Abnormal development of the inferior temporal region in fetuses with Down syndrome.

Down syndrome (DS) is a genetic condition associated with impairment in several cognitive domains. Previous evidence showed a notable neurogenesis reduction in the hippocampal region of DS fetuses, which may account for the impairment of declarative memory that characterizes DS starting from early life stages. The fusiform gyrus (FG) and the inferior temporal gyrus (ITG) play a key role in visual recognition memory, a function that is impaired in children and adults with DS. The goal of the current study was to establish whether fetuses with DS (17-21 weeks of gestation) exhibit neuroanatomical alterations in the FG and ITG that may underlie recognition memory impairment. We found that the FG and ITG of fetuses with DS had a reduced thickness and fewer cells in comparison with euploid fetuses. Moreover, DS fetuses had fewer cells expressing the neuronal marker NeuN than euploid fetuses, but a similar number of cells expressing the astrocytic marker GFAP and, consequently, a higher percentage of astrocytes. Immunohistochemistry for calretinin (CR), a marker of GABAergic interneurons, showed that in DS fetuses the ratio of CR-positive vs. CR-negative cells was greater than in euploid fetuses, both in the FG (+77%) and ITG (+61%). An increased ratio of CR-positive vs. CR-negative cells was also found in the entorhinal cortex, hippocampus and dentate gyrus. Results provide novel evidence that the FG and ITG of DS fetuses exhibit numerous developmental defects. These defects may underlie the functional alterations in visual recognition memory observed in children with DS.