Nucleolar size and shape is sexually dimorphic in neurons of the lateral magnocellular nucleus of the anterior neostriatum (LMAN) in birds.

The lateral magnocellular nucleus of the anterior neostriatum (LMAN) in birds plays an important role in songlearning processes. Recently, it has been shown that structural changes at the cellular level in males are causally related to vocal learning. Whereas males sing, females do not. This sexual difference in behavior is also reflected in sexual differences in the neuronal structure in adult birds, with males having larger neuronal somata than do females. In the present report, the size and shape of the nucleoli were investigated to determine if sexual differences were also present at the nucleolar level. The data demonstrated a strong sexual difference in nucleolar size in both juvenile and adult birds, the cross-sectional area of the nucleoli being significantly larger in males than in females (30% and 50% larger in juvenile and adult birds, respectively). This difference between males and female finches was also reflected by the cross-sectional area of a specific type of nucleolus exhibiting a central light area. In both sexes, nucleoli exhibiting a central light area were significantly larger in juvenile and adult birds than nucleoli that lacked a central light area. The percentage of neurons exhibiting a central light area was higher in adult males than in adult females, but not in juvenile birds. The time course of development of nucleoli exhibiting a central light area in males was very similar to the development of neuronal somata size in LMAN neurons. The larger size of the nucleoli in LMAN neurons in males and the developmental changes in the incidence of nucleoli exhibiting a central light area may be indicative of a high level of ribosome production necessary for song-learning processes to occur.