The use of AgNO3 staining, commonly applied in cytogenetics to identify nucleolar organizer regions, has made it possible to identify nucleoli in primary spermatocytes. One nucleolus was identified in each analyzed
cell. Considerable morphological differentiation of the nucleoli during the prophase of the first meiotic division, particularly in leptotene, unobserved in other farm animal species, was noticed. Dark-hued grain-like structures were found within the disintegrating nucleoli, corresponding approximately or exactly to the number of the nucleolar organizer regions in the domestic cattle karyotype. Dark areas were identified in the selected prometaphase chromosomes. Their number corresponded with the number of active NORs defined in the domestic
cattle karyotype
