Synaptic configuration of quadrivalents and their association with the XY bivalent in spermatocytes of Robertsonian heterozygotes of Mus domesticus

BACKGROUND: The nuclear architecture of meiotic prophase spermatocytes is based on higher-order patterns of spatial associations among chromosomal domains and consequently is prone to modification by chromosomal rearrangements. We have shown that nuclear architecture is modified in spermatocytes of Robertsonian (Rb) homozygotes of Mus domesticus. In this study we analyse the synaptic configuration of the quadrivalents formed in the meiotic pro- phase of spermatocytes of mice double heterozygotes for the dependent Rb chromosomes: Rbs 11.16 and 16.17. RESULTS: Electron microscope spreads of 60 pachytene spermatocytes from four animals of Mus domesticus 2n = 38 were studied and their respective quadrivalents analysed in detail. Normal synaptonemal complex was found between arms 16 of the Rb metacentric chromosomes, telocentrics 11 and 17 and homologous arms of the Rb metacentric chromosomes. About 43% of the quadrivalents formed a synaptonemal complex between the heterologous short arms of chromosomes 11 and 17. This synaptonemal complex is bound to the nuclear envelope through a fourth synapsed telomere, thus dragging the entire quadrivalent to the nuclear envelope. About 57% of quadrivalents showed unsynapsed single axes in the short arms of the telocentric chromosomes. About 90% of these unsynapsed quadrivalents also showed a telomere-to-telomere association between one of the single axes of the telocentric chromosome 11 or 17 and the X chromosome single axis, which was otherwise normally paired with the Y chromosome. Nucleolar material was associated with two bivalents and with the quadrivalent. CONCLUSIONS: The spermatocytes of heterozygotes for dependent Rb chromosomes formed a quadrivalent where four chromosomes are synapsed together and bound to the nuclear envelope through four telomeres. The nuclear configuration is determined by the fourth shortest telomere, which drags the centromere regions and heterochromatin of all the chromosomes towards the nuclear envelope, favouring the reiterated encounter and eventual rearrangement between the heterologous chromosomes. The unsynapsed regions of quadrivalents are frequently bound to the single axis of the X chromosome, possibly perturbing chromatin condensation and gene expression.

Robertsonian chromosomes and the nuclear architecture of mouse meiotic prophase spermatocytes

BACKGROUND: The nuclear architecture of meiotic prophase spermatocytes is based on higher-order patterns of spatial associations among chromosomal domains from different bivalents. The meiotic nuclear architecture depends on the chromosome characteristics and consequently is prone to modification by chromosomal rearrangements. In this work, we consider Mus domesticus spermatocytes with diploid chromosome number 2n = 40, all telocentric, and investigate a possible modification of the ancestral nuclear architecture due to the emergence of derived Rb chromosomes, which may be present in the homozygous or heterozygous condition. RESULTS: In the 2n = 40 spermatocyte nuclei random associations mediated by pericentromeric heterochromatin among the 19 telocentric bivalents ocurr at the nuclear periphery. The observed frequency of associations among them, made distinguishable by specific probes and FISH, seems to be the same for pairs that may or may not form Rb chromosomes. In the homozygote Rb 2n = 24 spermatocytes, associations also mediated by pericentromeric heterochromatin occur mainly between the three telocentric or the eight metacentric bivalents themselves. In heterozygote Rb 2n = 32 spermatocytes all heterochromatin is localized at the nuclear periphery, yet associations are mainly observed among the three telocentric bivalents and between the asynaptic axes of the trivalents. CONCLUSIONS: The Rb chromosomes pose sharp restrictions for interactions in the 2n = 24 and 2n = 32 spermatocytes, as compared to the ample possibilities for interactions between bivalents in the 2n = 40 spermatocytes. Undoubtedly the emergence of Rb chromosomes changes the ancestral nuclear architecture of 2n = 40 spermatocytes since they establish new types of interactions among chromosomal domains, particularly through centromeric and heterochromatic regions at the nuclear periphery among telocentric and at the nuclear center among Rb metacentric ones.

El trofoblasto vellositario en placentas con trisomía del cromosoma 16 / Villous trophoblastin placentae with chromosome 16 trisomy

Estudios citogenéticos de vellosidades coriales provenientes de abortos espontáneos, indican que la trisomía del cromosoma 16 es la de mayor incidencia entre las anomalías del cariotipo. Con el objeto de tener mas antecedentes que permitan un mejor diagnóstico histopatológico, se realizó un estudio morfológico y morfométrico del área trofoblástica en vellosidades coriales de intercambio. Se estudiaron 14 muestras de placentas con diagóstico de cariotipo normal y 12 muestras con diagnóstico de trisomía del cromosoma 16, obtenidas de abortos espontáneos de 7 a 12 semanas de gestación. Las muestras se fijaron en Bouin alcohólico, se incluyeron en parafina y se realizaron cortes seriados de 5 um que fueron teñidos con hematoxilina-eosina-azul de Alcián y ácido peryódico Schiff. Además se les aplicó un tratamiento enzimático con diastasa. Otros cortes fueron sometidos a la técnica de inmunoperoxidasa, utilizando el anticuerpo anticitoqueratina. Con un ocular milimetrado, se midió la altura del trofoblasto, sinciciotrofoblasto y citotrofoblasto, en 20 vellosidades de cada muestra. Los resultados se analizaron con el test t de Student. Los resultados indican que las vellosidades coriales trisómicas de fines del primer trimestre de gestación, presentan una hipoplasia del trofoblasto que se debe, principalmente, a una disminución del citotrofoblasto, al compararlas con las vellosidades de cariotipo normal. La morfología de las células citotrofoblásticas trisómicas de 7 a 9 semanas de gestación, presenta características similares a las de cariotipo normal. En cambio, a fines del primer trimestre manifiesta notorias diferencias que permiten deducir que las vellosidades coriales de intercambio, de placentas con trisomía del cromosoma 16, podrían tener un comportamiento indiferenciado para la edad gestacional que les corresponde