires fields in the 1-100 kHz frequency range. However, an isolated zona shows weak rotation in the opposite direction to that of a cell, and in response to much higher field frequencies (approx. 1 MHz). In zona-intact mouse oocytes, the rotation of cell and zona are not rigidly coupled: thus rotation of the cell can still be induced when the zona is held stationary. However, rotation of freely suspended zona-intact cells is much slower than that of zona-free cells and requires an optimum field frequency that is approximately 1.5 kHz higher. These observations show that the electrical properties of the oocyte that are measured by rotation are altered by the presence of the zona pellucida, even though no such influence has been detected using micro-electrodes. The data are consistent with the zona acting as a porous shell with a conductivity of 40 microS/cm (preliminary estimate made at a single medium conductivity of 26 microS/cm). Measurements on cells from which the zonae had been removed gave values for the membrane capacity and resistivity of 1.2-1.3 microF/cm2 and 400 omega.cm2, respectively. These values may reflect the presence of plasmalemma microvilli. The results strongly suggest that the technique may be useful for studies of cell maturation and for in vitro fertilization, because the cells may be further cultured after measurement.

Cytologic investigation of human in vitro fertilization failures

A cytogenetic study of unsuccessfully fertilized oocytes was done for information on the frequency and type of chromosomal abnormalities leading to preimplantation loss. Of 72 oocytes, 44 were assessed as mature, 19 as intermediate, and 9 as immature. The mean fertilization rate was 68%. This rate was significantly lower in immature oocytes than in mature ones (22% versus 79%). Immature oocytes completed maturation in vitro up to metaphase II of meiosis. Intermediate oocytes became fertilizable, however, a significant proportion showed morphologic abnormalities after fertilization. Thus, eggs with three pronuclei were hypotriploid or had three prophasic chromosome sets, together with polyspermy and with first polar body chromosomes. Eggs with developmental arrest showed marked asynchrony in pronuclear morphogenesis. One had a chromosome complement of 24,XX. It is concluded that insemination prior to completion of maturation leads to abnormal development.