Intraocular hypothalamic transplants containing histaminergic neurons: innervation of host iris and hippocampal cografts

Survival of grafted fetal neural cells in kainic acid lesioned CA3 region of adult hippocampus depends upon cell specificity

We hypothesize that the degree of graft cell survival within the damaged CNS correlates with the specificity of donor cells to the region of grafting. We investigated graft cell survival following transplantation of fetal micrografts into the CA3 region of the adult rat hippocampus at a time-point of 4 days after an intracerebroventricular administration of kainic acid (KA). Grafts consisted of 5'-bromodeoxyuridine (BrdU) labeled embryonic day (E) 19 cells from hippocampal fields CA3 and CA1 and E15 and E19 cells from the striatum. Absolute cell survival in these grafts was quantitatively analyzed at 1 month postgrafting, using BrdU immunostaining of serial sections and three-dimensional reconstruction of grafts. Absolute graft cell survival in lesioned CA3 was dramatically greater for cells having hippocampal origin (CA3 cells, 69% cell survival; CA1 cells, 42% cell survival) than those having nonhippocampal origin, such as striatal cells (E15 cells, 12% cell survival; E19 cells, 4% cell survival). This difference is in sharp contrast to survival of these cells in culture, where E19 cells from both hippocampal and nonhippocampal origins exhibited similar survival. Comparison of survival among hippocampal cell types indicated significantly greater survival for cells that are specific to the lesioned area (i.e., CA3 cells) than for those that are nonspecific to the lesioned area (i.e., CA1 cells). Graft cell survival in the intact CA3 region (contralateral to KA administration), however, did not differ either between cells having hippocampal and nonhippocampal origins or between CA3 and CA1 cells (CA3 cells, 26% cell survival; CA1 cells, 33% cell survival; and E15 striatal cells, 20% cell survival). These results underscore the finding that enhanced survival of fetal cell grafts in the lesioned CNS is critically dependent upon the specificity of donor fetal cells to the region of transplantation. Thus, grafting of cells that are specific to the lesioned area is a prerequisite for achieving maximal graft cell survival and integration in the lesioned host CNS.

Intracranial transplantation and survival of tuberomammillary histaminergic neurons

Investigations were undertaken to determine whether fetal histaminergic neurons in the tuberomammillary nucleus of the posterior hypothalamus survive intracranial transplantation to adult hosts. Two methods of transplantation were utilized. Grafts were placed either into the delayed cavity of a fimbria-fornix lesion or directly into the hippocampus using stereotaxic techniques. The tissue was taken from rat fetuses at embryonic days 16-17 and grafted into adult rats of either the Sprague-Dawley or the Fischer 344 strain. Routine histology and immunohistochemistry were used to evaluate the grafts. All transplants to Sprague-Dawley rats showed signs of rejection, while no signs of rejection were seen in any of the Fischer 344 rats. Transplants placed directly into the delayed fimbria-fornix cavity did not grow as well or contain as many surviving neurons as the intraparenchymal grafts. The largest number of surviving histamine-positive neurons was obtained with grafts of posterolateral blocks of hypothalamus from fetal day 17 placed directly into the CA1 region of the rostral hippocampal formation of Fischer 344 hosts. Histamine-immunoreactive cell bodies with neuritic outgrowth were found in all Fischer 344 rats that received hypothalamic grafts. Cell bodies exhibited histamine immunoreactivity evenly throughout the cytoplasm and had morphological characteristics resembling histaminergic neurons in situ. Axonal outgrowth extended throughout the grafted hypothalamic tissue, and was sometimes seen in the host hippocampal tissue as well. It is concluded that fetal histaminergic neurons survive transplantation to the adult hippocampal formation, and that this allograft procedure can supplement current strategies to investigate the function of histaminergic tuberomamillary neurons in the central nervous system.

Hypothalamic tissue stimulates hippocampal pyramidal neuron survival during development: evidence from intraocular double transplants

The present study was undertaken to evaluate innervation and possible growth promotion by posterior hypothalamic tissue on different areas that are, or are not, interactive with this brain region during development. Posterolateral hypothalamus was dissected from embryonic day 17 rat fetuses, and inserted into the anterior chamber of the eye of adult rat hosts. Two weeks postgrafting, a second transplant consisting of either fetal hippocampal, cerebellar, or lung tissue was placed adjacent to the first graft. Growth of the intraocular double transplants was monitored weekly by measurements through the cornea. Fetal hippocampal tissue grew significantly larger when placed together with a hypothalamic graft, as compared to single hippocampal transplants. Cerebellar or lung tissue growth was not stimulated by a hypothalamic cograft. Pyramidal neuron cell counts demonstrated a significantly higher final number of these neurons in growth-stimulated hippocampal grafts, as compared to non-stimulated single hippocampal grafts. Immunohistochemistry with antibodies directed against histamine or histidine decarboxylase revealed that hippocampal transplants received the most dense histaminergic innervation. Cerebellar transplants contained occasional histaminergic neurites, and lung tissue never exhibited any histaminergic innervation from the adjacent hypothalamic graft. Taken together, these results demonstrate a growth-promoting effect of posterior hypothalamic tissue on developing hippocampal tissue, as well as target specificity of histaminergic innervation patterns.

Explorations of otic transplantation

Embryonic rat inner ears were transplanted to the anterior chamber of the eyes of adult rats. While considerable development was evident, the structures present were limited to the vestibular division. We hypothesized that this selective survival could be due to the rate of vascularization. To test the effects of graft vascularization we made transplants in which the internal structures were exposed by removing the apex and base of the developing cochlea. The transplants were rapidly vascularized by the iris. Many of the soft labyrinthine structures of the cochlea from 1-day-old donors showed considerable development, including the spiral limbus, basilar membrane, and organ of Corti. To test the possibility that the cochlea requires inductive or trophic support beyond Embryonic Day 15 (E15), we cotransplanted the embryonic inner ear with developing brain stem. In these transplants, we observed improved development of the cochlea, with spiral ganglion cells and an organ of Corti possessing hair cells, Deiter's cells, and pillar cells. To further address the effect of developing CNS tissue on the development of grafted inner ear, we transplanted E15 inner ears to either the cortex or the brain stem of neonatal rats. In these experiments we have seen evidence of both vestibular and cochlear sensory surfaces. In the cochlea, an organ of Corti-like structure can be seen. The possibility of neural connections with the host brain has yet to be investigated.