Expression pattern of neurocan in the human fetal neostriatum

The mammalian neostriatum is organised as a heterogenous mosaic of structurally and functionally different compartments ; striosomes and matrix, distinguishable by their time of neurogenesis and neurodiferentiation, afferenet and efferent connections and distribution of neuroactive polypeptides. This mosaical organization is most apparent during fetal and early postnatal development, and differs from the one described in the adult brain. The exact relationship between the fetal and adult mosaic organization, as well as the molecular mechanisms that rely under it, are still unknown. Studies show that corticostriatal afferents connect with the neostriatum selectively with the striosome or matrix region and that striosome and matrix cells can segregate in the absence of dopamine afferents. Also, during neostriatum formation, newly arrived striatal neurons which will form the striosomes tend to selectively aggregate. This suggests that the cell surface and extracellular matrix (ECM) molecules play a crucial role in the formation of neostriatal compartments. In a present study, we evaluated at the histological level the expression pattern of the ECM neurocan (NCAN), a chondroitine-sulphate proteoglycan that can modulate neuronal migration and adhesion, in the human fetal neostriatum. We used indirect immunohystochemical staining on postmortal brain tissue from 12 to 40 gestation weeks (GW). NCAN immunoreactivity is first recognized in the fetal neustriatum at 16 GW in cell islands that match the cytoarchitectinical cell islands recognized on Nissl sections. The peak of NCAN immunoreactivity is observed between the 19th – 23rd GW in a tight cell-free zone that surrounds the striosomes – the peristriosomes or perimeters, which are most pronounced during fetal and early postnatal development. At GW 21 NCAN shows a shift in immunoreactivity – it is visible in the matrix. After the 23rd GW neurocan expression declines untill GW 27 when its expression ceases. We propose that the selective expression of neurocan in the different modules of the fetal striatum reflects the differences in the sequence of growth and maturation of striatal afferents, and/or that neurocan has a role in forming the boundaries between striatal modules.