Developmental expression of glial fibrillary acidic protein mRNA in the rat brain analyzed by in situ hybridization.

Glial fibrillary acidic protein (GFAP) accumulates in astrocytes during development. We have characterized the increase in GFAP mRNA during development of the rat brain by using Northern blotting and in situ hybridization histochemistry and have found a caudal to rostral gradient of expression, consistent with overall brain maturation. GFAP mRNA was first observed at embryonic day 16 (E16) in the glial limitans of the ventral hindbrain. During brain development message levels increased rostrally and by postnatal day 5 (P5) the entire glial limitans showed a positive signal which persisted into adulthood. GFAP mRNA was also found to accumulate in a caudal to rostral direction within the Purkinje cell layer of cerebellum beginning shortly after birth. By P5 the entire layer was positive and signal in this region could be localized to Bergmann glia by P15. A transient elevation in GFAP mRNA was apparent during the second postnatal week in cerebellum and cerebrum. Using in situ hybridization, a peak in message levels was observed at P15 and could be localized primarily to the deep white matter of cerebellum, to the corpus callosum, and to certain hippocampal fiber tracts. The pattern of GFAP expression in these regions is consistent with the differentiation of interfascicular glia and the appearance of type-2 astrocytes during the initial events of myelination. GFAP mRNA levels in white matter were greatly reduced in the adult. The pronounced regional differences in GFAP mRNA expression during development may reflect the differentiation of subpopulations of astrocytes.