Thyroxin dependency of the developing locus coeruleus. Evidence from intraocular grafting experiments

Human fetal tissues grafted to rodent hosts: structural and functional observations of brain, adrenal and heart tissues in oculo

The potential for growth and development of human tissue grafts was explored by transplantation to the anterior chamber of the eye of rats and mice. Tissues were obtained from therapeutic abortions, performed in the eighth to twelfth week of gestation, using a slight modification of routine vacuum aspirations. Recipients were either adult rats immunosuppressed with cyclosporin A and protected with antibiotics, or nude immunodeficient Balb C mice. Catecholamine-rich tissues such as chromaffin cells from the adrenal medulla, sympathetic ganglia, central dopamine neuroblasts from the substantia nigra, and noradrenaline neuroblasts from the locus coeruleus all survived grafting, and in many cases formed nerve fibers that invaded the host iris. Similarly, central serotonin neurons from developing raphe nuclei grafts were able to innervate host irides. Human fetal cerebellar and cerebral cortical transplants continued their development in rat host eyes. Extracellular recordings from such cerebellar and cortical grafts revealed spontaneously active cells with immature action potential waveforms. Spinal cord grafts also survived and contained substance P-immunoreactive neurons. Dorsal root ganglia were able to form nerve fibers invading the host iris, as evidenced by neurofilament immunohistochemistry. Heart tissue survived and manifested spontaneous rhythmic contractions in oculo. Both human cortex cerebri and heart tissue grafts became innervated by sympathetic adrenergic nerve fibers from the rat host iris. Thus both graft-to-host and host-to-graft neuronal connections may be established between man and rat. Taken together, these data suggest that transplantation of human fetal nervous tissues to the anterior chamber of immunosuppressed or immunodeficient rodent hosts yields a unique model system for studies of human brain development, developmental disturbances, connectivity, and the action of drugs.

The effect of GM1 ganglioside on coerulospinal, noradrenergic, adult neurons and on fetal monoaminergic neurons transplanted into the transected spinal cord of the adult rat

GM1 ganglioside, thyroxine and hydrocortisone were tested for their ability to improve the survival and growth of fetal locus coeruleus noradrenergic neurons in the transected, adult spinal cord. GM1 alone was also tested for its effect on fetal mesencephalic dopaminergic neurons implanted into a small dorsolateral cavity at the L2 region of the cord previously transected at the T9-T10 region. None of the substances tested had any measurable effect on either of the fetal implants. However, in the GM1- and thyroxine-treated animals the somatic dendrites of the axotomized, noradrenergic, coerulospinal neurons appeared more robust, and more intensely fluorescent, compared to their appropriate controls. GM1 also caused a pronounced sprouting of the axotomized monoaminergic (catecholaminergic and serotonergic) fibres in the rostral region of the cord adjacent to the transection site. All of the mesencephalic dopaminergic implants survived in both the GM1-treated animals and their saline-injected controls. However, their development was apparently not influenced by GM1. The results indicate that GM1 and thyroxine can enhance those aspects of the reactive mechanisms of mature, axotomized, noradrenergic coerulospinal neurons that promote their regeneration. As such, GM1 could become a useful tool in current attempts to foster the regeneration of damaged monoaminergic neurons in the mammalian CNS.

Control of the development of the ipsilateral retinothalamic projection in Xenopus laevis by thyroxine: results and speculation

The ipsilateral retinothalamic projection of the frog Xenopus laevis is formed by the axons of a subset of retinal ganglion cells which are found throughout peripheral and non-nasodorsal retina. Unlike the crossed retinotectal and retinothalamic projections, which begin to form during early embryonic stages, the ipsilateral projection does not begin to develop until late in tadpole life, at stages when thyroxine first becomes detectable in the circulation. Blocking the production of thyroid hormone in tadpoles prevents the development of the ipsilateral projection, in a reversible manner. Intraocular injection of thyroxine can "rescue" the development of the projection in tadpoles which otherwise remain premetamorphic. In addition, the projection from one eye of a metamorphically-blocked tadpole can be induced to form by an intraocular injection of thyroxine at a dose which has no detectable effect on retinal development in the other, untreated eye. These results indicate that the development of the ipsilateral retinothalamic projection is dependent upon thyroxine, and strongly suggest that the hormone acts at the level of the eye, rather than at the optic chiasm or thalamic target, to bring about the development of a new pathway. A number of ways in which thyroxine might act in the system are discussed.

Fetal locus coeruleus transplanted into the transected spinal cord of the adult rat: some observations and implications

The 16-day-old fetal locus coeruleus can survive and grow, when transplanted into the spinal cavity created by transection of the adult spinal cord. The implants usually break up into smaller cell groups which become lodged in the host spinal cord, mainly in the caudal region. There was a marked proliferation of the damaged coerulospinal noradrenergic fibers in the rostral, ventral horn region of the cord. A neurotrophic substance produced by the locus coeruleus implants is probably responsible for this effect. Evidence for vigorous axonal growth of noradrenergic fibers derived from the implants in the caudal region of the cord was obtained. In 2/12 cases, the surviving locus coeruleus did not grow into the host tissue. Neosympathetic innervation of the caudal region of the cord occurred both in cases in which the implants survived, or did not survive. The survival rate of the transected, implanted animals is greater than 90%. The success rate of fusion of the implant with the host tissue is 40%. Based on these results, and other reports published recently, it is concluded: (1) that the purely technical difficulties of transplantation and survival of the fetal locus coeruleus in the transected spinal cord of the young adult rat on a long-term basis can be successfully solved. (2) For optimum reinnervation, the transplant should be placed in the lumbar intumescence, and not in the spinal cavity created by transection. (3) The spinal cavity created by transection must be bridged by some other method. The use of fetal spinal cord tissue on fetal mesencephalic tissue may prove to be useful for this purpose. (4) Transection of the cord must be done sub-pially, in order to minimize retraction and compression of the cord. The damaged pia must be resutured. (5) A second fetal locus coeruleus implant should be placed adjacent to the exposed surface of the rostral region of the cord. The results are discussed both in the context of recent efforts to try to restore functional activity to the damaged spinal cord, and in terms of efforts to try to understand the problems involved in fostering the growth of fetal brain tissue in the damaged spinal cord of an adult host animal.

Structural and Functional Impairment of Adrenergic Input to Intraocular Cerebellar Grafts During Thyroid Hormone Deficiency

Embryonic cerebellae were transplanted to the anterior eye chamber of normal and thyroidectomized adult rats and were left in the eye to mature for 5 weeks. The cerebellar grafts received adrenergic innervation from the sympathetic ground plexus of the host iris. The density of adrenergic fibers innervating the grafts was reduced by approximately 50% in all thyroidectomized groups, without any effect on the fluorescence intensity or the morphology of individual nerve fibers. The reduction in adrenergic ingrowth was entirely prevented in grafts raised in similarly thyroidectomized recipients, which were substituted daily with thyroxine (100 micrograms/kg, SC). Electrophysiologically, there was no difference between the groups in terms of spontaneous firing rate of Purkinje neurons, recorded extracellularly from cerebellar grafts. However, there was a 10-fold decrease in the potency of catecholamines to inhibit the spontaneous neuronal activity in thyroidectomized hosts as compared to controls when applied either locally by micropressure ejection or by superfusion. Similar to the observed histologic changes in catecholamine innervation, the abnormal responsiveness of hypothyroid cerebellar neurons to locally applied catecholamines could be prevented by daily substitution with thyroxine in these animals. It is thus concluded that there is both structural and functional impairment of the adrenergic innervation of intraocular cerebellar grafts that underwent development under conditions of thyroid hormone deficiency.