Immunolabelling of spliceosomes in sections and cultured astrocytes of human fetal brain tissue

In the cell nucleus first large pre-mRNAs are synthesized which contain protein coding as well as non-coding sequences. The latter are removed in a process called splicing which takes place in nuclear spliceosomes. These spliceosomes consist among others of protein factors, such as the splicing factor SC35 being abundant in speckled regions of the cell nucleus. This study aims at determining immunostaining patterns using anti-SC35 in sections of the human fetal prosencephalon and cultured human astrocytes. Within the allocortical entorhinal region of the fifth gestational month the number, size and distribution of SC35-positive speckles varies considerably among the laminae which can, thus, clearly be delineated. The immature isocortical plate, however, does not display a laminar arrangement at this developmental stage. Differential immunostaining patterns can be seen in subcortical areas. Cultured human astrocytes reveal numerous speckles occupying a large portion of the nucleoplasm. On account of the SC35-immunostaining patterns no distinction of subpopulations of astrocytes is possible. The results demonstrate that SC35-immunoreactive speckles show lamina and area-specific characteristics of human fetal brain sections. Conspicuous differences in number, size and distribution of speckles are visible in different cytoarchitectonic structures; thus, architectonic borders stand out clearly in SC35-immunopreparations. The occurrence of area-specific immunolabelling of nuclear speckle domains reflects neuronal differentiation at the pre-translational level. It may be assumed that a distinct set of proteins, generated by a definite nerve cell type, can be correlated with a distinct morphology of spliceosomes. The in vitro finding indicates that anti-SC35 may well be used as a tool to study possible alterations of the speckles after, for instance, application of growth factors.