Transplantation of brain tissue in the brain of rat. II. Growth characteristics of neocortical transplants in hosts of different ages.

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.