No abstract

We describe analyses of the structure and expression of the rat fibronectin gene with particular attention to the 40‐kb stretch from the center of the gene which encodes 17 type‐III repeating units. Each repeat is precisely separated from its neighbors by introns and most are encoded by pairs of exons. Three repeats are encoded precisely by single exons and two of these (EIIIA and EIIIB) are alternatively spliced in a cell type‐specific fashion. A third site of alternative splicing (EIIIB) reported here is similar in expression to the previously described EIIIA segment. Both are excluded from mRNA in liver cells and are, therefore, absent from plasma fibronectin. These two alternative splices, plus a third one (V) reported previously, can occur in all possible combinations giving 12 fibronectin mRNAs from a single gene. These splicing variations account for most but not all of the known fibronectin subunit variants. We report investigations designed to detect other regions of alternative splicing. We also show that the pattern of alternative splicing is somewhat altered on oncogenic transformation.

We have previously provided evidence that the rat optic nerve contains three types of macroglial cells that develop as two distinct lineages: one lineage comprises type 1 astrocytes, which develop before birth, while the other comprises oligodendrocytes and type 2 astrocytes, which develop after birth from a common, bipotential glial progenitor cell. In the present study we have examined the influence of axons on the development of these two glial cell lineages by cutting the optic nerve at birth so that the retinal ganglion cell axons in the nerve degenerate. Using antibodies to distinguish the different types of glial cells in suspensions and semithin frozen sections of cut and uncut optic nerves, we show that neonatal transection results in a striking decrease in the total number of oligodendrocytes, type 2 astrocytes and their progenitor cells but has much less effect on the number of type 1 astrocytes. Since the [3H]thymidine labelling indices of oligodendrocytes and their progenitor cells were not significantly decreased in cut nerves, our results suggest that the progenitor cells and/or their progeny die in large numbers following neonatal nerve transection. We conclude that axons are required for the survival of cells of the oligodendrocyte-type 2 astrocyte lineage, at least during postnatal development.