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PubMed2004

Regulation of isthmic Fgf8 signal by sprouty2.

Fgf8 functions as an organizer at the mes/metencephalic boundary (isthmus). We showed that a strong Fgf8 signal activates the Ras-ERK signaling pathway to organize cerebellar differentiation. Sprouty2 is expressed in an overlapping manner to Fgf8, and is induced by Fgf8. Its function, however, is indicated to antagonize Ras-ERK signaling. Here, we show the regulation of Fgf8 signaling in relation to Sprouty2. sprouty2 expression was induced very rapidly by Fgf8b, but interfered with ERK activation. sprouty2 misexpression resulted in a fate change of the presumptive metencephalon to the mesencephalon. Misexpression of a dominant negative form of Sprouty2 augmented ERK activation, and resulted in anterior shift of the posterior border of the tectum. The results indicate that Fgf8 activates the Ras-ERK signaling pathway to differentiate the cerebellum, and that the hyper- or hypo-signaling of this pathway affects the fate of the brain vesicles. Sprouty2 may regulate the Fgf8-Ras-ERK signaling pathway for the proper regionalization of the metencephalon and mesencephalon.

Adaptor Proteins, Signal Transducing↗

PubMed2001

Inductive signal and tissue responsiveness defining the tectum and the cerebellum.

The mes/metencephalic boundary (isthmus) has an organizing activity for mesencephalon and metencephalon. The candidate signaling molecule is Fgf8 whose mRNA is localized in the region where the cerebellum differentiates. Responding to this signal, the cerebellum differentiates in the metencephalon and the tectum differentiates in the mesencephalon. Based on the assumption that strong Fgf8 signal induces the cerebellum and that the Fgf8b signal is stronger than that of Fgf8a, we carried out experiments to misexpress Fgf8b and Fgf8a in chick embryos. Fgf8a did not affect the expression pattern of Otx2, Gbx2 or Irx2. En2 expression was upregulated in the mesencephalon and in the diencephalon by Fgf8a. Consequently, Fgf8a misexpression resulted in the transformation of the presumptive diencephalon to the fate of the mesencephalon. In contrast, Fgf8b repressed Otx2 expression, but upregulated Gbx2 and Irx2 expression in the mesencephalon. As a result, Fgf8b completely changed the fate of the mesencephalic alar plate to cerebellum. Quantitative analysis showed that Fgf8b signal is 100 times stronger than Fgf8a signal. Co-transfection of Fgf8b with Otx2 indicates that Otx2 is a key molecule in mesencephalic generation. We have shown by RT-PCR that both Fgf8a and Fgf8b are expressed, Fgf8b expression prevailing in the isthmic region. The results all support our working hypothesis that the strong Fgf8 signal induces the neural tissue around the isthmus to differentiate into the cerebellum.

Animals↗

PubMed1981

Brain stem origins of spinal projections in the lizard Tupinambis nigropunctatus.

In order to study brainstem origins of spinal projections, ten Tegu lizards (Tupinambis nigropunctatus) received complete or partial hemisections of the spinal cord at the first or second cervical segment. Their brains were processed for conventional Nissl staining. The sections were surveyed for the presence or absence of retrograde chromatolysis. Based on analysis and comparison of results from lesions in the various spinal cord funiculi, the following conclusions were reached: The interstitial nucleus projects ipsilaterally to the spinal cord via the medial longitudinal fasciculus, as does the middle reticular field of the metencephalon. The red nucleus and dorsal vagal motor nucleus both project contralaterally to the spinal cord via the dorsal part of the lateral funiculus. The superior reticular field in the rostral metencephalon and the ventrolateral vestibular nucleus project ipsilaterally to the spinal cord via the ventral funiculus. The dorsolateral metencephalic nucleus and the ventral part of the inferior reticular nucleus of the myelencephalon both project ipsilaterally to the spinal cord via the dorsal part of the lateral funiculus. Several brainstem nuclei in Tupinambis project bilaterally to the spinal cord. The ventrolateral metencephalic nucleus, for example, projects ipsilaterally to the cord via the medial longitudinal fasciculus and contralaterally via the dorsal part of the lateral funiculus. The dorsal part of the inferior reticular nucleus projects bilaterally to the spinal cord via the dorsal part of the lateral funiculus. The nucleus solitarius complex projects contralaterally via the dorsal part of the lateral funiculus but ipsilaterally via the middle of the lateral funiculus. The inferior raphe nucleus projects bilaterally to the spinal cord via the middle part of the lateral funiculus. These data suggest that supraspinal projections in reptiles, especially reticulospinal systems, are more highly differentiated than previously thought. On the other hand, recent findings in cat, opossum, and monkey reveal that the organization of supraspinal pathways in the Tegu lizard bears a striking resemblance to that observed in mammals.

Animals↗

PubMed1993

Orderly migration of neurons to the principal sensory nucleus of the trigeminal nerve of the rat.

As nuclei in the central nervous system develop, neurons actively migrate from their site of generation to their permanent residence. This study examines the spatiotemporal sequence of the migration of neurons to the principal sensory nucleus of the trigeminal nerve (PSN) of the rat. Tritiated thymidine autoradiography and bromodeoxyuridine immunohistochemistry were used to examine the spatiotemporal patterns of migration of PSN neurons born on gestational day (G) 12 (early-generated neurons) and of those born on G14 (late-generated neurons). The final residence of early- and late-generated neurons was determined by injecting a thymidine analog into a pregnant rat on G12 or G14 and sacrificing the pups on postnatal day (P) 30. Early- and late-generated neurons were distributed medially and laterally, respectively. The schedule of the migration of PSN neurons was also determined. A few pioneer neurons born on G12 reached the PSN by G14; however, the last of the neurons born on G12 arrived in the PSN by G18. The migration of neurons born on G14 was completed 2-6 days later than that of the early-generated neurons. The path followed by migrating neurons was delineated by radial glial fibers. These processes were identified in the developing metencephalon by RAT-401 immunohistochemistry. Radial glial fibers extended from the lateral part of the ventricular zone through the tegmentum and the PSN to the surface of the metencephalon external to the sensory tract of the trigeminal nerve. RAT-401-immunoreactive processes were detected during the period of neuronal migration, but disappeared by P5. Thus, the migration of PSN neurons follows an inside-to-outside sequence, which apparently is organized by radial glial fibers. The inside-to-outside sequence of neuronal migration directly opposes the outside-to-inside gradient of synaptogenesis.

Animals↗

PubMed1982

The organization of the reptilian formation: a comparative study using Nissl and Golgi techniques.

The brainstem reticular formation has been studied in 16 genera representing 11 families of reptiles. Measurements of Nissl-stained reticular neurons revealed that they are distributed along a continuum, ranging in length from 10 micrometer to 95 micrometer. Reticular neurons in crocodilians and snakes tend to be larger than those found in lizards and turtles. Golgi studies revealed that reticular neurons possess long, rectilinear, sparsely branching dendrites. Small reticular neurons (less than 31 micrometers length) possess fusiform or triangular somata which bear two or three primary dendrites. These dendrites have a somewhat simpler ramification pattern when compared with those of large reticular neurons ( greater than 30 micrometers length). Large reticular neurons generally possess perikarya which are triangular or polygonal in shape. The somata of large reticular neurons bear an average of four primary dendrites. The dendrites of reptilian reticular neurons ramify predominantly in the transverse plane and are devoid of spines or excrescences. The dendritic ramification patterns observed in the various repitilian reticular nuclei were correlated with known input and output connections of these nuclei. Nissl and golgi techniques were used to divide the reticular formation into seven nuclei. A nucleus reticularis inferior (RI) is found in the myelencephalon, a reticularis medius (RM) in the caudal two-thirds of the metencephalon, and a reticularis superior (RS) in the rostral metencephalon and caudal mesencephalon. Reticularis inferior can be subdivided into a dorsal portion (RID) and a ventral portion (RIV). All reptilian groups possess RID and RM but RIV is lacking in turtles. Reticularis superior can be subdivided into a large-celled lateral portion (RSL) and a small-celled medial portion (RSM). All reptilian groups possess RSM and RSL, but RSL is quite variable in appearance, being best developed in snakes and crocodilians. The myelencephalic raphe nucleus is also quite variable in its morphology among the different reptilian families. A seventh reticular nucleus, reticularis ventrolateralis (RVL), is found only in snakes and in teiid lizards. It was noted that the reticular formation is simpler (fewer numbers of nuclei) in the representative of older reptilian lineages and more complex (greater numbers of nuclei) in the more modern lineages. Certain reticular nuclei are present or more extensive in those families which have prominent axial musculature.

Alligators and Crocodiles↗

PubMed1990

Do CNS anlagen have plasticity in differentiation? Analysis in quail-chick chimera.

Heterotopic transplantations of brain vesicles of a quail embryo into a chick embryo were carried out in order to elucidate if CNS anlagen have plasticity in differentiation at the 7-10 somite stage. Quail cells are distinguished from chick cells due to the difference in nuclear morphology. The prosencephalon did not differentiate into the cerebellum when transplanted into the metencephalon, although previous study showed that the prosencephalon has the capacity to differentiate into the optic tectum. The mesencephalon differentiated as an optic tectum when transplanted into the prosencephalon or into the rhombencephalon. The metencephalon differentiated as a cerebellum in the telencephalon. It is concluded that only the prosencephalon has limited plasticity, but the mesencephalon and rhombencephalon are determined by the 7-10 somite stage.

Animals↗

PubMed1995

Cerebellar volume asymmetries are related to handedness: a quantitative MRI study.

Four cerebellar subregions were delineated (left, right; anterior, posterior), and their volumes measured on contiguous 3.1 mm coronal MR images in 15 dextral and 8 nondextral healthy control subjects. (1) Left and right cerebellar hemisphere volume asymmetries interacted with anterior-posterior level (anterior: right > left; posterior: left > right), following a pattern commonly found in the neocortex; (2) A significant handedness effect was found (P < or =0.01) on a composite index of cerebellar asymmetry, such that dextrals showed more asymmetry than nondextrals. These data suggest that the same pattern of asymmetries observed at the neocortical level is also present in the metencephalon. These asymmetries, possibly resulting from multiple developmental growth gradients acting on the metencephalon early in gestation, are associated with handedness differences in adulthood.

Adult↗

PubMed1992

The midbrain-hindbrain phenotype of Wnt-1-/Wnt-1- mice results from stepwise deletion of engrailed-expressing cells by 9.5 days postcoitum.

Mice homozygous for null alleles of the putative signaling molecule Wnt-1 have a reproducible phenotype: loss of the midbrain and adjacent cerebellar component of the metencephalon. By examining embryonic expression of the mouse engrailed (En) genes, from 8.0 to 9.5 days postcoitum, we demonstrate that Wnt-1 primarily regulates midbrain development. The midbrain itself is required for normal development of the metencephalon. Thus, the observed neonatal phenotype is explained by a series of early events, within 48 hr of neural plate induction, that leads to a complete loss of En domains in the anterior central nervous system. Wnt-1 and a related gene, Wnt-3a, are coexpressed from early somite stages in dorsal aspects of the myelencephalon and spinal cord. We suggest that functional redundancy between these two genes accounts for the lack of a caudal central nervous system phenotype.

Animals↗

PubMed1987

Behavioral determination of refractory periods of the brainstem substrates of self-stimulation.

The objective of this study was to estimate the refractory periods of the brainstem neurons responsible for self-stimulation behavior in the rat. In a first experiment, we tested the robustness of the double pulse technique used to estimate the refractory periods of reward-relevant neurons. We obtained estimates of the relative T-pulse effectiveness at a wide range of stimulation frequencies. The results of this experiment suggest that the refractory period estimates obtained with the behavioral version of the double pulse technique are not dependent on the arbitrary choice of the stimulation frequency. However, the use of stimulation frequencies higher than 100 Hz should preferably be avoided. In a second experiment, we applied the double pulse technique using C-pulse intensity higher than T-pulse intensity to estimate the refractory periods of the brainstem reward-relevant neurons. Using moveable electrodes, we tested 9 metencephalic and 7 mesencephalic sites in 4 animals. In the metencephalon, the most excitable reward-relevant neurons have absolute refractory periods of less than 0.6 and 0.8 ms and have a supernormal period that occurs at least between 5 and 10 ms after the initial excitation. The mesencephalic reward-relevant neurons were found to have more heterogeneous physiological characteristics. The most excitable cells in the mesencephalon have absolute refractory periods of less than 0.4 ms and have a supernormal period occurring as soon as 2.4 ms after the initial excitation. At some mesencephalic sites, we observed first an abrupt initial recovery followed by a plateau, followed by a renewed and continuous recovery, a pattern that was never observed in the metencephalon. The hypothesis of the contribution of two distinct sub-populations of reward-relevant neurons is proposed and the implication of monoaminergic pathways in reward is discussed.

Animals↗

PubMed1996

Engrailed, Wnt and Pax genes regulate midbrain--hindbrain development.

The mouse Engrailed, Wnt and Pax genes, which are homologues of Drosophila segmentation genes, have provided a critical genetic entry point for dissecting the molecular and cellular control of mesencephalon and metencephalon development in vertebrates. Mutant phenotypes and gene expression data suggest that six members of these gene families are required for early formation of these brain regions. Ectopic transplantation studies have shown that the midbrain-hindbrain-junction protein can act as an organizer and recruit certain host cells to re-establish parts of the entire region. Taken together, these studies indicate that the mesencephalon and metencephalon develop as one independent unit, and that the genetic network regulating development of this region involves conserved genes that control segmentation in Drosophila. By analogy, segmentation of the rest of the brain might best be described in terms of 'genetic units' defined by genetic and transplantation data.

Animals↗

PubMed1987

Sex steroids promote neurite growth in mesencephalic tyrosine hydroxylase immunoreactive neurons in vitro.

The influence of steroid hormones on the differentiation of catecholaminergic and serotonergic (5-HT) neurons was studied in dissociated cell cultures from embryonic day 14 (E14) rat diencephalon, mesencephalon and metencephalon treated for 6 days with 17 beta-estradiol (E), testosterone (T), 5 alpha-dihydrostestosterone (DHT), progesterone (P), dexamethasone (DEX), or E + T. The effects of these hormones on morphologic differentiation were determined by morphometric measurements of total length of neurites of immunocytochemically identified neurons in culture, which were stained with antisera against tyrosine hydroxylase (TH) or 5-HT. A significant increase in neurite length was observed in cultures of TH-immunoreactive (TH-IR) neurons from the mesencephalon treated with E, T, E + T, but not with P, DHT or DEX. Based on labeling with [3H]dopamine (DA) uptake and competition with specific inhibitors, these mesencephalic TH-IR cells appear to represent DA neurons of the A8-A10 groups (which includes the substantia nigra). No statistically significant effects of these steroids were observed on TH-IR neurons from the diencephalon (assumed to be precursors of the tuberoinfundibular and incertohypothalamic dopaminergic groups). The 5-HT neurons of the raphe nuclei (metencephalon) showed no statistically significant response to steroids. We conclude that during the early fetal period, sex steroids can affect the morphologic differentiation of mesencephalic DA neurons in vitro, indicating that these hormones are capable of selectively influencing the development of a specific population of monoamine neurons during this critical period.

Animals↗

PubMed1991

Induction of a mesencephalic phenotype in the 2-day-old chick prosencephalon is preceded by the early expression of the homeobox gene en.

The homeobox gene en, homologous to the gene en-grailed of Drosophila, is expressed in the metencephalic-mesencephalic segment of the vertebrate neural tube. Using quail-chick chimeras, an antibody against en proteins, and cytoarchitectonic techniques, we demonstrate that metencephalon transplanted to prosencephalon, at E2, maintains a high level of en proteins and its presumptive cerebellar fate. The ectopic metencephalon induces in the contiguous host prosencephalon the expression of en and, subsequently, a mesencephalic phenotype. These related genetic and phenotypic expressions indicate that the transcriptional regulatory en gene is involved in cerebellar and mesencephalic cyto-differentiation. The expression of en can also be induced in chick prosencephalon by a mammalian metencephalic graft, indicating that the factors regulating the transcription of en are phylogenetically well conserved.

Animals↗

PubMed1999

The leukemic oncogene tal-2 is expressed in the developing mouse brain.

tal-1 (T-cell acute leukemia-1; also known as SCL) and tal-2 genes belong to a family of basic helix-loop-helix transcription factors and were originally isolated from the breakpoints of chromosomal translocations in human T-cell leukemia cell lines. tal-1 is expressed not only in hematopoietic cells but also in several endothelial structures and the central nervous system during development. On the other hand, the detailed function and the sites of expression of tal-2 have remained obscure. We cloned the tal-2 cDNA from a mouse embryonic cDNA library and examined its expression pattern in the mouse, comparing with that of tal-1. In situ analyses revealed that tal-2 transcripts are detected at embryonic day 12.5 in the following regions; 1) the diencephalon-the zona limitans intrathalamica and the pretectum, 2) the mesencephalon-the tectum, and the anterior and posterior tegmentum, 3) the metencephalon-the isthmus and the anterior pons. In the diencephalon and the mesencephalon, the expression sites of tal-2 gene were similar to those of tal-1, and its expression was stronger than that of tal-1. In the metencephalon, tal-2 expression was observed in the anterior pons, whereas tal-1 transcripts were detected in the entire pons, and showed stronger expression than tal-2. The tal-2 messages were barely detectable in the brain at birth. These results suggest that tal-1 and tal-2 are involved in the development of specific areas of the central nervous system.

Amino Acid Sequence↗

PubMed1999

Pax2/5 and Pax6 subdivide the early neural tube into three domains.

The nested expression patterns of the paired-box containing transcription factors Pax2/5 and Pax6 demarcate the midbrain and forebrain primordium at the neural plate stage. We demonstrate that, in Pax2/5 deficient mice, the mesencephalon/metencephalon primordium is completely missing, resulting in a fusion of the forebrain to the hindbrain. Morphologically, in the alar plate the deletion is characterized by the substitution of the tectum (dorsal midbrain) and cerebellum (dorsal metencephalon) by the caudal diencephalon and in the basal plate by the replacement of the midbrain tegmentum by the ventral metencephalon (pons). Molecularly, the loss of the tectum is demonstrated by an expanded expression of Pax6, (the molecular determinant of posterior commissure), and a rostral shift of the territory of expression of Gbx2 and Otp (markers for the pons), towards the caudal diencephalon. Our results suggest that an intact territory of expression of Pax2/5 in the neural plate, nested between the rostral and caudal territories of expression of Pax6, is necessary for defining the midbrain vesicle.

Animals↗

PubMedDate not supplied

Patterns of age-related shrinkage in cerebellum and brainstem observed in vivo using three-dimensional MRI volumetry.

This study investigates the time course and regional differences in age-related volume loss in cerebellum and brainstem. Three-dimensional (3D) magnetic resonance imaging (MRI) volumetry was used to measure the volumes of 11 regions in the cerebellum and three regions in the brainstem in 48 healthy volunteers (age 19.8-73.1 years). Landmark-adjusted lattices were used to divide the cerebellum into three radial (lobules I-V = lingula/lobulus/culmen, lobules VI-VII = declive/folium/tuber, lobules VIII-X = pyramis/uvula/nodulus) and three transverse subdivisions (vermis, medial, lateral hemisphere). The radial sectors extended laterally throughout the vermis and the medial hemisphere. The brainstem was divided into midbrain, metencephalon (pons and tegmentum pontis) and medulla. Total cerebellar volume marginally declined with age using a linear regression model. An exponential model better described the age dependency of total cerebellar volume. The curve predicted that the volume remained stable until age 50 years and declined thereafter. Volume loss in the cerebellar vermis was striking. Shrinkage in the medial hemisphere was markedly less and only the inferior sector showed a trendwise negative association with age. The lateral hemisphere was not affected by age. No age effects were found for total brainstem volume, metencephalon and medulla. Only the mid-brain showed a trend for age-related shrinkage. The mediolateral gradient of decreasing age effects is similar to the histological pattern of alcoholic cerebellar atrophy (although our subjects were non-alcoholics according to DSM-IIIR criteria and laboratory data) suggesting that a common factor is involved in both processes. In search for a cause of the regional vulnerability, vascular, functional, structural and molecular/genetic factors may be considered.

Adult↗

PubMed1993

Tracing neuroepithelial cells of the mesencephalic and metencephalic alar plates during cerebellar ontogeny in quail-chick chimaeras.

The quail-chick chimaera system was used to investigate the origin of the various neuronal cell types of the cerebellum from the mesencephalic and metencephalic brain vesicles at the 11- to 14-somite stage in the avian embryo. We have already demonstrated that the cerebellum is derived from both the mesencephalic and metencephalic brain vesicles. The mesencephalic contribution to the cerebellum is restricted to a mediodorsal territory inserted as a V-shaped area into the primitive metencephalic vesicle through complex morphogenetic movements taking place from day 2 to day 4 of embryonic development. Here we report that the cerebellar presumptive territory extends along the anteroposterior axis, over the caudal half of the mesencephalon and the rostral half of the metencephalon. Along the dorsoventral axis, the cerebellar anlage is located in the alar plates at the exclusion of the roof and basal plates, i.e. lies in the lateral walls of the neuroepithelium in the area included between approximately 25 and 120 degrees with respect to the sagittal plane. We also report that the neuroepithelium corresponding to the cerebellar presumptive territory also yields other brain structures (e.g. part of the optic tectum in the mesencephalon). The external granular layer (EGL) arises only from the rostral metencephalon, undergoing extensive tangential movements which we have analysed in detail: the more ventral the position of cells in the metencephalic alar plates the more rostral and lateral is their position in the EGL. Finally, we discuss the fact that the cerebellar cortex, an integrative structure of the brain, arises from the alar plates of the neural tube. This is consistent with the general spatial organization of the neural anlage of the vertebrate embryo, in which this part of the neuroepithelium is devoted to the production of interneurons, whereas the basal plate and the neural folds yield motor structures and primary sensory neurons respectively.

Animals↗

PubMed2006

Immunohistochemical localization and biological activity of the steroidogenic enzyme cytochrome P450 17alpha-hydroxylase/C17, 20-lyase (P450C17) in the frog brain and pituitary.

It is now clearly established that the brain has the capability of synthesizing various biologically active steroids including 17-hydroxypregnenolone (17OH-Delta(5)P), 17-hydroxyprogesterone (17OH-P), dehydroepiandrosterone (DHEA) and androstenedione (Delta(4)). However, the presence, distribution and activity of cytochrome P450 17alpha-hydroxylase/C17, 20-lyase (P450(C17)), a key enzyme required for the conversion of pregnenolone (Delta(5)P) and progesterone (P) into these steroids, are poorly documented. Here, we show that P450(C17)-like immunoreactivity is widely distributed in the frog brain and pituitary. Prominent populations of P450(C17)-containing cells were observed in a number nuclei of the telencephalon, diencephalon, mesencephalon and metencephalon, as well as in the pars distalis and pars intermedia of the pituitary. In the brain, P450(C17)-like immunoreactivity was almost exclusively located in neurons. In several hypothalamic nuclei, P450(C17)-positive cell bodies also contained 3beta-hydroxysteroid dehydrogenase-like immunoreactivity. Incubation of telencephalon, diencephalon, mesencephalon, metencephalon or pituitary explants with [(3)H]Delta(5)P resulted in the formation of several tritiated steroids including 17OH-Delta(5)P, 17OH-P, DHEA and Delta(4). De novo synthesis of C(21) 17-hydroxysteroids and C(19) ketosteroids was reduced in a concentration-dependent manner by ketoconazole, a P450(C17) inhibitor. This is the first detailed immunohistochemical mapping of P450(C17) in the brain and pituitary of any vertebrate. Altogether, the present data provide evidence that CNS neurons and pituitary cells can synthesize androgens.

3-Hydroxysteroid Dehydrogenases↗

PubMed1988

Comparison of [14C]glucose and [14C]deoxyglucose as tracers of brain glucose use.

Because glucose metabolism and functional activity in brain regions are normally coupled, knowledge of regional brain glucose use can yield insights into regional functional activity. The deoxyglucose (DG) method is widely used for this purpose in experimental animals and humans but questions have arisen regarding its limits and accuracy. Therefore an experiment was designed to compare the DG method on a structure-by-structure basis with another tracer of glucose use, [6-14C]glucose, in normal rats. The cerebral metabolic rates obtained using the two tracers were similar in the telencephalon, but the results using DG were substantially lower in the midbrain and hindbrain (diencephalon, 18%; mesencephalon, 20%; metencephalon, 29%; and myelencephalon, 35%). The primary DG metabolite, DG 6-phosphate (DG-6-P) was found to disappear in a non-uniform manner from the major brain structures: telencephalon less than diencephalon less than mesencephalon = metencephalon less than myelencephalon. Thus a correlation was found between the rate of DG-6-P loss and the extent to which the DG method gave lower values of glucose use. Thus this may explain, at least in part, the discrepancies between the two methods.

Animals↗