Various technical details on the transplantation of the embryonic neural tissues in the brains of the neonatal and adult rats are presented. Conditions determining successful or leading to unsuccessful survival, growth and differentiation of these transplants are critically examined.
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.
In this study the growth of neural transplants was analyzed in relation to the age of the host animals and the site of transplantation. The influence of these two host parameters on the growth of neural transplants with high growth potential (tissue from 15-day-old embryos) and low growth potential (tissue from 18-day-old embryos) was investigated. Neocortical neural tissues of constant volume, obtained from 15- and 18-day-old embryos, were transplanted into the forebrain or cerebellum of 5-, 10-, 20-, 25-, 30-, 35- and 180-day-old host animals and analyzed, quantitatively well as qualitatively, 90 days after transplantation. The transplants grew larger in volume in the cerebellum than in the forebrain region of the hosts of all ages. In both sites, tissue from 15-day-old embryos yielded larger transplants than tissue from 18-day-old embryos. Transplants from 15-day-old embryos grew most extensively in 5-day-old host animals (33-fold in the cerebellum, and 23-fold in the forebrain region.) In older host animals it grew less extensively, and without much variation in size that could be attributed to the age of host animals. Tissue from 18-day-old embryos grew little, regardless of site of transplantation or age of host. Apparently the age of the host animals and the site of transplantation had greater influence on the growth of the neural transplants with high growth potential than on those with low growth potential. Histologically, the neural transplants in all cases contained normal-looking and fully differentiated neurons and were anatomically integrated with the host brain.
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