Ultrastructure and synaptology of the initial axon segment of cat spinal motoneurons during early postnatal development

Theoretical relation between axon initial segment geometry and excitability

In most vertebrate neurons, action potentials are triggered at the distal end of the axon initial segment (AIS). Both position and length of the AIS vary across and within neuron types, with activity, development and pathology. What is the impact of AIS geometry on excitability? Direct empirical assessment has proven difficult because of the many potential confounding factors. Here, we carried a principled theoretical analysis to answer this question. We provide a simple formula relating AIS geometry and sodium conductance density to the somatic voltage threshold. A distal shift of the AIS normally produces a (modest) increase in excitability, but we explain how this pattern can reverse if a hyperpolarizing current is present at the AIS, due to resistive coupling with the soma. This work provides a theoretical tool to assess the significance of structural AIS plasticity for electrical function.

Maturation Dynamics of the Axon Initial Segment (AIS) of Newborn Dentate Granule Cells in Young Adult C57BL/6J Mice

Newborn dentate granule cells (DGCs) are generated in the hippocampal dentate gyrus (DG) of rodents through a process called adult hippocampal neurogenesis, which is subjected to tight intrinsic and extrinsic regulation. The use of retroviruses encoding fluorescent proteins has allowed the characterization of the maturation dynamics of newborn DGCs, including their morphological development and the establishment and maturation of their afferent and efferent synaptic connections. However, the study of a crucial cellular compartment of these cells, namely, the axon initial segment (AIS), has remained unexplored to date. The AIS is not only the site of action potential initiation, but it also has a unique molecular identity that makes it one of the master regulators of neural plasticity and excitability. Here we examined the dynamics of AIS formation in newborn DGCs of young female adult C57BL/6J mice in vivo Our data reveal notable changes in AIS length and thickness throughout cell maturation under physiological conditions and show that the most remarkable structural changes coincide with periods of intense morphological and functional remodeling. Moreover, we demonstrate that AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.SIGNIFICANCE STATEMENT The hippocampal dentate gyrus (DG) of rodents generates newborn dentate granule cells (DGCs) throughout life. This process, named adult hippocampal neurogenesis, confers a unique degree of plasticity to the hippocampal circuit, and it is crucial for learning and memory. Here we studied, for the first time, the formation of a key cellular compartment of newborn DGCs, namely, the axon initial segment (AIS) in vivo Our data reveal remarkable AIS structural remodeling throughout the maturation of these cells under physiological conditions. Moreover, AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.

Nonreciprocal mechanisms in up- and downregulation of spinal motoneuron excitability by modulators of KCNQ/Kv7 channels

KCNQ/K_v7 channels form a slow noninactivating K⁺ current, also known as the M current. They activate in the subthreshold range of membrane potentials and regulate different aspects of excitability in neurons of the central nervous system. In spinal motoneurons (MNs), KCNQ/K_v7 channels have been identified in the somata, axonal initial segment, and nodes of Ranvier, where they generate a slow, noninactivating, K⁺ current sensitive to both muscarinic receptor-mediated inhibition and KCNQ/K_v7 channel blockers. In this study, we thoroughly reevaluated the function of up- and downregulation of KCNQ/K_v7 channels in mouse immature spinal MNs. Using electrophysiological techniques together with specific pharmacological modulators of the activity of KCNQ/K_v7 channels, we show that enhancement of the activity of these channels decreases the excitability of spinal MNs in mouse neonates. This action on MNs results from a combination of hyperpolarization of the resting membrane potential, a decrease in the input resistance, and depolarization of the voltage threshold. On the other hand, the effect of inhibition of KCNQ/K_v7 channels suggested that these channels play a limited role in regulating basal excitability. Computer simulations confirmed that pharmacological enhancement of KCNQ/K_v7 channel activity decreases excitability and also suggested that the effects of inhibition of KCNQ/K_v7 channels on the excitability of spinal MNs do not depend on a direct effect in these neurons but likely on spinal cord synaptic partners. These results indicate that KCNQ/K_v7 channels have a fundamental role in the modulation of the excitability of spinal MNs acting both in these neurons and in their local presynaptic partners.

Synaptic rearrangement following axonal injury: Old and new players

Following axotomy, the contact between motoneurons and muscle fibers is disrupted, triggering a retrograde reaction at the neuron cell body within the spinal cord. Together with chromatolysis, a hallmark of such response to injury is the elimination of presynaptic terminals apposing to the soma and proximal dendrites of the injured neuron. Excitatory inputs are preferentially eliminated, leaving the cells under an inhibitory influence during the repair process. This is particularly important to avoid glutamate excitotoxicity. Such shift from transmission to a regeneration state is also reflected by deep metabolic changes, seen by the regulation of several genes related to cell survival and axonal growth. It is unclear, however, how exactly synaptic stripping occurs, but there is substantial evidence that glial cells play an active role in this process. In one hand, immune molecules, such as the major histocompatibility complex (MHC) class I, members of the complement family and Toll-like receptors are actively involved in the elimination/reapposition of presynaptic boutons. On the other hand, plastic changes that involve sprouting might be negatively regulated by extracellular matrix proteins such as Nogo-A, MAG and scar-related chondroitin sulfate proteoglycans. Also, neurotrophins, stem cells, physical exercise and several drugs seem to improve synaptic stability, leading to functional recovery after lesion. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Activity-dependent plasticity of spinal circuits in the developing and mature spinal cord

Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.

Reprogramming axonal behavior by axon-specific viral transduction

The treatment of axonal disorders, such as diseases associated with axonal injury and degeneration, is limited by the inability to directly target therapeutic protein expression to injured axons. Current gene therapy approaches rely on infection and transcription of viral genes in the cell body. Here, we describe an approach to target gene expression selectively to axons. Using a genetically engineered mouse containing epitope-labeled ribosomes, we find that neurons in adult animals contain ribosomes in distal axons. To use axonal ribosomes to alter local protein expression, we utilized a Sindbis virus containing an RNA genome that has been modified so that it can be directly used as a template for translation. Selective application of this virus to axons leads to local translation of heterologous proteins. Furthermore, we demonstrate that selective axonal protein expression can be used to modify axonal signaling in cultured neurons, enabling axons to grow over inhibitory substrates typically encountered following axonal injury. We also show that this viral approach also can be used to achieve heterologous expression in axons of living animals, indicating that this approach can be used to alter the axonal proteome in vivo. Together, these data identify a novel strategy to manipulate protein expression in axons, and provides a novel approach for using gene therapies for disorders of axonal function.

Axon initial segment ensheathed by extracellular matrix in perineuronal nets

Perineuronal nets of extracellular matrix are associated with distinct types of neurons in the cerebral cortex and many subcortical regions. Large complexes of aggregating proteoglycans form a chemically specified microenvironment around the somata, proximal dendrites and the axon initial segment, including the presynaptic boutons attached to these domains. The subcellular distribution and the temporal course of postnatal formation suggest that perineuronal nets may be involved in the regulation of synaptic plasticity. Here we investigate structural and cytochemical characteristics of the extracellular matrix around axon initial segments virtually devoid of synaptic contacts. Wisteria floribunda agglutinin staining, the immunocytochemical detection of aggrecan and tenascin-R, as well as affinity-labeling of hyaluronan were used to analyze perineuronal nets associated with large motoneurons in the mouse superior colliculus. The molecular composition of perineuronal nets was divergent between neurons but was identical around the different cellular domains of the individual neurons. The axon initial segments largely devoid of synapses were covered by a continuous matrix sheath infiltrating the adjacent neuropil. The periaxonal zone penetrated by matrix components often increased in diameter along the initial segment from the axon hillock toward the myelinated part of the axon. The axonal and somatodendritic domains of perineuronal nets were concomitantly formed during the first three weeks of postnatal development. The common molecular properties and major structural features of subcellular perineuronal net domains were retained in organotypic midbrain slice cultures. The results support the hypothesis that the aggrecan-related extracellular matrix of perineuronal nets provides a continuous micromilieu for different subcellular domains performing integration and generation of the electrical activity of neurons.

LIM kinase 1 accumulates in presynaptic terminals during synapse maturation

LIM kinase 1 (LIMK1) is a cytoplasmic protein kinase that is highly expressed in neurons. In transfected cells, LIMK1 binds to the cytoplasmic tail of neuregulins and regulates the breakdown of actin filaments. To identify potential functions of LIMK1 in vivo, we have determined the subcellular distribution of LIMK1 protein within neurons of the rat by using immunomicroscopy. At neuromuscular synapses in the adult hindlimb, LIMK1 was concentrated in the presynaptic terminal. However, little LIMK1 immunoreactivity was detected at neuromuscular synapses before the 2nd week after birth, and most motoneuron terminals were not strongly LIMK1 immunoreactive until the 3rd week after birth. Thus, LIMK1 accumulation at neuromuscular synapses coincided with their maturation. In contrast, SV2, like many other presynaptic terminal proteins, can be readily detected at neuromuscular synapses in the embryo. Similar to its late accumulation at developing synapses, LIMK1 accumulation at regenerating neuromuscular synapses occurred long after these synapses first formed. In the adult ventral spinal cord, LIMK1 was concentrated in a subset of presynaptic terminals. LIMK1 gradually accumulated at spinal cord synapses postnatally, reaching adult levels only after P14. This study is the first to implicate LIMK1 in the function of presynaptic terminals. The concentration of LIMK1 in adult, but not nascent, presynaptic terminals suggests a role for this kinase in regulating the structural or functional characteristics of mature synapses.

Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat

The aim of the present study was to provide a quantitative analysis of the synapses made onto the axon hillock and initial segment of rat abducens motoneurons retrogradely or intracellularly stained with HRP. GABA-immunoreactive terminals contacting these axons were visualized using a postembedding procedure. The presynaptic terminals contained either spherical or pleomorphic vesicles. gamma-Aminobutyric acid (GABA)-immunoreactive axon terminals, which belonged to this last category, were distributed both onto axon hillocks and the proximal part of initial segments. The percentage of axonal membrane covered by synapses ranged from 44.1 to 68.2%. A quantitative analysis performed on a series of ultrathin sectioned terminals contacting the axon of an intracellularly labeled motoneuron revealed a significant correlation between the length of membrane apposition of the terminals and their perimeter or surface area, and also between the area of membrane apposition and terminal volume. GABA-immunoreactive terminals had a mean perimeter and volume that were larger than those of unlabeled axon terminals. The number of active zones was correlated with the area of apposition. Some hypotheses concerning the functional role of the GABAergic innervation of this particular part of the neuron are discussed.

Postnatal development of alpha- and gamma-peroneal motoneurons in kittens: an ultrastructural study

Motoneurons innervating the peroneus brevis muscle of 1 week- and 3 week-old kittens were retrogradely labelled by HRP and examined by electron microscopy. At 1 week the distribution of mean cell body diameters was unimodal. Consequently alpha- and gamma-motoneurons could not be identified by their size. The aim of this study was to see whether the alpha- and gamma-motoneurons of kittens could be identified using the combination of ultrastructural criteria previously defined in the adult cat. Using these three criteria it was not possible to distinguish all the motoneurons as either alpha- or gamma in the kitten and a fourth criterion (frequency of F bouton profiles) was added to aid identification. However, with these four criteria, at 1 week six of 21 motoneurons and at 3 weeks two of 18 could still not be clearly identified as alpha or gamma (four were tentatively considered to be gamma, and four could not be identified). The maturation of alpha-motoneurons between 1 week and the adult was accompanied by an increase in somatic membrane area and a significant decrease in the somatic packing density of F boutons. On gamma-motoneurons there was a decrease in the somatic packing density of F boutons between 1 and 3 weeks. However, the numbers of F and S boutons remained stable for both motoneuron types. Age-related changes in apposition and active zone lengths of F and S boutons characterize the synaptic rearrangements which are occurring during the postnatal development of motoneurons.

Activity-dependent development of spinal cord motor neurons

Patterned neuronal activity in early postnatal life can regulate the acquisition of the mature morphological and electrophysiological properties of neurons. Many properties of motor neurons are developmentally regulated and may be influenced by epigenetic factors. The pattern of activation of motor neurons can regulate axon terminal morphology and synaptic efficacy at the neuromuscular junction. Motor neuron morphology and synaptic connections can also be modified by exposure to specific hormones in the early postnatal period. The acquisition of mature physiological and anatomical properties is paralleled by the acquisition of specific molecular properties. Recent experiments using molecular markers for motor neuron differentiation indicate that motor neurons undergo activity-dependent development during a circumscribed period in early postnatal life. Normal motor neuron differentiation requires a normal pattern of neuronal activity in early postnatal life. Differentiation also requires activation of the NMDA receptor over the same time period. The activity-dependent development of morphological, electrophysiological and molecular properties of motor neurons is similar to activity-dependent development in the vertebrate visual system. The neuromuscular system may provide an accessible system for characterizing the molecules subserving the translation of patterned neuronal activity into mature neuronal phenotype.

Microtubule bundles in fish cerebellar Purkinje cells

The initial axon segments and the cell bodies of Purkinje cells were examined in electron microscopic serial sections and toluidine blue semithin sections of goldfish cerebellum. We observed two characteristic cytoplasmic features different from those of other vertebrate neurons, 1. The areas of Nissl substance and Golgi apparatus are sharply divided in the periphery and center of the cytoplasm, 2. Microtubules fasciculated by cross-bridges in the axon hillock and initial axon segment remain bundled in the perikaryon, pass near the eccentric nucleus, and enter into the Golgi area of the central cytoplasm, where they are surrounded by mitochondria. We suggest that the intracellular fasciculated microtubules may establish a prepared pathway for fast anterograde and retrograde transport to and from the Golgi area of the cell body.

Induction of plasticity in the isolated spinal cord in paraplegia

In adult life a severe injury of the spinal cord results in total loss of locomotor functions of the hind limbs, i.e., paraplegia. However, after similar injury in neonatal life most hind limb functions are retained unaffected into adult life. Can such survival of locomotor function be produced in an adult paraplegic? Observations based on our previous studies suggest that sparing of function in the neonate is due to: 1) incomplete development of descending cord tracts 2) the presence of polyneuronal control of limb muscles by spinal motoneurons and 3) active growth of synaptic connections occurring in the cord while limbs are polyneuronally innervated. Such growth and remodelling ceases once mononeuronal (= adult) control of limb muscles is established. We suggest that recreation of conditions similar to neonatal life would be able to revive lost locomotor functions in the adult paraplegic. Experimental animal models are outlined here which may form a basis for future research.

Immunolocalization and quantitation of a novel nerve terminal protein in spinal cord development

In the adult spinal cord, the neuron-specific protein NT75 is located in nerve terminals synapsing in the superficial laminae of the dorsal horn. The present study examines the occurrence of NT75 in the developing rat spinal cord. NT75 immunoreactivity is detectable in primary afferent axons at the dorsal root entry zone on embryonic day 15. Subsequently, staining of presumptive nerve terminals appears in the deeper laminae of the dorsal horn, expanding into the superficial laminae during the first postnatal week. NT75 staining also appears in developing corticospinal tract axons in the brainstem at birth, and at lumbosacral levels by postnatal day 5. As NT75-positive nerve terminals approach the adult distribution, staining of primary afferent and corticospinal axons decreases, becoming undetectable by postnatal day 30. Dense transient staining of presumed nerve terminals in the ventral horn is also apparent during early postnatal development. Quantitative analysis of developing spinal cord shows a low level of NT75 immunoreactivity at birth. NT75 activity then increases substantially, reaching values by the third and fourth postnatal weeks up to 2.5 times that seen in adults. The occurrence of NT75 immunoreactivity correlates with the reported time course of synaptic development in the spinal cord. In addition, the results suggest that NT75 immunoreactivity is maintained at high levels in the nerve terminals of certain neural pathways into adulthood, whereas in other systems NT75 immunoreactivity may be detectable only during development.

Large diameter primary afferent input is required for expression of the Cat-301 proteoglycan on the surface of motor neurons

The expression of a cell surface proteoglycan, recognized by monoclonal antibody Cat-301, is regulated by neuronal activity in early life. Here we report that the expression of the Cat-301 proteoglycan on motor neurons depends on primary afferent input in the early postnatal period. Previously we showed that in two different systems, Y-cells in the cat lateral geniculate nucleus and motor neurons in the hamster spinal cord, the expression of the Cat-301 antigen requires neuronal activity during a circumscribed period in development. Disrupting the activity of Y-cells (by dark rearing or by monocular lid suture) or of motor neurons (by sciatic nerve crush or by spinal cord lesion) during the early postnatal period prevents Cat-301 expression. Disrupting neuronal activity in adults has no effect on Cat-301 expression. The onset of Cat-301 expression corresponds to the end of the period of activity-dependent development. In order to further dissect the components of the segmental reflex are required for the expression of Cat-301 on motor neurons, here we evaluated the effect of deafferentation by dorsal rhizotomy. In adult animals two weeks after deafferentation all sciatic motor neurons continue to express Cat-301. In contrast, in neonates two weeks after deafferentation the normal developmental expression of Cat-301 is reduced and less than 50% of sciatic motor neurons express Cat-301. We next selectively lesioned the small diameter afferents using the neurotoxin capsaicin. In contrast to rhizotomy, neonatal deletion of small diameter afferents has no effect on the development of Cat-301 expression on motor neurons. These results imply that input relayed by large diameter primary afferents (probably those conveying muscle and/or joint information) is required for normal maturation of motor neuronal properties during early life. They also provide further evidence for activity-dependent maturation of motor neurons.

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons

The postnatal development of length, branching structure, and spatial distribution of dendrites of triceps surae motoneurons, intracellularly stained with horseradish peroxidase, was studied from birth up to 44-46 days of postnatal (d.p.n.) age in kittens and compared with corresponding data from adult cats. The number of dendrites of a triceps surae motoneuron was about 12, and the arborization of each dendrite generated an average of 12-15 terminal branches. There was no net change in the number of dendrites of a neuron or in the degree of branching of the dendrites despite the occurrence of both a transient remodeling of the dendritic branching structure and changes of the spatial distribution of the dendritic branches during postnatal development. The perisomatic territory in the transverse plane occupied by the dendritic branches of a motoneuron increased in parallel with the overall growth of the spinal cord. Thus, the relative size of the dendritic territory in this plane was kept almost constant, whereas dendritic branches projecting in the rostrocaudal direction grew much faster than the spinal cord and also became more numerous. At birth the rostro-caudal dendritic span of individual motoneurons bridged 1:6 to 1:5 of the L7 spinal cord segment length; this figure was 1:3 at 22-24 d.p.n. Hence, in this direction, the growing dendritic branches invaded novel dendritic territories. The change in dendritic branch length from birth to 6 weeks of age corresponded to an average growth rate of 2 to 4 microns per dendritic branch and day, which implies that the total increase in length of the dendrites of a neuron could amount to 1 mm/day. The increase in branch length did not occur in a uniform or random manner; instead, it followed a spatiotemporal pattern with three phases: From birth to 22-24 d.p.n., growth was particularly prominent in greater than or equal to 3rd order preterminal and 2nd through 6th order terminal branches. From 22-24 to 44-46 d.p.n., a large increase in branch length confined to terminal branches of greater than or equal to 3rd branch orders was observed. As indicated by topological analysis, this length increase was probably due in part to a resorption of peripheral dendritic branches during this stage of development. From 44-46 d.p.n. to maturity, the increase of dendritic branch length was restricted to preterminal branches of low (less than or equal to 4th) branch order.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of transient cortical projections from auditory, somatosensory, and motor cortices to visual areas 17, 18, and 19 in the kitten

We have examined the anatomical features of ipsilateral transient cortical projections to areas 17, 18, and 19 in the kitten with the use of axonal tracers Fast Blue and WGA-HRP. Injections of tracers in any of the three primary visual areas led to retrograde labeling in frontal, parietal, and temporal cortices. Retrogradely labeled cells were not randomly distributed, but instead occurred preferentially at certain loci. The pattern of retrograde labeling was not influenced by the area injected. The main locus of transiently projecting neurons was an isolated region in the ectosylvian gyrus, probably corresponding to auditory area A1. Other groups of transiently projecting neurons had more variable locations in the frontoparietal cortex. The laminar distribution of neurons sending a transient projection to the visual cortex is characteristic and different from that of parent neurons of other cortical pathways at the same age. In the frontoparietal cortex, transiently projecting neurons were located mainly in layer 1 and the upper part of layers 2 and 3. In the ectosylvian gyrus, nearly all the neurons are located in layers 2 and 3. In addition, a few transiently projecting neurons are found in layer 6 and in the white matter. Transiently projecting neurons have a pyramidal morphology except for the occasional spindle-shaped cell of layer 1 and multipolar cells observed in the white matter. Anterograde studies were used to investigate the location of transient fibers in the visual cortex. Injections of WGA-HRP at the site of origin of transient projections gave rise to few retrogradely labeled cells in areas 17, 18, and 19, demonstrating that transient projections to these areas are not reciprocal. Although labeled axons were found over a wide area of the posterior cortex, they were more numerous over certain regions, including areas 17, 18, and 19, and absent from other more lateral cortical regions. Transient projecting fibers were present in all cortical layers at birth. Plotting the location of transient fibers in numerous sections and at all ages showed that these fibers are not more plentiful in the white matter than they are in the gray matter. We found no evidence that the white/gray matter border constituted a physical barrier to the growth of transient axons. Comparison of the organization of this transient pathway to that of other transient connections is discussed with respect to the development of the cortex.

An ultrastructural study of the synaptology of gamma-motoneurones during the postnatal development in the cat

The postnatal development of cat triceps surae gamma-motoneurones, retrogradely labelled with horseradish peroxidase (HRP), was studied light and electron microscopically. The mean diameter of the cell bodies of the gamma-motoneurones increased by about 25% from birth to the adult stage, which was much less than the increase in cell body diameter of alpha-motoneurones (about 45%). Throughout development the only bouton types apposing the gamma-motoneurones were the F- and S-types, with flattened and spherical synaptic vesicles, respectively. Thus, the C-, M- and T-types of boutons seen on a alpha-motoneurones. The number of boutons on the gamma-motoneurone cell bodies seemed to decrease postnatally. This decrease was only moderate for S-type boutons but substantial for F-type boutons. In contrast, the number of boutons on the proximal dendrites appeared to increase and this was most evident for S-type boutons. The mentioned postnatal changes in synaptology were more differentiated with regard to bouton type and part of the neurones under study than what could be inferred from earlier studies on the postnatal development of alpha-motoneurones. These changes also occurred later than in alpha-motoneurones. The relative dominance of F-type boutons with probable inhibitory actions on the immature gamma-motoneurone may explain the previously demonstrated poor encoding of muscle length by muscle spindles during the first postnatal weeks in the kitten.

An ultrastructural study of cat lumbosacral gamma-motoneurons after retrograde labelling with horseradish peroxidase

Twelve retrogradely horseradish peroxidase (HRP)-labelled triceps surae motoneurons of gamma size (mean cell body diameter less than 38 micron) were studied ultrastructurally. The contours of the cell bodies, as observed in the transverse midnucleolus plane, were elongated to rounded. The axons identified all originated from the cell body. The mean diameter of the stem dendrites was 4.5 micron. A substantial part of the cell membrane was covered by glial extensions. The boutons and synaptic contacts apposing the gamma-motoneurons could be classified into two categories on the basis of the type of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles. In each neuron, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S+F-type boutons were estimated on the cell body and in two dendritic compartments. In comparison with alpha-motoneurons and Renshaw cells, the cell bodies of the gamma-motoneurons were covered by smaller and strikingly fewer boutons of both the S- and F-types. The values for percentage synaptic covering and packing density of boutons on the proximal dendrites were also lower for gamma-motoneurons than for both alpha-motoneurons and Renshaw cells, although the differences were less pronounced than on the cell body. No boutons of the C-, M-, and T-types described for alpha-motoneurons were found on the gamma-motoneurons.

Early postnatal development of the monkey visual system. II. Elimination of retinogeniculate synapses

Profiles of retinal terminals, and of their synaptic and non-synaptic contacts, were measured in electron micrographs from magnocellular and parvocellular laminae of the dorsal lateral geniculate nucleus (LGNd) in newborn, 1-,4-,8- and 17-week-old rhesus monkeys. Morphologic criteria, i.e., the presence of pale mitochondria and large round vesicles, were used to identify the profile of retinal origin. Size-frequency histograms were stereologically reconstructed and used to calculate the density of retinal boutons and synaptic and non-synaptic plaques. The density values were adjusted for laminar growth to yield estimates of total numbers of these elements. Numerical estimates indicate bouton proliferation during the first week, followed by substantial reductions in bouton number accompanied by profound decreases in synapse number and cumulative synaptic area. In magnocellular layers, the reduction in synapse number is more pronounced after the eighth week, whereas the decrements in both features in the parvocellular laminae occur before this time. This synapse elimination process may be due entirely to retinal bouton retraction in parvocellular layers, but involves additional retinal synapse loss in the magnocellular segment. The parvocellular division shows a further size contraction of the remaining synapses. Immature synapses predominate in the LGNd throughout the 4-month period, and no quantitative evidence for direct transformation of immature to mature contacts is obtained. Non-synaptic junctions are stable in number but of increasing size in magnocellular layers, whereas substantial increases in number and area are found in parvocellular laminae. The preceding modifications in synaptic organization of the monkey LGNd occurring during the initial postnatal period may provide morphologic bases for the physiological and behavioral changes observed in this species during the same interval. Our data underscore the conclusion that synaptic reorganization occurs over a prolonged period, probably extending beyond 4 months, and involving the process of synapse elimination.

Postnatal changes in the termination pattern of recurrent axon collaterals of triceps surae alpha-motoneurons in the cat

alpha-Motoneurons innervating the triceps surae and short plantar muscles were stained intracellularly with horseradish peroxidase (HRP) in 0-44-day-old kittens and adult cats. The terminal arborizations of the recurrent axon collaterals in the spinal cord were studied in the light microscope (LM). The short plantar motoneurons lacked axon collaterals in all age groups. With a few exceptions in the youngest kittens (0-1 days of age), the projection field of the axon collaterals of triceps surae motoneurons did not change during development. The exceptional motoneurons had axon collaterals projecting ventromedial to the adult termination areas in Rexed's laminae VII and IX. Within all parts of the projection field, there was a substantial postnatal reduction in the number of axon collateral swellings, interpreted as synaptic terminals, and a total elimination of short and thin axonal processes without swellings. The findings are discussed in relation to earlier demonstrated loss of synaptic terminals on the motoneurons and elimination of polyneuronal innervation of muscle fibers postnatally.

Normal postnatal development of retinogeniculate axons and terminals and identification of inappropriately-located transient synapses: electron microscope studies of horseradish peroxidase-labelled retinal axons in the hamster

Axons from the eyes reach the dorsal lateral geniculate nucleus of the hamster at birth and both crossed and uncrossed axons spread throughout the nucleus within which they overlap extensively between postnatal days 2-6, before segregating to terminate in different parts of the nucleus by days 8-10 [So, Schneider and Frost (1978) Brain Res. 142, 343-352]. We have labelled retinal axons and their terminations between the day of birth (day 0) and day 6 by injecting one eye with horseradish peroxidase a few hours prior to sacrifice. Labelled profiles were then systematically sought, identified and their position determined, by electron microscope study of large frontal thin sections of both dorsal lateral geniculate nuclei. Labelled crossed and a few labelled uncrossed axons were present at day 0 and became progressively more common over the following few days; appropriately-located labelled uncrossed axons and terminals in the centromedial part of the nucleus (future ipsilateral sector) were relatively less common than labelled crossed axons in the ventrolateral part of the nucleus (part of the future contralateral sector), particularly between days 0 and 3. Synaptic contacts established by such labelled axons were characterized by predominantly electron-lucent spherical presynaptic vesicles and a prominent postsynaptic density. At day 4, labelled uncrossed axons made synaptic contact in the future contralateral sector (which is devoid of uncrossed input after days 8-10) and a few crossed axons made synaptic contacts in the future ipsilateral sector (devoid of crossed input after days 8-10). Such terminals and their synaptic contacts, were identical to appropriately-located ones in the same material. Inappropriately-located terminals were not found in the future contralateral sector at day 6, or in adults. No specialized contacts were observed between inappropriately-located axons or terminals and either other axon terminals or glial cell processes. Thus, during the development of the hamster retinogeniculate projection, inappropriately-located axons establish transient synaptic contacts with geniculate cells, and these contacts are lost as the segregated adult pattern of projections is established.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural and electrophysiological properties of accessory abducens nucleus motoneurones: an intracellular horseradish peroxidase study in the cat

The physiological and morphological (light and electron microscopy) properties of six retractor bulbi motoneurones were analysed using the technique of intracellular recording and intracellular labelling with horseradish peroxidase. The retractor bulbi motoneurones were identified by antidromic invasion and orthodromic responses following stimulation of trigeminal afferents were studied. Two of these motoneurones were examined ultrastructurally. Terminal boutons forming synapses with labelled soma, labelled proximal and distal dendrites were characterized. Serial sections allowed the axon hillock to be analyzed and the initial segment of a presumed motoneurone to be observed in the section where the injected motoneurone was described. The ultrastructure of unidentified elements observed in the accessory abducens nucleus is stressed.

A theory on the lability and stability of spinal motoneuron soma size and induction of synaptogenesis in the adult spinal cord

There exists a dynamic relationship between the soma size of a motoneuron and its motor unit size. Adult motoneuron soma size can be experimentally increased if the neuron is allowed to innervate more muscle fibers than it normally innervates. In postnatal mammals a transition from polyneuronal to mononeuronal innervation of limb muscle fibers occurs which is temporally related to a plastic change in the perikaryal size. This lability of postnatal motoneuron size is temporally related to growth of synaptic connections on the motoneuron. In adult mammal, regenerating motor axons polyneuronally innervate the muscle fibers for a transient period. This hypothesis proposes that a plastic change in soma size occurs in these adult motoneurons. This short-lived labile state may revive the embryonic properties and evoke growth of synaptic boutons. Experimentally induced labile state in motoneuron pools and spinal ganglion neurons in the adult mammal should offer a basis for the study of mechanisms of synaptogenesis in the spinal cord.

An ultrastructural study of serially sectioned Renshaw cells. III. Quantitative distribution of synaptic boutons

The quantitative distribution of synaptic boutons on 17 presumed Renshaw cells has been studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons and were located in lamina VII, ventromedially to the main motor nuclei. In each of the presumed Renshaw cells, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S + F-type boutons were estimated on the cell body and in two dendritic compartments. The F/S percentage synaptic covering ratio was also calculated. The previously demonstrated differences within the present group of neurons, with respect to the site of axonal origin, were not accompanied by any corresponding differences in the quantitative distribution of synaptic boutons. However, it is suggested that the presumed Renshaw cells may possibly fall into two categories with respect to the F/S percentage synaptic covering ratio. The results are discussed in relation to previous studies on the neuronal architecture and synaptic types on the same presumed Renshaw cells, as well as in relation to earlier observations on the quantitative distribution of boutons on central neurons, particularly spinal alpha-motoneurons.

Trophism between C-type axon terminals and thoracic motoneurones in the cat

1. Quantitative ultrastructural examinations of axon terminals synapsing with normal alpha-motoneurones in segment T9 of cat spinal cord provided estimates of their numbers, sizes and synaptic structure. One synapse, the C type, derived from short-axon propriospinal segmental interneurones, was studied in detail.2. The numbers, sizes and post-synaptic structure of normal C-type synapses at T9 were compared with similar estimates from material provided by cats subjected to partial central deafferentation by double spinal hemisection at T5 and T10 between 7 days and 2 years previously.3. The proportion of C-type synapses present progessively increased from 1% in normal cats to 8.8% 200 days following hemisection, and had still attained a level of 3.1% by 2 years; these increases imply that the absolute number of C-type synapses underwent increase.4. Mean sizes of C-type synapses increased from 4.0 mum (normal) to 5.8 mum (200 days) and retained their enlarged sizes up to 2 years (5.9 mum). Furthermore, while 84% of C-type synapses were under 6 mum in length in normal motoneurones, 48% were over 6 mum long 200 days post-operatively.5. The unique post-synaptic structure of C-type synapses also proliferated following partial central deafferentation of the motoneurones. The elongated cistern, increased numbers and individual lengths of lamellae of the associated underlying rough endoplasmic reticulum indicated a trophic interaction between the presynaptic C terminal and its post-synaptic motoneurone.6. Counts of ribosomes ;bound' to lamellae of the subsynaptic rough endoplasmic reticulum, and of the lamellae-associated polyribosomes interposed between individual lamellae for normal and 200 day post-operative C-type synapses indicated an over-all post-operative increase in capacity for local subsynaptic protein synthesis topographically directed towards this type of axon terminal.7. The observed greater increase in frequency of ribosomes ;bound' to the rough endoplasmic reticulum, together with an over-all proliferation of this structure, specificially indicated an increased capacity for synthesis of protein for utilization in sites remote from those of synthesis (e.g. a trans-synaptic passage of protein).8. A hypothesis is advanced on the basis of the above results relating both pre- and post-synaptic changes in structure to an increased functional activation of the segmental short-axon propriospinal interneurones forming the C-type synapses, as a compensatory response to partial central deafferentation of spinal motoneurones.

Evidence for a postnatal elimination of terminal arborizations and synaptic boutons of recurrent motor axon collaterals in the cat

Triceps surae alpha motoneurons in cats of different postnatal ages were stained intracellularly with horseradish peroxidase (HRP). The recurrent axon collateral trees of the neurons were studied light microscopically. A large reduction of the number of axon collateral end branches and swellings, interpreted as synaptic boutons, was found to occur during the first two weeks of postnatal life.

An ultrastructural study of serially sectioned Renshaw cells. II. Synaptic types

Boutons and synaptic contacts on 17 presumed Renshaw cells were studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps and surae alpha-motoneurons and located in lamina VII, ventromedially to the main motor nuclei. The boutons and synaptic contacts could be classified into two main categories on the basis of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles, the alpha-motoraxon collateral boutons falling into the S-category. In addition, some S-type boutons containing neurofilaments and some being apposed by small presynaptic boutons were observed. The results are discussed to earlier observations on the synaptology of central neurons, particularly spinal alpha-motoneurons.

Sequential alterations of neuronal architecture in nucleus magnocellularis of the developing chicken: a Golgi study

Nucleus magnocellularis in the chicken consists predominantly of a population of medium-sized cells which receive large, axosomatic endings from the auditory nerve. The morphological development of these cells and their auditory input were studied with the Golgi methods. At 7 1/2-9 days of incubation (embryonic days 7 1/2-9, staged according to the Hamburger-Hamilton series), cells in nucleus magnocellularis have several long, branched dendrites, which often end in bulbous swellings. By embryonic day 10, efferent axons have already grown out from the cells and characteristic terminal plexuses of these axons are seen in nucleus laminaris bilaterally. The dendrites of cells in nucleus magnocellularis have been replaced by a multitude of long somatic processes, giving the cell body a shaggy appearance. This arrangement is maintained up to embryonic day 15, when a remarkable second transformation occurs. The cells lose their somatic processes and present bald, round profiles. Around embryonic days 17-18 a primitive-looking process with a tip like a growth cone emerges from the cell body and somatic spines are evident. By days 19-20, one or two thin, frail dendritic processes can be seen. Correlated with this dramatic series of changes in the cells is a fixed sequence of transformations of the incoming axons. Around embryonic day 10, primary sensory axons in nucleus magnocellularis end in swellings resembling growth cones. Between days 11 and 13, following the explosive growth of somatic processes there is a corresponding expansion and ramification of the auditory nerve endings. On embryonic day 14, there is a condensation of the terminal axon branches, which now form a compact, highly branched plexus. Between days 16 and 17, the plexus coalesces into a calycine structure, now approaching its final form, the end-bulb of Held, which is achieved by embryonic days 19-20. The transformation of the plexus to the calycine form occurs around the same that the cell loses its somatic processes. The parallel sequence in the morphogenetic stages of the assembly of the end-bulbs and their target cells evinces a correlation, if not a causal relationship between the sensory axons and the developing neurons. The arrangement of the somatic processes and axonal branches during the early, multipolar stage would provide an opportunity for optimum interactions between the synaptogenetic processes of the afferent axons and the target cells. The later morphological transformations could orchestrate the specific, cell-to-cell interactions which accompany, or even depend on the activity of the definitive end-bulb synapse.

An ultrastructural study of serially sectioned Renshaw cells. I. Architecture of the cell body, axon hillock, initial axon segment and proximal dendrites

Seventeen neurons which were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons were studied ultrastructurally. All 17 neurons were situated in lamina VII, ventro-medially to the main motor nuclei. This and other facts support the assumption that the observed neurons are morphological correlates to the physiologically defined Renshaw cells. The contours of the cell bodies, as observed in the midnucleolus plane, were elongated. The axons originated either from the cell bodies or from dendrites. The number of dendrites of each neuron varied between 3 and 7. The appearance of the presumed Renshaw cells was also compared with that of a larger sample of neurons from the ventral part of lamina VII which was studied light microscopically in semithin sections. It was suggested that the Renshaw cells belong to the larger and more elongated neurons in the area.

Neurons of the basolateral amygdala: a Golgi study in the opossum (Didelphis virginiana)

The cytoarchitecture of the opossum basolateral amygdala was studied using Golgi techniques. The neuronal morphology was similar in all nuclei of the basolateral complex, an three distinct cell classes were recognized. Class I neurons, which vary in size in different nuclei, have spiny dendrites and long, projection axons. Axon hillocks and initial axonal segments often have spinous protrusions, while more distal portions of the axon give off several beaded collaterals that arborize primarily in the vicinity of the cell. Class II neurons are smaller, spine-sparse cells that are found in all nuclei of the basolateral amygdala but are greatly outnumbered by class I neurons. Axons branch and give off beaded collaterals which form a moderate to dense arborization within the dendritic field of the cell. Class II neurons exhibit considerable morphologic variability including one subtype that resembles the chandelier cell of the cerebral cortex. Varicosities (1.0 - 1.5 micrometers swellings) found along the axonal collaterals of these amygdaloid chandelier cells do not have a uniform distribution but tend to be aggregated. Segments of the collaterals displaying such clustered varicosities sometimes form nest-like entanglements. Clusters of varicosities have been observed forming multiple contacts with initial segments of class I axons. Class III neurons are neurogliaform cells which have many short, varicose dendritic branches that contact dendrites of class I neurons. Only the initial portions of their axons were impregnated. This study indicates that many of the cell types seen in the generalized, metatherian opossum are similar to those described in more specialized, placental mammals. This is the first description of amygdaloid chandelier cells and their contacts with the spiny initial segments of class I projection neurons.

An ultrastructural study of the synaptic contacts of alpha 1-motoneuron axon collaterals. II. Contacts in lamina VII

Horseradish peroxidase (HRP) was injected intracellularly in triceps surae alpha-motoneurons. The axons and axon collaterals of these neurons were traced light and electron microscopically. Synaptic boutons of collaterals in the ventral part of Rexed's lamina VII were studied ultrastructurally. The boutons exhibited spherical synaptic vesicles and made synaptic contacts with cell bodies and proximal dendrites of neurons assumed to be Renshaw cells and with dendrites of unknown origin. The observations are discussed in relation to earlier qualitative and quantitative studies on the other known synaptic contacts of the alpha-motor axons, both in the central and peripheral nervous system.

An ultrastructural study of the synaptic contacts of alpha-motoneurone axon collaterals. I. Contacts in lamina IX and with identified alpha-motoneurone dendrites in lamina VII

Horseradish peroxidase (HRP) was injected intracellularly in triceps surae alpha-motoneurones. The axons and axon collaterals of these neurones were traced light and electron microscopically. Synaptic boutons of collaterals, in Rexed's lamina IX or in synaptic contact with HRP-stained motoneurone dendrites in lamina VII, were studied ultrastructurally. The boutons exhibited spherical synaptic vesicles and made synaptic contacts of two different types with HRP-stained alpha-motoneurone dendrites in lamina IX and VII, dendrites and cell bodies of large neurones in lamina IX, dendrites of unknown origin in lamina IX and with one cell body of a medium size neurone in lamina IX. The observations are discussed in relation to earlier qualitative and quantitative studies on the synaptology of cat spinal alpha-motoneurones.

The axon initial segment as a synaptic site: ultrastructure and synaptology of the initial segment of the pyramidal cell in the rat hippocampus (CA3 region)

The axon initial segments (ISs) of pyramidal cells in the rat hippocampus (CA3 region) were studied by means of light microscopy of Golgi-impregnated material and electron microscopy of random and serial thin sections. The ISs display three distinguishing characteristics; fascicles of microtubules, membrane undercoating and clusters of ribosomes. The ISs contain cisternal organelles which are often associated with synapses and are in continuity with smooth and rough endoplasmic reticulum. Small spines are recognized on the ISs both in the light and electron microscope. There are 10-25 on each IS and they are usually concentrated on the proximal 30 micrometers of the IS. Axonic spines contain spine apparatuses, clusters of ribosomes, multivesicular bodies and other organelles. Several collaterals are also recognized to originate from the axon proximal to the start of a myelin sheath. The IS receives many synapses both on its shaft and spines. Almost all of them are of the symmetrical type with flattened vesicles but a few asymmetrical synapses with spherical vesicles occur. Pyramidal cell ISs are very rarely presynaptic at asymmetrical synapses with spherical vesicles. Based on serial sectioning studies, the number of synapses on one IS is estimated at 100-200. These abundant synaptic contacts on the IS suggest that it is an important synaptic site. The possibility that there are two different inhibitory systems controlling the output of the pyramidal cell is discussed.

Elimination of synapses in the developing nervous system

Reduction of the number of axons that contact target cells may be a general feature of neural development. This process may underlie the progressively restricted malleability of the maturing nervous system.

Presence of the ruffed cell in the olfactory bulb of the catfish, Parasilurus asotus, and the sea eel, Conger myriaster

The ruffed cell is present in the olfactory bulb of the catfish, Parasilurus asotus, and of the sea eel, Conger myriaster. Its morphological features have been studied by light microscopy, high-voltage electron microscopy, and conventional electron microscopy. The ruffed cell of the catfish is very similar to that of the goldfish in its location and in its structural features. It has a spheroidal or ovoid cell body about 15 to 30 micrometer in diameter. Several dendrites arise from the cell body to form a rounded dendritic field about 100 micrometer in diameter near the cell body. The initial unmyelinated portion of its axon (IP) consists of a shaft and many protrusions arising from it. The shaft, about 1 micrometers in diameter, located extends for about 100 to 200 micrometer, where it acquires a myelin sheath. The protrusions intermingle with one another in a complex manner to form a rather discrete spheroidal field about 20 to 50 micrometers in diameter, located in the vicinity of the cell body. In contrast, the ruffed cell of the sea eel differs rather significantly from that of the goldfish in its morphological features. The ruffed cell of the sea eel is of a bipolar type. One thick dendrite arises from the cell body and extends for about 50 to 100 micrometers, where it gives rise to many thread-like dendritic branches. The IP arises from the cell body as a smooth thin process. However, about 30 to 70 micrometers distant from its origin, many elaborate protrusions arise from the axonal shaft. These intermingle with one another to form a spheroidal or ovoid field about 20 to 40 micrometers in diameter. Distal to this protrusion-bearing part, the axon continues as a smooth, thin process. In spite of these differences in structural features, the ruffed cell of the catfish and that of the sea eel are very similar in their synaptic features in the nest (the special synaptic field around the ruffed cell IP, composed of its protrusions, of granule cell dendrites, and of other neuronal processes); that is, synapses between the ruffed cell IP (its shaft and protrusions) and granule cell dendrites and serial synapses made by the ruffed cell IP, granule cell dendrites, and perinest cell dendrites. These results suggest that the ruffed cell is generally present in the teleostean olfactory bulb, although its detailed structural features may vary from species to species. Moreover, the neuronal organization of the olfactory bulb seems to be fundamentally similar in various species of teleosts.

Relations between cell body size, axon diameter and axon conduction velocity of triceps surae alpha montoneurons during the postnatal development in the cat

Triceps surae alpha-motoneurons in cats of different postnatal ages were stained intracellularly with horseradish peroxidase (HRP) and studied light microscopically. In individual neurons, the mean diameter of the cell body and the intramedullary axon diameter were measured and related to the axon conduction velocity. The mean diameter of the cell body grew from 39.6 micrometer at birth to 57.6 micrometer in the adult cat, while the corresponding figures for the intramedullary axon diameters were 2.4 micrometer and 6.7 micrometer. During the same period of time, the axon conduction velocity increased from 11.3 m/s to 93.5 m/s, and the ratio between the conduction velocity and the intramedullary diameter of the axon (CV/d ratio) increased from 4.6 to 14.1. The results indicate that the growth of the cell body is smaller and completed earlier than the growth in diameter of the intramedullary and, in particular, the peripheral parts of the axon. The considerable change of the CV/d ratio during the postnatal development may be explained by previously described immature morphological properties of the axons in very kittens, and by a changing relation between the dimensions of the intramedullary and peripheral parts of the axon.

Development of neocortical circuitry: quantitative ultrastructural analysis of putative monoaminergic synapses

The distribution, numbers and morphology of presumed monoaminergic (MA) synapses were examined in somatosensory cortex of neonatal rats and mice (newborn to 16 days of age). MA synapses were identified using an ultrastructural cytochemical marker, 5-hydroxydopamine (5-OHDA), which results in the appearance of small granular vesicles (SGV) in their presynaptic terminals. From birth to 7 days of age, 20--30% of all synapses sampled in somatosensory cortex contain SGVs. However, few SGV synapses are seen in 8-day-old cortex and by 12 days of age, SGVs are no longer detectable in cortex. A specific laminar distribution for these SGV synapses -- which is distinct from the overall synaptic distribution -- is first seen at 3 days of age and is essentially unchanged until 7 days postnatally. During this entire period, the SGV synapses predominate in the primordium of layer IV, where they account for 50--70% of all synapses. Morphometric analysis of SGV synapses indicates that there are differences in junctional symmetry, vesicle shape and configuration of the contact zone between SGV and non-SGV synapses, as well as between SGV synapses themselves in the various cortical layers. The laminar distribution and morphological characteristics of SGV synapses suggest that the MA projection to neocortex exhibits a high degree of spatial specificity during its ingrowth. Also, the relatively high proportion of SGV synapses in the first postnatal week may reflect a potent influence exerted by the MA inputs on immature neocortex. The decreased numerical density of SGV synapses after 7 days of age is probably due to the development of the blood-brain barrier to 5-OHDA.

Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus) I. Golgi impregnation and serial thin sectioning studies

A new type of neuron was recognized in the olfactory bulb of the goldfish (Carassius auratus) by means of light and high voltage electron microscopy of Golgi-impregnated material, combined Golgi-electron microscopy, and electron microscopy of serial thin sections. The neuron is located in the layer between the olfactory nerve layer and the anterior olfactory nucleus. It has a spherical cell body, about 10--20 microns in diameter, and several dendrites which form a spherical dendritic field, about 70--100 microns in diameter, in the vicinity of the cell body. The most remarkable structural feature of this neuron is that its initial unmyelinated portion of the axon (IP) has elaborate protrusions with many synapses. The IP can be divided into three parts, parts 1, 2 and 3, based on its structural features. Part 1 is the initial part of the IP, about 20--40 microns in length. Many elaborate protrusions arise from the shaft and intermingle with one another to constitute a spherical field, about 20--40 microns in diameter, around the shaft. Part 2 is the middle part of the IP, about 10--20 microns in length. There are several collateral-like protrusions, which are scattered along the shaft and extended laterally about 5--15 microns. Part 3 is the last part of the IP, and is cylindrical without protrusions. The length of part 3 varies from 20 to more than 100 microns. The axon acquires a myelin sheath at distance of 70--250 microns from its origin. Protrusions make synaptic contacts mainly with granule cell dendrites. Some of them are of the reciprocal type. Protrusion are presynaptic in asymmetrical synapses, and postsynaptic in symmetrical synapses with granule cell dendrites. The shaft of the IP also has synapses similar to those on protrusions. The neuron described is a new type of neuron in the vertebrate central nervous system. We propose for it the name "ruffed cell."

Regional differences in the timing of synapse elimination in skeletal muscles of the neonatal rabbit

The time course of disappearance of polyneuronal innervation during development was studied electrophysiologically in skeletal muscles of the rabbit. There were differences of up to a week in the estimated time of onset of synapse elimination in various muscles, with a tendency for the process to begin earlier in muscles situated more anteriorly in the body. There were also differences among muscles in the peak rate of synapse loss during maturation, but these differences do not appear to be related to position in the body or to contractile properties of the muscle.

Ultrastructure and quantitative synaptology of the sacral parasympathetic nucleus

This study examines the anatomical substrate for the spinal micturition reflex. Light microscopy of pyridine silver-stained sections revealed that the sacral parasympathetic nucleus (SPN) exists as a broken column or chain of cell clusters located along the intermediolateral portion of the dorsal horn in sacral segments S2-S4. Quantitative analysis of neuropil components in electron micrographs provides data for each type of bouton identified in this nucleus. On the somata of these neurons, boutons containing clear spherical vesicles (S type) comprise 70% of the bouton population. Terminals containing three or more dense core vesicles (GS boutons) account for 26% and boutons containing flattened vesicles (F boutons) comprise 4% of the population. F boutons are more common on large dendrites where they comprise 10% of the total bouton population. The actual population density of each bouton type is most evident when the number of boutons is expressed as boutons per 100 micron of membrane length (btn/100 micron). S type boutons are the most frequently encountered type. The population density of S boutons is the same on soma and dendrites at 6.66 btn/100 micron. F boutons are more numerous on large (greater than 2 micron) dendrites (1.28 btn/100 micron) than on small dendrites (0.63 btn/100 micron) or on somata (0.36 btn/100 micron). GS boutons occur more frequently on small dendrites (3.66 btn/100 micron) than on somata (2.29 btn/100 micron), large dendrites (2.88 btn/100 micron) or medium dendrites (2.27 btn/100 micron). These data suggest that the dense core vesicle-containing boutons are applied primarily to small (less than 1 micron) dendrites and that F boutons are associated mostly with large or proximal dendrites. These results provide a quantitative profile of the synaptic input to the sacral autonomic (parasympathetic) neurons which innervate the urinary bladder and demonstrate specific population differences on various postsynaptic structures in this nucleus.

Spontaneous phagocytosis of C-type synaptic terminals by spinal alpha-motoneurons in newborn kittens. An electron microscopic study

The cell bodies and proximal dendrites of spinal alpha-motoneurons were studied electron microscopically with the aid of serial sections during the first postnatal week in the cat. The observations suggested that some of the synaptic terminals of the so-called C-type on the cell bodies and dendrites are phagocytosed by the motoneurons during the first few days after birth. This finding is discussed in relation to the earlier demonstrated postnatal loss of synaptic terminals on the motoneurons after birth and differences demonstrated between different functional types of spinal motoneurons with respect to the number and distribution of C-type terminals in the adult cat.