ASPECTS OF 3-DIMENSIONAL CHROMOSOME REORGANIZATION DURING THE ONSET OF HUMAN MALE MEIOTIC PROPHASE

The three-dimensional morphology and distribution of human chromosomes 3 were studied in nuclei of spermatogonia and spermatocytes I from fo rmaldehyde-fixed human testis sections. Chromosome arms, pericentromer es and telomeric regions were painted by a three-color, five-probe flu orescence in situ hybridization protocol. Light optical serial section s of premeiotic and meiotic nuclei obtained by confocal laser scanning microscopy revealed that premeiotic chromosomes 3 are separate from e ach other and occupy variably shaped territories, which are sectored i n distinct 3 p- and q-arm domains. Three-dimensional reconstructions o f the painted chromosome domains by a Voronoi tessellation approach sh owed that mean chromosome volumes did not differ significantly among t he premeiotic and meiotic stages investigated. A significant increase in surface area and reduction of dimensionless 'roundness factor' esti mates of arm domains indicated that the restructuring of spatially sep arate chromosome territories initiates during preleptotene, Telomeric regions, which in meiotic stem cells located predominantly in arm-doma in chromatin, showed a redistribution towards the domain surface durin g this stage. At leptotene homologues were generally misaligned and di splayed intimate intermingling of non-homologous chromatin, Pairing in itiated at the ends of bent zygotene chromosomes, which displayed a co mplex surface structure with discernible sister chromatids, The result s indicate that, in mammals, homology search is executed during leptot ene, after remodeling of chromosome territories.