The mechanisms that govern the development of the neural circuitry are still largely unknown. Actually, it is not clearly established to what extent areal cortical differentiation depends on environmental or genetic factors. The first part of the present report deals with the findings of several transplantation studies, performed in newborn recipient rats, in which these developmental issues were addressed. In humans, the cerebral cortex can be affected by a variety of diseases and, therefore, several experimental studies have been undertaken to determine to what extent transplantation of cortical neurons could be proved as a useful treatment for cerebral cortical damage. The second part deals with the functional effects of transplantations performed in adult recipient rats.

The first experiments with neural transplantation into the brain (for a review, see Bjorklund and Stenevi 1985) were reported at the end of the last century by Thompson (1890), who took cortical tissue from adult cats and implanted the tissue pieces into the neocortex of adult dogs. The neurons did not survive, but the transplanted mass did not disintegrate entirely. Successful graft­ ing into the brain was reported later by Ranson (1909), who implanted spinal ganglia into the cerebral cortex of developing rats, and by Dunn (1917), who reported survival of implanted neonatal cortex into the cortex of newborn recipients. Another important step was made by Le Gros Clark (1940) who reported that, in the rabbit, embryonic (E) cortical tissue could be successfully grafted into the cortex of young recipients. Ex­ periments by Wallace and Das (1982), and Bjorklund and Stenevi (1984), showing behavioral effects of grafts, subsequently stimu­ lated a growing interest in neurotransplantation research. Intrac­ erebral grafting is now considered a powerful tool for addressing fundamental questions about development, regenerative, or re­ storative phenomena in the central nervous system (CNS) and is a potential therapy for neurodegenerative diseases.

1 Transplantation of Embryonic Cortex into Newborn Host Cortex: A Means to Study Neural Development.- 1.1 Introduction.- 1.2 Materials and Methods.- 1.2.1 Dissection and Preparation of the Graft Tissue.- 1.2.2 Transplantation of Tissues.- 1.2.3 Transplant Survival and Histology.- 1.2.4 Tract-Tracing Techniques.- 1.3 Early Specification of Neocortical Efferent Projections: The Homo- or Heterotopic Transplantation Paradigm.- 1.3.1 Development of Efferents from Frontally Placed Frontal or Occipital Transplants.- 1.3.1.1 Introduction.- 1.3.1.2 Results.- 1.3.1.3 Discussion.- 1.3.2 Development of the Spinal Cord Projection.- 1.3.2.1 Introduction.- 1.3.2.2 Materials and Methods.- 1.3.2.3 Results.- 1.3.2.4 Discussion.- 1.3.3 Development of the Tectal Projection.- 1.3.3.1 Introduction.- 1.3.3.2 Results.- 1.3.3.3 Discussion.- 1.4 Conclusion.- 2 Transplantation of Embryonic Cortex into the Damaged Cortex of Adult Rats.- 2.1 Introduction.- 2.2 Transplantation into the Frontal Cortex: Anatomical, Metabolic, and Behavioral Aspects.- 2.2.1 Anatomical Study: Transplant Efferents.- 2.2.1.1 Introduction.- 2.2.1.2 Materials and Methods.- 2.2.1.3 Results.- 2.2.1.4 Discussion.- 2.2.2 Metabolic Study: 2-Deoxyglucose Uptake.- 2.2.2.1 Introduction.- 2.2.2.2 Materials and Methods.- 2.2.2.3 Results.- 2.2.2.4 Discussion.- 2.2.3 Behavioral Study: Forelimb Usage.- 2.2.3.1 Introduction.- 2.2.3.2 Materials and Methods.- 2.2.3.3 Results.- 2.2.3.4 Discussion.- 2.2.4 Conclusion.- 2.3 Transplantation into the Occipital Cortex: Electrophysiological and Anatomical Aspects.- 2.3.1 Introduction.- 2.3.2 Materials and Methods.- 2.3.3 Results.- 2.3.3.1 Electrophysiological Observations.- 2.3.3.2 Anatomical Observations.- 2.3.4 Discussion.- 2.4 Conclusion.- 3 Summary.- 4 References.