The early phase of vascularization in intraocular telencephalic transplants

The present study focused on the early events of vascularization of intraocular cerebral transplants. Telencephalic pieces of rat embryos (E15) were transplanted into the anterior eye chamber of adult rats in deep ketamine-xylazine narcosis. At 3-, 4-, 5-, 6-, or 7-day postoperative survival periods, the rats were perfused and the transplants, with their iridic beds, were processed into serial, semi-thin sections. In 3- and 4-day transplants, neither dilated (perfused) nor collapsed blood vessels were found, but tissue defects, without proper wall and filled by non-nucleated (mature, host) erythrocytes, were seen. On post-operative day 5, large sinusoids were seen lines by endothelium and free of blood cells (as a consequence of perfusion). On days 6 and 7, the usual, although large, blood vessels were found. Our results suggest that the critical period of transplant vascularization is between postoperative days 4 and 5, and that the original vessels of donor tissue degenerate and disappear during the first postoperative days and thus, do not participate directly in transplant vascularization. Our hypothesis is that vascular invasion begins with the opening of host blood vessels into clefts formed by degeneration of graft tissue. For a period, a hemostasis occurs in these blood-filled lacunae, and then endothelium invasion from host vessels forms the proper wall. The transplant vasculature develops from these large sinusoids. The results challenge the role of the pre-existing donor vessels in transplant vascularization. A possible explanation of such paradoxical results is that the donor tissue must reach a stage of maturation to receive the ingrowing vessels, either host vessels, and the presence of vessels in the donor brain is the sign of this stage of maturation but has no direct role in transplant vascularization.