Electron microscopic observations on the development of the median eminence in perinatal rats

Hypothalamic innate immune reaction in obesity

Findings from rodent and human studies show that the presence of inflammatory factors is positively correlated with obesity and the metabolic syndrome. Obesity-associated inflammatory responses take place not only in the periphery but also in the brain. The hypothalamus contains a range of resident glial cells including microglia, macrophages and astrocytes, which are embedded in highly heterogenic groups of neurons that control metabolic homeostasis. This complex neural-glia network can receive information directly from blood-borne factors, positioning it as a metabolic sensor. Following hypercaloric challenge, mediobasal hypothalamic microglia and astrocytes enter a reactive state, which persists during diet-induced obesity. In established mouse models of diet-induced obesity, the hypothalamic vasculature displays angiogenic alterations. Moreover, proopiomelanocortin neurons, which regulate food intake and energy expenditure, are impaired in the arcuate nucleus, where there is an increase in local inflammatory signals. The sum total of these events is a hypothalamic innate immune reactivity, which includes temporal and spatial changes to each cell population. Although the exact role of each participant of the neural-glial-vascular network is still under exploration, therapeutic targets for treating obesity should probably be linked to individual cell types and their specific signalling pathways to address each dysfunction with cell-selective compounds.

Ontogenesis of pyroglutamyl peptidase II activity in rat brain, adenohypophysis and pancreas

Pyroglutamyl peptidase II (PPII; E.C. 3.4.19.-) is a highly specific membrane-bound ectoenzyme degrading thyrotropin releasing hormone (TRH). The ontogenesis of this enzyme was measured in rat brain regions, adenohypophysis and pancreas. In hypothalamus PPII activity was maximal at day 8 postnatal, decreasing to adult values at day 45. The postnatal ontogenic patterns in posterior cerebral cortex and hypothalamus were similar. In olfactory bulb, two peaks of activity were observed (3th and 22nd day) while in adenohypophysis it appeared only at day 8, increased to day 30, decreasing thereafter to adult values.

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.

The fetal baboon median eminence as a circumventricular organ: I. Transmission electron microscopy

This investigation has focused upon a set of neuroanatomical correlates that underscore functional changes in the median eminence of the fetal baboon Papio anubis. Eight fetal primate brains were harvested at mid-gestation (100 days post-coitus) and prepared for routine light and transmission election microscopic examination following ventriculo-cisternal perfusion with high osmolarity aldehyde fixatives. The median eminence and other adjacent circumventricular organs (CVO) were blocked and embedded in epon. Routine transmission electron microscopic examination revealed discreet regional differentiation and zonal maturation of the fetal baboon median eminence at 100 days post-coitus. The ependymal and hypendymal zones were anatomically separate from the underlying fibrous and palisade zones of the median eminence. The dominant cell type of the ependymal and hypendymal zones were tanycytes whose apical surfaces constituted the floor of the third cerebral ventricle. The distal processes of tanycytes terminated upon the abluminal basal lamina of well developed perivascular spaces in the contact zone of the fetal median eminence. Numerous axon profiles that harbored both dense core and microvesicles were also observed to terminate upon the system of perivascular spaces that contained numerous fenestrated capillaries. Sharing this common perivascular system were cells of the pars tuberalis that contained numerous secretory inclusions. These data strongly suggest that by mid-gestation the fetal primate median eminence is highly differentiated and may be capable of a wide range of functional activities in response to changes in the maternal-placental environment. The ultrastructural correlates of an active neuroendocrine axis are apparent at this period of development.

Axovascular relationships in developing median eminence of perinatal rats with special reference to luteinizing hormone-releasing hormone projections

Topographical relationships of neurosecretory axons with the capillaries of the primary portal plexus were studied in the median eminence of rats from the 14th fetal till the 9th postnatal day by means of electron microscopy combined with morphometric analysis. Special attention was given to the light and electron microscopic immunocytochemical examination of luteinizing hormone-releasing hormone projections to the median eminence. Neurosecretory axons possessing secretory granules and clear microvesicles were first observed in the median eminence at the 14th fetal day. However, all of them were situated at a distance from the primary portal plexus. By the 20th fetal day, neurosecretory axons reached the external basal lamina of the primary portal plexus giving rise to so-called axovascular contacts. Some axons even penetrated into the perivascular space, apparently facilitating the neurohormone delivery into the hypophysial portal circulation. From that time on, both the number of the axons abutting on the external basal lamina and the entire area of axovascular contacts increased gradually. As to luteinizing hormone-releasing hormone axons, they grew into the median eminence from the 18th fetal day concentrating in older fetuses and neonates either over the primary portal plexus, or around the infundibular recess of the 3rd ventricle. After birth, the concentration and distribution of luteinizing hormone-releasing hormone axons within the median eminence became similar to those of adults. Luteinizing hormone-releasing hormone axons were found to arise from the neurons of septopreoptic area including the diagonal band of Broca. These data suggest the onset of neurohormone release in the median eminence from the 14th fetal day followed by the establishment of the hypothalamic control over the pituitary functions during the perinatal period in rats.

A quantitative electron microscopic study on the frequency of exocytosis in the anterior pituitary of perinatal rats

Granule exocytosis was quantitatively investigated in the perinatal rat anterior pituitary at the electron microscopic level. Both the number of cells in the process of exocytosis and the number of extruded granules per cell profile found in a standardized area of section were counted. The first distinct figure of exocytosis was detected in the anterior pituitary of fetal rats on day 18.5 of gestation, although occasional cells on day 17.5 had structures resembling granule extrusion. The frequency of cells showing granule discharge was very low on day 18.5 of gestation, but it sharply increased on day 19.5; a similar level was maintained up to day 21.5 of gestation. While the number of exocytosed granules per cell profile was almost unchanged during the fetal and neonatal period up to day 3 after birth. The frequency of cells undergoing exocytosis decreased near the time of birth, after which it transiently increased and dropped again to a minimum at 12 h after delivery. During days 1 to 3 of postnatal life, cells in the process of exocytosis were less frequent compared to fetuses between day 19.5 and 21.5. Both the number of cells undergoing exocytosis and the number of discharged granules per cell profile first exceeded the fetal values on the 6th postnatal day and were remarkably augmented between days 9-20 of the neonatal period. These data are discussed in relation to the hormone secreting activity of the anterior pituitary gland of perinatal rats.

Differing postnatal development of the somatostatin- and luliberin- systems in the male and female rat

By means of light-microscopic immunohistochemistry the perikarya of the luliberin-(LRF-) and somatostatin systems of neonate rats were found to be in differing stages of development. At a time point when the LRF-producing neurons had obviously attained their final shape and size, the somatostatin-immunoreactive perikarya were still in a postnatal phase of maturation. Whereas the number of the latter perikarya increases with advancing age, the number of LRF-immunoreactive perikarya decreases significantly from postnatal day 7 onward. Both peptide-hormone systems do not project concomitantly and to the same extent to their principal neurohemal regions in the organum vasculosum laminae terminalis (OVLT) and the median eminence (ME). In all presently studied stages of development, despite considerable individual variations in one age group, among the components of the LRF-system the OVLT displays a more intense immunoreactivity than the ME. The somatostatin system, however, projects to the OVLT with a conspicuous temporal delay compared to the ME, and, furthermore, in the OVLT the pattern of immunoreactivity characteristic of adult rats is not yet attained at postnatal day 21. Evidence for differences in the immunoreactivity between male and female animals was restricted to the LRF-system. Finally, the results obtained on the stria terminalis speak in favour of the fact that the long-range extrahypothalamic projections of the somatostatin system also undergo postnatal maturation. In the stria terminalis, somatostatin-immunoreactive fibers can be demonstrated initially on postnatal day 7. They attain their full immunoreactivity on postnatal day 21. Furthermore, in the bed nucleus of the stria terminalis an intermittent cytoplasmic immunoreactivity is observed, which is limited to the animals of postnatal day 7 and disappears completely during the further course of development.

Prepubertal ontogeny of responsiveness to estradiol in female rat central nervous system

The physiological response to systemic estrogens changes dramatically during the period from birth to puberty. With the onset of puberty, the rat reaches a critical developmental plateau with regard to endocrinological responsiveness to estradiol. Since the appearance of the pubertal response pattern appears to be less a consequence of some intrinsic "trigger' than the natural continuation of a developmental sequence that begins prenatally, its ontogeny should be examined in a broad context that will take account of the impact of each of the dynamic components influencing the interactions between estradiol and the central nervous system on the functional development of the organism as a whole. The prepubertal ontogeny of endocrinological responsiveness to estradiol in the central nervous system of the female rat is examined in the context of several of the important factors that are known to influence the functional development of the hypothalamo-pituitary-gonadal circuit:the rapidly changing hormonal environment of the morphologically and physiologically immature juvenile rat, the shifting predominance of alphafetoprotein and "adult" estradiol-binding protein, sexual differentiation of the neural substrate, and the development of mature pituitary-gonadal feedback mechanisms. The availability of ever more sensitive techniques for the measurement of the actions of estradiol in the central nervous system of the immature organism has necessitated a re-evaluation of existing data. This, in turn, suggests that new approaches should be applied to the examination of problems related to the development of reproductive maturity of the central nervous system.

The fetal development of the luteinizing hormone-releasing hormone (LHRH) neuronal systems of the guinea pig brain

We have studied the distribution of LHRH-like immunoreactive material in fetal guinea pig brains beginning at day 25 of gestation. Cells and processes were first detected throughout the peripheral, intracranial and central course of the nervus terminalis at 28 (but not 25) days of gestation. The localization of LHRH in this structure preceded its appearance in the hypothalamus and coincided with the initial detection of immunoreactive LH in the pituitary gland. The possible role of the LHRH neuronal network within the nervus terminals in the development of reproductive function is discussed. Comparisons between the brains of littermates of both sexes were made at each age (days 28 through 60 of gestation) to determine possible differences between the sexes in the development of the LHRH neurosecretory systems. No sexually dimorphic features were evident in these systems throughout the prenatal period except at days 40 and 45. At these ages, differences in the number of LHRH neurons in the arcuate nucleus were found between the sexes in some but not all of the brains examined. These differences in LHRH concentrations may reflect the onset of testicular activity as indicated by an increase in serum testosterone levels. Increased serum testosterone concentrations were observed in the male fetuses beginning at 45 days of gestation. However, cell counts made within this nucleus from days 40 through 60 of gestation indicated no comparable sexual dimorphism in the total neuronal population which appeared to be relatively stable throughout this period of brain growth. The number of immunoreactive LHRH neurons visible throughout the brain increased from days 30 through 45 and fewer LHRH cells were seen on days 50 and 60 of gestation, particularly in the arcuate nucleus. The apparent decrease in visible LHRH neurons was concomitant with an increase in number and more extensive distribution of immunoreactive processes throughout the hypothalamus and in certain extrahypothalamic areas of the brain.

Prenatal and early postnatal development of the glial cells in the median eminence of the rat

The development of the glial cells of the rat median eminence (ME), including the supraependymal cells, was investigated from embryonic day (ED) 14 through postnatal day (PD) 7, and pituicyte development from ED 12 through ED 17. The anlage of the ME and neurohypophysis shows a neuroepithelial-like structure at ED 12. From ED 13 to 15, the cells of both regions start to differentiate. At the ultrastructural level, only one cell type appears. At the beginning of ED 16, glioblasts of the oligodendrocyte and astrocyte series migrate laterally (from the region of the arcuate nucleus) into the ME. Also at this time the first distinctive structural features appear in the neurohypophysial anlage, the cells of which later develop into pituicytes. Starting at ED 18, tanycytes and astrocytic tanycytes arise in the ME from local glial cells, and somewhat later oligodendroblasts and astroblasts are formed from immigrant glioblasts. Due to their common features, the pituicytes, tanycytes and astrocytic tanycytes apparently represent different forms of the same parent cell type. Microglial and supraependymal cells are first seen at ED 12. Initially, they resemble the prenatal phagocytic connective tissue cells and mature in fetus into typical electron-dense microglia and macrophage-like supraependymal cells. Both cell types are apparently of mesodermal origin. The microglial elements of the ME probably migrate from the mesenchyma through the basement into the nervous tissue. The intraventricular macrophages of the infundibular region may originate from microglia, epiplexal cells and subarachnoid macrophages.

Electron microscopic immunocytochemical investigation on the postnatal development of the vasopressin system in the rat

The present ultrastructural results indicate that, in the rat, the vasopressin-synthesizing perikarya of the supraoptic nucleus (NSO) attain a certain degree of maturity earlier than those of the paraventricular nucleus (NPV). In the neonate rat, the stainability of the nuclear areas is very weak; in the perikarya of the NSO a few labeled granules can be found, whereas the perikarya of the NPV often display only a labeled Golgi area, the cytoplasm being devoid of granules. At the end of the first (NSO) and the second (NPV) postnatal weeks, the filling of the neurosecretory granules with vasopressin is inhomogeneous with irregular spots of reaction product distributed on the granules. This feature is less obvious during the following week and has nearly disappeared after the third and fourth postnatal weeks. Already in the neonate two types of vasopressin-positive fibers are observed in the median eminence, characterized by the different diameters of their granules and by their typical location in the internal and the external pericapillary contact zone. Especially in one and two week-old animals, in the internal zone of the median eminence and, to a lesser degree in the neural lobe, the immuncytochemical reaction product is deposited on an axonal tubular network. Judging from the presence of very few vasopressin-negative fibers in the neural lobe of the neonate, the development of the oxytocin system appears to be delayed. A characteristic relationship between pituicytes and the neurosecretory fibers can be observed during the first two postnatal weeks. After the third postnatal week the immunocytochemical features of the vasopressin system correspond approximately to that in adult rats.

Development of the diencephalon in the rat. II. Correlation of the embryonic development of the hypothalamus with the time of origin of its neurons

The development of the nuclei of the hypothalamus was examined in normal and X-irradiated embryos from day 13 (E13) to the day before birth (E22). The diencephalic neuroepithelium was subdivided into three lobes (dorsal, medial, and ventral) and two lobules (superior and inferior). The hypothalamus is derived from the ventral lobe and the inferior lobule. The ventral neuroepithelial lobe generates the neurons of most of the early arising hypothalamic structures, including those of the lateral tier nuclei associated with the medial forebrain bundle, and the heterogeneous intermediate tier nuclei. A specialized neuroepithelial region lining the diamond shaped ventricle produces the early neurohypophysial magnocellular neurons; the neurons of the paraventricular nucleus remain at this site, whereas the neurons of the supraoptic nucleus could be traced migrating laterally. The neurons of the late arising hypophysiotropic area of the posterior hypothalamus are derived from components of the inferior neuroepithelial lobule: the dorsomedial and ventromedial nuclei apparently from a shared matrix in the main portion of the inferior lobule; the tuberomammillary-arcuate complex from its posteroventral recess. The triple-decked and sequentially produced components of the mammillary system may arise from separate neuroepithelial sites. The autoradiographic results of the previous study (Altman and Bayer, '78a) showed that the structural and functional heterogeneity of the mature hypothalamus is paralleled by cytogenetic heterochronicity; the present embryonic observations indicate that many of the distinguishable components of the hypothalamus arise from a mosaic of heterogeneous neuroepithelial sites.

Ultrastructure of synaptosomes from fetal rat brain

The crude mitochondrial fraction P2 and subfractions of P2 were prepared from the brain stem, hemispheres and whole brain of 19-day-old fetal rats. Samples were fixed in glutaraldehyde-osmium, NaMnO4 or by Tranzer's triple fixation method (aldehydr-chromate-dichromate-osmium) and examined by electron microscopy. The C-fraction from whole brain was the main synaptosome fraction, containing 3.2% presynaptic terminals as counted from all membrane bound particles. The brain stem showed more presynaptic terminals than the hemisphere (2.8% versus 0.9%) suggesting a caudal-rostral maturation gradient for synaptogenesis. The maturity of the nerve endings obtained was very variable in contrast to the rather uniform synaptosomes derived from adult tissue. They varied from profiles without any substructures to mature synaptosomes displaying asymmetric synaptic junctions. Monoamine synaptosomes containing small granular vesicles were not detected in the present study, suggesting immaturity of the granular monoamine pool at this stage of development.

An electron microscope study of the differentiating capillaries of the mouse neurohypophysis

The developing capillaries of the mouse neurohypophysis were studied in the electron microscope to elucidate the fine structural differentiation of the vascular component of the neuro-hemal contact zones in the external median eminence and the neural lobe. In the embryo the growth of the superficial net of the primary plexus of the hypophysial portal system is largely manifested by the presence of proliferation areas located within the capillary plexus covering the surface of the median eminence. Presumptive shallow capillary loops diverge from these ares in the first postnatal week. Differentiation of the capillary wall follows the pattern outlined for continuous capillaries. A few fenestrae appear in the endothelium of immature, superficial vessels at the 17th gestational day, increase in frequency during the following embryonic days, and occur regularly in the postnatal animal. In the neural lobe the internal capillaries proliferative by vascular sprouts emanating from the vessels on the surface of the gland. At the end of embryonic time an extensive net has developed, composed of capillaries with immature characteristics. Proliferation is largely finished by the end of the third postnatal week, when mature capillaries dominate the picture. Formation of attenuated, porous areas is a postnatal process, apart from single fenestrae appearing in the walls of a few immature capillaries in late fetal life. The structural possibilities for an onset of neurohypophysial function in the mouse is discussed.

The ultrastructure of the developing medial preoptic nucleus in the postnatal rat

The postnatal maturation of the medial preoptic area (MPOA) of male and female rats was studied by means of light and electron microscopy. In Golgi-Kopsch preparations, bi- and multipolar cells were present at birth in both sexes; the perikarya of most of these cells, as seen with the electron microscope, exhibited an immature appearance characterized by a sparse organelle content. During the next 5-10 days, an increase in cell size and cytoplasmic differentiation occurred in each sex. By 10 days, many of the MPOA neurons displayed an increased organelle density with the most prominent change being a proliferation of rough endoplasmic reticulum. Further neuronal development was also evidenced by a greater frequency of dendritic spines than seen at either birth or 5 days. Lastly, the number of synapses per unit area increased rapidly in each sex during the first two weeks of postnatal life. Thus, a period of increasing cytological maturation and synaptic organization closely paralleled previously demonstrated stages of functional differentiation of gonadotrophin regulatory mechanisms in the rat brain.