Electrophysiological and Morphological Properties of Embryonic Neocortical Grafts Developing in Different Regions of the Host Rat Brain

Parallel morphological and electrophysiological studies of embryonic neocortical tissue (primordia of anterior parietal--presumptive sensorimotor--cortex) grafted into different regions of the host adult brain (sensori-motor cortex, caudate-putamen, septum or thalamus) were carried out to investigate to what extent the properties of transplanted embryonic neocortex--an advanced organizational form of neuronal tissue--are affected by homotopic or heterotopic surroundings. The results point to the importance of the host tissue environment as it influences both the size, the morphological and functional properties of the implanted embryonic cortical plate tissue. The cortical grafts were smaller in size when developed homo-topically in sensorimotor cortex and grew larger in heterotopic environment (caudate-putamen, septum, thalamus). The orientation of neuronal dendrites in the grafts tends to follow that seen in the surroundings. The homotopic grafts showed a better structural integration into the host brain. Differences were seen between intracortical and subcortical transplants in the spontaneous firing rate of neurons and in the ratio of units with various types of spontaneous discharge.