PACHYTENE CHROMOSOMES IN TRISOMY-19 MALE-MICE WITH ROBERTSONIAN TRANSLOCATIONS

Three male mice with trisomy 19 induced by a Robertsonian translocatio n system were used for the study of meiotic prophase cells and germ ce ll differentiation. Present in these males were two Robertsonian chrom osomes each with a chromosome 19 arm in common, two acrocentric chromo somes corresponding to the second arms of the two Rbs and one acrocent ric chromosome 19. These five chromosomes showed a wide range of meiot ic pairing configurations. One particular observation was the formatio n of a true double synaptonemal complex (SC) with three lateral axes a nd two central elements, which joined the three chromosomes 19 togethe r. Integration of the acrocentric chromosome 19 in a complex pentavale nt configuration was seen in 45% of the pachytene nuclei. The proporti on of spermatocytes showing association between a quadrivalent and the acrocentric no. 19 was 26%, In 29% of the nuclei, the acrocentric no. 19 was free, integrated or associated with the XY complex, paired wit h the X chromosome or associated with a bivalent. Finally, in 57% of p achytene cells, the meiotic multivalents or the free univalent 19 were associated with the proximal part of the X chromosome or integrated i n the sex chromatin. Therefore, the question arises with regard to the fate of these spermatocytes. The testicular histology shows an arrest of germ cell development at the spermatocyte stage. Several mechanism s seem to be the cause of germ cell depletion in a sequence of differe nt, impaired developmental processes.