Afferent and efferent connections of cortical transplants implanted into the damaged sensorimotor area of the cerebral cortex of adult rats.

Abstract

Intensive investigations are currently being carried out into the transplantation of human embryonal brain tissue into the brain of mammals [5], and a number of successful operations have been carried out to transplant embryonal nervous tissue in clinical practice. At the same time, further development of the clinical use of the method is impossible without numerous experiments on the transplantation of the nervous tissue of animals, and without the study of various aspects of the interaction of the graft with the brain of the recipient. A part of the problem which is under the most intense debate is the capacity of the implanted nervous tissue to form interneuronal connections with the host brain. Only in the presence of such interaction can one speak of the morphological integration of the graft with the host brain which is the structural basis of the positive influence which neurotransplantation exerts on the restoration of disrupted functions of the CNS [2, 6, 7, 17]. When the results of the transplantation of embryonal anlagen (of the retina, the tegmentum of the midbrain, the occipital region of the cortex) into the damaged brain of newborn and adult rats, with the subsequent study of the interneuronal connections of the graft with the host brain [12, 14], it has been noted that an increase in the age of the embryo and the recipient limits the growth of axons and the formation of specific interneuronal connections. On the other hand, many investigations [1, 8, 18] point to the presence of a sufficient number of specific reciprocal connections between the graft and the brain of the recipient in adult animals. As a result of the study of the interneuronal connections between the transplant and the brain of the animal, it has been demonstrated that there exists a few reciprocal connections, identified on the basis of the presence of anterogradely labeled fibers and retrogradely labeled neurons in the transplant when a marker is introduced into the recipient's brain [4], between a graft placed in the neocortex and adjacent areas of the cortex. The purpose of the present study was the investigation of capacity of embryonal tissue of the neocortex, implanted into a cavity formed following the preliminary ablation of the sensorimotor cortex of the adult rat, to restore their specific connections, those which occur in the normal brain. Material and Methods. The experiments were carried out on 35 adult Wistar rats, in whom tissue of the neocortex of 17-day old embryos were transplanted into the damaged region three to six days following the preliminary removal of an area of the sensorimotor cortex of the left hemisphere, under nembutal anesthesia, following the method previously described [4]. Four series of experiments were carried out four to six months following the transplantation. In series I, in five rats, a 0.4% solution of horseradish peroxidase (HP) (produced by the Olaine Company) was introduced iontophoretically into the graft. In the remaining three series, 0.2-0.3 ~tliters of HP was injected into the caudatoputamen (10 rats, series II), into the ventroposterior complex of the thalamus (10 rats, series III), and into the cervical divisions of the spinal cord (10 animals, series IV). The injection into the subcorfical structures was accomplished stereotactically [16]. Two days after the microinjections, the brain of the animals was perfused, at first with a 10% isotonic solution of sodium chloride, and then with a mixture of a 1% solution of paraformaldehyde and a 1.25% solution of glutaraldehyde in phosphate buffer (pH 7.4). Then the brain was immersed in a 30% solution of sucrose for two days. Frontal sections, 30-40 ~tm, were made up, and stained following Mesulam's technique [15] with tetramethylbenzidine or benzidine.