THE DISTRIBUTION OF ASTROCYTES, OLIGODENDROGLIA AND MYELIN IN NORMAL AND TRANSPLANTED RAT SUPERIOR COLLICULUS - AN IMMUNOHISTOCHEMICAL STUDY

Immunohistochemical methods have been used to determine the distributi on of macroglia and myelin in the normal rat superior colliculus (SC) and in grafts of fetal tectal tissue. The fetal tissue was derived fro m 15 day-old (E15) rat embryos and was transplanted onto the midbrain of newborn host rats of the same (PVG/c) strain. Antibodies to glial f ibrillary acidic protein (GFAP) and carbonic anhydrase II (CAII) were used to visualize astrocytes and oligodendroglia respectively. Myelin was immunostained with antibodies to either proteolipid protein (PLP) or myelin basic protein (MBP). In the normal SC, GFAP positive astrocy tes were found scattered throughout the SC, particularly in the superf icial layers. They were especially prominent at the pial surface, arou nd major blood vessels and at the midline between the two colliculi. C AII immunoreactive oligodendroglia and associated myelin were also fou nd throughout the SC; by far the lowest density was seen in the stratu m griseum superficiale (SGS). Both types of macroglial cell were found in abundance in tectal transplants, indicating that the precursors of these glial types were present in the E15 rat mesencephalon. In matur e grafts, large numbers of fibrous astrocytes were found throughout th e neuropil and the level of GFAP immunoreactivity was consistently gre ater than in host SC. Astrocytes seemed to be maintained in a reactive , perhaps immature state within the grafted tissue. Tectal transplants possessed large numbers of fully differentiated CAII-positive oligode ndroglia and the grafts contained a dense network of myelinated axons. However the distribution of CAII and PLP immunoreactivity was not hom ogeneous; there were localized, well-defined regions that contained fe w oligodendroglia and relatively little myelin. These areas stained in tensely for acetylcholinesterase (AChE) and were almost certainly homo logous to the SGS of normal SC. The relative lack of oligodendroglia i n the AChE stained patches in grafts and in SGS in situ suggests that local factors influencing the proliferation and distribution of oligod endroglia in normal SC may have been operating in a similar manner wit hin the tectal transplant neuropil.