Centromere and telomere redistribution precedes homologue pairing and terminal synapsis initiation during prophase I of cattle spermatogenesis

Alterations in nuclear topology associated with meiotic chromosome pairing were studied in premeiotic cells and spermatocytes I of adult bovine males. To this end, we performed FISH with chromosome, pericentromeric satellite- DNA and telomere-specific probes in combination with immunostaining of syna ptonemal complex proteins (SCP3., SCP1) on testis tissue sections. Nuclei o f premeiotic cells (spermatogonia) exhibited a scattered telomere distribut ion while pericentromeres formed a few intranuclear clusters. We observed t hat the chromosome pairing process in cattle prophase I is preceded by repo sitioning of centromeres and telomeres to the nuclear periphery during prel eptotene. Clustering of chromosome ends (bouquet formation) was observed du ring the transition from leptonema to zygonema and coincided with pairing o f a sub-centromeric marker of bovine chromosomes 7. Dissollution of bouquet topology during zygonema left perinuclear telomeres scattered over the nuc lear periphery at pachynema. SCP3 staining in frozen tissue sections reveal ed the appearance of this axial element protein in intranuclear aggregates during preleptotene, followed by extensive axial element formation during l eptotene. Synapsis as revealed by SCP1 staining initiated peripherally at e arliest zygotene, at this stage nuclei still contained numerous SCP3 cluste rs. Our observations reveal prominent non-homologous satellite-DNA associat ions in spermatogonia and indicate the conservation of topological features of the meiotic chromosome pairing process among mammals. The comparison of telomere dynamics in mouse and cattle prophase I suggests that a larger nu mber of chromosomes prolongs the duration of the bouquet stage. Copyright ( C) 2001 S. Karger AG, Basel.