Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus.

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Sex difference in cellular proliferation within the telencephalic ventricle zone of Bengalese finch.

Cellular proliferation within the ventricular zone (VZ) may contribute to sex differences through the net addition of neurons in song control nuclei. To address this issue, we administered [(3)H]thymidine to Bengalese finches of both sexes, and estradiol benzoate (EB) to females 15 days post hatching. The birds were killed 2h later to examine thymidine labeled cells within the VZ at three brain levels, HVC, anterior commissure and Area X. Our results indicated that: (1) cell proliferation in the VZ was significantly higher in the three studied brain levels in males and EB implant females relative to intact or empty implant females, respectively; (2) proliferation in the dorsal half of the VZ, in proximity to HVC, was notably higher than that in the ventral half of the VZ; (3) proliferation in the ventral VZ (VVZ), which is relatively close to Area X was higher relative to other subregions of VZ (dorsal and intermediate). Our study suggests that sex differences in cell proliferation in the VZ may contribute to the net growth of HVC and Area X in males, and estradiol may play an important role in sexual difference in cellular proliferation within the VZ.