Transplantation of brain tissue in the brain of rat. I. Growth characteristics of neocortical transplants from embryos of different ages.

Differential growth of neural transplants as related to the age of the donor embryos was investigated in this study. Neocortical tissue of constant volume, obtained from embryos of 15, 16, 17, 18, 19, 20, and 21 days' gestational age, was transplanted into the cerebellum of 10-day-old rats. The fully grown transplants were analyzed quantitatively and qualitatively 90 days after transplantation. The ultimate volume of the transplants and the estimated total number of neurons in them followed a gradient in relation to the age of the donor embryos. At one extreme, the neural transplants from 15-day-old embryos grew very large, showing a 21-fold increase in size, and at the other extreme, those from 21-day-old embryos grew less than two-fold in volume. These differences were determined by the developmental history of the transplants. Neural tissue obtained from 15-day-old embryos contained predominantly neuroepithelial cells which continued to proliferate even after transplantation. This resulted in the large size of these transplants. At the other extreme, neural tissue from 21-day-old embryos contained predominantly preformed neuroblasts, and they simply differentiated afte transplantation. Due to this, the transplants were small in size. Neural tissues obtained from other embryos of different gestational ages between these two extremes contained neuroepithelial cells and preformed neuroblasts in differential ratios. The number of neuroepithelial cells in the transplants and their differential proliferative activity after transplantation, and the number of neuroblasts present, determined the differential sizes of these transplants. In histological preparations, all transplants were seen to contain normal-looking and well-differentiated neurons, and normal-looking neuropil. The transplants were integrated with the host brain, in that there was neither any gap nor any scar tissue between the transplants and the host neural tissue surrounding them. Neither the transplants nor the host brains showed any pathological reaction or neoplastic growth.