Transplantation of embryonic occipital cortex to the brain of newborn rats: a Golgi study of mature and developing transplants.

Segments of the occipital cortex were taken from rat embryos (E16-E19) and transplanted to the cerebral cortex or the tectal region of a newborn rat host. With the aid of Golgi impregnation techniques, neuron morphology was studied in cortical transplants which had survived for 1 week or more in the host brain. In mature transplants (greater than 4 weeks) three main groups of neurons, termed groups I-III, were identified. Group I neurons resembled pyramidal neurons of the intact cerebral cortex. No preferential orientation of either soma or dendrites of group I neurons was observed in the transplants, and some group I neurons had curved apical dendrites. Group II neurons had predominantly stellate form and their dendrites were densely covered with spines. Paucity or absence of dendritic spines characterized group III neurons which exhibited various dendritic topologies. Different neuron types were also recognized in immature transplants growing for 1 and 2 weeks in the host brain. The sequence of dendritic maturation of transplanted cortical neurons is similar to that seen in intact cortex, although the stage reached related more to the actual age of the transplant than to that of the host. Thus, group I neurons in the 1-week-old transplants taken from E16 embryos had not attained the same complexity of branching as pyramidal neurons in the surrounding host cortex, but rather resembled slightly younger cells more like those found in the cerebral cortex of the newborn rat. These results show, therefore, that at least the basic cell classes identified in intact visual cortex can also be recognized in the cortical transplants. This will provide a foundation for studies defining which cells project to the host brain and which are involved in particular intrinsic connections.