Developmental expression of monocyte chemoattractant protein-1 in the human cerebellum and brainstem

The developmental expression of monocyte chemoattractant protein-1 (MCP-1) in the cerebellum, medulla oblongata and pons was investigated in 26 normal human brains, ranging from 20 weeks of gestation (GW) to adulthood by means of an immunohistochemical method. Immunoreactivity to MCP-1 was observed in neurons of the cerebellum and brainstem, and was mainly distributed in the cytoplasm and dendrites. MCP-1-positive Purkinje cells appeared at 27 GW, reached a peak at 36 GW, and then decreased after 1 month of age, almost completely disappearing by 1-2 years of age. MCP-1-reactive neurons in the dentate nucleus and inferior olivary nucleus showed temporal increases similar to that of Purkinje cells. In the pons, however, MCP-1 reactivity was low in neurons of the pontine nuclei persisting from the fetal to the adult period, and was very low and short in the reticular formation and cranial nerve nuclei during the fetal and/or neonatal period. MCP-1 Western blotting of the cerebellar cortex confirmed the specificity of the immunohistochemistry. Our results suggest that MCP-1 may be related to the maturation of Purkinje cells, the dentate nucleus, the inferior olivary nucleus, and their network, promoting the growth of dendrites and synapses. Furthermore, MCP-1 may also be useful for the study of abnormal neuron development and ischemic damage.

The origin and development of the vagal and spinal innervation of the external muscle of the mouse esophagus

Retrograde and anterograde tracing and immunohistochemical techniques were used to examine the origin of the extrinsic innervation, and the development of the vagal innervation to the mouse esophagus. Cholinergic nerve terminals were localised using an antiserum to the vesicular acetylcholine transporter and cholinergic cell bodies were localised using an antiserum to choline acetyltransferase. Cholinergic nerve terminals, which also contained calcitonin gene-related peptide, were present at the motor end plates in the external (striated) muscle of the esophagus. Following injection of Fast Blue into subdiaphragmatic or cervical levels of the esophagus, the only retrogradely-labelled cholinergic nerve cell bodies that also contained calcitonin gene-related peptide were found in the nucleus ambiguus. Neurons in the dorsal motor nucleus of the vagus, the nodose ganglia and dorsal root ganglia gave rise to a number of different types of nerve terminals within the myenteric plexus. Retrogradely-labelled neurons in the dorsal motor nucleus of vagus contained cholinergic markers only, nitric oxide synthase only or cholinergic markers plus nitric oxide synthase, retrogradely-labelled neurons in the dorsal root ganglia contained calcitonin gene-related peptide only, and a small number of retrogradely-labelled neurons in the nodose ganglia contained tyrosine hydroxylase. The development of the vagal innervation to the esophagus was examined following application of DiI to the vagus nerve of fixed mouse embryos. Anterogradely-labelled nerve fibres, which arose from both nodose ganglia and the medulla, were already present in the esophagus of embryonic day 12 (E12) mice. Some of the DiI-labelled vagal nerve fibres were present in among the smooth muscle cells of the external muscle layer prior to their transdifferentiation to striated muscle. We conclude that the neurons in the nucleus ambiguus that project to the esophagus differ from other extrinsic neurons in their chemistry as well as their targets within the esophagus. The development of the extrinsic innervation precedes the transdifferentiation of the external muscle to striated muscle, raising the possibility that, during development, smooth muscle of the esophagus is innervated transiently by vagal neurons.

Hyperpolarization-activated inward currents contribute to spontaneous electrical activity and CO2/H+ sensitivity of cultivated neurons of fetal rat medulla

Neurons growing out from cultivated fetal medullary slices that exhibited spontaneous electrical activity after blockade of synaptic transmission were investigated by the patch-clamp technique for their response to decreases in the extracellular pH. Increases in the [H+], induced by increases in pCO2, resulted in a decrease in spike frequency associated with a decrease in the rate of depolarization preceding each action potential. The type of ion channel, contributing to interspike depolarization, and which may therefore be the site of CO2/H+ action, was identified by application of agents that inhibited the hyperpolarization-activated cation, IH, channel (Cs+ and ZD7288). Application of Cs+ and ZD7288 slightly hyperpolarized the cell membrane, decreased the interspike slope and inhibited CO2/H+-induced modulations of spike frequency in one group of CO2-inhibited medullary neurons, suggesting that IH contributes to spontaneous neuronal activity and to CO2/H+-sensitivity. CO2/H+ effects on IH were further confirmed in voltage-clamp experiments. Increasing the bath CO2 from 2% to 9% reduced the IH amplitude, shifted the mean EH from -54 to -60 mV, lengthened the voltage-dependent delay of current activation and increased the time-constants of activation at all potentials studied. It is concluded that depolarizing inward currents through IH channels participate in the gradual ramp-like change in membrane potential which depolarizes the cell up to the threshold of Na+ spike generation. CO2/H+-induced inhibition of IH reduces the contribution of this ion current to the interspike depolarization and accounts for the CO2/H+-induced decrease in spike frequency in one type of CO2/H+-inhibited medullary cells.

EphA4 (Sek1) receptor tyrosine kinase is required for the development of the corticospinal tract

Members of the Eph family of tyrosine kinase receptors have been implicated in the regulation of developmental processes and, in particular, axon guidance in the developing nervous system. The function of the EphA4 (Sek1) receptor was explored through creation of a null mutant mouse. Mice with a null mutation in the EphA4 gene are viable and fertile but have a gross motor dysfunction, which is evidenced by a loss of coordination of limb movement and a resultant hopping, kangaroo-like gait. Consistent with the observed phenotype, anatomical studies and anterograde tracing experiments reveal major disruptions of the corticospinal tract within the medulla and spinal cord in the null mutant animals. These results demonstrate a critical role for EphA4 in establishing the corticospinal projection.

Cell-specific survival motor neuron gene expression during human development of the central nervous system: implications for the pathogenesis of spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive disorder characterized by the progressive loss or degeneration of the motor neurons. To investigate the expression of survival motor neuron (SMN), the spinal muscular atrophy-determining gene, and its relationship with the pathogenesis of the disease, we analyzed by means of in situ hybridization the location of SMN mRNA in fetal, newborn, infant, and adult human central nervous system tissues. The large motor neurons of the spinal cord are the main cells that express SMN together with the neurons of the medulla oblongata, the pyramidal cells of the cortex, and the Purkinje cells of the cerebellum. Some sensory neurons from the posterior horn and dorsal root ganglia express SMN to a lesser degree. Furthermore, strong SMN expression is detected in the ependymal cells of the central canal. The expression is present in the spinal cord at 8 weeks of fetal life throughout postnatal and adult life. The sharp expression of SMN in the motor neurons of the human spinal cord, the target cells in spinal muscular atrophy, suggests that this gene is implicated in neuronal development and in the pathogenesis of the disease. The location of the SMN gene expression in other neuronal structures not clearly or directly associated with clinical manifestations or pathological findings of spinal muscular atrophy may indicate a varying sensitivity to the absence or dysfunction of the SMN gene in motor neurons.

Anatomic relationships of the human arcuate nucleus of the medulla: a DiI-labeling study

The arcuate nucleus (ARC) at the ventral surface of the human medulla has been historically considered a precerebellar nucleus. More recently, it has been implicated in central chemoreception, cardiopulmonary coupling and blood pressure responses. A deficiency of the ARC has been reported in a subset of putative human developmental disorders of ventilatory function. To investigate anatomic relationships of the ARC with brainstem regions involved in cardiorespiratory control, we applied crystals of DiI, a lipophilic dye which labels cells and cell processes by lateral diffusion along cell membranes, to 23 paraformaldehyde-fixed human fetal brainstems at 19 to 22 weeks postconceptional age. After 7 to 15.5 months diffusion, serial frozen sections were examined by florescence microscopy. DiI diffusion from the ARC labeled fibers and cell bodies in the medullary raphé, and the external arcuate fibers. Diffusion from the medullary raphé [corrected] labeled the reticular formation, medullary raphé, and the ARC. Diffusion from the pyramid and the basis pontis (negative control) labeled the corticospinal tract, with no labeling of the medullary raphé or ARC. The results suggest the existence of cellular connections between the ARC and the caudal raphé, a region implicated in cardiorespiratory control.

Growth and death of rohon-Beard cells in Rana pipiens and Ceratophrys ornata

Rohon-Beard (R-B) neurons of the medulla oblongata and spinal cord of anurans originate during gastrulation, become distinguishable just after closure of the neural tube, and are present in maximum numbers at the end of the embryonic period, just before feeding begins. Cell deaths are first seen in the earliest larval stages; in Rana pipiens and Ceratophrys ornata, they may not be complete until the very end of larval development or a day or two later, in the juvenile froglet. This is in sharp contrast with Xenopus laevis, in which the last R-B cells die well before the onset of metamorphic climax. Cell losses tend to reach completion in the trunk in a craniocaudal progression, that is, first in the medulla oblongata, then sequentially at brachial, postbrachial, and lumbar levels. Nuclei and cells increase in size through embryonic and early larval stages, reaching maxima at stages X-XIV (of 25 larval stages), then shrinking before cell death occurs. While Ceratophrys produces only two-thirds as many R-B cells as does R. pipiens, its rate of cell death is slower, gauged by attained stage, and at every stage, X-XXIV, Ceratophrys displays a greater number of surviving cells. In hypophysectomized Rana pipiens larvae some 7-15% of the peak numbers of R-B cells are still present after 400 days, more than 4 times the length of the usual larval period. Most or all of these surviving cells are in the tail. The extreme persistence of R-B cells in hypophysectomized larvae is consistent with the view that the R-B cell population can be characterized as being divided into those cells whose death occurs relatively early and those in which cell destruction occurs late, presumably dependent upon different factors. The critical factor for onset of cell death in late larvae may well be the surge in thyroid hormone concentration, which characterizes metamorphic climax.

[A quantitative analysis of the development of the auditory centers of the medulla oblongata in the hen]

Development of three auditory nuclei in medulla oblongata was studied in the chicken (Gallus gallus). Quantitative description of the volume growth of the magnocellular nucleus neurons and linear growth of the auditory nuclei is given from the formation of differentiated neuroblasts (Day 9 of embryogenesis) until the 180th day of postembryonic development. Growth of the embryos was described according to the Bertalanffy's model. Growth of the neurons proceeded at the highest rate at the rostral level of magnocellular nucleus (mN = 1.204 microns/day) and the lowest rate of the caudal level (mN = 1.045 microns/day). The curve of linear growth of the auditory nuclei is similar with the growth curve of the neurons and follows the Bertalanffy's equations. The magnocellular nucleus is characterized by the fastest growth (mN = 75.6 microns/day) and the angular nucleus by the slowest growth (mN = 52.0 microns/day). The growth of the neurons and auditory nuclei decreases before hatching.

Expression of cytochromes P450aldo and P45011 beta in rat adrenal gland during late gestational and neonatal stages

The development of the rat adrenal gland during late gestational and neonatal stages was studied by following the expression of aldosterone synthase cytochrome P450 (P450aldo) and glucocorticoid-synthesizing cytochrome P450 (P45011 beta). Cells expressing P450aldo, a functional marker for the mineralocorticoid-synthesizing zona glomerulosa, were not detected until day 20 of fetal age, i.e., 2 days before birth, although the zona glomerulosa cells were histologically recognizable at the 18th day of gestation. The intensity of P450aldo staining thereafter became stronger with age in the outer portion of the cortex. Cells expressing P45011 beta, a marker for the glucocorticoid-producing zona fasciculata, were present in the fetal adrenals on the 18th day. P45011 beta-positive cells were distributed over the whole adrenal gland and intermingled with the cells containing tyrosine hydroxylase, a marker enzyme for medullary cells. The P45011 beta-positive and tyrosine hydroxylase-positive cells began to separate on the 20th day, and were completely resolved from each other around the third day after birth. Expression of P450aldo and P45011 beta, together with that of tyrosine hydroxylase, thus serves as a suitable marker for studying the development of the adrenal gland.

Requirement for Brn-3.0 in differentiation and survival of sensory and motor neurons

Specific families of transcription factors mediate events in the sequential maturation of distinct neuronal phenotypes. Members of one such family, the class IV POU domain transcription factor Brn-3.0, and two highly related factors Brn-3.1 and Brn-3.2, are differentially expressed in the developing and mature mammalian nervous system. The expression pattern of Brn-3.0 suggested that it has an important role in the development of sensory ganglia, as well as red nucleus, inferior olive, and nucleus ambiguus. Analysis of mice null for the Brn-3.0 locus shows that Brn-3.0 is required for the survival of subpopulations of proprioceptive, mechanoreceptive and nociceptive sensory neurons, where deletion of the gene affects neurotrophin and neurotrophin-receptor gene expression. Deletion of Brn-3.0 also alters either differentiation, migration or survival of specific central neuronal populations.

Guidance of circumferentially growing axons by netrin-dependent and -independent floor plate chemotropism in the vertebrate brain

Netrin-1, a diffusible signal secreted by floor plate cells at the ventral midline of the vertebrate CNS, can attract ventrally migrating axons and repel a subset of dorsally migrating axons in the spinal cord and rostral hindbrain in vitro. Whether netrin-1 can act as a global cue to guide all circumferentially migrating axons is, however, unknown. Here, we show that netrin-1 can attract alar plate axons that cross the floor plate along its entire rostrocaudal axis. Dorsally directed axons forming the posterior commissure are, however, repelled by the floor plate by a netrin-independent mechanism. These results suggest that netrin-1 functions as a global guidance cue for attraction to the midline. Moreover, floor plate-mediated chemorepulsion may also operate generally to direct dorsal migrations, but its molecular basis may involve both netrin-dependent and -independent mechanisms.

Properties of spontaneous inhibitory synaptic currents in cultured rat spinal cord and medullary neurons

1. To identify the type(s) and properties of inhibitory postsynaptic receptor(s) involved in synaptic transmission in cultured rat embryonic spinal cord and medullary neurons, we have used whole cell patch-clamp techniques to record miniature inhibitory postsynaptic currents (mIPSCs) in the presence of tetrodotoxin, DL-2-amino-5-phosphonovaleric acid, and 6-cyano-7-nitroquinoxaline-2,3-dione. 2. The mIPSCs recorded from both spinal cord and medullary neurons had skewed amplitude distributions. 3. The glycinergic antagonist strychnine and the GABAergic antagonist bicuculline each decreased both the frequency and mean peak amplitudes of mIPSCs. We conclude that both glycine and gamma-aminobutyric acid (GABA) are neurotransmitters at inhibitory synapses in our cultured cells. 4. Most (approximately 96-97%) mIPSCs decay with single-exponential time constants, and decay time distributions were consistently best fitted by the sum of four Gaussians with decay constants as follows: D1 = 5.8 +/- 0.1 (SE) ms (n = 63), D2 = 12.2 +/- 0.2 ms (n = 61), D3 = 23.2 +/- 0.4 ms (n = 54), and D4 = 44.7 +/- 1.0 ms (n = 57). We conclude that the four classes of decay times represent kinetically different inhibitory postsynaptic receptor populations. 5. Strychnine and bicuculline usually had one of two different effects on the mIPSC decay time constant distributions; either selective decreases in the frequency of mIPSCs with decay times in certain classes (i.e., the D1 class was reduced by bicuculline, the D2 class by strychnine, and the D3 and D4 classes by both antagonists) or a nonselective depression in the frequency of mIPSCs with decay times in all four classes. The particular effect observed in a given neuron was correlated with the presence or absence of ATP and guanosine 5'-triphosphate (GTP) in the patch pipette. Namely, in 71% of the antagonist applications where the pipette contained ATP and GTP, the result was a nonselective decrease in mIPSCs in all decay time constant classes. Conversely, in 54% of the antagonist applications in their absence, the result was a selective decrease in the frequency of mIPSCs in specific decay time constant classes. 6. In some experiments, mIPSCs reappeared in antagonist solution after an essentially complete block. Recovery from block in the continued presence of antagonist was never observed in the absence of ATP and GTP (8 neurons), and, at the same time, 5 of 9 neurons patched with ATP and GTP in the pipette did show recovery (56%).

Inhibitory synaptic transmission in isolated patches of membrane from cultured rat spinal cord and medullary neurons

1. To quantify the variability in the characteristics of inhibitory glycinergic and GABAergic currents at single synaptic connections between cultured rat embryonic spinal cord or medullary neurons, we have used patch-clamp techniques to record miniature inhibitory postsynaptic currents (mIPSCs) in cell-attached patches. Experiments were performed with the patch pipette containing either a low-calcium internal saline to allow comparison with subsequent whole cell recordings or external saline with tetrodotoxin, DL-2-amino-5-phosphonovaleric acid, and 6-cyano-7-nitroquinoxaline-2,3-dione, a solution that is more appropriate for bathing a nerve terminal. 2. The mIPSCs recorded from the synapses restricted to the cell-attached patches were characterized by their times to peak, amplitudes, and time constants of decay. The degree of variability in these characteristics was quantified with the use of the following model-independent parameters: the coefficient of variation, skewness, and kurtosis. The distribution of time to peak values has a mean value of 5.6 +/- 0.5 (SE) ms, has the lowest coefficient of variation (0.33 +/- 0.01), is fairly symmetrical, and has a Gaussian shape with respect to peakedness. On the other hand, both the amplitude and decay time constant distributions are highly skewed and more peaked than Gaussian distributions. The mean amplitude is -6.6 +/- 0.6 pA with a coefficient of variation of 0.60 +/- 0.05, whereas the mean decay time constant is 22.8 +/- 1.0 ms with a coefficient of variation of 0.81 +/- 0.03. 3. The amplitude distributions for spontaneous inhibitory currents recorded from cell-attached patches are best fitted by the sum of multiple Gaussians. The coefficient of variation for the first Gaussian peak fitted to the amplitude distributions is 0.290 +/- 0.028. 4. Decay time distributions were consistently best fitted by the sum of four Gaussians with decay constants as follows: D1 = 5.7 +/- 0.2 ms (n = 12), D2 = 11.2 +/- 0.7 ms (n = 11), D3 = 20.6 +/- 0.8 ms (n = 12), and D4 = 43.8 +/- 2.3 ms (n = 16). These mean values are essentially identical to those reported in the preceding paper for mIPSCs recorded in the whole cell configuration. 5. In eight neurons we were able to record mIPSCs both in cell-attached patches and in subsequent whole cell configurations. The properties of mIPSCs recorded from single synapses (i.e., times to peak, amplitude, and time constants of decay) show as much variability as those of mIPSCs recorded subsequently in the whole cell mode; that is, there are no statistically significant differences in the coefficients of variation, skewness, or kurtosis for the three different distributions.

Oligodendrocyte progenitors are generated throughout the embryonic mouse brain, but differentiate in restricted foci

Recent evidence from studies mapping the expression of putative oligodendrocyte progenitor specific mRNAs has suggested that oligodendrocyte progenitors arise during embryogenesis, in specific foci of the neuroectoderm. In order to test this hypothesis, we have assayed different regions of the embryonic central nervous system for their ability to generate oligodendrocytes following transplantation into neonatal cerebrum. To allow identification of donor-derived oligodendrocytes in wild-type host brain, we used the MbetaP transgenic mouse, which expresses lacZ in oligodendrocytes, as donor tissue. We found that tissue fragments derived from several levels of the anterior-posterior axis of the neural tube at E14.5 and E12.5, chosen to include (hindbrain, cervical and lumbar spinal cord), or exclude (dorsal telencephalon) putative foci of oligodendrocyte progenitors, all produced oligodendrocytes following transplantation. In addition, these same regions taken from E10.5, prior to the appearance of putative oligodendrocyte progenitor markers, also all yielded oligodendrocytes on transplantation. This indicates that precursor cells that can generate oligodendrocytes are widespread throughout the neuroectoderm as early as E10.5. We have also used the oligodendrocyte lineage-specific glycolipid antibodies O4, R-mAb and O1 to identify those regions of the developing brain that first support the differentiation of oligodendrocytes from their progenitor cells. We found that the first oligodendrocytes arise in prenatal brain at E14.5, in a restricted zone adjacent to the midline of the medulla. These cells are mitotically inactive, differentiated oligodendrocytes and, using light and electron microscopy, we show that they become functional, myelin-bearing oligodendrocytes. We have mapped the subsequent appearance of differentiated oligodendrocytes in the prenatal brain and show that they appear in a restricted, tract-specific manner. Our results suggest that oligodendrocytes are generated from neuroectodermal cells positioned throughout the rostrocaudal axis of the neural tube, rather than at restricted locations of the neuroectoderm. By contrast, the differentiation of such cells into oligodendrocytes does occur in a restricted manner, consistent with local regulation of oligodendrocyte progenitor differentiation.

Vitamin A-deficient quail embryos have half a hindbrain and other neural defects

BACKGROUND

Retinoic acid (RA) is a morphogenetically active signalling molecule thought to be involved in the development of severely embryonic systems (based on its effect when applied in excess and the fact that it can be detected endogenously in embryos). Here, we adopt a novel approach and use the vitamin A-deficient (A-) quail embryo to ask what defects these embryos show when they develop in the absence of RA, with particular reference to the nervous system.

RESULTS

We have examined the anatomy, the expression domains of a variety of genes and the immunoreactivity to several antibodies in these A- embryos. In addition to the previously documented cardiovascular abnormalities, we find that the somites are smaller in A- embryos, otic vesicle development is abnormal and the somites continue up to and underneath the otic vesicle. In the central nervous system, we find that neural crest cells need RA for normal development and survival, and the neural tube fails to extend any neurites into the periphery. Using general hindbrain morphology and the expression patterns of Hoxa-2, Hoxb-1, Hoxb-4, Krox-20 and FGF-3 as markers, we conclude that segmentation in the myelencephalon (rhombomeres 4-8) is disrupted. In contrast, the dorsoventral axis of the neural tube using Shh, islet-1 and Pax-3 as markers is normal.

CONCLUSIONS

These results demonstrate at least three roles for RA in central nervous system development: neural crest survival, neurite outgrowth and hindbrain patterning.

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Perinatal developmental changes in respiratory activity of medullary and spinal neurons: an in vitro study on fetal and newborn rats

Experiments were performed in vitro on fetal and newborn rat brainstem-spinal cord preparations to analyse the perinatal developmental changes in inspiratory motor output. The amplitude of the inspiratory bursts of the whole C4 ventral root (global extracellular recording), the firing patterns of 80 medullary inspiratory neurons (unitary extracellular recording) and the firing and membrane properties of 71 respiratory neurons in the C4 ventral horn (whole-cell recording) were analysed at embryonic day 18 (E18), 21 (E21) and post natal days 0 to 3 (P0-3). At E18, the amplitude of the C4 bursts was weak and variable from one respiratory cycle to the next, as well as the discharge pattern of most of the medullary inspiratory neurons. C4 motoneurons were immature, very excitable and displaying variable inspiratory discharges, but already able to deliver sustained bursts of potentials when depolarised. At E21 and P0-3, the amplitude of the C4 bursts was increased and stable, most of the medullary inspiratory neurons already were able to generate a stable firing pattern and C4 motoneurons showed maturational changes in terms of the resting potential, spike amplitude and input membrane resistance. This work suggests that the short period extending from E18 to E21 is a critical maturational period for the medullary respiratory network which becomes able to elaborate a stable respiratory motor output.

Sudden infant death syndrome: postnatal changes in the volumes of the pons, medulla and cervical spinal cord

The brainstem and cervical spinal cord were sampled from 45 cases of sudden infant death syndrome (SIDS), from 17 control cases without neurological disease, and from three negative control cases with abnormal growth of the central nervous system (36-98 postconceptional weeks). Morphometric analyses were performed on serial Nissl sections to determine the total volumes of the pons, nucleus pontis, medulla and cervical spinal cord. Normal development was characterized by a linear increase in the volumes of these regions during the first postnatal year. Regression analysis revealed that in SIDS cases the rates of increase in the volumes of the pons and nucleus pontis were significantly greater than in controls (56% and 83%, respectively), while growth rates did not differ significantly for the medulla and cervical spinal cord. By direct comparison, there was a significant increase in the mean volumes of the pons (33%), nucleus pontis (38%) and medulla (19%) in SIDS cases when compared to controls. No evidence of excessive edema or gliosis was noted in the brainstem by light and electron microscopy to account for the increased volumes. Subtle morphological abnormalities in brainstem neurons from SIDS cases, including an increased size of Nissl bodies in the cytoplasm of large motor neurons and the presence of paranucleolar coiled bodies, were consistent with an increased synthesis and transport of ribosomal RNA, an increased synthesis of cellular proteins and neuronal hypertrophy.

Development of intrinsic electrophysiological properties in neurons from the gustatory region of rat nucleus of solitary tract

There is no current understanding of the nature or time course of maturation of intrinsic electrophysiological properties for neurons in the gustatory region of the nucleus of the solitary tract (NST). Therefore, we used whole cell recordings in an in vitro slice preparation of the rat brainstem to characterize development of resting membrane, action potential and repetitive discharge properties of cells in gustatory NST at postnatal days 5, 10, 15, 20, and 30, and adult ages. Neurons were filled with Biocytin to verify location and characterize morphology. Membranes from younger neurons demonstrated a steeper current-voltage relation or higher input resistance, and a longer time constant than mature cells. Action potentials in younger cells had a slower rate of rise and were longer in duration. The afterhyperpolarization that typically follows the spike discharge usually had one phase in younger neurons, but was characterized by two or more phases in an increasing proportion of older cells. The repetitive discharge frequency in response to a range of depolarizing current pulses increased during development, and frequency/current plots were steeper in older compared with younger neurons. However, in all age groups there was clear accommodation of the discharge frequency. The greatest changes in resting membrane, action potential, and discharge properties were observed between P5 and P15, and mature values were generally reached by P20. At each postnatal age, neurons could be categorized in four neuron groups, based on the discharge pattern in response to a hyperpolarizing/depolarizing current protocol. Anatomical reconstructions indicated that although cells increased in overall dendritic expanse during development, neurons became less complex as illustrated by decreases in number of dendritic branch points, and in number and density of spines. The timing of major developmental differences in intrinsic electrical characteristics observed here is associated with a period of previously reported maturational changes in extracellular taste responses to number and concentration of chemical stimuli. However, further alterations in extracellular taste responses proceed after apparent maturation of intrinsic neural properties.

Early development of efferent projections from the chick tectum

The early development of the uncrossed tectobulbar and the crossed tectospinal tracts was studied. These two projections arise from the same structure, the mesencephalon, and develop during the same time period, but follow divergent courses. We have traced the pathways followed by these projections and identified the positions at which axon guidance decisions are made. The first neurons differentiate either side of the entire rostrocaudal extent of the dorsal midline and initiate axons that extend dorsoventrally across the surface of the tectum. At the ventral edge of the tectum these axons turn abruptly and fasciculate to form a caudal descending projection to the hindbrain. These axons extend to the caudal hindbrain and do not project to the periphery along cranial nerve roots. We therefore consider this tract to be the tectobular, rather than the mesencephalic division of the trigeminal. While the tectobulbar projection is still developing, a second wave of axons is initiated, which arises from only the rostral part of the tectum. These axons grow beyond the tectobulbar turn point and continue toward the ventral midline, where they cross the floor plate, before turning caudally at the lateral edge of the main descending hindbrain tract, the ventrolateral tract. We discuss the development of these tracts with reference to possible guidance cues mediating their course.

Competition and position-dependent targeting in the development of the Drosophila R7 visual projections

The R7 photoreceptor neuron projections form a retinotopic map in the medulla of the Drosophila optic lobe. The more inner photoreceptors mutation, an allele of gap1, results in the differentiation of excess R7s in the eye, whose axons invade the brain and establish functional connections. We have used this hyperinnervation phenotype to explore the roles of photoreceptor-target regulation, competitive interactions, and chemoaffinity in map formation. We show that the extra axons are supported in a wild-type brain, with all R7s from a single ommatidium sharing a single termination site, and thus there is no evidence that the target regulates the size of the presynaptic population. In mosaic eyes, in which ommatidia containing extra R7s are surrounded by ommatidia lacking all R7 cells, R7 axons still target to appropriate retinotopic locations in a largely empty R7 terminal field. Axons at the edges of the projection, however, send collaterals into vacant areas of the field, suggesting they are normally restrained to share single termination sites by competitive interactions. In contrast, no sprouts are seen when the vacant sites are juxtaposed with singly innervated sites. In the third instar, R7 and R8 axons transiently display halos of filopodia that overlap adjacent terminals and provide a means to assess occupancy at adjacent sites. Finally, in sine oculis larvae in which only a small number of ommatidia develop, the R7/R8 axons target to predicted dorsoventral portions of the medulla despite the absence of their neighbors, suggesting that position in the eye field determines their connectivity in the brain. We suggest that the mechanisms used to set up this insect map are formally similar to strategies used by vertebrates. The availability of a genetic model for these events should facilitate studies aimed at understanding the molecular bases of retinotopic map development.

Involvement of the rostral ventro-lateral medulla in respiratory rhythm genesis during the peri-natal period: an in vitro study in newborn and fetal rats

The involvement of the rostral ventro-lateral medulla (RVLM) in respiratory rhythm genesis was analysed on brain stem-spinal cord preparations from newborn and fetal rats in which the ability to generate central respiratory activity in vitro persists. The respiratory rhythm (around 5 per min) was stable in preparations from newborn and pre-term fetal (D20-21) rats but very variable in young fetuses (D18). In newborn and D20-21 fetal rats, RVLM electrical stimulation delivered during mid-expiration initiated premature inspiration while RVLM electrolytic lesions suppressed the respiratory rhythm. In D18 fetuses, RVLM stimulation had no effect and strong stimulations evoked only diffuse activation. Extracellular recordings of the activity of 423 RVLM neurons showed that this area contained numerous inspiratory neurons in all the age groups studied. Low Ca(2+)-high Mg2+ medium bathing (assumed to block synaptic transmission) abolished the inspiratory bursts in the cervical roots and most of the 99 RVLM inspiratory neurons investigated. In newborn and D20-21 rats, however, 7 of the 68 RVLM inspiratory neurons tested behaved like respiratory pacemakers, since they continued to fire with a bursting pattern, while in the D18 preparations, none of 31 did so. These experiments confirm that the RVLM is a crucial site in respiratory rhythm genesis in newborn rats, and suggest that this may also be the case in D20-21 fetuses, but probably not in D18 fetuses.

The relationship between age-related heart rate changes and developing brain function: a model of anencephalic human fetuses in utero

We attempted to identify the brain segment which controls heart rate changes in human fetuses with advancing gestation. Twelve anencephalic and 165 normal fetuses (control-group fetuses) between 25-32 weeks' gestation were studied. The instantaneous fetal heart rate (FHR) data were obtained from each fetus for a continuous 90-120 min period, using an external cardiotocograph. Calculations included the 'individual probability distribution matrices' in which the FHRs at 1 beat/min intervals between 110 and 180 beats/min, the beat-to-beat differences (DFHRs) between +/- 5 beats/min and the probability values were arranged in rows, columns and the corresponding elements, respectively. Using 2-gestational-week intervals probability distribution matrices (age-group probability distribution matrices) obtained from 335 normal fetuses in our previous study as a reference, the difference between a given 'individual probability distribution matrix' and the corresponding age-group probability distribution matrix' was quantified as the 'difference rate' according to the formula in the text. From 25-26 to 27-28 weeks' gestation, the 'difference rates' in four anencephalic fetuses, with only the spinal cord preserved, were significantly higher in value than those of control-group fetuses, whereas the rates in four fetuses with both the spinal cord and medulla oblongata preserved, indicated no significant differences. From 29-30 to 31-32 weeks' gestation, the rates of the four fetuses with the spinal cord and medulla oblongata preserved, showed significant differences from the control-group fetuses. These findings suggest that there is a critical period between 27-28 and 29-30 weeks' gestation with regard to the developing brain function pertaining to FHR changes. In the early stage, the medulla oblongata plays a role in FHR changes, whereas, in the latter stage, the brain cephalad to the medulla also appears to take on the role of FHR regulator.

Neurochemical differentiation of human bulbospinal monoaminergic neurons during the first trimester

The neurochemical differentiation of bulbospinal noradrenergic and serotonergic neurons has been followed in first trimester human fetuses. Analysis of microdissected CNS regions revealed detectable levels of noradrenaline (NA) and serotonin (5-HT) in pons, medulla oblongata and throughout the spinal cord from 5-6 weeks of gestation. In all regions there was a pronounced increase in tissue levels of the monoamines, especially from 8-9 weeks on. 5-HT levels were lower than NA levels except for pons, where the opposite was true. With increasing fetal age, the results seemed less consistent because of considerable interindividual variations. Using immunohistochemical localization of tyrosine hydroxylase (TH), a marker for noradrenergic neurons, immature cell bodies were seen in the brain stem at the earliest stage studied, that is at 4 weeks of gestation. Several TH and 5-HT-immunoreactive (IR) cell groups were found in pons and medulla oblongata at 5 weeks. Significant structural differentiation of TH- and 5-HT-IR cell bodies was seen during the first trimester. Immunoreactive fibers began to appear at 5 weeks in the cervical spinal cord. At 6 weeks both types of fibers could be found in the white matter throughout the entire spinal cord while fibers in gray matter appeared at 9 weeks. The number of TH-IR fibers was considerably larger than the number of 5-HT-IR fibers. This is the first time the biochemical development of human bulbospinal monoaminergic neurons during the first trimester has been described. Continued investigations of the ontogenetic growth and differentiation of these human bulbospinal monoaminergic neurons will gain necessary insight into the genetically determined capacity for plasticity, potentially possible to activate later in life in response to spinal cord injury. Further, intraspinal transplantation of CNS tissue relevant to the severed spinal cord would by necessity entail selection of embryonic cell populations. Using such therapeutic strategies, detailed knowledge of the inherent capacities of the donor tissues will be crucial.

Endothelin concentrations in respiration-related structures of the medulla during the perinatal period of the rat

Endothelin-like immunoreactivity (ET-LI) was quantified in the developing (foetal and postnatal) rat brain stem and cerebellum using radioimmunoassay. The brain stem structures chosen for this study were (a) dorsal medullary region (DMR) including the region of nucleus tractus solitarius where the peripheral chemoreceptor afferents are known to terminate, (b) ventral medullary region (VMR) where the central chemoreceptors are thought to be located and (c) cerebellum (CER), as a control area. Compared to the prenatal period, significantly elevated concentrations of ET-LI were detected in the early postnatal period and thereafter the concentrations decreased: DMR and VMR: in comparison to the prenatal concentrations, a two-fold increase was found on the day of birth which further increased significantly (P < 0.001) on postnatal day 1 only in the region of DMR; CER: low concentrations of ET-LI were found in the early postnatal period which were not significantly different from the prenatal values. No ET-LI could be detected in any of the three regions in the adult rats. The results are discussed in view of the hypothesis that (1) endothelin appears to play an important role in the perinatal period and (2) it is involved in the chemoreceptor pathway.