Experiments on chiasmata and nondisjunction in mice.

Abstract

Meiosis in the female mammal has both an intrinsic interest and a practical one. Intrinsically, it is interesting because of the complexity of its control and because the whole of its prophase, including recombination of linked genes by crossing over, occurs in utero and is followed by a long dormant stage before final maturation of the ova, in readiness for fertilization. Its practical interest lies in the frequency with which errors at meiotic divisions are the source of numerical chromosome aberrations in man and other mammals. Though meiotic errors are documented also in the male, probably more commonly errors occur at first meiotic division in the female by chromosomal nondisjunction and are incremental with her age. There are two alternative explanatory sets of hypotheses for the maternal age relationship; the first, of postnatal aging (for example, the terminalization hypothesis), and the second, which is one of prenatal aging, and is in essence a production line hypothesis. Whichever of these two hypotheses proves in the end to be correct, the important mechanism for the origin of numerical chromosome anomalies through nondisjunction at first meiotic division is failure of formation, or of persistence, of chiasmata: the mechanical properties of chiasmata are essential for normal disjunction. These general points, including older ideas concerning the nature of chiasmata, will be discussed briefly.Because of the largely cryptic nature of the meiotic prophase in female mammals, a technique had to be developed for its experimental study, which would allow a more direct approach to the developing oogonia and oocytes than is possible in vivo during fetal life. More direct control was achieved by tackling oogenesis and meiosis in vitro during embryonic gonadogenesis, well before the final maturation of the ova in vivo. The technique consists in removing the fetal ovaries during early development (often at day 14, taking as day 1 the day of plug detection), and allowing these to mature in vitro, in an organ culture system, for 8 days at 37° C. Subsequently the fetal ovaries, which have by now grown considerably in size, are transplanted under the renal capsule of a previously spayed young adult female. After 19 days, to allow for maturation of the ovaries in vivo, the gonads are removed, the maturing oocytes are harvested and chromosome preparations are made for analysis at 1st or at 2nd meiotic metaphase, at will. The technique we have developed for the experimental study of mammalian female meiosis is versatile in many ways, and one of its applications is exemplified in Polani et al. (1979)KeywordsMeiotic DivisionMeiotic ProphaseRenal CapsuleOrgan Culture SystemFinal MaturationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.