DNA methyltransferase-1 (Dnmt1) is involved in the maintenance of DNA 
methylation patterns and is crucial for normal mammalian development. The aim of 
the present study was to assess the localization of Dnmt1 in cow, during the 
latest phases of oocyte differentiation and during the early stages of 
segmentation. Dnmt1 expression and localization were assessed in oocytes 
according to the chromatin configuration, which in turn provides an important 
epigenetic mechanism for the control of global gene expression and represents a 
morphological marker of oocyte differentiation. We found that the initial 
chromatin condensation was accompanied by a slight increase in the level of 
global DNA methylation, as assessed by 5-methyl-cytosine immunostaining followed 
by laser scanning confocal microscopy analysis (LSCM). RT-PCR confirmed the 
presence of Dnmt1 transcripts throughout this phase of oocyte differentiation. 
Analogously, Dnmt1 immunodetection and LSCM indicated that the protein was 
always present and localized in the cytoplasm, regardless the chromatin 
configuration and the level of global DNA methylation. Moreover, our data 
indicate that while Dnmt1 is retained in the cytoplasm in metaphase II stage 
oocytes and zygotes, it enters the nuclei of 8-16 cell stage embryos. As 
suggested in mouse, the functional meaning of the presence of Dnmt1 in the 
bovine embryo nuclei could be the maintainement of the methylation pattern of 
imprinted genes. In conclusion, the present work provides useful elements for 
the study of Dnmt1 function during the late stage of oocyte differentiation, 
maturation and early embryonic development in mammals.