Immunocytochemical localization of vasotocin and mesotocin in the hypothalamus of lacertilian reptiles

Summary The hypothalamic magnocellular neurosecretory system of lizards was studied with the unlabeled antibody peroxidase-antiperoxidase complex (PAP) technique at the light microscopic level. It was shown that vasotocin and mesotocin are synthesized in separate neurons. The vasotocinergic as well as the mesotocinergic perikarya are of different sizes. Both cell types occur in close juxtaposition, but without a distinct pattern of distribution. The external zone of the lacertilian median eminence contains numerous immunoreactive vasotocinergic fibers and only few immunoreactive mesotocinergic fibers. The general organization of the hypothalamic magnocellular neurosecretory system of lizards, as revealed by immunocytochemistry, is essentially similar to that revealed with unspecific staining methods.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

The extravascular channel system in the rostral pituitary of Mugil cephalus (Teleostei) as revealed by use of horseradish peroxidase

Summary Using the unlabeled antibody peroxidase-antiperoxidase (PAP) technique at the light microscopic level, it was demonstrated that, in the amphibian magnocellular hypothalamo-hypophysial neurosecretory system, vasotocin and mesotocin are synthesized in separate neurons. A tendency to preferential location of the two kinds of neuronal perikarya is described. The neurosecretory perikarya are the origin of separate vasotocinergic and mesotocinergic axons. In the neural lobe, the pattern of distribution of the two types of axons is different. The coarse ventricular dendrites of both kinds of neurons are hormone-containing processes. Staining with anti-bovine neurophysin I serum suggested that the vasotocinergic and the mesotocinergic neurons synthesize different neurophysins.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Mesotocin and vasotocin in the brain of the lizard Gekko gecko

Summary The distribution of mesotocin and vasotocin was studied in the brain of the lizard Gekko gecko with antisera specific for either peptide. Both mesotocinergic and vasotocinergic perikarya are found in the paraventricular and supraoptic nuclei of the hypothalamus, whereas vasotocinergic neurons are exclusively present in the bed nucleus of the stria terminalis and in a cell group of the rhombencephalon. The distributional pattern of the mesotocinergic fibers corresponds closely to that of the vasotocinergic fibers. However, throughout the entire brain the mesotocinergic innervation is less dense than the vasotocinergic innervation. No sex differences are present in the mesotocinergic fiber system.Abbreviations acc nucleus accumbens - bst bed nucleus of the stria terminalis - bv blood vessel - dB diagonal band of Broca - dc dorsal cortex - dth dorsolateral thalamic nucleus - lc lateral cortex - me median eminence - oc optic chiasma - ot optic tract - pag periaqueductal grey - pvn paraventricular nucleus - rc rhombencephalic cell group - sep septum - son supraoptic nucleus - tect mesencephalic tectum - vth ventrolateral thalamus     

The topography of mesotocin and vasotocin systems in the brain of the domestic mallard and japanese quail: Immunocytochemical identification

Summary The neurosecretory systems producing mesotocin (MT) and vasotocin (VT) (the avian homologues of oxytocin and vasopressin, respectively) were characterized in the brains of the domestic mallard and Japanese quail by means of indirect immunofluorescence techniques using specific antisera. In the anterior preoptic region, including the organum vasculosum of the lamina terminalis, and at different levels of the supraoptic and paraventricular nuclei, separate mesotocin- and vasotocin-producing neurons were identified. Mesotocinergic and vasotocinergic neurons were also located in the tuberomammillary area, among the ectomammillary tract fibers. The supraoptico-neurohypophysial tract, formed by vasotocin- and mesotocincontaining axons, enters the internal zone of the median eminence and ends in the posterior lobe of the pituitary. The external zone of the rostral median eminence appears to contain vasotocin and mesotocin fibers, which terminate in close contact with the capillaries of the hypophysial portal system.With contributions by Dr. B. Kerdelhué, Laboratoire des Hormones Polypeptidiques du CNRS, 91190 Gif-sur-Yvette, France     

Vasotocin-immunoreactive elements and neuronal typology in the suprachiasmatic nucleus of the chicken and Japanese quail

Summary A Golgi study of the suprachiasmatic nucleus (SCN) of the chicken and Japanese quail revealed in this area a complex neuronal pattern and typology, including specialized dendritic patterns. Immunocytochemical studies provided evidence for the existence of a vasotocinergic system within the SCN, mainly in its rostral portion. Other clusters of immunoreactive elements are located in the lateral and dorsal divisions of this nucleus; they show a different distribution in the chicken and Japanese quail. The present results confirm, in birds, the existence of a morphologically defined SCN, the complex cytoarchitecture of which suggests specialized functions.This study was supported by grants from CNR (83.00447.04, 84.01769.04 and 84.00797.04) and MPI (60%)     

The reticular formation of lampreys (Petromyzonidae) — A target area for exohypothalamic vasotocinergic fibres

Summary The octapeptide vasotocin, which is formed in the classical neurosecretory nuclei of lampreys (Petromyzonidae), is transported, bound to the carrier protein neurophysin, not only to the neurohypophysis but also to various other regions of the brain via exohypothalamic fibres. A target area of this exohypothalamic vasotocinergic system is, in the brook lamprey (Lampetra planeri Bloch), a relatively well circumscribed area in the isthmus region of the rhombencephalic tegmentum motoricum, which is called area lateralis tegmenti. In this area, which belongs to the reticular formation, the vasotocinergic fibres form synaptic contacts with nerve cell perikarya and processes. The vesicles contained in the fibres were identified, ultrahistochemically, as neurophysin vesicles. They correspond to the neurophysin vesicles observed in the neurohypophysis of the same species. The functional significance of the vasotocinergic supply to portions of the reticular formation in lampreys is open to discussion.Supported by grants from the Ministry for Science and Technology of the German Democratic Republic. Acknowledgements: For skilled technical assistance the authors are indebted to their colleagues Mrs. S. Mehnert, E. Siebert, C. Schneider, and I. Seifert.     

Specific activation of estrogen receptor alpha and beta enhances male sexual behavior and neuroplasticity in male Japanese quail

Two subtypes of estrogen receptors (ER), ERα and ERβ, have been identified in humans and numerous vertebrates, including the Japanese quail. We investigated in this species the specific role(s) of each receptor in the activation of male sexual behavior and the underlying estrogen-dependent neural plasticity. Castrated male Japanese quail received empty (CX) or testosterone-filled (T) implants or were daily injected with the ER general agonist diethylstilbestrol (DES), the ERα-specific agonist PPT, the ERβ-specific agonist DPN or the vehicle, propylene glycol. Three days after receiving the first treatment, subjects were alternatively tested for appetitive (rhythmic cloacal sphincter movements, RCSM) and consummatory aspects (copulatory behavior) of male sexual behavior. 24 hours after the last behavioral testing, brains were collected and analyzed for aromatase expression and vasotocinergic innervation in the medial preoptic nucleus. The expression of RCSM was activated by T and to a lesser extent by DES and PPT but not by the ERβagonist DPN. In parallel, T fully restored the complete sequence of copulation, DES was partially active and the specific activation of ERα or ERβ only resulted in a very low frequency of mount attempts in few subjects. T increased the volume of the medial preoptic nucleus as measured by the dense cluster of aromatase-immunoreactive cells and the density of the vasotocinergic innervation within this nucleus. DES had only a weak action on vasotocinergic fibers and the two specific ER agonists did not affect these neural responses. Simultaneous activation of both receptors or treatments with higher doses may be required to fully activate sexual behavior and the associated neurochemical events.     

The vasotocinergic system in the hypothalamus and limbic region of the budgerigar (Melopsittacus undulatus)

We report a morphological and biochemical analysis on the presence, distribution and quantification of vasotocin in the hypothalamus and limbic region of the budgerigar Melopsittacus undulatus, using immunohistochemistry on serial sections and competitive enzyme linked immunoadsorbent assay measurements on tissue extracts. Analysis of the sections showed large vasotocin-immunoreactive neurons in three main regions of the diencephalon, of both male and female specimens. Vasotocinergic cell bodies were located in the ventral and lateral areas of the hypothalamus, dorsal to the lateral thalamus and medial to the nucleus geniculatus lateralis. Immunoreactive neurons were placed also periventricularly, close to the walls of the third ventricle, at the level of the magnocellular paraventricular nucleus. Well evident bundles of immunoreactive fibers were placed ventral to the anterior commissure in the same regions of the hypothalamus and thalamus where vasotocinergic perikarya are localized. Fibers were identified close to the third ventricle, and in the lateral hypothalamic area along the lateral forebrain bundle. In contrast to what reported for other oscine and non-oscine avian species, we were not able to identify immunopositive neurons in any region above the anterior commissure, or detect relevant differences on the distribution of the vasotocin immmunoreactivity between sexes. Competitive enzyme linked immunoadsorption assay and image analysis of the extension of immunoreactivity in the tissue sections were consistent with the qualitative observations and indicated that there is no statistically significant dimorphism in the content of vasotocin or in the location and distribution of vasotocinergic elements in the investigated areas of male and female parrot brains.     

Immunohistochemical demonstration of alpha-1-antitrypsin in the islet cells of human pancreas

Summary Using the unlabeled antibody peroxidase-antiperoxidase complex (PAP) technique at the light microscopic level, it has been shown that, in the dipnoan preoptico-hypophysial neurosecretory system, vasotocin and mesotocin are synthesized in separate neurons. In the preoptic nuclei, the perikarya of these two types of neurosecretory neurons are not located preferentially. The two types of neurosecretory perikarya give rise to separate vasotocinergic and mesotocinergic axons, respectively. The dipnoan median eminence and neural lobe contain separate vasotocinergic and mesotocinergic nerve fibres, the general distribution of which is described. In the pars distalis and the pars intermedia of the hypophysis, neurohypophysial hormone-containing nerve fibres have not been found.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk OnderzoekThe authors are greatly indebted to Prof. Dr. Hyder, Department of Zoology, University of Nairobi, Kenya for kindly supplying us with the fixed material used in this study     

Intracerebral Sex Differences in the Vasotocin System in Birds: Possible Implication in Behavioral and Autonomic Functions

The brain vasotocinergic system demonstrates clear sexual dimorphism in birds investigated so far. This paper examines the evidence obtained in studies on gallinaceous (domestic fowl, Japanese quail) and passerine (canary, junco, zebra finch) birds. Vasotocin (VT)-immunoreactive parvocellular neurons are present in the nucleus of stria terminalis of males, but they are less abundant or absent in the corresponding structure of females. A similar difference has been observed in the dorsal paraventricular area of domestic fowl. Sex-related differences in VT-gene expression have been confirmed byin situhybridization. Moreover, overall brain content of VT mRNA in cockerels is about twice that of hens, suggesting that VT synthesis may also be sexually dimorphic in other brain areas where morphological sex differences have not yet been revealed. The vasotocinergic system in birds is implicated in body fluid homeostasis, and during ontogeny it starts to respond to osmotic challenges in a sexually dimorphic way. Photoperiod, aging, or castration—all associated with changes in circulating testosterone levels—affect sexually dimorphic VT pathways and cell clusters. Sexually dimorphic vasotocinergic circuits are distributed in regions containing steroid-concentrating cells and are closely intermingled with aromatase-containing neurons that may mediate activational effects of gonadal steroids on this peptidergic system. However, it remains undetermined whether the observed neuroanatomical sex differences are related to sexually dimorphic autonomic and behavioral effects induced by VT. Most likely, VT in birds has a modulatory rather than a specific regulatory function in control of male sexual behavior and vocalization.     

毛茛科金莲花亚科植物的地理分布

本文对毛茛科金莲花亚科各属的地理分布作了分析,该亚科植物除了少数属的一些种分布到南半球的温带地区,一些种分布或延伸到亚热带山地、非洲东部和北部的干旱、半干旱的地区外,绝大部分的属、种均分布于泛北极区域。根据其17个属的地理分布式样,把它们划分为8个分布区类型:(1)北温带分布类型4属;(2)北温带和非洲分布类型1属;(3)北半球温带和南半球间断分布类型1属;(4)欧洲和东亚间断分布类型1属;(5)西亚分布类型1属;(6)地中海分布类型3属;(7)欧亚和温带亚洲分布类型1属;(8)东亚分布类型5属。本文以形态特征为主,结合花粉和染色体的性状分析,认为东亚特有的鸡爪草属、Megaleranthis和铁破锣属可能分别是联系驴蹄草属和金莲花属,鸡爪草属和金莲花属以及金莲花族和升麻族的中间类型。另外,文中详细地统计了该亚科的不同等级分类群及特有种在各个植物区的分布,并从系统发育的观点讨论了各个植物区所具有的原始类群和进化类群,提出了如下论点,即东亚植物区(特别是中国西南部)不但是金莲花亚科植物分布的多度和多样性中心以及特有类群的分布中心,而且还是原始类群的保存中心,伊朗-土兰区及地中海周围是第二分布中心。     

Development of arginine vasotocin innervation in two species of anuran amphibian:Rana catesbeiana andRana sylvatica

Arginine vasotocin (AVT) is a neurotransmitter in the amphibian central nervous system and is released from the neurohypophysis in the regulation of hydromineral balance and other homeostatic functions. Many amphibians experience drastic changes in habitat with respect to water availability during their transformation from aquatic larvae to terrestrial adults. To examine whether metamorphosis is accompanied by a reorganization of central vasotocinergic neurons, the developmental organization of vasotocin neurons and nerve fibers was studied with immunocytochemistry in the brains of bullfrogs (Rana catesbeiana) and woodfrogs (R. sylvatica). In bullfrogs, early limb-bud-stage tadpoles had AVT-immunoreactive neurons and nerve fibers in the lateral septal nucleus, amygdala, preoptic hypothalamus, suprachiasmatic nucleus, and posterodorsal tegmentum. Woodfrog larvae showed similar patterns of hypothalamic AVT immunoreactivity, although neuronal staining in the amygdala did not appear until metamorphic climax, and never appeared in septal neurons or in the posterodorsal tegmentum. Whereas the highly terrestrialR. sylvatica adults must adapt to an adult habitat with prolonged periods of dehydration,R. catesbeiana adults remain semiaquatic and, as such, need not develop extreme mechanisms for water retention. Nonetheless, vasotocinergic pathways showed developmental similarities in the two species. The early appearance of AVT innervation in bothRana suggests that AVT has neuroregulatory functions well before metamorphosis.     

Immuno-cytochemical demonstration,in the external region of the amphibian median eminence,of separate vasotocinergic and mesotocinergic nerve fibres

Summary By means of single and double immuno-enzyme cytochemical staining techniques, it was shown that the external region of the amphibian median eminence contains separate vasotocinergic and mesotocinergic nerve fibres. Moreover, it was demonstrated that the vasotocinergic fibres also contain neurophysin. In animals in which the hypothalamic magnocellular neurosecretory preoptic nuclei had been completely removed, the immuno-reactive vasotocinergic and mesotocinergic fibres of the median eminence had disappeared. From this result, it is concluded that, at least the great majority of the vasotocinergic and mesotocinergic fibres of the external region of the amphibian median eminence are processes of neurosecretory perikarya located in the hypothalamic magnocellular preoptic nuclei. On the other hand, our results do not exclude the possibility that a minority of these neurosecretory fibres originate from small immuno-reactive perikarya which were found in the tuber cinereum. The observation that both kinds of processes accumulate around blood capillaries of the hypophysial portal system strongly suggests that they play a role in the control of the activity of the pars distalis of the hypophysis.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Comparison of male antennal morphology and sensilla physiology for sex pheromone olfactory sensing between sibling moth species: Ectropis grisescens and Ectropis obliqua (Geometridae)

Ectropis grisescens and Ectropis obliqua (Lepidoptera: Geometridae) are sibling pest species that co‐occur on tea plants. The sex pheromone components of both species contain (Z,Z,Z)‐3,6,9‐octadecatriene and (Z,Z)‐3,9‐cis‐6,7‐epoxy‐octadecadiene. E. obliqua has (Z,Z)‐3,9‐cis‐6,7‐epoxy‐nonadecadiene as an additional sex pheromone component, which ensures reproductive segregation between the two species. To ascertain the detection mechanism of olfactory organs for sex pheromone components of E. grisescens and E. obliqua, we applied scanning electron microscopy and single sensillum recording to compare antennal morphology and sensillum physiology in the two species. There was no apparent morphological difference between the antennae of the two species. Both species responded similarly to all three sex pheromone components, including, E. obliqua specific component. The distribution patterns of antennal sensilla trichodea differed between the two species. Sex pheromone olfactory sensing in these sibling species appears to be determined by the density of different types of olfactory sensing neurons. Dose‐dependent responses of sensilla trichodea type 1 to (Z,Z)‐3,9‐cis‐6,7‐epoxy‐octadecadiene, the most abundant component, showed an “all or none” pattern and the other two components showed sigmoidal dose‐response curves with a half threshold of 10^?4 (dilution equal to the concentration of 10 μg/μl). These results suggest that the major sex pheromone component functions as an on–off controller while secondary components function as modulators during olfactory transmission to the primary olfactory center.     

TGF-β receptor type II and fetuin in the developing sheep neocortex

Fetuin shows a characteristic pattern of distribution in the developing neocortex in many mammalian species. Its expression is confined to early-appearing cortical-plate and later subplate neurons. A short 19 amino-acid sequence of fetuin shows a degree of homology to an 18 amino-acid sequence of the TGF-β type II receptor (TβR-II) and in vitro fetuin binds to members of the TGF-β family of cytokines. It has been suggested that fetuin is the biologically significant antagonist of these cytokines. We have compared, using immunocytochemistry, the distribution pattern of TβR-II and fetuin in the developing neocortex of foetal sheep. TβR-II immunoreactivity first appears at around 40 days of gestation in the fetal sheep (E40, term in sheep is 150 days from conception), localised in two discreet bands: one just outside the cortical plate in the inner part of the marginal zone and one deep in the cortical plate in what becomes the transient subplate zone. By E70–E80, TβR-II is prominent in a population of subplate cells, whereas, by E120 only small patches of TβR-II-positive cells are visible, principally in pyramidal cells in layer VI. The developmental sequence of the staining pattern for TβR-II in the neocortex is complementary to that for fetuin, rather than overlapping with it. Double-labelling of fetuin and TβR-II shows some cellular co-localisation, especially at E60, but most fetuin-positive cells are not immunoreactive for TβR-II. Thus, fetuin’s proposed role as an antagonist of TGF-β cytokines and mimic of TβR-II is not consistent with the observed distribution of these two molecules in the developing neocortex of the foetal sheep. Received: 20 March 1997 / Accepted: 12 May 1997     

鄂西南亚热带山地常绿落叶阔叶混交林物种多度分布格局

基于七姊妹山自然保护区内6 hm~2监测样地多度数据,通过累计经验分布曲线(ECDF)表征该样地内不同生活型功能群的物种-多度分布格局,并采用6种模型对各功能群不同取样尺度物种等级-多度曲线进行拟合并检验其拟合效果,分析多度格局与模型拟合在不同尺度间的差异,探讨其背后的生态学过程与机制。结果表明:(1)各尺度下落叶种比常绿种的物种数多,物种多样性指数更大,但个体数相对较少;不同功能群稀有种比例排序为:落叶种所有种常绿种。(2)6种模型中的断棍模型的拟合效果较差;中大尺度(50 m×50 m、100 m×100 m)上不同生活型树种多度分布能接受的模型较少,除大尺度的常绿树种外,拟合最优模型均为对数正态分布模型,大尺度的常绿树种拟合最优模型为中性模型;小尺度上(20 m×20 m)常绿树种的最优模型为对数正态分布模型,落叶树种最优模型为生态位优先模型,所有树种在小尺度最优模型为Zipf-Mandelbrot模型。研究认为,随着尺度逐渐扩大,中性过程较生态位过程对物种-多度格局的解释力度更大,落叶树种物种多度格局的形成机制较常绿树种更接近于样地所有树种物种-多度格局的形成机制。     

中国有尾两栖类地理分布格局与扩散路线

通过绘制我国有尾两栖类的地理分布图,分析该类群的地理分布格局与可能的起源和扩散路线。研究结果表明,有尾两栖类在我国的地理分布格局,有三个关键区域:1.位于长江上游的、有尾两栖类物种丰富度最高的横断山区,是有尾两栖类的分布中心,该区域很可能亦为有尾两栖类的起源中心及主要特化中心;2.三个次级分布与特化中心,大别山-淮河流域区、南岭-珠江流域区、浙闽山地-江南水乡区;3.四个小型的三级分布与特化中心,台琼海岛区、东北长白山-三江流域区、秦巴山地-汉江渭河流域区、天山-伊犁河流域区。另外,该类群可能的迁移和扩散路线可归纳为:沿着各水系,从分布中心分别向东西南北四个方向迁移和扩散,其中以向北和向东为主,向南和向西扩散的种类较少。向北扩散的主要是小鲵科(Hynobiidae)的一些种类,向东扩散的种类则以蝾螈科(Salamandridae)物种为主。广布于我国东部的隐鳃鲵科(Cryptobranchidae)种类,极可能是该类群向北和向东这两个方向扩散的结果。研究结果表明,东亚两栖动物区系具有明显的整体性,即东亚两栖动物区系是单源的、本地起源的。本项研究结果支持中国日本界假说。     

红松存在/不存在数据的多尺度空间分布格局

红松属小兴安岭地区地带性植被优势种,该地区也是其分布的北缘。在景观尺度上开展红松的分布格局研究有利于进一步了解红松分布机理、未来迁移过程等问题,对其经营和保护有重要意义。将景观指数法与点格局分析法结合,设定8个空间尺度,利用红松存在/不存在数据,通过计算各空间尺度上红松聚集程度和景观指数,分析小兴安岭地区红松种群在多尺度上的分布格局。研究结果表明,小尺度上红松聚集分布明显,随机分布区多处于其聚集分布区的边缘,均匀分布区则散布在其聚集分布区内。景观指数研究表明,通过景观指数可判断红松聚集分布格局趋势,而不能判断均匀分布、随机分布格局趋势,因为它们在多尺度下景观指数波动大,不能用景观指数来描述分布格局。研究得出如下结论:1)红松主要分布在其分布区的核心区域内,在分布区边缘和过渡带上呈随机分布,2)存在/不存在数据能够用来分析种群的多尺度空间分布格局,3)空间尺度的变化会引起树种分布格局的变化,随机分布随尺度增加,边缘化程度加强,4)单一尺度上,景观格局指数不能完全描述种群分布格局;而在多尺度上,变化趋势稳定的景观指数表明聚集分布存在,而波动剧烈的景观指数常与随机分布和均匀分布联系在一起,5)地形因子中,红松对坡度和海拔两个因子变化敏感。     

The contribution of common and rare species to species abundance patterns in alpine meadows: The effect of elevation gradients

Ecological niche modeling uses environmental variables associated with species distribution points to simulate species distribution and its importance in biodiversity conservation. This study aimed to quantify plant community composition and species abundance distribution (SAD) in alpine meadows at different elevations and to assess the contribution of rare and common species to SAD. We established a permanent study plot of 210 hm² in Gannan Tibetan Autonomous Prefecture, China, surveyed 315 sample squares (0.5 m × 0.5 m), and calculated the Hill numbers. The results showed that (1) a total of 72 species were surveyed at different altitudes, with Kobresia humilis and Kobresia macrantha as the main dominant species; (2) the SADs of overall and common species fit the ecological niche model (GSM (Geometric Sequence Model)), indicating that ecological niche differentiation is the main factor influencing SAD. The fitted model for rare species SAD varied with elevation, suggesting that various ecological processes influence rare species SAD. (3) Hill numbers showed a “single peak” pattern with increasing elevation. The number of rare species was higher than that of common species. Still, the distribution frequency of common species was significantly higher than rare species. The correlation between common-rare species sequences and cumulative species distribution was high. This indicates that common species dominate the species diversity pattern of the community, are the main contributors to the SAD pattern, and should be protected first. Rare species are also important carriers of community function and include much spatial information. Rare and common species work together in different ways to influence and maintain the species diversity patterns of alpine meadow plant communities.     

Electron microscopic immunocytochemical demonstration of separate vasotocinergic and mesotocinergic nerve fibres in the median eminence of the frog hypophysis

Summary With the use of the unlabelled antibody peroxidase-antiperoxidase complex (PAP) technique at the electron microscopic level, it was shown that both the internal and the external regions of the median eminence of the frog contain separate vasotocinergic and mesotocinergic nerve fibres. This observation confirms the results of previous immunocytochemical studies at the light microscopic level. The mean size of the neurohypophysial hormone-containing granules in the external region of the median eminence is significantly smaller than that of the neurohypophysial hormone-containing granules in the internal region of the median eminence. No significant difference could be found between the mean granule size of the vasotocinergic and mesotocinergic fibres of either the internal or the external region of the frog median eminence.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek