To detect structural alterations in human oocytes that may give rise to predisposition to aneuploidy, unfertilized human oocytes from an IVF programme were processed for indirect anti-tubulin immunofluorescence. The spindle of oocytes aged for 2 days is rather small, and bi- or multipolar. Chromosomes are no longer aligned at the spindle equator but are scattered all over the degenerating spindle. This implies that human oocytes aged for 2 days may no longer be able to develop into a chromosomally balanced, normal embryo. In oocytes aged for 3-4 days the chromosomes become more decondensed and form a restitution nucleus. Microtubules radiate out from the latter towards the cell periphery and form a network of fibres in the cytoplasm. A similar alignment of tubules is found in unfertilized, activated oocytes. Oocytes with an aberrant cytoskeleton and chromosomal array were predominantly obtained from aged females. They include two binucleated oocytes with two sets of chromosomes and two oocytes with displaced chromosomes one of which had a tripolar spindle.

Summary. The development of the sexually indifferent gonads and ovaries of the mouse was studied in embryos, fetuses, newborns and prepuberal animals from day 10 post-coitum through post-natal day 32. During the phase of compartmentalization, the mesonephric tubule-ovigerous cord continuum becomes subdivided into independent structures. This is due to maturation of the primitive stroma surrounding the various components of the continuum and its progressive transformation into a thick, fibrous connective tissue. The mesonephric tubules become separated from the ovigerous cords and condensed into an irregular epithelioid cell mass which remains confined at the hilus of the ovary. The ensuing compartmentalization of the various segments of the ovigerous cords results in the individualization of the follicles and the transformation of the somatic-cell-only portions of the cords into islets of interstitial (steroidogenic) cells.The above findings demonstrate that the follicle and interstitial (steroidogenic) cells of the mouse ovary are of mesonephric origin.Introduction.

Pachytene analysis was undertaken in a male patient heterozygous for a 14q22q Robertsonian translocation. The relatively low rate of XY autosome association led us to examine the relationships existing between the chromosomes involved in the translocation, the rate of XY-autosome association and the degree of spermatogenic failure. Cytogenetic investigations in infertile men and the results of the meiotic studies suggest a direct correlation between the frequency of XY-autosome association at pachytene and the degree of spermatogenic failure. Whether associations arise as a consequence or cause of germ cell failure is still not certain.

A rapid fluorescence in-situ hybridization (FISH) technique was used for direct chromosomal analysis on germ cells from an infertile male with large-headed spermatozoa. The interphase chromosomes were fluorescently-labelled using an extremely bright cyanine dye during a 5-15 min FISH procedure. Germ cells were analysed using a battery of chromosome-specific DNA probes in several consecutive rapid FISH experiments. It was found that the majority of large-headed spermatozoa contained a diploid chromosome number probably due to errors in meiosis I or II divisions, whereas the majority of spermatozoa with normal sized heads are haploid and may be utilized for selective in-vitro fertilization procedures. Rapid FISH may be useful for the detection of major chromosomal aneuploidies in germ cells as an alternative technique to standard or multicolour FISH, and may find an additional application for the chromosomal analysis of human preimplantation embryos.

The spatial relationships of acrocentric chromosomes were studied during prophase I of meiosis in human oocytes and spermatocytes by using cytogenetic techniques, electron microscopy, and in situ hybridization. Ultrastructural investigations revealed an ordered arrangement of nucleolar bivalents at the zygotene and pachytene stages. The end of the bivalent corresponding to the cytological satellite was consistently attached to the nuclear envelope. The fibrillar center of the nucleolus always contained rDNA chromatin fibers emanating from the secondary constriction region. Association of ribosomal genes from two bivalents in the same fibrillar center was frequently observed. Ultrastructural studies demonstrated the close proximity of chromatids in the short arm region of the involved nonhomologous acrocentrics. A breakage/reunion model based on our data can explain the formation of all observed types of Robertsonian translocations: monocentrics and dicentrics with or without rDNA.Robertsonian translocations are the most frequent form of chromosome rearrangement in man, their incidence being about 1% in the general population. They result from different modes of exchange occurring within the centromeric region of two acrocentric chromosomes, thus producing a metacentric or submetacentric element (1). In man, the chromosomal segment above the centromere is divided into three parts: the proximal short arm, the secondary constriction or stalk, which is the nucleolus organizer region (NOR), and the distal cytological satellite. The five pairs of acrocentric chromosomes share multiple copies of rRNA genes variably dispersed among the NORs (2, 3). Nonhomologous chromosomes are involved in 90% of Robertsonian translocations, and the most frequently observed are the 13;14 and the 14;21 translocations (4). Most translocations between heterologs are dicentric and devoid of Ag-positive NOR (4-8).The formation of Robertsonian translocations has been ascribed to chromatid exchange after a break between acrocentric chromosomes associated with the same nucleolus, during spermatogonial and oogonial mitosis (9). It has also been suggested that Robertsonian rearrangements result from an orderly, nonrandom process during meiotic pairing and exchange (10). Indeed, nonhomologous acrocentric chromosomes are often associated with the same nucleolus at meiotic prophase I in human spermatocytes and oocytes (11)(12)(13)(14). The nonrandom involvement of chromosomes in translocations can also be explained by an accidental meiotic recombination between partially homologous sites on nonhomologs. If the pairing is "end-to-end," crossing-over results in two recombinants, one acentric and the other dicentric (15). The two centromeres are so close to one another that they may act as a single centromere (16).Electron microscope studies provide more precise information about the relationship between the nucleolus and the acrocentric chromosomes. A connecting region of the nucleolus was reported in close association with the nucleolar chro...