Ectopic expression of telencephalin in brains with holoprosencephaly.

Telencephalin (TLN), a telencephalon-specific glycoprotein, is exclusively expressed in neurons of the mammalian telencephalon. In the normally developing human brain, TLN immunoreactivity appeared and increased from 35 gestational weeks (GW) in the temporal cortex, and reached adult level at 5 months of postnatal age, being strong in the molecular layer, and weak in the external and internal granular layers. TLN expression corresponded with the development of neuronal dendrites and synapses. In brains with holoprosencephaly TLN immunoreactivity was already strong from as early as 28 GW. Staining was weak in the molecular layer, but strong in the external sparse and middle cellular layers in most cases. Notably, TLN was abundant in the glomerular structures in the internal pyramidal and multiform layers of fetal brains with alobar holoprosencephaly, which disappeared with increasing age. These results indicate premature and ectopic development of the dendrites and synaptic network in holoprosencephaly.