Fetal hypothalamic transplants in the third ventricle of the adult rat brain. Correlative scanning and transmission electron microscopy

Stereotaxic Coordinates and Morphological Characterization of a Unique Nucleus (CSF-Contacting Nucleus) in Rat

A unique nucleus, the cerebrospinal fluid (CSF)-contacting nucleus, has recently been recognized in the brain parenchyma. The outstanding feature of this nucleus is that the neural somas are located in the parenchyma, but their processes stretch into the CSF, implying that it may be a key structure bridging the nervous and body fluids-regulating systems and may play a pivotal role in modulating physiological activities. However, the true biological significance of this nucleus needs to be uncovered. The morphology of a nucleus is one of the most important parameters for neuroscience studies. For this reason, a common experimental animal, Sprague-Dawley (SD) rats, was chosen. The position, adjacent structures, neuronal distribution, size, three-dimensional reconstruction, and core coordinates of the CSF-contacting nucleus in SD rats of different weights (90–400 g) were illustrated for the first time. Furthermore, the formulas for calculating the core coordinates of the CSF-contacting nucleus in rats of different weights were revealed. Finally, the possible biological functions uncovered by past research are reviewed in this paper. This study provides an indispensable methodology and a significant reference for researchers interested in this unique nucleus.

Neurotransmitter and amino acid analysis and ultrastructural observations of fetal brain cortex transplantation to adult rat brain under the effect of dexamethasone

OBJECTIVE

To conduct an investigation of fetal cortical tissue graft survival using transmission electron microscopy and analyzing neurotransmitters and amino acids and their function, with special reference to the effect of dexamethasone.

METHODS

Transplantation of fetal cortical brain tissue to 100 adult Wistar albino rats weighing 170 to 220 g was performed. The rats were divided into three groups. Only transplantation of fetal cortical brain tissue was performed in the first group (n=36). In the second group (n=48), dexamethasone was administered in addition to fetal cortical tissue transplantation. The third group (n=16) was used as the surgical control group. The rats were allowed to live for 6 weeks and were then decapitated. The grafts were examined by electron microscopy. Additionally, quantitative analyses of the neurotransmitters and amino acids of the grafts were conducted using high-pressure liquid chromatography.

RESULTS

Electron microscopic observations revealed that the grafts were still surviving at the end of the 6th week in both groups. However, in the group that received dexamethasone, neurons and their organelles were better developed than in the group that did not receive dexamethasone. Concommitantly, results of quantitative analysis in the dexamethasone group revealed statistically extremely significant higher amino acid values for glutamic acid, aspartic acid, beta-alanine, and lysine and significantly higher values for gamma-aminobutyric acid, glutamine, glycine, and serine when compared to the nondexamethasone group.

CONCLUSION

Dexamethasone is effective in increasing the survival and in developing the ultrastructural and functional outcome of transplanted neurons in fetal grafts.

Scanning electron microscopy of central nervous system cerebrospinal-fluid-contacting surfaces: a bibliography (1963-1995)

This bibliography is compiled to assist in locating papers related to the application of scanning electron microscopy (SEM) to cerebrospinal-fluid-contacting surfaces in vertebrates. The use of SEM by neuroscientists has continued apace since the publication of the first bibliography in 1980. SEM studies now include more than 50 species of vertebrates and range from cyclostomes to humans; they encompass development from embryo to senescence and concern both normal and pathologic morphology. Although remarkable strides have been made toward a greater understanding of many aspects of the structure and function of cerebrospinal-fluid-contacting surfaces, many significant problems await the judicious application of scanning electron microscopy.

Differential timing and sexual dimorphism in the expression of the vasopressin gene in the developing rat brain

The ontogeny of vasopressin (VP) gene expression in extrahypothalamic neurons of the bed nucleus of the stria terminalis (BNST) and medial amygdala (MA) was investigated using in situ hybridization. In both the BNST and MA, VP mRNA-positive cell profiles were only detected a few days after birth. In males, VP mRNA-expressing cell profiles appear in the BNST on day 3 and in the MA on day 5. In contrast, hybridization signals could not be detected in the female brain until day 14 in the BNST and day 35 in the MA. Adult VP mRNA levels are attained by day 35 in the BNST and day 60 in the MA for both sexes. From this data, it is apparent that development of the VP system is delayed in females compared to the males. Therefore, these data suggest that the sexual dimorphic pattern in the expression of VP mRNA in these nuclei of the adult brain is evident early in the development process.

Appearance of neurons with glucocorticoid receptors and neurovascular links in the embryonal rat hypothalamus grafted in the third ventricle

We have investigated the appearance of the transmitter phenotypes of hypothalamic neurons in grafts transplanted into the third ventricle of adult female rats. The grafts were the mediobasal hypothalamus and the preoptic area of 12.5-day-old rat embryos, and were examined 40-100 days later. Wheat germ agglutinin (WGA) was injected into the jugular vein of several animals for the examination of the existence of neurovascular associations. Three days after the injection, WGA appeared to have been incorporated into the neurons in the paraventricular, periventricular, and arcuate nuclei of the host animals. In the grafts, WGA was also seen incorporated in certain neurons which were found immunoreactive for tyrosine hydroxylase (TH), rat corticotropin-releasing factor (rCRF), substance P (SP), or somatostatin (SRIH). Neurons immunoreactive for neuropeptide Y (NPY) and ACTH did not seem to incorporate WGA. These findings suggest that the neurons containing TH, rCRF, SP, or SRIH link with fenestrated capillaries developed in the grafts. The immunoreactivity for glucocorticoid receptor (GR) was detected mainly in the nucleus of certain neurons and glial cells in the grafts as well as in the host hypothalamic neurons. In the grafts, strong GR immunoreactivity was detected in the cells immunoreactive for TH, NPY, and rCRF as in the host animals. It is concluded that the undifferentiated hypothalamic neurons differentiate to synthesize GR as well as definitive peptides and TH in the grafts.

Allografts of CNS tissue possess a blood-brain barrier. I. Grafts of medial preoptic area in hypogonadal mice

This study represents the first part of a three-part investigation of blood vessels supplying CNS tissue transplanted within the brains of adult mammalian hosts. The results emphasize that blood vessels in solid CNS grafts contribute a blood-brain barrier to that of the host. Neurosecretory cells in basal forebrain grafts placed intraventricularly on the dorsal surface of the host median eminence, a neurosecretory site containing fenestrated blood vessels, do not stimulate similar blood vessels to inhabit the transplanted tissue. Solid grafts of the medial preoptic area containing neurons that synthesize and secrete gonadotropic hormone-releasing hormone (GnRH) were obtained from AKR mice and placed into the third cerebral ventricle of hypogonadal (HPG) mice genetically incapable of synthesizing GnRH. GnRH neurons in the allografts were confirmed immunohistochemically. Blood vessels supplying the host median eminence and the allograft at 10 days to 3 months post-transplantation were analyzed with peroxidase cytochemistry applied in three ways: to HPG mice injected systemically with native horseradish peroxidase; to HPG mice infused into the aorta with peroxidase subsequent to perfusion fixation; and to HPG mice brains fixed by immersion and incubated for endogenous peroxidase activity in red cells retained within blood vessels. The median eminence of the HPG mouse was innervated by GnRH neurons residing within the graft, and blood vessels traversing the median eminence-allograft interface were seen rarely. The allografts contained no fenestrated endothelia, and no extravasations of blood-borne HRP were related directly to leaky blood vessels supplying the grafted tissue. Endothelial cells throughout the CNS grafts were similar morphologically to blood-brain barrier endothelia; they were nonfenestrated, exhibited interendothelial tight junctional complexes and an endomembrane system of organelles, and they endocytosed blood-borne HRP that eventually was sequestered within dense body lysosomes. The results support the belief that blood vessels supplying CNS tissue transplanted to a host brain manifest endothelial characteristics identical to those of the tissue in normal life and to those of the host CNS.

Correlative scanning-immunoelectromicroscopic analysis of neuropeptide localization and neuronal plasticity in the endocrine hypothalamus

Thirty-two Sprague-Dawley rats were divided into four groups, eight rats per group. Animals were hypophysectomized with removal of both the pars distalis and the neural lobe of the neurohypophysis. Groups of eight rats were euthanized 1, 2, 4 and 8 weeks following hypophysectomy and prepared for routine scanning electron microscopy (SEM) and correlative immunoelectron microscopy employing antisera against arginine vasopressin (AVP). Eight normal rats served as controls. In experimental rats that survived one to eight weeks posthypophysectomy, remarkable neuroanatomical alterations were notable in the median eminence and adjacent third cerebral ventricular lumen. In contrast to normal control rats, large numbers of neurites were observed with SEM to insinuate from the lateral recess into the cerebral ventricular lumen and as early as one week following hypophysectomy they overgrew the apical surfaces of ependymal cells that constitute the lining of the cerebral ventricle. Immunoelectron microscopy revealed that a significant proportion of these neurites were magnocellular in origin in that they harbored AVP-positive neurosecretory vesicles. In addition to large numbers of invading magnocellular neurites, neuronal perikayria with apparent axosomatic synapses were observed to emerge upon the thick feltwork of invading axons, the latter of which appeared to freely terminate within the ventricular lumen. AVP-positive axon profiles were, in addition, seen to terminate upon the basal lamina of portal perivascular spaces in the zona externa of the median eminence. These data are consistent with the idea that following hypophysectomy (to include high stalk section of the neurohypophyseal system), that there is rapid, and dynamic sprouting and regrowth of AVP-positive axons into the adjacent third cerebral ventricular lumen and to the contact zone of the median eminence as well. This phenomenon may represent a compensatory physiological response to injury of the neurohypophyseal system characterized by a highly plastic neuroanatomical reorganization of magnocellular elements which appear to utilize the CSF of the third cerebral ventricle as a functional terminus for the neurocisternal secretion of AVP which ultimately enters the systemic circulation.

Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

The present review summarizes how the nervous system responds to trauma. The goal is to provide an introduction to the problems, techniques, experimental paradigms, current issues, and future promise. The review is especially designed for basic scientists and clinicians who are not currently involved in research on CNS reorganization, and for students just entering the field. The review characterizes the secondary degenerative events that occur after trauma, and the types of growth that commonly occur. A standard terminology is set forth with criteria for differentiating between related phenomena. Experimental methods are described that can be used documenting reorganization of circuitry. The principles that determine whether a given process will or will not occur are summarized, and some of the factors that may regulate the nature and extent of growth are considered. Research strategies are outlined that have been used to evaluate whether reorganization of circuitry is functionally significant. Finally, future directions in research and clinical application are discussed, focusing especially on the efforts to facilitate regeneration, and the work on transplants of CNS tissue to facilitate growth of surviving connections, and to replace tissue destroyed by trauma.

The effect of fetal hypothalamus grafts on weight gain resulting from lesions of the ventromedial hypothalamus

Bilateral ventromedial hypothalamic lesions in female adult rats which resulted in hyperphagia and rapid weight gain were followed by placement of fetal brain tissue in the anterior third ventricle. The treatment group received fetal hypothalamus grafts, and fetal cortical tissue of identical age was grafted into the control group. A significant reduction in average daily weight gain was noted from 4 to 12 weeks following transplantation in the treatment group. At 12 weeks posttransplantation, the animals were sacrificed for histological analysis. Examination of the hypothalamus grafts revealed neurons, ependymal clusters, and axonal processes which appeared to infiltrate the surrounding hypothalamic parenchyma.

Ultrastructural organization of normal and transplanted rat fascia dentata: I. A qualitative analysis of intracerebral and intraocular grafts

Few studies have dealt with the general ultrastructure and synaptic organization of grafted brain tissue. This study was therefore performed to extend current light microscopic observations on intracerebral and intraocular grafts of hippocampal tissue to the ultrastructural level. Blocks of tissue containing the hippocampus and fascia dentata from day 21 embryonic rats were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains or eyes were processed for electron microscopy. Tissue containing the graft dentate molecular layer and adjacent granule cell layer was selected for ultrastructural analysis, together with a few samples of the hilus and CA3. Normal dentate tissue was analyzed as control. At the light microscopic level most intracerebral and intraocular grafts displayed an organotypic organization with clearly recognizable cell and neuropil layers. Under the electron microscope the grafted granule cells had normal-appearing dendrites bearing the normal types of spines and forming the normal types of synapses. This was the case even in the absence of the normal major extrinsic afferents like the perforant path. The graft dentate granule cells formed axons and terminals with characteristics of the normal mossy fiber system in the hilus and CA3, in addition to aberrant supragranular mossy fiber terminals known from light microscopic studies of dentate transplants. Abnormal structures included a few dendritic growth cones and an increased occurrence of polyribosomes in spines. Their occurrence indicates ongoing dendritic plasticity even 100 days after transplantation. There was also an increased density of glial elements, particularly in the intraocular grafts. In some of these grafts the granule cells displayed immature traits in terms of nuclear indentations. Dentate interneurons of the basket cell type were present in both the intracerebral and the intraocular grafts. We conclude that grafted dentate granule cells, in different surroundings and without the normal, major perforant path input, can develop a basically normal cellular morphology, which includes the normal ultrastructural characteristics of the dendrites with spines and synapses, and the mossy fibers and its terminals.

Characteristics of vasculature and neurovascular relations in intraventricular anterior hypothalamic transplants

Fetal hypothalami obtained from normal Long-Evans rats were transplanted to the lateral, third or fourth ventricle of adult male Brattleboro rats, homozygous for diabetes insipidus. The density of the capillary plexuses within the grafts did not vary as a function of their intraventricular location; all transplants exhibited a capillary density equivalent to that of the in situ hypothalamus. Intravascular injections of HRP resulted in retrograde neuronal labeling only in grafts that were attached to circumventricular organs of the host brain, especially the median eminence. Typically, HRP-associated label was confined to vascular and perivascular elements and not diffusely distributed within the graft parenchyma, indicating that capillaries within the transplants developed barrier properties similar to those of the native hypothalamus. Neural integration of the transplant with the recipient brain was limited; at most points of apposition the neuropil of graft and host were separated by an intervening ependymal layer or glia limitans. All surviving grafts contained neurophysin and/or vasopressin-immunoreactive (VP-ir) neurons. Three anatomically distinct populations of VP-ir neurons were identified. Magnocellular VP-ir neurons were identified in less than half of the grafts, and when present they were few in number distinct, but sparse vasopressinergic innervation of median eminence capillaries was observed in all cases where grafts containing magnocellular neurons were apposed to this structure. Most grafts contained numerous, parvicellular VP-ir neurons arranged in aggregations which resembled the suprachiasmatic nucleus (SCN). SCN-like cell groups projected to neural targets within the graft and the host brain, but they did not project onto blood vessels. A second, distinct class of parvicellular VP-ir neuron also was identified in a majority of transplants. In contrast to SCN-like ('type' I) cells, these 'type II' parvicellular neurons were somewhat larger and found in less discretely organized groups, and they projected to vascular targets; usually locally elaborated capillary plexuses intrinsic to the transplants. In the present study, there was no amelioration of the symptoms of diabetes insipidus in the host animals despite the presence of numerous VP-ir neurons in virtually all grafts. This was probably related to the limited survival of magnocellular VP-ir neurons, which appear to be the principal source of vasopressinergic projections from the graft to fenestrated capillary plexuses of the host brain.

Ultrastructural organization of normal and transplanted rat fascia dentata: II. A quantitative analysis of the synaptic organization of intracerebral and intraocular grafts

As part of an ultrastructural analysis of the normal rat fascia dentata and intracerebral and intraocular dentate transplants the synapses in the dentate molecular layer were quantified. Hippocampal and dentate tissue from 21-day-old rat embryos were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains and the recipient eyes were processed for electron microscopy, and the graft dentate molecular layer with the adjacent granule cell layer selected for ultrastructural analysis. Tissue from the dentate molecular layer of normal adult rats served as controls. The dentate synapses were classified as asymmetric (Gray's type 1) or symmetric (Gray's type 2), and according to the postsynaptic element (cell body, dendritic shaft, dendritic spine). The spine synapses were further classified into simple and complex types according to the spine-terminal configuration. Also, the length of synaptic contacts of the individual synaptic types was measured in some grafts, just as the percentage of the cross sectional area of the neuropil covered by blood vessels. The results showed that the synaptic density, expressed as number per unit area of neuropil, to a large extent was the same within the different parts of the normal dentate molecular layer. Compared with this the synaptic density was reduced with 16.4% in dentate molecular layer of the intracerebral graft, primarily because of a 17.6% reduction of simple synapses on dendritic spines and almost halving of the symmetric synapses on dendritic shafts. The synaptic density was independent of the age of the recipient, the intracerebral location of the graft, and the survival time. Although the synaptic length of some of the individual synaptic types increased, this did not compensate for the loss of synapses. In the intraocular grafts the synaptic density was lower than in the intracerebral grafts. Despite the reduced synaptic density, which mainly involved two synaptic types, we conclude that grafted dentate granule cells can develop a remarkably normal, ultrastructural synaptic organization even in the absence of major afferent inputs. This outcome must accordingly be achieved by reorganization of the available intrinsic afferents.

Fetal hypothalamic transplants in diabetic rats analysed by scanning electron microscopy

Adult male Brattleboro rats with chronic diabetes insipidus underwent stereotaxic surgery wherein minced fragments of anterior hypothalamus from fetal rats, 17 days post-coitus, were stereotaxically positioned into the lumen of the host third cerebral ventricle. Host rats with fetal donor tissue were killed at various times following surgery and were prepared for correlative scanning-transmission electron microscopy. Examination with this technique revealed the presence of large neurografts which grew to occupy the entire lumen of the host third ventricle. Grafts were well vascularized and in addition exhibited remarkable numbers of supraependymal, cerebrospinal fluid-contacting neurons. The physical emergence of this cell line in proximity to viable grafts is discussed with respect to the biochemical influences that a neuropeptide producing fetal transplant has upon a peptide-deficient host.

A Golgi study of hypothalamic transplants in young and old host rats

The supraoptic nucleus of the F344 rat shows an age-related dendritic regression. In order to determine whether this previously observed dendritic regression may have been related to extrinsic (to the cell) hormonal, neurotoxic, or other circulating factors unique to the hypothalamus of older brains, we conducted a quantitative Golgi study of F344 embryonic anterior hypothalamic transplants into the third ventricle of young adult (5 months) and older (25 months) male F344 rats. Three months following transplantation there were no qualitative effects of host age on neuronal morphology, nor were there quantitative effects on transplant size, dendritic length or branching frequency within the transplanted tissue. These results suggest that either (a) there were no age-related changes in factors in the host brain which were sufficient to significantly affect dendritic extent or, (b) intrinsic connections or other properties within the transplant may be important in moderating the effect of the milieu of the aged brain on the transplanted tissue.

Neural transplantation: a review of recent developments and potential applications to the aged brain

Mammalian neural transplantation has recently been recognized to be a valuable technique for studying normal development and regeneration in the central nervous system. In addition, the ability of grafted neurons to reinnervate damaged regions of the host brain and to ameliorate some neuroendocrine deficits, cognitive disorders and motoric dysfunctions in young adult rodents has suggested that transplantation therapy may be effective in treating human neurodegenerative diseases and neurotransmitter deficiencies related to aging. It is of particular interest that initial studies of neuron transplants in aged rodents indicate that cholinergic, dopaminergic and noradrenergic neurons all integrate to some extent with the aged brain, and that the product of this graft-host interaction is improved behavioral performance of aged subjects. The present paper critically reviews the present domain of neural transplantation, its application to studies on the properties of the aged mammalian brain and discusses the possible therapeutic use of transplants in ameliorating transmitter-specific abnormalities associated with Parkinson's disease and Alzheimer's disease.

Functional capacity of solid tissue transplants in the brain: evidence for immunological privilege

The capacity of the mammalian brain to support the physiological function of allografts was assessed in parathyroidectomized Fischer strain rats bearing either isografts or immunogenic DA allografts of parathyroid glands implanted in their cerebral cortices. Established isografts and allografts survived indefinitely in the brain, maintaining normal serum calcium levels, with equal numbers of spontaneous failures (18-21%) in each group. Similarly, both MHC-compatible and incompatible skin allografts survived and were 'functional' at 40-50 days postgrafting as assessed by: continued formation of keratin; the presence of differentiated hair follicles and sebaceous glands; and frequent mitotic figures. No serum alloantibodies were induced by either MHC-incompatible parathyroid glands or skin in this site. However, both types of allografts were promptly rejected or failed to become established in the brains of specifically presensitized hosts. Furthermore, when Fischer hosts with long-established intracerebral DA parathyroid grafts received orthotopic DA skin grafts, their parathyroid grafts were rejected along with first-set rejection of the skin grafts. The tempo of this cellular immune response and the primary alloantibody response that accompanied it indicate that although the intracerebral grafts failed to induce detectable host sensitization or suppression, they remained susceptible to immune effectors. Thus, by using strongly immunogenic, adult tissues, we have established that the rat cerebral cortex is an immunologically privileged site, and the privilege is not dependent on lack of graft immunogenicity or alterations in host responsiveness. Furthermore, Ia+ (possible antigen-presenting) cells were rare in the cortical parenchyma sites used for transplantation though numerous in the choroid plexus of the ventricles and in certain areas of white matter. Therefore, privilege probably reflects deficient graft antigen presentation related to the paucity of Ia+ cells as well as to the brain's poor lymphatic drainage.

Cytoarchitectonics of substantia nigra grafts: a light and electron microscopic study of immunocytochemically identified dopaminergic neurons and fibrous astrocytes

Maturation of dopaminergic (DA) neurons and astroglia was studied in transplants of the substantia nigra grown for up to 7 months in the brain of rats. The investigation had three specific aims. The first was to observe effects of different transplant positions on the longevity of DA neurons. Second, the grafts were examined for changes of synaptic interactions and associations between DA neurons and astroglia. Third, an answer was sought to the question whether transplanted DA neurons migrate into the adjacent host brain. The grafts were taken from the ventral mesencephalon of rat embryos of different ages (day 14 to 18 of gestation) and placed into the cerebral cortex, tectum, cerebellum, or ventricles of newborn host animals. Following different times of survival the immunocytochemical localization of tyrosine hydroxylase (TH) and of glia filament protein (GFA) in the transplants were observed. In all of the transplantation sites, except for one, neurons of different morphologies that contained TH were found in the grafts. The cerebellar white matter of the host brain failed to support the long-term survival of DA neurons. The overall structure of mature substantia nigra grafts had some resemblance to intact substantia nigra (SN). On the ultrastructural level, it was found that morphological expression of some immature features of DA neurons, such as glial sheaths, somatic spines, and lack of oligodendroglia, persisted in mature grafts. Specific associations of DA neurons and astroglia in the grafts suggested that the cytoarchitectonic appearance of a given brain region may be related to the existence of particular neuron glia relationships. In contrast to intact SN, transplants revealed deficiencies in unlabeled pleomorphic boutons and contained some TH-immunoreactive terminals. Migration of DA neurons and their processes into the adjacent host brain was rarely observed.

Embryonic hypothalamic tissue transplanted to the IVth ventricle of newborn Brattleboro rats

The hypothalamic region, dissected from normal rat embryos, was transplanted into the IVth ventricle of newborn mutant Brattleboro rats. Water intake and urine osmolality were measured in both the recipient animals and unoperated littermate controls during a 7-week period following weaning. No differences were found between operated and unoperated animals. Ten weeks after transplantation, host animals were fixation perfused and the brains prepared for either catecholamine fluorescence or vasopressin immunohistochemistry. Well-developed grafts were found in the IVth ventricle of the hosts. They received innervation from the host locus coeruleus and contained many neurons with vasopressin-like immunoreactivity. Vasopressin-containing fibers were found running from the grafts into the host medulla.

Fetal hypothalamic transplants: neuronal and neurovascular interrelationships

This investigation is one of a series aimed at developing an understanding of the neuroanatomical correlates of neuronal and vascular interactions that occur between heterografts of normal fetal hypothalamic tissue transplanted into the third cerebral ventricle of adult Brattleboro rats with homozygous autosomal diabetes insipidus (DI). Rapid vascular invasion of fetal neurografts occurred within 3 days and arose from the mantle plexus of portal capillaries in the underlying host median eminence. Host vessels also invaded from the periventricular stratum and preoptic area. In the ventral regions of heterografts neurites were observed to terminate upon perivascular spaces that surrounded local capillaries. Despite the development of apparently normal neurovascular zones, none of the DI host recipients demonstrated an amelioration of polydipsia or polyuria characteristic of diabeties insipidus.

Neuronal and neurovascular integration following transplantation of the fetal hypothalamus into the third cerebral ventricle of adult Brattleboro rats. Neurological transplants: I

This investigation has confronted some very basic questions of neurobiology and specifically deals with the neurovascular and neuroanatomical interactions that occur between graft and host following neural transplantation. Host Long-Evans rats with chronic autosomal diabetes insipidus (DI) received stereotaxic implants of normal 17 day post-coitus fetal hypothalamic fragments from the rostral (anterior) hypothalamus of normal Long-Evans pups. Following stereotaxic surgery DI hosts were killed 60 or 90 days later and their brain prepared for correlative microangiography-immunocytochemistry coupled with transmission electron microscopy. Explants were rapidly invaded by host vessels from several routes. (1) Vessels appeared to arise from portal capillaries in the underlying median eminence and (2) from adjacent vessels from the paraventricular nucleus and surrounding endocrine hypothalamus and (3) possibly from intrinsic vessels of the graft. The former remained fenestrated and established bonafide neurovascular zones in the ventral regions and in actively growing explants. Small clusters of arginine vasopressin-positive fibers and neurophysin positive neurons were noted throughout the parenchyma of explants. Despite the presence of neurosecretory neurons and neurovascular (neurohemal) zones, none of the host rats exhibited a physiological return to normal parameters of water balance. However the active growth and development of explants in the third cerebral ventricle of DI host rats coupled with emergence of neurovascular zones lends support to a potential model for analyzing the development of anatomical substrates for the central delivery of neuropeptide hormones.

Naltrexone effects on pituitary neurointermediate lobe and median eminence

The long-acting opiate antagonist naltrexone hydrochloride was administered by intraperitoneal injection, in a dose response protocol, to adult rats. The drug was used to observe effects of opiate receptor blockade on cells of the pituitary gland and adjacent hypothalamus. At higher drug doses (5 mg/kg or 10 mg/kg), neurites directly innervating pars intermedia cells contained swollen vesicles and disrupted membranous elements. Fibers within the median eminence of the hypothalamus appeared swollen, and contained myelin figures. Despite the consistent degenerative changes appearing in neurites, measurements of levels of dopamine, norepinephrine, and epinephrine in striatum, and hypothalamus did not differ significantly between naltrexone-treated or control animals, although there was a significant elevation of norepinephrine in the pituitary after drug treatment. At all drug dose levels administered, supraependymal neuron-like cells appeared atop the ependyma of the third ventricle above the median eminence. These observations suggest that naltrexone produces specific "neurotoxic" effects on neurites of the tuberoinfundibular system, and may induce changes in the ventricular environment which stimulate the appearance of supraependymal neurons.

The neuroanatomical and neurovascular organization of normal fetal hypothalamic explants in the third cerebral ventricle of Brattleboro rats with homozygous diabetes insipidus

This investigation has combined microangiography, immunocytochemistry, coupled with transmission and scanning electron microscopy to discuss the neuroanatomical interactions that occur in the brains of Brattleboro rats with diabetes insipidus, following stereotaxic placement of normal fetal hypothalamic fragments into the third cerebral ventricle. Following surgical placement of 17 day post-coitus hypothalamic fragments, host rats with chronic autosomal homozygous diabetes insipidus were killed and their brains were prepared for analysis. A significant degree of explants (68%) flourished and grew in the lumen of the third cerebral ventricle of recipient hosts. Explants were rapidly invaded by host vessels from two routes. Vessels arose from the underlying mantle plexus of portal capillaries which remained fenestrated in the lower one-third of the explants and developed neurovascular (neurohemal) zones. The second source of vessels arose from bed capillaries of the adjacent paraventricular nucleus and adjacent hypothalamus. In contrast to vessels arising from the contact zone, these latter vessels remained unfenestrated. Small clusters of immunocytochemically positive neurons (neurophysin positive) were seen throughout the explants. Numerous healthy magnocellular neurons harboring numerous dense core vesicles and exhibiting multiple axosomatic and axodendritic synapses were seen throughout the neuropil of explants. Axon profiles were noted to terminate upon the abluminal basal lamina of perivascular spaces surrounding fenestrated capillaries in the lower one third of explants. None of the host animals exhibited physiological return to normal parameters of urine output, drinking behavior, and/or urine osmolarity. However the growth and development of explants in the third cerebral ventricle of DI hosts coupled with the emergence of bonafide neurovascular zones supports a potential anatomical substrate for the central delivery of neuropeptide hormones in this experimental model.

In vitro and in vivo studies on development and regeneration of vasopressin neurons

The present paper reviews recent work conducted in our laboratory on vasopressin neurons either grown in culture or transplanted into vasopressin deficient rats. The in vitro model of reaggregated cell culture used the anterior hypothalamus, including vasopressin neurons of the SON from normal timed-pregnant LE rats of similar ages used in our in vivo model. Various cells were co-cultured with their known target tissue, the posterior pituitary to analyze further the influence of the target tissue on hormone production. At a designated end point, cultured cells were fixed and stained immunocytochemically for vasopressin and neurophysin. Radioimmunoassay of the samples was performed for vasopressin quantification. Hypothalamic cells from all ages produced vasopressin (VP). The co-culturing of hypothalamus with posterior pituitary produced a significant increase in VP. Correlative transplantation studies were conducted using timed-pregnant Long-Evans (LE) rats at various days post coitus (dpc) and neonatal tissue from 0- and 5-day old rat pups. Animals survived about 40 days then were perfused and their brains processed for vasopressin and neurophysin thick-section immunocytochemistry. The results showed that the capability for survival of younger grafts was much greater than that of older tissue. With this paper, we have shown that the reaggregation of anterior hypothalamic cells in a culture system can be used for microassay of neurosecretory activity. These data suggest a close correlation between the ability of a neuron to survive transplantation and its stage of development. With the present studies, we have shown that neurons not fully differentiated maintain a greater degree of plasticity than older tissue and are better able to survive the rigors of transplantation and that various manipulations of environmental factors have an effect on brain development at critical times.

CSF-contacting pinealocytes in the pineal recess of the Mongolian gerbil: a correlative scanning and transmission electron microscope study

The pineal recess of the Mongolian gerbil was studied using correlative scanning and transmission electron microscopy. The surface of the pineal recess can be subdivided into three distinct zones: (1) central, (2) transitional, and (3) peripheral. In the gerbil, the deep pineal gland is located deep to the central and transitional zones. The ependyma of the peripheral zone is densely ciliated and resembles that of the main ventricular lining. Ependymal cells of the transitional zone are sparsely ciliated but possess numerous microvilli on their apical surfaces. Supraependymal neurons were identified in the transitional zones. These cells appear to make a synaptic-like contact with the underlying ependymal cells. Of the three zones, the central zone demonstrated the greatest amount of morphological variability. Although a number of supraependymal structures could be identified in the central zone, the most remarkable feature was the presence of protruding cells that possessed no significant surface features. Employing correlative transmission electron microscopy, the protruding cells were shown to be CSF-contacting pinealocytes. The number of CSF-contacting pinealocytes present in the central zone varied from one cell to large clusters that covered the entire zone. The results of this investigation demonstrate the presence of a direct contact and the potential for interaction between the deep pineal gland and the CSF of the pineal recess in the gerbil.

An ultrastructural analysis of the development of foetal rat retina transplanted to the occipital cortex, a site lacking appropriate target neurons for optic fibres

Foetal retina was removed from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats. The purpose of this procedure was to examine the development of retinal neurons and photoreceptors, and document synaptic patterns during maturation of the transplanted retina in an environment lacking a normal target for optic axons. Host animals were sacrificed at 5, 10, 15, 20 and 30 days and samples of cortex containing the transplant were subjected to a light and electron microscopic analysis. During early stages of development, (5 days) the retina assumes a radial orientation with the scleral (outer) surface located centrally and the vitreal (inner) surface occupying the periphery. Numerous mitotic figures are found at the centre of the transplant and columns of primitive neuroblasts appear to radiate out from this zone. By 10 to 15 days after transplantation the retinal tissue contains numerous small rosettes each of which displays a histotypic organization with recognizable layers of sensory cells and their centrally-projecting processes, an outer limiting membrane, made up of a network of zonulae adherentes, and a rudimentary outer and inner plexiform layer which delineate the cells of the inner nuclear layer. Ultrastructural analysis of such rosettes confirmed the presence of typical bipolar, amacrine, horizontal and ganglion cells, but revealed that while the plexiform layers were occupied by numerous processes from these neurons, few if any, of these exhibited synaptic vesicles. By 20 to 30 days following transplantation sensory cells have completely differentiated, giving rise to prominent inner and outer segments which display typical cilia, centrioles and basal bodies, together with numerous stacked lamellae of photoreceptors which were contorted, presumably due to growth in an abnormal site. It should be further emphasized that these structures developed in the absence of pigment cells. Synaptic development ensues during this period to form characteristic dyads within the outer and inner plexiform layers. Additionally, clusters of amacrine to amacrine contacts occurred in the inner plexiform layer and were found to be increased relative to other types of junctions. In general, synaptogenesis takes place in the outer and inner plexiform layers and all categories of retinal synapses are established, but the process was found to be significantly delayed in comparison to normal retina at the same stage of development. Quantitative analysis revealed a reduced number of presumptive ganglion cells in proportion to the other categories of neurons. Optic fibres remained small and failed to myelinate. It is suggested that lack of an appropriate target for optic axons induced this alteration and may be indirectly related to the delay in the onset of synaptic development.

Brain grafts reverse hypogonadism of gonadotropin releasing hormone deficiency

Hypogonadism in the mutant hpg mouse is characterized by a deficiency of hypothalamic gonadotropin releasing hormone (GnRH). Affected male mice exhibit immature reproductive organs, small abdominal testes and low pituitary and plasma gonadotropin concentrations. Recent studies have demonstrated the potential of fetal brain transplants to establish functional connections with host tissues. We therefore sought to use this approach to correct the hpg deficit. Fetal preoptic area (POA) (a site of GnRH production) from unaffected animals of the hpg strain was transplanted into the anterior third ventricle of adult hpg mice. We report that in such implanted animals, killed 2 months post-implantation, the POA grafts contained GnRH neurones, from which GnRH-positive fibres could be traced to capillaries of the median eminence. Hypothalamic GnRH and pituitary and plasma gonadotropin concentrations were increased compared with levels in untreated (hpg) animals. The testes were enlarged and had descended into the scrotum. Evidence of full spermatogenesis and interstitial cell development was present in testicular sections. No such effects were seen with transplants of cortical tissue.

Emergence of supraependymal cells in rat third ventricle after administration of p-chloroamphetamine

Administration of single or multiple 10 mg/kg doses of parachloroamphetamine hydrochloride, a serotonin-neurotoxic drug, to adult male rats, leads to emergence of clusters of supraependymal cells in the third ventricle above the median eminence. Transmission EM of specimens previously examined with scanning electron optics reveals that the cell clusters have characteristics of neurons. Two weeks after the initial drug injection, the ventricular surfaces of some animals lack the cell clusters, suggesting a "transience" in their appearance. The neuron-like elements may appear in response to alterations in brain serotonin levels, and their presence suggests an ability of adult nerve cells to migrate in response to chemical changes in neural tissue or cerebrospinal fluid.