Initial endocytosis of perioxidase or ferritin by growth cones of cultured nerve cells

ESCRT-II controls retinal axon growth by regulating DCC receptor levels and local protein synthesis

Endocytosis and local protein synthesis (LPS) act coordinately to mediate the chemotropic responses of axons, but the link between these two processes is poorly understood. The endosomal sorting complex required for transport (ESCRT) is a key regulator of cargo sorting in the endocytic pathway, and here we have investigated the role of ESCRT-II, a critical ESCRT component, in Xenopus retinal ganglion cell (RGC) axons. We show that ESCRT-II is present in RGC axonal growth cones (GCs) where it co-localizes with endocytic vesicle GTPases and, unexpectedly, with the Netrin-1 receptor, deleted in colorectal cancer (DCC). ESCRT-II knockdown (KD) decreases endocytosis and, strikingly, reduces DCC in GCs and leads to axon growth and guidance defects. ESCRT-II-depleted axons fail to turn in response to a Netrin-1 gradient in vitro and many axons fail to exit the eye in vivo. These defects, similar to Netrin-1/DCC loss-of-function phenotypes, can be rescued in whole (in vitro) or in part (in vivo) by expressing DCC. In addition, ESCRT-II KD impairs LPS in GCs and live imaging reveals that ESCRT-II transports mRNAs in axons. Collectively, our results show that the ESCRT-II-mediated endocytic pathway regulates both DCC and LPS in the axonal compartment and suggest that ESCRT-II aids gradient sensing in GCs by coupling endocytosis to LPS.

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Mobility and cycling of synaptic protein-containing vesicles in axonal growth cone filopodia

The spatial distribution and coordination of vesicular dynamics within growth cones are poorly understood. It has long been thought that membranous organelles are concentrated in the central regions of growth cones and excluded from filopodia; this view has dramatically shaped conceptual models of the cellular mechanisms of axonal growth and presynaptic terminal formation. To begin to test these models, we studied membrane dynamics within axonal growth cones of living rat cortical neurons. We demonstrate that growth cone filopodia contain vesicles that transport synaptic vesicle proteins bidirectionally along filopodia and fuse with the filopodial surface in response to focal stimulation, allowing for both local secretion of vesicular contents and rapid changes in the plasma membrane composition of individual filopodia. Our results suggest a new model in which growth cone filopodia are actively involved in both emitting and responding to local signals related to axon growth and early synapse formation.

TUC-4b, a novel TUC family variant, regulates neurite outgrowth and associates with vesicles in the growth cone

The TUC (TOAD-64/Ulip/CRMP) proteins are homologs of UNC-33, a protein that is required for axon extension and guidance in Caenorhabditis elegans. The TUC proteins are expressed in newly born neurons in the developing nervous system and have been implicated in semaphorin signaling and neuronal polarity. Here, we identify several new variants of the TUC family, each of which is expressed during distinct periods of neural development. We cloned and characterized TUC-4b, a variant of TUC-4a that includes a unique N-terminal extension. The functional relevance of this N-terminal domain is demonstrated by the finding that overexpression of TUC-4b, but not TUC-4a, results in increased neurite length and branching. Furthermore, whereas TUC-4a is expressed throughout life, TUC-4b is expressed exclusively during embryonic development. TUC-4b is localized to SV2 (synaptic vesicle protein 2)-positive vesicles in the central domain of the growth cone, suggesting a potential role in growth cone vesicle transport. Furthermore, TUC-4b interacts with the SH3A (Src homology 3A) domain of intersectin, an endocytic-exocytic adaptor protein. Together, these data suggest that TUC-4b can regulate neurite extension and branching through a mechanism that may involve membrane transport in the growth cone.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

Extracellularly applied horseradish peroxidase increases the number of dense core vesicles in leech sensory neurons

The uptake of horseradish peroxidase (HRP), applied as an extracellular tracer, is a classical method for studying endo/exocytosis of synaptic vesicles at the ultrastructural level. It is generally not considered that HRP may affect neuronal function. Reported here is the finding that extracellularly applied HRP (0.1%) perturbs dense core vesicles in the synaptic processes of leech neurons. The strength of the effect varies with neuronal class. In sensory afferents, the number of dense core vesicles increases 5-fold, while there is only a 2-fold increase in central neurons.

Membrane recycling in the neuronal growth cone revealed by FM1-43 labeling

Membrane dynamics within the chick ciliary neuronal growth cone were investigated by using the membrane-impermeant dye FM1-43. A depolarization-evoked endocytosis was observed that shared many properties with the synaptic vesicle recycling previously described at the presynaptic terminal. In addition, in the absence of depolarization a basal level of constitutive endocytotic activity was observed at approximately 30% of the rate of evoked endocytosis. This constitutive endocytosis accounted for large amounts of membrane retrieval: the equivalent of the entire growth cone surface area could be internalized within a 30 min period. Endosomes generated via constitutive and evoked processes were highly mobile and could move considerable distances both within the growth cone and out to the neurite. In addition to their different requirements for formation, evoked and constitutive endosomes displayed a significant difference in release properties. After a subsequent depolarization of labeled growth cones, evoked endosomes were released although constitutive endosomes were not released. Furthermore, treatment with latrotoxin released evoked endosomes, but not constitutive endosomes. Although the properties of evoked endosomes are highly reminiscent of synaptic vesicles, constitutive endosomes appear to be a separate pool resulting from a distinct and highly active process within the neuronal growth cone.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

Calcium phosphate-mediated transfection of primary cultured brain neurons using GFP expression as a marker: application for single neuron electrophysiology

We investigated the efficiency of transfecting primary cultured rat postnatal brain neurons (substantia nigra pars compacta neurons and locus coeruleus neurons) with cDNA encoding GFP (jellyfish green fluorescent protein) using a calcium phosphate method. The proportion of transfected neurons (transfection efficiency) was approximately 5%, when cultures from the substantia nigra pars compacta were transfected 3 days after plating. The transfection efficiency decreased when cultures were transfected 10 days after plating (1.7%). Neurons were cotransfected at a very high probability ( > 78%) with the muscarinic m2-receptor cDNAs together with GFP plasmids. Transfected neurons were very healthy as indicated by the zero-current potential and the microscopical appearance. Because the transfection efficiency is low, this method cannot be used for experiments involving the whole cell population. The transfection efficiency of 1.7% corresponded to approximately 20 transfected cells per dish in our culture conditions and these cells are sufficient in number for electrophysiological studies. Therefore, this is an excellent method for studying the influence of exogenous genes on single neurons using electrophysiological techniques.

Dynamics of axonal microtubules regulate the topology of new membrane insertion into the growing neurites

Nerve growth depends on the delivery of cell body-synthesized material to the growing neuronal processes. The cellular mechanisms that determine the topology of new membrane addition to the axon are not known. Here we describe a technique to visualize the transport and sites of exocytosis of cell body- derived vesicles in growing axons. We found that in Xenopus embryo neurons in culture, cell body-derived vesicles were rapidly transported all the way down to the growth cone region, where they fused with the plasma membrane. Suppression of microtubule (MT) dynamic instability did not interfere with the delivery of new membrane material to the growth cone region; however, the insertion of vesicles into the plasma membrane was dramatically inhibited. Local disassembly of MTs by focal application of nocodazole to the middle axonal segment resulted in the addition of new membrane at the site of drug application. Our results suggest that the local destabilization of axonal MTs is necessary and sufficient for the delivery of membrane material to specific neuronal sites.

A quantifiable model of axonal regeneration in the demyelinated adult rat spinal cord

Strategies to increase the extent of axonal regeneration in the adult CNS must address an array of intrinsic and environmental factors which influence neuritic outgrowth. In order to develop an in vivo model of axonal regeneration in which potential therapies may be assessed, we have quantified growth cones within demyelinated regions in the dorsal funiculus of the spinal cord, following a discrete axotomy. Demyelinated lesions were produced by the intraspinal injection of galactocerebroside antibodies plus serum complement proteins. Axonal integrity was not compromised by the demyelination protocol. Axonal injury was induced at the caudal extent of the demyelinated region using a micromanipulator-controlled Scouten knife. The severity of axonal injury was varied in different animals at the time of surgery and was quantified 8 days later by counting degenerate axons in transverse 1-microm resin sections. Evidence of axonal regeneration within these animals was assessed by an electron microscopic analysis of growth cone frequency and position relative to the site of axotomy. Growth cones were identified within the region of demyelination only; no growth cones were identified within the dorsal column white matter adjacent to the demyelinated region, or rostral or caudal to the region of demyelination, or in animals with an injury but no demyelination. Quantification of growth cones within regions of demyelination indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of axons severed. These findings indicate that demyelination facilitates axonal regeneration in the adult rat CNS and illustrate a quantifiable method of assessing axonal regeneration.

Dynamic organization of endocytic pathways in axons of cultured sympathetic neurons

Despite the wealth of information about endocytic pathways in non-neuronal cells, little is known about these crucial sorting, recycling, and degradative pathways in neurons. In this report, we analyzed in detail the dynamic steady-state organization of endocytically derived organelles as they progress through the endosomal-lysosomal pathway in axons of live cultured sympathetic neurons. By ratiometric imaging of neurons endocytically labeled with the pH indicator 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), we demonstrate a trimodal frequency distribution of endocytic organelle pH in axon shafts, indicating two rapid acidification steps in the progression from endocytosis to the lysosome. Axonal branch points display a unimodal organelle pH distribution (mean 6.4), implicating them as meeting places for endocytic organelles and Golgi-derived vesicles or as sorting sites. By following endocytic organelle traffic retrogradely from growth cone to soma, we identified significant transition points in the pathway. Growth cones exhibit a unimodal pH distribution comprised mainly of acidified recycling/sorting endosomes (mean 6.3). However, organelles in the axon shaft immediately adjacent to the growth cone display the distinct trimodal pH distribution of the axon, suggesting that important sorting events occur between these domains. An abrupt increase in organelle acidification occurs in the distal axon 50-150 microns from the growth cone, demonstrating a discontinuous spatial gradient of acidification along axons. Immunofluorescence microscopy reveals that the lysosomal glycoprotein LEP100 is present in axons and is concentrated in two important regions: the proximal axon where the endocytic organelle population is largely acidified, and the same region of the distal axon where substantial acidification occurs.

The alteration of ultrastructure and immunoreactivity of human embryonic organ of Corti tissue culture after exposure to aminoglycoside (neomycin) ototoxicity

Using electron microscopy and GABA immunohistochemistry we evaluated the effects on human embyronic organ of Corti tissue culture of exposure to the ototoxic aminoglycoside antibiotic neomycin at a dose of 1 mM for 48 to 96 hrs. Neomycin induces the formation of multilamellar myeloid structures. These lesions, found only in the basal coil but both in inner and outer hair cells, were characteristic of the membrane-associated neomycin-induced damage. A large amount of lipofucsin and numerous lipoid vacuoles as well as vesicle-filled mound-like protrusions were also observed after exposure to neomycin. It seems there is no obvious effect on GABAergic innervation.

Acceleration of membrane recycling by axotomy of cultured aplysia neurons

The rapid transition of a stationary axon into a motile growth cone requires the recruitment of membrane and its strategic insertion into the neurolemma. The source of membrane to support the initial rapid growth postaxotomy is not known. Using membrane capacitance measurements, we examined quantitative aspects of membrane dynamics following axotomy of cultured Aplysia neurons. Axotomy activates two processes in parallel: membrane retrieval and exocytosis. Unexpectedly, membrane retrieval is the dominant process in the majority of the experiments. Thus, while a growth cone is vigorously extending, the total neuronal surface area decreases. We suggest that the initial rapid extension phase of the newly formed growth cone postaxotomy is supported by a pool of intracellular membrane that is rapidly retrieved from the neurolemma.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Slice cultures of the imprinting-relevant forebrain area MNH of the domestic chick: quantitative characterization of neuronal morphology

The persistence of morphological features of neurons in slice cultures of the imprinting-relevant forebrain area MNH (mediorostral neostriatum and hyperstriatum ventrale) of the domestic chick was analysed at 7, 14, 21 and 28 days in vitro. After having been explanted and kept in culture the neurons in vitro have larger soma areas, longer and more extensively branched dendritic trees and lower spine frequencies compared to the neurons in vivo. During the analyzed culturing period, the parameters soma area, total and mean dendritic length, number of dendrites, number of dendritic nodes per dendrite and per neuron as well as the spine densities in different dendritic segments showed no significant differences between early and late periods. Highly correlated in every age group were the total dendritic length and the number of dendritic nodes per neuron, indicating regular ramification during dendritic growth. Since these morphological parameters remain stable during the first 4 weeks in vitro, this culture system may provide a suitable model to investigate experimentally induced morphological changes.

Gene delivery to spinal motor neurons

This study demonstrates the direct delivery of plasmid gene constructs into spinal motor neurons utilizing retrograde axoplasmic transport. The plasmid vectors contained the Lac Z gene under the control of both the Rous sarcoma virus (RSV) and Simian virus (SV)40 promoters. beta-Galactosidase expression was observed in alpha and gamma motor neurons by histochemical staining following direct injection into the sciatic nerve or gastrocnemius muscle. The presence of LacZ gene constructs was confirmed by the polymerase chain reaction (PCR). The ability to introduce gene constructs into motor neurons allows for the study of gene regulation and permits the development of gene therapy strategies for motor neuron diseases including the spinal muscular atrophies (SMA) and amyotrophic lateral sclerosis (ALS).

Sites of plasmalemmal expansion in growth cones

Studies on plasmalemmal expansion in isolated nerve growth cones identified large, clear vesicles characteristically found in growth cones as the plasmalemmal precursor. The present article examines these plasmalemmal precursor vesicles (PPVs) in greater detail in the intact cell. (a) Pulse-chase experiments with the phospholipid precursor, [3H]glycerol, followed by radioautographic analysis show that PPVs in distal neurites and growth cones are labeled prior to equilibration of the label with the plasmalemma. (b) Pulse-chase experiments with lectin-ferritin conjugates demonstrate that PPVs are not endocytotic, that they contain lectin receptors, and that, during growth, patches of lectin receptors appear on the plasmalemma covering PPV clusters. (c) Freeze-fracture studies show that this plasmalemma shares with PPVs a paucity of intramembrane particles. (d) Lectin labeling experiments and freeze-fracture analysis demonstrate, furthermore, that the plasmalemma forms a network of invaginations at the base of PPV clusters. (e) Correlative studies indicate that the refractive 'vacuoles' seen in growth cones by phase-contrast light microscopy correspond to the PPV clusters seen at the ultrastructural level. These results confirm the identity of the plasmalemmal precursor in the intact cell and demonstrate that PPV clusters form distinct, dynamic organelles specialized for plasmalemmal expansion in the growth cone.

Differences in growth of neurons from normal and regenerated teleost spinal cord in vitro

Explants and dissociated cells from normal adult spinal cord and regenerating cord of the teleost Apteronotus albifrons were grown in vitro for periods of 8 to 12 wk. During this time the neurons showed extensive neurite outgrowth. Neurite outgrowth from tissue explants and dissociated cells of regenerated spinal cord starts sooner and is more profuse than that from normal (unregenerated) cord. Neurite outgrowth is maximized by using adhesive substrata and a high density of explants or dissociated cells. Inasmuch as Apteronotus does regenerate its spinal cord naturally after injury, whereas mammals do not, this culture system will be useful to study factors that control (permit) regeneration of spinal neurons in this adult vertebrate.

Effect of monensin on the neuronal ultrastructure and endocytic pathway of macromolecules in cultured brain neurons

1. The endocytic pathway of horseradish peroxidase (HRP) was investigated in the perikarya of cultured neurons by electron microscopy and enzyme cytochemistry. The tracer was observed in endocytic pits and vesicles, endosomes, multivesicular bodies, and lysosomes. It took approximate 15 min for the transfer of HRP from the exterior of the cell to the lysosomes. 2. Monensin induced distension of the Golgi apparatus and formation of intracellular vacuoles. When neurons were incubated with both monensin and HRP for 30 to 120 min, the number of HRP-labeled endosomes was greater than that in the monensin-free group, whereas the reverse was seen for HRP-positive lysosomes. The formation of HRP-positive lysosomes in monensin-treated cells was blocked by 47 to 79%. 3. These results indicate that the intracellular transport of the endocytosed macromolecule is pH dependent. It is also possible that the export of lysosomal enzymes is inhibited by monensin, resulting in an accumulation of the endosomes and a reduction of the lysosomes.

B-50/GAP43 localization on membranes of putative transport vesicles in the cell body, neurites and growth cones of cultured hippocampal neurons

We conducted an electron microscopic immunocytochemical study to locate B-50/GAP43 in various cellular elements of hippocampal neurons grown in dissociated cell culture. B-50 was detected with pre- and post-embedding immunoincubation, using affinity-purified B-50 antibodies and secondary antibodies coated on gold probes. For the first time ultrastructural evidence is presented for the location of B-50 on the membranes of electron-lucent transport vesicles with a diameter of 99.4 +/- 2.9 nm, present in the trans region of the Golgi apparatus, in neurites and in growth cones of cultured hippocampal neurons.

Weak-base amines inhibit the anterograde-to-retrograde conversion of axonally transported vesicles in nerve terminals

Acidotropic weak-base amines were used to investigate the role of acidic compartments in the pathway of aterograde-to-retrograde conversion of axonally transported vesicles in axon terminals. A local concentrated population of nascent axon tips was produced by transecting the rat sciatic nerve in situ to allow local and direct exposure of the axon tips to test solutions. Immersion of the nascent axon tips in solutions containing 10 mM ammonium choloride or 10 mM propylamine caused the axon tips to become distended by an accumulation of elongated membranous tubules and occasional large vacuoles that were both distinct from retrograde organelles. To test whether this accumulation was the result of an impairment of anterograde-to-retrograde conversion, a radioactive pulse-labelling method was used together with a retrograde collection ligature, to quantify the proportion of anterogradely transported proteins that returned from the axon tips by retrograde transport. Exposure of the axon tips to 10 mM ammonium chloride caused the anterogradely transported membrane proteins to accumulate in the axon tips and reduced by about 50% the amount of protein that returned to the retrograde collection ligature. These observations implicate the involvement of acidic membranous compartments in the anterograde-to-retrograde conversion pathway that leads to the formation of retrograde organelles in axon tips. Exposure of nerves to Acridine Orange, which is a vital acidotropic fluorescent dye, confirmed the presence of acidic compartments in the axon tips. Based on these observations, we propose that the membranous tubules that accumulated in the axon tips in the presence of weak-base amines represent a transient intermediate in the pathway of anterograde-to-retrograde conversion of axonally transported vesicles in axon terminals, and that acidic membranous compartments within axon terminals are required for the conversion of these tubules into retrograde organelles.

Myotome and early neurogenesis in chick embryos

The present study was undertaken in order to verify the identification of profiles of presumptive growth cones in vivo. The developing spinal nerves of chick embryos were studied by light and electron microscopy. We traced the onset of efferent and afferent innervation of the myotome in 2- to 4-day-old chick embryos in order to be sure that we were examining the growing tips of axons. In the process of studying these growing axons, we were able to observe some unique relationships of neural tube, myotome, and differentiating spinal nerves. The neural tube tightly abuts the myotome in Hamburger and Hamilton's (HH) stage 14 chick embryos and cytoplasmic projections from the myotome directly abut the neural tube. The first ventral roots could be identified in HH stage 15 embryos and dorsal roots in HH stage 16 embryos, both under 2 1/2 days of age. The advancing spinal nerve courses toward the anterior or cranial half of the myotome, and growth cones directly contact the medial wall of the myotome. The spinal nerves continue to abut tightly the myotome during the succeeding day of embryonic life, and growth cones enter the substance of the myotome by 3 days, or HH stage 19 embryos. These dorsolaterally directed axons will form the dorsal ramus of the spinal nerves and the ventral ramus continues to be contiguous with the myotome. Invasion of the myotome by axons (putative innervation), and thus innervation of myotomal cells in the 3-day chick embryos, was a totally unexpected finding. The myotome and its potential derivatives thus have extensive neural contact by 3 days of embryonic life in the chick. These findings document a parallel differentiation of afferent and efferent elements of the nervous system and confirm previous accounts identifying growth cones in an intact organism. These findings suggest that afferent as well as efferent nerves may have critical roles in the differentiation of the mesodermal as well as ectodermal derivatives.

The response of the cerebral hemisphere of the rat to injury. I. The mature rat

The response to injury of the cerebrum of the mature rat was studied chronologically in stereotactically placed knife wounds by using both light and electron microscopical, and immunohistochemical, techniques. Immediately after injury haematogenous cells fill the lesion and ischaemic necrosis occurs along the margins and a zone of cell swelling occupies the surrounding area. This phase is transformed by the appearance of large numbers of macrophages and fibroblasts, and some reactive astrocytes in the zone of cell swelling at 4 days. Blood vessels grow into the lesion at this time. Collagen deposition begins in the subpial region of the wound and, with time, scarring progresses into the deeper parts of the wound. By 8 days, the lesion contains a matrix of collagen fibrils, capillaries, fibroblasts, macrophages and astrocytes. The wound margins are better defined as astrocytes become aligned and secrete the basement membrane of the glia limitans, initially in the subpial regions of the scar. By 16 days, a glia limitans is complete along the margins of the entire lesion and the scar tissue between is reduced in area and contain fibroblasts, scattered macrophages, collagen fibrils and a few extra- parenchymatous astrocytes. Subsequently the scar condenses to a thin layer and becomes less vascularized; few cells remain. The persistence of astrocytes within mesenchymatous scar tissue excluded from the cerebral neuropile is a new finding. No further changes are seen in the scar after 30 days. The progressive development and maturation of scar tissue from the pial surface of the wound into the deeper regions of the cerebrum suggests that the major source of fibroblasts is from the meninges. The appearance of macrophages before fibroblasts in the wound may indicate that macrophages secrete a substance that is a trophic stimulus for fibroblasts. The organization of a glia limitans by astrocytes also proceeds inwards from the pial surface. Within the neuropile, degeneration of damaged neural elements is the prominent feature in the first 8 days after injury. Macrophages and reactive astrocytes also appear among the debris and are numerous by 4 days at the junctions of viable and necrotic neuropile. Signs of a regenerative response of neural processes is first seen at 4 days as growth cones appear in the viable neuropile at the edges of the necrotic zone. Growth cones are most numerous at 8 days. Evidence for new synapse formation is seen over the surface of dendritic swellings from 16 days onwards. Synapses of varying maturity are present, the most mature are adjacent to the dendritic shaft. This observation may suggest-that these swellings are true growth cones, in which case, this new synaptogenesis is similar to that over dendritic growth cones during development. It is not possible to judge the relative importance of either collateral sprouting or true regeneration in the reorganization of connections after injury, but neurite growth and the associated synaptogenesis described here could contribute to the recovery process. If the swellings on dendritic processes are true growth cones, then this is evidence for the regeneration of dendritic processes.

Peripheral nerve regeneration

Schwann cell basal laminae were demonstrated to serve as efficient conduits for the growth of regenerating axons in frozen nerve grafts, and in in situ freezing experiments. Regenerating axonal sprouts usually emanated from the first node of Ranvier proximal to the site of damage, and grew out along the inner surface of the basal lamina. Early growth cones contained numerous clear vesicles of about 50 nm in diameter.

Sequential development of connections between striate and extrastriate visual cortical areas in the rat

In these experiments we have asked whether the projection from the rat's primary visual cortex, area 17, to the extrastriate visual cortical area 18a is formed in a sequence and whether that sequence resembles the pattern of inside-out cortical neurogenesis. For this purpose fluorescent retrograde tracers were injected into area 18a at different postnatal ages (P1, P5, adult). Animals survived until 3-4 weeks of age, after migration is complete and neurons have arrived at their final laminar location. In the ipsilateral cortex, P1 injections retrogradely labeled cells in layers 5 and 6 of area 17. Labeling after P5 injections extended into more superficial layers and included the bottom of layer 2/3 and layers 4-6. After P5, more labeled cells were found at the top of layer 2/3, producing the adult laminar pattern, where the projection originates predominantly from layer 2/3. A similar sequence of laminar labeling was observed in the transcallosal connection of area 18a. This sequence of labeling, deep layers before superficial, resembles the pattern in which cortical neurons are born and indicates that axons arrive at their cortical targets in the order the cells were generated.

Role of microtubules in the cytoplasmic compartmentation of neurons. II. Endocytosis in the growth cone and neurite shaft

We investigated the role of microtubules in the compartmentation of motility and endocytosis in the neurite shaft and growth cone of cultured chick sensory neurons. As reported previously by Letourneau and Ressler (J. Cell Biol., 98 (1984) 1355-1362), stimulating microtubule polymerization with taxol inhibits growth cone motility. In neurons that had grown for 18-30 h, taxol treatment caused growth cones to round up forming an obvious varicosity (taxol bulb) at the terminal. Removal of taxol allowed nearly immediate resumption of cortical motility in all 17 neurons observed by time-lapse videomicroscopy. However, only one of the 17 neurites was observed to elongate measurably even after 5 h of observation. In 14 cases taxol was rinsed out and the concentration of nerve growth factor was increased 10x, 11/14 neurites retracted within the next hour. Endocytic activity was investigated by incubating control and taxol treated neurons in either cationized ferritin or horseradish peroxidase for 30 min. The number and area of label-containing vesicles was measured along with the total area of the growth cone or taxol bulb. We found that taxol treatment caused a 7-fold decrease in the ratio of the area of the labeled vesicles to the area of growth cone or taxol bulb. Conversely, in neurite shafts, normally relatively quiescent with respect to endocytosis, those regions devoid of microtubules in both control and nocodazole-treated cells contained a high concentration of label-containing vesicles. We conclude that the presence of microtubules plays a role in regulating endocytic activity by the overlying cell cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

The ultrastructure of the neuronal growth cone: new insights from subcellular fractionation and rapid freezing studies

In this review I have discussed the ultrastructure of the growth cone in relation to two aspects of growth cone behaviour; motility and membrane recycling. There are obvious and severe limitations in studying such a dynamic entity as the growth cone with the static images produced by the electron microscope, but these notwithstanding, electron microscopy, as I have tried to show here, has made important contributions in this area. Notable amongst these contributions is the fairly complete catalogue we now have of the organelles within the growth cone and their spatial relations, in particular the cytoskeletal and membrane bounded elements. Among the important questions that remain unanswered are those relating to the source and destiny of plasma membrane components, especially those concerned with recognising extrinsic cues, and the control of the cytoskeleton in relation to neurite extension and growth cone guidance. These questions can be approached using electron microscopy especially the rapid freezing and deep-etching methods used in conjunction with specific probes such as antibodies and we can look forward to progress in these areas in the near future.

Differential growth of the branches of a regenerating bifurcate axon is associated with differential axonal transport of organelles

Axonal trees display differential growth during development or regeneration; that is, some branches stop growing and often retract while other branches continue to grow and form stable synaptic connections. In this study, an in vitro model of differential growth is examined to identify the intracellular events responsible for this phenomenon. When the giant cerebral neuron of Aplysia californica is placed in culture, vigorous growth occurs from the ends of both branches of its bifurcate axon. If an appropriate target neuron is placed next to one branch, growth from that branch is unabated while growth from the other branch is suppressed. The bidirectional fast transport of membranous organelles was examined in the two branches by the use of high-resolution video microscopy. Transport was similar in the branches in the absence of a target cell but was much greater in the growing than in the nongrowing branch when a target was present. Electron microscopic examination of fixed specimens confirmed these findings. Differential growth may be initiated or sustained by a diversion from certain branches of materials used in growth which are supplied by fast axonal transport.

A phosphatase activity and a synaptic vesicle antigen in multivesicular bodies of frog retinal photoreceptor terminals

Previous work has suggested that multivesicular bodies participate in endocytosis and membrane cycling at nerve terminals, including the presynaptic terminals of retinal photoreceptors. We now have found that multivesicular bodies located in the presynaptic terminals of photoreceptors in retinae of Rana pipiens show reaction product in preparations incubated to demonstrate phosphatase activity at pH 5, using cytidine monophosphate as the substrate. Evidently, multivesicular bodies in photoreceptors can possess at least some hydrolytic enzymes during their sojourn in the terminals. We have also found that the multivesicular bodies in frog retinal photoreceptor terminal stain, immunocytochemically, for the presence of SV2, an antigen of synaptic vesicles. This observation supports the suggestion that, along with the extensive, repeated reuse of membrane components for synaptic vesicle recycling, there is some incorporation of the components into structures that are potentially degradative.

The cytoskeletons of isolated, neuronal growth cones

We have examined by electron microscopy the cytoskeletons of growth cones isolated from neonatal rat forebrain by the method of Gordon-Weeks and Lockerbie [Gordon-Weeks and Lockerbie (1984) Neuroscience 13, 119-136]. When fixed in suspension with conventional fixatives, isolated growth cones contain a central region filled with a branching system of smooth endoplasmic reticulum and a cortical region immediately beneath the plasma membrane that is relatively free of organelles and is composed of an amorphous granular cytoplasm. The filopodia of isolated growth cones are also devoid of organelles and contain a cytoplasm that is similar in appearance to that in the cortical region. No microtubules or neurofilaments have been found in these growth cones. When isolated growth cones were prepared for electron microscopy by a method which preserves actin filaments [Boyles, Anderson and Hutcherson (1985) J. Histochem. Cytochem. 33, 1116-1128], microfilaments were found throughout the cortical cytoplasm. In the filopodia, the microfilaments were bundled together and oriented longitudinally. Filopodial microfilament bundles often extended into the body of the growth cone and could traverse it completely. Inclusion of Triton X-100 (1% v/v) in the fixative solubilized the membranes and soluble cytoplasmic proteins of growth cones, allowing an unobscured view of the microfilament cytoskeleton including the core bundle of microfilaments in filopodia. Suspended within the cytoskeleton were the coats of coated vesicles. These were particularly numerous at the broad bases of filopodia. Microfilaments bound heavy meromyosin and were cytochalasin B (2.0 X 10(-7) M) sensitive. Individual microfilaments branched and within filopodia they were extensively cross-linked by thin (7 nm) filaments. Microtubules and neurofilaments were not seen in these cytoskeletons despite the fact that the fixative contained a Ca2+ chelator. When growth cones were preincubated in taxol (14 microM) their cytoskeletons were found to contain microtubules. These were located mainly in the centre of the growth cone, were absent from the filopodia and were contiguous with microfilaments. We conclude that the cytoskeletons of isolated neuronal growth cones from neurones of the central nervous system are mainly composed of actin microfilaments. Although microtubules are not normally present, there is a pool of soluble tubulin which will form microtubules in the presence of taxol. This may imply that those microtubule-associated proteins that promote tubulin polymerization are absent in the growth cone or are below the concentration threshold for polymerization.

Lipoprotein uptake by neuronal growth cones in vitro

Macrophages that rapidly enter injured peripheral nerve synthesize and secrete large quantities of apolipoprotein E. This protein may be involved in the redistribution of lipid, including cholesterol released during degeneration, to the regenerating axons. To test this postulate, apolipoprotein E-associated lipid particles released from segments of injured rat sciatic nerve and apolipoprotein E-containing lipoproteins from plasma were used to determine whether sprouting neurites, specifically their growth cones, possessed lipoprotein receptors. Pheochromocytoma (PC12) cells, which can be stimulated to produce neurites in vitro, were used as a model system. Apolipoprotein E-containing lipid particles and lipoproteins, which had been labeled with fluorescent dye, were internalized by the neurites and their growth cones; the unmetabolized dye appeared to be localized to the lysosomes. The rapid rate of accumulation in the growth cones precludes the possibility of orthograde transport of the fluorescent particles from the PC12 cell bodies. Thus, receptor-mediated lipoprotein uptake is performed by the apolipoprotein B,E(LDL) (low density lipoprotein) receptors, and in the regenerating peripheral nerve apolipoprotein E may deliver lipids to the neurites and their growth cones for membrane biosynthesis.

Structural alteration and possible growth of afferents after kainate lesion in the adult rat thalamus

Afferents to the thalamic ventrobasal complex (VB) originating from the spinal cord, the dorsal column nuclei, and the somatosensory cortex were anterogradely labeled by WGA-HRP 30 days after an injection of kainic acid (KA), which produced a complete unilateral neuronal loss in the VB, the opposite side being used as a control. At the light microscopic level, there was no obvious rerouting of spinal afferents away from the lesioned areas towards unlesioned parts of VB. There was an apparent decrease in the number of lemniscal afferents to the lesioned side, which may indicate a progressive retrograde degeneration. At higher magnification, all three afferent systems studied demonstrated morphological changes, predominantly manifested by terminal swellings that reached up to 25 micron in diameter. Control experiments suggested that these morphological alterations were related neither to a direct action of the excitotoxin nor to the absence of a different afferent system but to the loss of neuronal postsynaptic targets. At the electron microscopic level, the normal ultrastructural features of VB were not observed after a KA lesion. No neuronal somata, dendrites, or normal presynaptic elements were observed. Neural elements, some of which were labeled from the somatosensory cortex or the dorsal column nuclei, were essentially of two types: varicosities and unmyelinated axonal profiles. Varicosities could be separated into two broad classes: The majority were large structures derived from large, sometimes myelinated, axons and containing a wealth of organelles. Since they were not completely surrounded by glial elements, we have denoted them unensheathed varicosities. Among the organelles, the most obvious features were vesicles and tubules of smooth endoplasmic reticulum, microtubules, mitochondria, and various lysosome-like inclusions. These unensheathed varicosities gave rise to large, mound-like protrusions containing large vacuoles and thin long protrusions either filled with neurofilaments or resembling unmyelinated axonal profiles. Others were completely surrounded by a glial sheet and were therefore called ensheathed varicosities. These ensheathed varicosities presented several characteristics typical of degenerating profiles, including neurofilamentous proliferation and morphological alterations of the mitochondria. Unmyelinated axonal profiles occupied a substantial territory in the lesioned area. They were most often grouped in bundles sometimes wrapped by glial processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Two modes of free migration of amacrine cell neuroblasts in the chick retina

The migration of amacrine neuroblasts toward the prospective amacrine cell layer in the chick embryo retina has been studied, in Golgi-stained sections, between days 5 and 9 of embryogenesis. Two distinct populations of presumptive amacrine neuroblasts have been identified on the basis of their shape and migratory behavior. One population (smooth amacrine neuroblasts) display smooth, monopolar or bipolar contours, moving freely across the retina without major changes in the original postmitotic shape, and give processes only after reaching the primitive inner plexiform layer. The second population (multipodial amacrine neuroblasts) includes multipolar neuroblasts with abundant filiform and/or lamelliform processes sprouting in various directions; these highly plastic cells begin modifying their shapes at the time of release from the ventricular lining and continue to do so as they move toward their definitive location. Thus, the well-known heterogeneity of adult amacrine cells seems to be preluded by differences in neuroblastic migratory patterns, suggesting the existence of at least two different subsets of amacrine cell precursors.

Synaptogenesis in the retina of the cat

We have studied the development of synapses in the retina of the cat from E(embryonic day)21 to adulthood. The inner plexiform layer (IPL) could be distinguished by E36, but at this age no synapses had formed, although compact processes had formed in the IPL and membrane specialisations had developed in adjacent processes. Conventional synapses form in the IPL from E45 and become increasingly numerous and differentiated over subsequent weeks. Extracellular space and cellular debris were prominent during the formation of these synapses. The conventional synapses appear to form principally between amacrine cells until E56, when ganglion cell dendrites could be identified as postsynaptic processes. Ribbon synapses characteristic of bipolar cells were identified around birth, suggesting that bipolar cells do not form synapses until that age. The outer plexiform layer (OPL) could be distinguished in central retina at E56. Extracellular space, debris of degenerating cells and mounds of agranular vesicles were prominent at this age but synapses were not observed until E59, when cone pedicles formed ribbon synapses onto horizontal cell processes. The first synapses clearly formed by spherules, also onto horizontal cells, were seen at E62. The central process of the postsynaptic triad, considered to be the dendrite of a bipolar cell, was first observed in both cone pedicles and rod spherules around birth, again suggesting that bipolar cells do not enter into synaptic arrangements until that age. Synaptogenesis in the OPL shows a strong centro-peripheral gradient; its initial stages were observed centrally in the late E50's but synapse formation was not complete in the retinal periphery until P(postnatal day)7 or later. We could not detect a centro-peripheral gradient in the formation of conventional synapses in the IPL, but the formation of ribbon synapses in this layer began centrally at birth and in the mid-periphery at P5. In summary, the first synapses to form in the retina are those which spread information laterally within the plexiform layers, between amacrine cells and from receptor to horizontal cells. The cells which carry information centrally, in particular bipolar cells, enter into synaptic arrangements considerably later. Further, retinal cells seem to form synapses in a distinct sequence: first amacrines, then receptors and lastly bipolar cells.

The early embryogenesis of papillary (sweat duct) ridges in primate glabrous skin: the dermatotopic map of cutaneous mechanoreceptors and dermatoglyphics

The present study documents the early innervation of the epidermis prior to the onset of differentiation of the papillary (sweat duct) ridge in glabrous digital skin of rhesus monkey embryos measuring 45, 50 and 55 mm (crown-rump) length. We observed small papillary ridges, spaced at a distance of approximately 40 microns, projecting into the dermis in the center of the distal glabrous digital pad of digits 2-5 in the 55-mm embryo. The other digital pads lacked any sign of ridge formation. A two-dimensional, approximately hexagonal grid of afferent nerves was present in the superficial dermis of all digital and palmar pads. At regular intervals of approximately 40 microns, afferent nerves ascended from the superficial dermal nerve plexus and innervated the overlying epidermis. By electron microscopy, axonal growth cones were identified contacting Merkel cells that projected several microns down into the superficial dermis in the digital pad of digit 3. Thus, the earliest wave of differentiated dorsal root ganglion neuroblasts innervates Merkel cells. Schwann cells partially encircled these growing axon tips and could be identified by the presence of rough endoplasmic reticulum and free ribosomes. The youngest embryo studied had no sign of ridge formation; however, axons ascended from the superficial dermal nerve net at 30-40-microns intervals to innervate the epidermis. We conclude that afferent nerve fibers provide a two-dimensional grid that could modulate the spacing and arrangement of the papillary or sweat duct ridges of successive digits. Such an interaction is possible between digits based on the overlapping dermatotopic maps of each rete ridge. An abnormal fingerprint could thus reflect abnormal dorsal root ganglion neuroblasts expressed through mesenchyme and epidermis.

Tissue culture analysis of neurite outgrowth in the presence and absence of serum: possible relevance for central nervous system regeneration

A tissue culture model has been developed to examine the hypothesis that axons can only regenerate when their growing tips are surrounded by extracellular fluid containing proteins derived from the blood. Fetal rat cerebral explants were cultured in serum medium for 10 days, followed by serum-free (SF) medium (from which serum had been removed) until 18 days in vitro (DIV). All explants cultured in serum medium for 0-10 DIV exhibited greater than 77% neurite viability (neurite viability ratio, NVR, 3.10). This degree of neurite viability was maintained for those explants exposed to serum until 18 DIV (NVR 2.82 at 18 DIV). By contrast, explants maintained in SF medium from 10-18 DIV had a much lower NVR, which, by 18 DIV, had declined to 0.30 (7.5% viability). Transmission electron microscopic analysis of explants fixed at 18 DIV confirmed these phase-contrast results and also showed a predominance of axonal profiles within the neurite population. In the center of explants, tissue viability was in excess of 75% in both the serum and SF media, suggesting that serum is of primary importance for axonal extension rather than neuronal survival. These data strengthen the hypothesis that blood-derived proteins may be needed for prolonged regeneration.

Polarized compartmentalization of organelles in growth cones from developing optic tectum

We have used computer-assisted reconstructions of continuous serial sections to study the cytoplasmic organization of growth cones in vivo. Optic tecta from 6.25-6.5-d-old chicken embryos were quick-frozen and then freeze-substituted in acetone-osmium tetroxide or, for comparison, prepared by conventional fixation. Images of eight freeze-substituted and two conventionally fixed growth cones were reconstructed from aligned serial micrographs. After freeze-substitution, numerous lumenless membrane-bound sacs arrayed in multilamellar stacks appear to replace the abundant smooth endoplasmic reticulum found after chemical fixation. Microtubule fascicles progressively diverge from their typical fascicular organization in the initial segment of the growth cone and are absent in the varicosity and the more distal segment. Mitochondria, in contrast, are concentrated in the proximal segment of the varicosity; multilamellar stacks and endosome-like vacuoles are in the distal segment; and coated pits and vesicles are concentrated near the terminal filopodium, which is the most distal and organelle-poor domain of the growth cone. These observations suggest that dilation and fusion of the lumenless, membrane-bound sacs that occurs during chemical fixation give rise to the network of smooth endoplasmic reticulum. The three-dimensional reconstructions show that the cytoplasmic components of growth cones, including the membrane-bound sacs and multilamellar stacks revealed by freeze substitution, are polarized along the axis of these growth cones, which suggests that they have a role in recycling of membrane during elongation of the growth cone.

Peroxidase activity at nodes of Ranvier in some commonly used laboratory animals after intramuscular administration of horseradish peroxidase

During retrograde transport of intramuscularly administrated horseradish peroxidase (HRP) nodal axon segments of rabbit, rat, and mouse peripheral nerve fibers became HRP-positive and showed a striking proximodistal segregation of certain axoplasmic organelles. HRP-positive bodies and HRP-negative vesiculotubular membrane profiles were seen just distal and proximal to the nodal midlevel, respectively. This appearance was similar to that found earlier in HRP-transporting feline muscle nerve fibers. Some mechanisms that might explain the segregated appearance are briefly discussed.

Cytoplasmic morphology weaver (wv) mouse cerebellar neurons at the culture substratum

The purpose of this study was to determine the structural basis for the hypermotility and impaired growth cone elongation of the homozygous weaver (wv/wv) mouse cerebellar granule cell neurons in culture. Two-day cultures of dissociated week-old normal (+/+) and wv/wv cerebellum were processed for electron microscopy of intact cells and cytoskeleton. Serial sections parallel to and starting from the substrate were examined. Fine-caliber neurites of normal granule cells are packed with parallel arrays of microtubules at all levels. Microfilament-packed microspikes are present at substrate level emanating from a cortical microfilament lattice at the terminus of neurites of varying length. Homozygous weaver granule cells at substrate level have lateral cytoplasmic extensions along the neurite. Microtubules that curve throughout the neurite are separated by cytoplasm. The lateral extensions and growth cone cytoplasmic projections contain microfilaments and occasionally microtubules. Microfilament-packed microspikes are not observed. Immunofluorescent detection of actin confirms the ultrastructural picture. A hallmark of the wv/wv cytopathology is the presence of large numbers of coated vesicles throughout the neurite shaft at the cell-substratum interface. These are rare at similar locations in +/+ neurites. We hypothesize that reduced tension in the growth cone and neurite owing to the presence of lateral extensions and absence of stable microspikes are responsible for the impaired elongation and hypermotility of mutant neurons.

Growth cone-target interactions in the frog retinotectal pathway

The growth cones of retinal ganglion cell axons were studied in the optic tract and tectum with horseradish peroxidase (HRP) histochemistry and electron microscopy. The ganglion cell growth cones has many morphological features similar to those described in vitro and in other in vivo systems. However, we found that some processes formed highly differentiated terminal arborizations, while retaining growth cones on many of their branches. In addition, ultrastructural examination of the tectal neuropil revealed that many ganglion cell axonal processes had characteristics of both growth cones and presynaptic endings. These findings are discussed in the context of the hypothesis of shifting connections and the evidence that retinotectal map formation involves several mechanisms, including a process that depends on the action potential activity in the optic fibers.

The possible role of "sibling neurite bias" in the coordination of neurite extension, branching, and survival

In this review we consider a novel mechanism, "sibling neurite bias," which may explain aspects of the coordination of elongation, branching, and resorption among different neurites growing from the same neuronal cell body. In this model, growing neurites which incorporate structural precursors at higher rates would deplete the cellular pool of precursors available to their "sibling" neurites; neurites would compete for survival, but in addition they would bias each other's behavior during active growth. Evidence is reviewed that "sibling neurite bias" may contribute to the establishment and stabilization of specific neural connections. Specific examples examined include the loss of polyinnervation at the developing neuromuscular junction, contextual mapping in the retino-tectal system, and selective neurite growth patterns and synaptic connections in nerve tissue culture model systems.

Poststimulatory endocytosis, microvesicle repopulation and changes in smooth endoplasmic reticulum in nerve endings of the median eminence superfused in vitro

Mediobasal hypothalami of adult rats were superfused in vitro. A single 5 min pulse of 60 mM KCl-containing medium was infused, followed by either 15, 30, 45, 60 or 75 min superfusions with standard medium. In some experiments, 5 or 10% dextran was added followed by a 15 min recovery. Morphologically, two recovery phases were recognized. The early phase (15-30 min) was characterized by two features: (1) A clear-cut increase in the quantity of large, pleomorphic vacuoles occupying the axoplasm of nerve endings; these vacuoles were observed to be connected to caveolae of the same diameter in the axolemma and they were either coated or uncoated. (2) Progressive increase in the quantity of microvesicles (synaptic vesicles) from an initial depleted state. The vacuoles were found to contain dextran aggregates. Microvesicle-like protrusions bulged from the membrane of vacuoles. The late phase, from 45 min poststimulation onward, was typically identified after the appearance of tubules of smooth endoplasmic reticulum at the most distal segments of the nerve terminals. During this period, large vacuoles tended to decrease in quantity. Granular vesicles remained scant during the entire observation period. Images suggesting formation of microvesicles from tubules of smooth endoplasmic reticulum were observed. These results open the possibility that endocytosis of patches of membranes forming large vacuoles be an important mechanism for retrieving the membranes belonging to microvesicles and granular vesicles. Some of these large vacuoles may contribute to the early regeneration of microvesicles. More microvesicles could later be produced from the smooth endoplasmic reticulum.

Use of a lectin-peroxidase conjugate (WGA-HRP) to assess the retinotopic precision of goldfish optic terminals

The lectin wheat-germ agglutinin (WGA), conjugated to horseradish peroxidase (HRP), is an effective tracer for goldfish optic axons. Under suitable conditions, WGA-HRP injected iontophoretically into the normal goldfish tectum is taken up by retinotopic optic terminals (but not by axons of passage) and labels a small, discoid patch of retinal ganglion cells. Under identical conditions, unconjugated HRP is mainly taken up by injured axons of passage. WGA-HRP injected into the tectum after regeneration of the contralateral optic nerve again labels a small patch of ganglion cells, demonstrating the extent to which regenerated terminals are also retinotopic.

Solubility properties of neuronal tubulin: evidence for labile and stable microtubules

The solubility properties of tubulin and microtubules in pure cultures of sympathetic neurons were examined by electron microscopic and biochemical techniques. For morphological analyses, neurons were extracted with Triton X-100 in the presence or absence of 1 mM CaCl2, and the resulting detergent-extracted residues were examined for microtubules. In parallel experiments, the solubility of tubulin was determined under various solution conditions. Detergent-extracted residues of neurons prepared without Ca2+ contained many microtubules. Neurite residues prepared in the presence of Ca2+ also contained microtubules, but at substantially lower numbers than in residues prepared without Ca2+. The biochemical data parallel the morphological observations. Following detergent-extraction under microtubule stabilizing conditions, 30% of the tubulin was detergent-soluble (i.e. unpolymerized), while 70% was detergent-insoluble (i.e. polymerized). A more detailed examination of the solubility properties of tubulin indicated that 62% was detergent-insoluble but soluble in buffers containing mM CaCl2, while 5-8% was detergent and Ca2+-insoluble. A variety of control experiments indicated that non-specific adsorption of tubulin onto detergent-insoluble components of the cultures, assembly of tubulin onto pre-existing microtubules, and incomplete extraction of tubulin from cells contributed minimally to the levels of Ca2+-soluble and insoluble tubulin obtained with the extraction conditions used. These results indicate that (a) the majority of neuronal tubulin is assembled into microtubules which disassemble upon treatment with Ca2+ and (b) a portion of the neuronal tubulin is assembled into microtubules which show the unusual property of Ca2+-stability.

Ultrastructural networks in growth cones and neurites of cultured central nervous system neurons

We have examined growth cones and neurites of cultured central nervous system neurons by high-voltage electron microscopy. Embryonic chicken retina cells were cultured on polylysine-treated and Formvar-coated gold grids for 2-6 days, fixed, and critical point dried. Growth cones and neurites were examined as unembedded whole mounts. Three-dimensional images from stereo-pair electron micrographs of these regions showed a high degree of ultrastructural articulation, with distinct, non-tapering filaments (5-9 nm in diameter) joining both cytoskeletal and membranous components. In the central regions of growth cones, interconnected structures included microtubules, large membranous sacs (up to 400 nm), and irregular vesicles (25-75 nm). A denser filamentous network was prevalent at the edges of growth cones. This network, which frequently adjoined the surface membrane, linked vesicles of uniform size (35-40 nm). Such vesicles often were seen densely packed in growth cone protrusions that were about the size of small synaptic boutons. Prevalent structural interconnections within growth cones conceivably could play a logistic role in specific membrane assembly, intracellular transport, endocytosis, and secretion. Because such processes are not unique to growth cones, the extensive linkages we have observed may have implications for cytoplasmic structure in general.