Dentatorubrothalamic tract localization with postmortem MR diffusion tractography compared to histological 3D reconstruction.

Diffusion-weighted imaging (DWI) tractography is a technique with great potential to characterize the in vivo anatomical position and integrity of white matter tracts. Tractography, however, remains an estimation of white matter tracts, and false-positive and false-negative rates are not available. The goal of the present study was to compare postmortem tractography of the dentatorubrothalamic tract (DRTT) by its 3D histological reconstruction, to estimate the reliability of the tractography algorithm in this specific tract. Recent studies have shown that the cerebellum is involved in cognitive, language and emotional functions besides its role in motor control. However, the exact working mechanism of the cerebellum is still to be elucidated. As the DRTT is the main output tract it is of special interest for the neuroscience and clinical community. A postmortem human brain specimen was scanned on a 7T MRI scanner using a diffusion-weighted steady-state free precession sequence. Tractography was performed with PROBTRACKX. The specimen was subsequently serially sectioned and stained for myelin using a modified Heidenhain-Woelke staining. Image registration permitted the 3D reconstruction of the histological sections and comparison with MRI. The spatial concordance between the two modalities was evaluated using ROC analysis and a similarity index (SI). ROC curves showed a high sensitivity and specificity in general. Highest measures were observed in the superior cerebellar peduncle with an SI of 0.72. Less overlap was found in the decussation of the DRTT at the level of the mesencephalon. The study demonstrates high spatial accuracy of postmortem probabilistic tractography of the DRTT when compared to a 3D histological reconstruction. This gives hopeful prospect for studying structure-function correlations in patients with cerebellar disorders using tractography of the DRTT.

Sex Differences in Presynaptic Density and Neurogenesis in Middle-Aged ApoE4 and ApoE Knockout Mice.

Atherosclerosis and apolipoprotein E ε4 (APOE4) genotype are risk factors for Alzheimer's disease (AD) and cardiovascular disease (CVD). Sex differences exist in prevalence and manifestation of both diseases. We investigated sex differences respective to aging, focusing on cognitive parameters in apoE4 and apoE knockout (ko) mouse models of AD and CVD. Presynaptic density and neurogenesis were investigated immunohistochemically in male and female apoE4, apoE ko, and wild-type mice. Middle-aged female apoE4 mice showed decreased presynaptic density in the inner molecular layer of the dentate gyrus of the hippocampus. Middle-aged female apoE ko mice showed a trend towards increased neurogenesis in the hippocampus compared with wild-type mice. No differences in these parameters could be observed in middle-aged male mice. Specific harmful interactions between apoE4 and estrogen could be responsible for decreased presynaptic density in female apoE4 mice. The trend of increased neurogenesis found in female apoE ko mice supports previous studies suggesting that temporarily increased amount of synaptic contacts and/or neurogenesis is a compensatory mechanism for synaptic failure. To our knowledge, no other studies investigating presynaptic density in aging female apoE4 or apoE ko mice are available. Sex-specific differences between APOE genotypes could account for some sex differences in AD and CVD.

DHA and cholesterol containing diets influence Alzheimer-like pathology, cognition and cerebral vasculature in APPswe/PS1dE9 mice.

Cholesterol and docosahexenoic acid (DHA) may affect degenerative processes in Alzheimer's Disease (AD) by influencing Abeta metabolism indirectly via the vasculature. We investigated whether DHA-enriched diets or cholesterol-containing Typical Western Diets (TWD) alter behavior and cognition, cerebral hemodynamics (relative cerebral blood volume (rCBV)) and Abeta deposition in 8- and 15-month-old APP(swe)/PS1(dE9) mice. In addition we investigated whether changes in rCBV precede changes in Abeta deposition or vice versa. Mice were fed regular rodent chow, a TWD-, or a DHA-containing diet. Behavior, learning and memory were investigated, and rCBV was measured using contrast-enhanced MRI. The Abeta load was visualized immunohistochemically. We demonstrate that DHA altered rCBV in 8-month-old APP/PS1 and wild type mice[AU1]. In 15-month-old APP/PS1 mice DHA supplementation improved spatial memory, decreased Abeta deposition and slightly increased rCBV, indicating that a DHA-enriched diet can diminish AD-like pathology. In contrast, TWD diets decreased rCBV in 15-month-old mice. The present data indicate that long-term dietary interventions change AD-like pathology in APP/PS1 mice. Additionally, effects of the tested diets on vascular parameters were observed before effects on Abeta load were noted. These data underline the importance of vascular factors in the APP/PS1 mouse model of AD pathology.

A study in male and female 5-HT transporter knockout rats: an animal model for anxiety and depression disorders.

Human studies have shown that a reduction of 5-HT transporter (SERT) increases the vulnerability for anxiety and depression. Moreover, women are more vulnerable to develop depression and anxiety disorders than men. For that reason we hypothesized that homozygous 5-HT transporter knockout rat (SERT(-/-)) models, especially female, are valuable and reliable animal models for humans with an increased vulnerability for anxiety- and depression-related disorders. As rats are extensively used in neuroscience research, we used the unique 5-HT transporter knockout rat, that was recently generated using N-ethyl-N-nitrosurea (ENU) -driven mutagenesis, to test this hypothesis. Behavioral testing revealed that male and female SERT(-/-) rats spent less time in the center of the open field and spent less time on the open arm of the elevated plus maze compared with wild-type 5-HT transporter knockout rats (SERT(+/+)). In the novelty suppressed feeding test, only male SERT(-/-) rats showed a higher latency before starting to eat in a bright novel arena compared with SERT(+/+) controls. Both male and female SERT(-/-) rats showed a higher escape latency from their home cage than SERT(+/+) littermates. Moreover, SERT(-/-) rats were less mobile in the forced swim test, and sucrose consumption was reduced in SERT(-/-) rats relative to SERT(+/+) rats. Both effects were sex-independent. Neurochemically, basal extracellular 5-HT levels were elevated to a similar extent in male and female SERT(-/-) rats, which was not influenced by the selective 5-HT reuptake inhibitor citalopram. 5-HT immunostaining revealed no difference between SERT(+/+) and SERT(-/-) rats in the dorsal raphe nuclei, in both males and females. These findings demonstrate that SERT(-/-) rats show anxiety and depression-related behavior, independent of sex. Genetic inactivation of the SERT has apparently such a great impact on behavior, that hardly any differences are found between male and female rats. This knockout rat model may provide a valuable model to study anxiety- and depression-related disorders in male and female rats.

Changes in cerebral blood volume and amyloid pathology in aged Alzheimer APP/PS1 mice on a docosahexaenoic acid (DHA) diet or cholesterol enriched Typical Western Diet (TWD).

High dietary cholesterol and low dietary docosahexaenoic acid (DHA) intake are risk factors for Alzheimer's disease (AD). However, it is unclear how these components influence the course of the disease. We investigated the effects of dietary lipids on beta-amyloid deposition and blood circulation in the brains of 18-month-old APP/PS1 mice. Starting at 6 months of age, mice were fed a regular rodent chow, a Typical Western Diet (TWD) containing 1% cholesterol, or a diet with a high (0.5%) level of DHA for 12 months. Relative cerebral blood volume (rCBV) and flow (CBF) were determined with (2)H MR spectroscopy and gradient echo contrast enhanced MRI. Deposition of beta-amyloid was visualized in fixed brain tissue with immunohistochemistry. The TWD diet increased plaque burden in the dentate gyrus of the hippocampus, but did not significantly reduce rCBV. In contrast, the DHA-enriched diet increased rCBV without changing blood flow indicating a larger circulation in the brain probably due to vasodilatation and decreased the amount of vascular beta-amyloid deposition. Together, our results indicate that the long-term intake of dietary lipids can impact both brain circulation and beta-amyloid deposition, and support the involvement of hemodynamic changes in the development of AD.

Effects of chronic paroxetine pretreatment on (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin induced c-fos expression following sexual behavior.

Chronic treatment with the selective serotonin reuptake inhibitor paroxetine impairs the functioning of 5-HT(1A) receptors involved in ejaculation. This could underlie the development of delayed ejaculation often reported by men treated with paroxetine. The neurobiological substrate linking the effects of selective serotonin reuptake inhibitor-treatment and 5-HT(1A) receptor activation with ejaculation was investigated. Male Wistar rats that were pretreated with paroxetine (20 mg/kg/day p.o.) or vehicle for 22 days and had received an additional injection with the 5-HT(1A) receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.4 mg/kg s.c.) or saline on day 22, 30 min prior to a sexual behavior test, were perfused 1 h after the sexual behavior test. Brains were processed for Fos-, and oxytocin immunohistochemistry. The drug treatments markedly changed both sexual behavior and the pattern and number of Fos-immunoreactive cells in the brain. Chronic pretreatment with paroxetine caused delayed ejaculation. Acute injection with 8-OH-DPAT facilitated ejaculation in vehicle-pretreated rats, notably evident in a strongly reduced intromission frequency, whereas 8-OH-DPAT had no effects in paroxetine-pretreated rats. Chronic treatment with paroxetine reduced Fos-immunoreactivity in the locus coeruleus, and prevented the increase in Fos-immunoreactive neurons induced by 8-OH-DPAT in the oxytocinergic magnocellular part of the paraventricular nucleus as well as in the locus coeruleus. Since oxytocin and noradrenalin facilitate ejaculation, the alterations in Fos-IR in these areas could connect selective serotonin reuptake inhibitor treatment and 5-HT(1A) receptor activation to ejaculation. Chronic paroxetine treatment and 8-OH-DPAT changed c-fos expression in a number of other brain areas, indicating that Fos-immunohistochemistry is a useful tool to find locations where selective serotonin reuptake inhibitors and 8-OH-DPAT exert their effects.

Induction of c-fos expression in cervical spinal interneurons after kainate stimulation of the motor cortex in the rat.

The expression of the immediate-early gene c-fos was used as a marker of neuronal activity to investigate the cervical spinal interneuron populations involved in the corticomotoneuronal pathway. Adult rats received unilateral kainate injections in the forelimb area of the primary motor cortex. After a survival period of 90 min, during which the animals showed vehement twitching of the contralateral forelimb, the rats were perfused and their brains and cervical spinal cords processed for Fos-like immunoreactivity. In the cervical spinal cord Fos-like immunoreactive neurons were found bilaterally in the dorsal horn and in the intermediate zone, though contralaterally significantly more labelled nuclei were encountered in two different areas. One area closely resembles the corticospinal terminal field as demonstrated with anterograde horseradish-peroxidase tract-tracing and the other reflecting primary afferent and noxious sensory neurons in the dorsal horn. Thus by monitoring the evoked expression of the immediate-early gene c-fos, structural components of the rat motor system can be identified.

Direct cortico-motoneuronal synaptic contacts are present in the adult rat cervical spinal cord and are first established at postnatal day 7.

In order to demonstrate direct cortico-motoneuronal synaptic contacts in the cervical spinal cord of the rat and to determine at which postnatal age these contacts are established, an electron microscopic study using double labelling was performed. Corticospinal axons were anterogradely labelled after horseradish peroxidase (HRP)-gel implantation into the cerebral motor cortex and motoneurons were retrogradely labelled after cholera toxin subunit B conjugated to HRP (CTB-HRP) injections into the distal forelimb flexor muscle. With the histochemical procedures used, both tracers yield similar needle-like crystalline deposits. Labelled axons, however, can be well differentiated from labelled motoneuronal dendrites and somata on morphological grounds. In adult rats, direct cortico-motoneuronal contacts were encountered. Experiments in developing postnatal rats demonstrated that these synapses are first present on postnatal day 7.

Transient functional connections between the developing corticospinal tract and cervical spinal interneurons as demonstrated by c-fos immunohistochemistry.

Previous research on the rat corticospinal tract (CST) which develops mainly postnatally revealed that some CST axons grow transiently into the spinal gray matter and are subsequently eliminated. In the present study the question was addressed whether these fibres also form transient functional connections. Rats aged 14 and 60 days postnatally received unilateral injections of the potent glutamate agonist kainate into the cerebral motor cortex. After a survival period of 90 min. the rats were perfused and their brains and spinal cords processed for the immediate early gene c-fos by immunohistochemistry. Increased levels of c-fos as opposed to sham-operated animals was observed in several brain nuclei as well as in the cervical spinal cord. In the spinal gray one population of labelled interneurons in particular appeared to correlate well with the CST projection field. A decrease was noted in the number of c-fos positive neurons from postnatal day 14 to 60, suggesting that during development transient functional connections are formed between the CST and its target.

Retrograde neuronal tracing with cholera toxin B subunit: comparison of three different visualization methods.

In this report a comparison is made of three different visualization methods of rat cervical motoneurons retrogradely labelled with cholera toxin B subunit (CTb). CTb conjugates such as CTb-HRP and CTb-FITC or CTb-TRITC, which can be visualized after histochemical detection and by fluorescence microscopy, respectively. The following results were obtained. (1) Immunochemical detection of CTb with peroxidase and DAB-Ni incubation provides the best labelling of the cell bodies and their processes, whereas immunochemical detection with FITC produces less effective labelling of the dendrites. (2) Histochemical visualization of CTb-HRP conjugate gives results similar to those of CTb immunochemistry but produces a much more granular appearance of the label, which may affect the identification of distal dendrites. In addition, direct electron-microscopic analysis of labelled structures can be achieved. (3) CTb-FITC and CTb-TRITC visualization permit double-labelling experiments but the labelled cells exhibit fluorescence only in their somata and proximal dendrites. (4) Factors other than labelling intensity, e.g. double-labelling, preservation of the label, compatibility with other techniques and even economic reasons must be taken into consideration when a selection of visualization methods is to be made.

Selective elimination of transient corticospinal projections in the rat cervical spinal cord gray matter.

In the present paper a description is given of the development of the rat corticospinal tract (CST) in the lower cervical spinal cord. This area contains, among other cells, the motoneurons innervating the distal forelimb muscles. HRP gels were implanted in the sensorimotor cortex of Wistar rats varying in age from postnatal day 0 (P0) to P60. After a survival period of 48 h, the rats were transcardially perfused, the spinal cords transversely sectioned at 30 microns and the sections reacted for HRP. Labelled CST axons in the dorsal funiculus were first detected at P2, and after a delay of 2 days the first fibres were found in the adjacent gray matter (P4). More labelled fibres were gradually added until maximal number and extension was reached at P10. By then the entire gray matter and large parts of the white matter were covered by labelled CST axons. From P10 onwards, the number of labelled CST fibres as well as their extension decreased. In the adult rat, some areas such as the lateral part of the ventral and dorsal horn and large parts of the ventral and lateral white matter ultimately became devoid of labelled CST axons. It is concluded that a massive overshoot occurs during the development of the terminal field of the rat CST. The results are discussed in conjunction with our previous findings on the development of the motoneurons innervating the rat distal forelimb muscles. The concurrent selective elimination of both CST axons and motoneuron dendrites is suggested to be correlated with progressively more mature, coordinated movements and with high digital skills especially.

Postnatal maturation of the dendritic fields of motoneuron pools supplying flexor and extensor muscles of the distal forelimb in the rat.

In the rat cervical spinal cord the corticospinal projection on motoneurons either direct or indirect (via interneurons) comes about postnatally making it accessible for experimental research. Therefore, the postnatal developmental changes of motoneurons and in particular their dendritic fields were examined. Motoneurons innervating the two antagonistic muscles in the distal forepaw, the m. flexor digitorum profundus and the m. extensor digitorum communis, were retrogradely labelled by intramuscular injections of cholera toxin subunit B conjugated with horseradish peroxidase in rats of various postnatal ages. Following a 48-72 hour survival period the motoneurons and their dendritic fields were studied in the seventh and eighth cervical spinal cord segments. Both the number and the position of motoneurons were found to remain constant throughout postnatal development. Extensor motoneurons were positioned dorsolaterally in the ventral horn at the border of grey and white matter, flexor motoneurons were in general medial to extensor motoneurons. The results on the dendritic field demonstrate firstly, that during postnatal development the extension of the dendrites of both flexor and extensor motoneurons changes from spreading out in all directions at postnatal day 2 to spreading in only a few, specific directions from postnatal day 21 onwards, with the restriction that both motoneuron pools follow a different time scale to achieve this. Secondly, both pools have a temporal dendritic component extending into the white matter of the lateral funiculus.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of the ipsilateral corticospinal component in rat spinal cord: a light and electron microscopic anterograde HRP study.

Most developmental and regeneration studies using the corticospinal tract (CST) as a model are focussed on its major contralateral component in the ventralmost part of the dorsal funiculus. However, for a correct interpretation of the data in these and future studies, a complete anatomical description of the rat CST, including its ipsilateral uncrossed component, is a prerequisite. In this study we used anterograde horseradish-peroxidase (HRP) tract-tracing at the light and electron microscopic level to describe the development of the uncrossed ipsilateral CST component. In the far caudal portions of the medulla oblongata, most of the CST fibers decussate dorsally and medially to assume positions in the ventralmost part of the dorsal funiculus. However, a small number of labeled fibers continue uncrossed in a position just lateral to the ventral median fissure. Labeled ipsilateral CST fibers caudally extend into the ventral funiculus of the third thoracic segment at postnatal day 1 (P1), into the seventh thoracic segment at P3, and into the first lumbar segment at P5 and P7. After the first postnatal week labeled ipsilateral CST fibers gradually disappear from upper lumbar, lower-, and mid-thoracic spinal cord levels, respectively. From P14 through adulthood the ipsilateral CST projection can not be visualized below upper thoracic spinal cord levels. Quantitative analysis of the amount of label along the length of the outgrowing ipsilateral CST revealed a characteristic staggered pattern of outgrowth: a small number of fibers take the lead and additional fibers are successively added. The outgrowth of labeled ipsilateral CST fibers is mainly restricted to spinal cord white matter. Occasionally a few outgrowing fibers into the adjacent cervical spinal gray can be observed, however, only between P3 and P7. Electron microscopic observations at the cervical enlargement (fifth cervical segment) demonstrate that the first labeled ipsilateral CST fibers, at P1, are characterized by the presence of growth cone-like enlargements at their distal endings. The process of myelination of labeled ipsilateral CST axons starts at about day P14. Although myelination seems to be largely completed at P28, labeled unmyelinated axons are still present in the adult ipsilateral CST component.

Collateralization of the cervical corticospinal tract in the rat.

Anterograde staining with Phaseolus vulgaris leucoagglutinin (PHA-L) revealed the spinal arborization pattern of corticospinal tract (CST) fibers in the cervical enlargement of the rat. Within the confines of the pyramidal tract local nets of small fibers are present in addition to the rather large CST fibers with varicosities. CST termination is primarily located in lamina IV and extends into lamina V and VI. Extensive collateralization of CST axons was found interconnecting neurons located both in different horizontal laminae and in subsequent spinal cord segments. This complex pattern of CST collateralization is suggested to add a coordinative role in motor control to this tract both through serial axo-dendritic contacts in the spinal gray and through axo-axonal contacts in the white as well as the gray matter.

A triple-labeling method: HRP anterograde tract tracing combined with double immunofluorescent cell staining in developing neural tissue of the rat.

In this paper we describe a triple-labeling technique on a single section of developing nervous tissue. This labelling consists of a histochemical visualization of anterogradely transported horseradish peroxidase (HRP), within outgrowing rat corticospinal tract fibers and a double immunofluorescence labelling against glial fibrillary acidic protein (GFAP) and vimentin, being specific markers for astrocytes and their precursors, respectively. For the visualization of the anterogradely transported HRP, tetramethylbenzidine (TMB) is used as a chromogen with ammonium heptamolybdate (AHM) as a stabilizer followed by an additional diaminobenzidine-cobalt incubation to prevent washing out the HRP-TMB-AHM reaction product during the subsequent immunohistochemistry. Thereafter, double immunofluorescence labelling is carried out on the same section by adding simultaneously polyclonal anti-GFAP and monoclonal anti-vimentin which are then visualized with antisera conjugated with tetramethylrhodamine-isothiocynate (TRITC) and fluorescein-isothiocyanate (FITC) respectively as markers. Since the above mentioned visualization of the anterogradely transported HRP does not affect the immunofluorescence labelling, the methods enables within a single cryosection of developing neural tissue, the localization and identification of both outgrowing axons and their adjacent differentiating glial cells. In addition the method can easily be adapted for triple labelling of HRP tract tracing in conjunction with double immunofluorescence histochemistry of neurotransmitters.

Postnatal development of the corticospinal tract in the rat. An ultrastructural anterograde HRP study.

Horseradish-peroxidase was used to anterogradely label and thus to trace the growth of corticospinal axons in rats ranging in age from one day to six months. Three to eight HRP-gels were implanted in the left cerebral hemisphere of the cortex. In each spinal cord three levels were studied, the cervical intumescence (C5), the mid-thoracic region (T5) and the lumbar enlargement (L3). The methodology employed for the electron microscopic visualization of HRP has been described previously (Joosten et al. 1987a). The outgrowth of labelled unmyelinated corticospinal tract axons in the rat spinal cord primarily occurs during the first ten postnatal days. The outgrowth of the main wave of these fibres is preceded by a number of pathfinding axons, characterized by dilatations at their distal ends, the growth cones. By contrast, later appearing unmyelinated axons, which presumably grow along the pathfinding axons, do not exhibit such growth cones. The first labelled pioneer axons can be observed in the cervical intumescence at postnatal day one (P1), in the mid-thoracic region at day three (P3) and in the lumbar enlargement at day five (P5). Prior to the entrance of the axons, the prospective corticospinal area or the pre-arrival zone is composed of fascicles consisting of unlabelled, unmyelinated fibres surrounded by lucent amorphous structures. During the outgrowth phase of the corticospinal fibres some myelinated axons could be observed within the outgrowth area even before day 14. These axons, however, were never labelled. These findings strongly suggest that the outgrowth area, which is generally denoted as the pyramidal tract, contains other axons besides the corticospinal fibres (and glial cells).(ABSTRACT TRUNCATED AT 250 WORDS)

An anterograde tracer study of the developing corticospinal tract in the rat: three components.

Light microscopic analysis of anterogradely transported wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) has been used to study the developing corticospinal tract (CST) in the rat. This study was carried out to examine the relationship between the site of injection within the cortex and the pattern of labelling of the developing CST in the spinal cord from postnatal day 1 (P1) through postnatal day 10 (P10). For this purpose the cortex was subdivided into 3 equal areas along the rostrocaudal axis: anterior, intermediate and posterior. After the operation the animals were allowed to survive for 24 h. The caudal extension of labelled CST axons originating in the anterior cortical area was restricted (L1 at P7 or P10) as compared with that of the CST fibres originating in the intermediate cortical area (S3 at P10). The axons of the posterior corticospinal (CS) neurones reach their most caudal extension in the spinal cord (T5) at P7 but then gradually disappear up till P14. Quantitative analysis of the amount of label along the length of the outgrowing CST fibres revealed the formation of a large stable peak at the level of the cervical enlargement after labelling of either the anterior or the intermediate cortical area. The formation of a second 'running' peak which moves caudally from mid-thoracic levels at P5 to mid-lumbar levels at P10 was only accomplished by labelling the intermediate cortical area and is probably caused by the accumulation of label in the growth cones at the distal ends of the outgrowing CST fibres. After labelling the posterior cortical area, no peaks could be detected, neither at the cervical nor at the lumbar intumescence. The major spinal grey termination field of the anterior CS neurones appeared to be the cervical intumescence, whereas the major spinal grey termination field of the intermediate CS neurones is the lumbar enlargement. By contrast, axons of posterior CS neurones never showed any outgrowth into the spinal grey matter at any level. Concluding, the developing CST in the rat consists of 3 components: the first having its originating neurones in the anterior part of the cortex and its termination field in the cervical intumescence; the second with its originating neurones in the intermediate part of the cortex and its termination field predominantly in the lumbar enlargement, and a third transient one, originating in the posterior cortex and gradually disappearing from spinal cord levels. Research using anterograde tracing techniques in combination with electron microscopy is necessary to further analyse these 3 different components.

Ultrastructural visualization of anterogradely transported horseradish peroxidase in developing corticospinal tract of rat.

Until now a satisfactory method for electron microscopic (EM) detection of anterogradely transported horseradish peroxidase (HRP) in developing neural tissue, using sensitive chromogen tetramethylbenzidine (TMB), has not been described. Use of the stabilizing agent ammoniumheptamolybdate (AHM), in combination with a modified prolonged osmication [4 hr at pH 5.0 in 0.1 M phosphate buffer (PB)] made possible visualization of HRP-TMB-(AHM) reaction product at the ultrastructural level in outgrowing corticospinal tract (CST) fibers of young postnatal rat. This reaction product appeared to be very distinctive and clearly detectable, making ultrastructural identification of HRP-labeled outgrowing CST fibers in rat spinal cord rather easy. In addition, the procedure described in this report preserves the ultrastructural details of the developing neural tissue.

On the development of the pyramidal tract in the rat. II. An anterograde tracer study of the outgrowth of the corticospinal fibers.

An anterograde tracer study has been made of the developing corticospinal tract (CST) in the rat using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Analysis of normal Rager stained material revealed that corticospinal axons reach upper cervical spinal cord levels at the day of birth (PO). Postnatal rats ranging in age from one (P1) to fourteen (P14) days received multiple WGA-HRP injections into the cortex of their left hemisphere and were allowed to survive for 24 h. The first labeled CST fibers caudally extend into the third thoracic spinal cord segment at P1; into the eighth thoracic segment at P3; into the first or second lumbar segment at P7 and into the second to third sacral segment at Pg. Thus the outgrowth of the leading 'pioneer' fibers of the CST is completed at P9 but later developing axons are continuously added even beyond P9. Quantitative analysis of the amount of label along the length of the outgrowing CST revealed a characteristic pattern of labeling varying with age. The most striking features of that pattern are: the formation of two standing peaks at the level of the cervical and lumbar enlargements respectively and the transient presence of a smaller running peak which moves caudally with the front of the outgrowing bundle. The standing peaks are ascribed to the branching of the axon terminals at both intumescences, whereas the running peak probably arises by the accumulation of tracer within the growth cones at the tips of the outgrowing CST axons. Factors such as the number of axons, the varying axon diameters, the branching collaterals, the presence of varicosities, the transport rate of the tracer, the uptake of the tracer at the injection site, which possibly may affect the amount of label present in both the entire bundle and in the individual axons are discussed. Current research is focused upon an analysis of the relation between the site of injection within the cortex and the pattern of labeling of the CST. A delay of two days was found between the arrival of the CST axons at a particular spinal cord level and their outgrowth into the adjacent spinal gray. However, combined HRP and electronmicroscopic experiments are necessary to determine the factors behind the maturation of the CST as well as the maturation of the spinal gray.

Cerebellar efferents in the lizard Varanus exanthematicus. II. Projections of the cerebellar nuclei.

The projections of the cerebellar nuclei have been studied in the lizard Varanus exanthematicus with various experimental anatomical techniques. In anterograde degeneration experiments (lesions of the cerebellar peduncle) both ascending and decending contralateral projections were found. Ascending fibers which could be traced from the cerebellar commissure ventralward decussated at the level of the trochlear and oculomotor nuclei. These fibers coursed rostralward to the mesodiencephalic junction. With anterograde tracing techniques (3H-leucine and HRP) this tract was found to terminate in the nucleus ruber and the interstitial nucleus of the fasciculus longitudinalis medialis. Moreover, retrograde tracer studies (HRP, "Fast Blue") showed that this tract appeared to arise mainly in the lateral cerebellar nucleus. With both anterograde degeneration and tracing techniques (3H-leucine and HRP) a bundle of fibers could be followed, which decussates in the basal part of the cerebellum and passes dorsally around the contralateral medial cerebellar nucleus to the lateral side of the brainstem. This contralaterally descending projection system was found, lateral to the vestibular nuclear complex, and as far caudally as the descending vestibular nucleus, to terminate on various vestibular nuclei. Horseradish peroxidase studies showed that this contralaterally descending projection system originates mainly in the medial cerebellar nucleus, but ipsilaterally descending projections were also found. With the fluorescent double labeling technique ("Fast Blue" and "Nuclear Yellow") the projections of the cerebellar nuclei described above were confirmed. Furthermore, double labeling revealed neurons in both cerebellar nuclei (especially the medial nucleus) that project to both the mesencephalon and the cervical spinal cord. The present results indicate that the efferent connections of the cerebellar nuclei in the lizard Varanus exanthematicus are organized as two main projections, an ascending projection comparable to the mammalian brachium conjunctivum arising in the lateral cerebellar nucleus, and a descending projection comparable to the mammalian hook bundle (fasciculus uncinatus), originating mainly in the medial cerebellar nucleus. Such projections are common for terrestrial vertebrates.