An electron microscopic study of the development of synapses in cultured fetal mouse cerebrum continuously exposed to xylocaine

Activity-Dependent Inhibitory Synaptogenesis in Cerebellar Cultures

Inhibitory synapses on Purkinje cell somata in organotypic cerebellar cultures derived from newborn mice were increased after chronic exposure post explantation to agents that enhance neuronal activity. Inhibitory synaptogenesis was reduced in similar cultures after continuous blockade of spontaneous neuronal discharges. By contrast, excitatory synapses developed fully in the absence of neuronal activity. The reduction of inhibitory synaptogenesis was prevented by the simultaneous application of activity blocking agents and neurotrophins BDNF or NT-4, which are TrkB receptor ligands, but not with NT-3, a TrkC receptor ligand. The effect of endogenous neurotrophins was evaluated by continuously exposing cerebellar cultures to antibodies to BDNF and NT-4, which caused a significant reduction in the development of inhibitory Purkinje cell axosomatic synapses. These combined results indicated a role for TrkB receptors in activity-dependent inhibitory synaptogenesis. This concept was supported by the promotion of inhibitory synaptogenesis by specific antibody activation of TrkB receptors.

The changeable nervous system: studies on neuroplasticity in cerebellar cultures

Circuit reorganization after injury was studied in a cerebellar culture model. When cerebellar cultures derived from newborn mice were exposed at explantation to a preparation of cytosine arabinoside that destroyed granule cells and oligodendrocytes and compromised astrocytes, Purkinje cells surviving in greater than usual numbers were unensheathed by astrocytic processes and received twice the control number of inhibitory axosomatic synapses. Purkinje cell axon collaterals sprouted and many of their terminals formed heterotypical synapses with other Purkinje cell dendritic spines. The resulting circuit reorganization preserved inhibition in the cerebellar cortex. Following this reorganization, replacement of the missing granule cells and glia was followed by a restitution of the normal circuitry. Most of these developmental and reconstructive changes were not dependent on neuronal activity, the major exception being inhibitory synaptogenesis. The full complement of inhibitory synapses did not develop in the absence of neuronal activity, which could be mitigated by application of exogenous TrkB receptor ligands. Inhibitory synaptogenesis could also be promoted by activity-induced release of endogenous TrkB receptor ligands or by antibody activation of the TrkB receptor.

Synapse formation and spontaneous activity in rat brainstem neurons in primary culture

The correlation between synaptogenesis and onset of spontaneous action potentials was assessed in rat brainstem cells up to 29 days in primary culture. Cells exhibited different stages of maturation followed by electron microscopy and patch clamp recordings. Terminal boutons with no preferential orientation of presynaptic vesicles appeared after 2 days in culture. After 5 days, preferential orientation of presynaptic vesicles and thickening of postsynaptic membranes were observed. The spontaneous discharge of action potentials, single or bursting, was observed after 7 days in vitro. This was followed by the expression of a 128-pS K(+) channel starting at 13 days in vitro. A 69-pS K(+) channel was also present throughout the duration of the cultures. These results suggest that spontaneous discharge of action potentials does not occur before synapses are formed and K(+) channel types develop differentially in brainstem neurons in vitro.

Development of specific synaptic network functions in organotypic central nervous system (CNS) cultures: implications for transplantation of CNS neural cells in vivo

This article provides a broad overview of the significant roles that morphophysiologic analyses of organotypic cultures of neural tissues explanted in vitro-initiated during the 1950s-have played in stimulating the more recent development of techniques for transplantation of neural cells and tissues into specific regions of the central nervous system (CNS) in vivo. The demonstrations by Crain and co-workers in the 1950s and 1960s that fetal rodent and human CNS neurons can continue to develop a remarkable degree of mature structure and function during many months of complete isolation in culture provided crucial evidence that development of many organotypic properties of nerve cells is regulated by epigenetic factors that ensure rather stereotyped expression despite wide variations in environmental conditions. These in vitro studies strongly suggested that fetal neural cells should, indeed, be capable of even more highly organotypic development after transplantation in vivo, as has been elegantly demonstrated by many of the successful CNS transplantation studies reviewed here.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Structure of the embryonic primate spinal cord at the closure of the first reflex arc

Early development of the spinal cord was studied in macaque monkey embryos, using light- and electron microscopy, Golgi impregnation and [3H]thymidine radioautography. All neurons engaged in the first reflex arc are generated before E27 in the 165 day gestation period. The earliest-generated cells differentiate into motoneurons while the commissural and association neurons are generated later. Synaptogenesis starts at E27 in the basal plate, and 2 days later in the alar plate. The first synapses appear as symmetrical junctions situated on the neuronal perikarya and proximal dendrites. Closure of the first spinal reflex are is established within 2 days and follows an antidromic pattern as related to the physiological spread of nerve impulses: synapses on motoneuronal somata and primary dendrites in the basal plate appear first and are followed by synapses in the alar plate, between dorsal root axon collaterals and somata of borderline (commissural/association) neurons. Commissural axons grow towards the floor plate, cross the midline and proceed caudo-rostrally, while association fibers remain ipsilateral. The first wave of apoptosis (programmed cell death) thins out dense populations of nerve and glial cells by E30. Some of the early-generated borderine cells that form commissural and association interneurons, seem to play the role of transient target cells and die once the definitive axonal pathways are established. Since transient cells form numerous synapses, deprivation from the afferent impulses is not a likely cause of their elimination. The present results indicate that the initial developmental events, including formation of the first reflex arc in the primate spinal cord, occur considerably earlier in respect to birth than in other mammals, but that the schedule of cellular events and cellular mechanisms seem to be the same.

Reduced cortical inhibitory synaptogenesis in organotypic cerebellar cultures developing in the absence of neuronal activity

Organotypic cerebellar cultures derived from newborn mice were continuously exposed to medium containing tetrodotoxin and elevated levels of magnesium to block all electrical activity. After 2 weeks in vitro, no activity was evident during the first 15-20 minutes following transfer to a recording medium without blocking agents. Thereafter, cortical discharge rates increased until a state of sustained hyperactivity was reached. Ultrastructural examination of such cultures revealed a reduction of inhibitory Purkinje cell somatic synapses to half the control value along with an even greater reduction of axodendritic synapses (largely inhibitory) in the cortical neuropil. No loss of axospinous synapses (excitatory) was evident. These results support the concept that spontaneous neuronal activity is necessary for the full development of inhibitory circuitry.

Eliminating afferent impulse activity does not alter the dendritic branching of the amphibian Mauthner cell

In the developing amphibian, the formation of extra vestibular contacts on the Mauthner cell (M-cell) enhances dendritic branching, while deprivation reduces it (Goodman and Model, 1988a). The mechanism underlying the interaction between afferent fibers and developing dendritic branches is not known; neural activity may be an essential component of the stimulating effect. We examined the role of afferent impulse activity in the regulation of M-cell dendritic branching in the axolotl (Ambystoma mexicanum) embryo. M-cells occur as a pair of large, uniquely identifiable neurons in the axolotl medulla. Synapses from the ipsilateral vestibular nerve (nVIII) are restricted to a highly branched region of the M-cell lateral dendrite. We varied the amount of nVIII innervation and eliminated neural activity. First, unilateral transplantation of a vestibular primordium deprived some M-cells of nVIII innervation and superinnervated others. Second, surgical fusion of axolotls to TTX-harboring California newt (Taricha torosa) embryos paralyzed the Ambystoma twin: voltage-sensitive Na+ channel blockade by TTX eliminated action potential propagation. Reconstruction of M-cells in 18 mm larvae revealed that dendritic growth was influenced by in-growing axons even in the absence of incoming impulses: impulse blockade had no effect on the stimulation of dendritic growth by the afferent fibers.

Effect of potassium depolarization on phosphate-activated glutaminase activity in primary cultures of cerebellar granule neurons and astroglial cells during development

The cerebellar granule cells are believed to be glutamatergic neurons. During the normal development of granule cells grown in a chemically defined medium, the specific activity of phosphate-activated glutaminase increased from 60 at 3 days to 150 (nmol/h/mg protein) at 15 days in vitro. Treatment with 25 mM K+ for the last 2 days elevated glutaminase activity in an age-dependent manner: about 100% at 3 and 6 days, 75% at 10 days, and 40% at 15 days in vitro. The enhancement of glutaminase in granule cells was dose-dependent. The half-maximal effect was obtained at about 20 mM K+, whereas the maximum concentration, which produced about a 2.5-fold increase in 3-day-old cultures was about 40 mM K+. The voltage-sensitive Na+ channel inhibitor tetrodotoxin had no effect on the depolarization-induced activity in granule cells. However, the increase in glutaminase by 25 mM K+ was significantly blocked by both organic (nifedipine) and inorganic (Ni2+ and Mg2+) calcium antagonists, indicating that elevation in activity may be mediated through transmembrane Ca2+ entry into granule cells. In contrast to neurons, in cultured cerebellar astrocytes, the activity of glutaminase slightly decreased during development, and treatment with 25 mM K+ had no significant effect on this enzyme activity. The present findings, together with previous observations, would indicate that depolarization with K+, which is believed to mimic in vivo presynaptic stimulation, could be one of the mechanisms that selectively controls the development and function of neurons, when measured in terms of the activity of the enzymes involved in the synthesis of cell-specific neurotransmitters.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Effects of polyunsaturated fatty acids and hormones on synaptogenesis in serum-free medium cultures of mouse fetal hypothalamic cells

The effects of soluble factors on synaptogenesis by mouse fetal hypothalamic cells cultured in chemically defined conditions have been examined using transmission electron microscopy. Hypothalami taken on the 16th day of gestation were mechanically dissociated and cells were seeded in a minimum serum-free medium supplemented or not with the following components: triiodothyronine, corticosterone and a mixture of polyunsaturated fatty acids (arachidonic acid plus docosahexaenoic acid bound to defatted bovine serum albumin). In the minimum serum free medium synapses were found after 10 days in culture. However, the development of synaptic vesicles was very limited, whereas that of the presynaptic and postsynaptic densities was apparently normal. Supplementation of the minimum serum-free medium with triiodothyronine, corticosterone and polyunsaturated fatty acids added simultaneously, permitted a full development of synapses as attested to by the increase in number and the regular shape and diameter of synaptic vesicles as well as by the complexity and diversity of synapse configurations. Among those three factors, polyunsaturated fatty acids clearly played a key role. The ability of synapses formed in culture to respond to potassium evoked depolarization was examined on cultures grown for 12 days in the simultaneous presence of the three above mentioned supplements. Exposure for 3 min to 60 mM potassium chloride induced in synaptic boutons vesicular depletion, apposition of vesicle clusters onto the presynaptic grid, appearance of a rich filamentous network and of some coated vesicles. Return to 3mM potassium chloride induced in 3 min a massive restoration of the population of vesicles which slightly differed from synaptic vesicles in control cultures. These results show that: (1) the formation of synaptic vesicles in this system is regulated by soluble factors among which polyunsaturated fatty acids play a major role, and (2) synapses formed de novo in chemically defined conditions of culture display the same ability to respond to and to recover from potassium evoked depolarization as adult axon terminals. Thus, they offer a suitable model for analysis of the mechanisms involved in membrane traffic in central neurons.

Synaptogenesis in rat cerebral cortex cultures is affected during chronic blockade of spontaneous bioelectric activity by tetrodotoxin

Reaggregated occipital cortex cells of 19-day-old fetal rats were grown in a serum-free, chemically defined medium, and chronically exposed to impulse-blocking levels of tetrodotoxin (TTX) in order to study the role of bioelectric activity in synaptogenesis. As judged by phase-contrast microscopy, no differences were noticed in the development of neuronal networks in the TTX-treated vs control cultures. In addition, when TTX was withdrawn from experimental cultures at any stage of development, bioelectric activity qualitatively comparable to that of the control cultures appeared within 1 min. However, quantitative stereological EM analysis revealed a significant retardation in synapse formation and ultrastructural maturation of synaptic junctions during the first 3 weeks. Around 23 days in vitro, the central zone of the reaggregates in control cultures started to degenerate, but not earlier then day 27 in TTX-treated cultures. During this time, the control, but not the experimental cultures showed (in intact tissue regions mainly situated at the outside of the aggregates) a large and selective loss of spine synapses. It is concluded that functional blockade not only retards the early growth and maturation of synaptic networks but also prevents the later occurring selective loss of spine synapses.

Interaction between trophic action and electrical activity in spinal cord cultures

The effect of conditioned medium (CM) on tetrodotoxin (TTX)-mediated neuronal cell death was investigated in dissociated spinal cord-dorsal root ganglion (SC-DRG) cultures. Nutrient medium was collected from donor SC-DRG cultures at 3--4-day intervals throughout development. The presence of survival-promoting neurotrophic material (NTM) in the conditioned medium was tested on TTX-treated cultures which were within the critical period of vulnerability to electrical blockade (days 7-21 in culture). Cultures were assayed by neuronal cell counts, choline acetyltransferase (ChAT) activity or fixation of [125I]tetanus toxin, a neuronal surface marker. Evidence of NTM was found in CM collected prior to day 8 and after day 21 in culture. Increasing the percentage of CM in fresh nutrient medium resulted in a dose-dependent increase in [125I]tetanus toxin fixation and ChAT activity in TTX-treated cultures. In addition, CM plus TTX treatment produced a 25% increase in neuronal cell counts as compared to controls. TTX treatment alone resulted in 20-25% decrease in neuronal number from that of control cultures. Cultures treated with CM alone had neuronal counts that were similar to controls. When electrical activity was blocked with TTX, NTM was not detectable in CM collected from donor cells during days 1-5. CM obtained from control cultures during the same interval had NTM activity. The existence of the critical period for electrical blockade-associated neuron death is associated with a decrease in the availability of NTM. Furthermore, the release of NTM from donor cells and the survival response of target neurons to NTM are apparently dependent on ongoing electrical activity.

Systems analysis of the integrative activity of the neuron (1974)

This article is aimed at revising the traditional concept of neuronal activity based on pre-eminence of transmembrane potentials and "electric summation" on the neuron surface. It presents a historical survey of the emergence of the prevailing concept on propagation of potentials along conductive structures and reveals the psychological situation that determined the transfer of this concept to dendrites and the neuronal soma. Structural and biophysical properties of the neuron which do not permit information propagation along the neuronal membrane without crude distortion are critically discussed in detail.

Ultrastructural studies on synaptic formations in dissociated fetal mouse brain cultures

Sequential electron microscopic studies of cultures of neurons derived from dissociated fetal mouse brain on the eleventh day of gestation revealed the formation of well-developed synapses during the second and third weeks of growth in vitro. Synaptic junctions were associated with synaptic vesicles nd dense synaptic membranes. Dense-core vesicles were also observed frequently in presynaptic terminals.

Developmental and neurochemical specificity of neuronal deficits produced by electrical impulse blockade in dissociated spinal cord cultures

Blockade of spontaneous electrical activity in dissociated fetal spinal cord cultures produced neuronal deficits as measured by biochemical and morphological techniques. Spinal cord cultures exhibited an age-dependent vulnerability to impulse blockade with tetrodotoxin (TTX) or xylocaine. Neuronal cell counts, [125I]tetanus toxin fixation and [125I]scorpion toxin binding indicated that TTX application produced neuronal deficits during the second or third week in culture. Application of TTX during the first or fourth week did not produce a difference in tetanus toxin fixation from controls. Radioautography of [125I]tetanus toxin revealed no obvious change in the label distribution after TTX treatment. Suppression of electrical activity during the first 6 days in culture had no effect on choline acetyltransferase (CAT) activity and no apparent effect on the appearance of the cultures. Application of TTX during the seventh day in culture decreased CAT activity to 68% of control. Chronic electrical blockade produced a progressively greater loss of CAT activity through 21 days in culture. GABAergic neurons, as indicated by high-affinity GABA uptake, glutamic acid decarboxylase activity and [3H]GABA radioautography, were not affected by electrical blockade. These data indicate that there is developmental and neurochemical specificity in the neuronal death produced by blocking spontaneous electrical activity in dissociated spinal cord cultures.

Junctions in rat neocortical explants cultured in TTX-, GABA-, and Mg++-environments

The occurrence of various cell-to-cell contacts and membrane specializations was quantitated in neocortical explant cultures prepared from 18-day-old rat embryos and exposed continuously to tetrodotoxin (TTX), gamma-aminobutyric acid (GABA) and an elevated level of magnesium ions (Mg++), respectively. Chronic TTX treatment resulted in a decrease in the number of synapses, paired neuronal membrane thickenings and nerve terminals with synaptic vesicles; the area of the neuronal compartment also decreased. By contrast, the gap junctions between glial cells increased, although the glial paired membrane thickenings decreased in number per unit area. Long-term GABA and Mg++ exposures did not alter significantly the occurrence of any of the cell contacts and membrane specializations analyzed when compared to control values. The results suggest an inhibitory effect of TTX on neuronal maturation and synapse formation in explant cultures of rat neocortex; this may lead secondarily to an increased demand for glial cell-to-cell communication.

Decreased synaptic areas on Purkinje spines in the cerebellar cortex of the mutant Japanese quail, Coturnix coturnix japonica

'Dark frayed feather nervous disorder' (dn) is a neurological mutation in quails in which the cerebellar cortex is abnormally organized. Purkinje cells are not aligned in a single row and show hypoplasia of the dendrites. The synapses between the parallel fibers and the spines of the Purkinje cell dendrites were examined with the technique of serial sections in electron microscopy. The postsynaptic thickenings were obviously decreased in the mutant quail despite the same density and size of dendritic spines of Purkinje cells. In addition, ectopic spines and postsynaptic differentiations free of parallel fibers were not found on the dn Purkinje cell. Because of the poor dendritic arborization, the total number of spines and the total synaptic area are, therefore, reduced in the dn Purkinje cell. According to the results obtained the dn mutant genetic locus is considered to affect primarily Purkinje cells.

The noradrenergic system in cultured aggregates of fetal rat brain cells: morphology of the aggregates and pharmacological indices of noradrenergic neurons

Spherical aggregates formed rapidly in culture by re-aggregation of trypsin-dissociated brain cells from the 17-day-old fetal rat. Over about 10 days in initially random distribution of cells evolved into a 3-layered arrangement; cells with characteristics of neurons were found largely in the intermediate layer. The survival of neuronal and glial cell types was evaluated histologically and verified by electron microscopy, which revealed synaptic and myelin structures that rapidly increased in number after 18 days in culture. Levels of norepinephrine (NE) and dopamine (DA) reached peaks of 9.5 and 4.4 ng/mg protein, respectively, at culture day 21. Uptake of [3H]NE paralleled these amine levels and was blocked by desipramine or pretreatment with either reserpine or 6-OH-DA. Autoradiographs of aggregates labeled with [3H]NE showed a high density of silver grains over cells, apparently neurons, with branching processes traced for 120 micrometer. Previously accumulated [3H]NE was released under depolarizing conditions (high [K+] or vertridine) only in the presence of Ca2+. Release was induced to a lesser extent by kainic greater than glutamic acid. Thus, such aggregates appear to contain catecholaminergic neurons capable of synthesis, uptake and release of NE. The time course of development of these functions supports suggestions that aggregate preparations might be useful in studying neurochemical or morphological aspects of brain development and function in vitro.

A quantitative electron microscopic study on synapse formation in dissociated fetal rat cerebral cortex in vitro

Occipital cortex of 19-day-old fetal rats was dissociated and cultured in vitro for 2-5 weeks in horse serum supplemented Eagle's MEM. The outgrowing neurons rapidly formed a dense network which started to degenerate after 3 weeks in vitro. By means of electron microscopy the numerical development of 6 different categories of synapses was followed during the time in culture. This approach revealed a sigmoid growth curve emerging over the first 3 weeks for the category of axo-dendritic synapses (which comprised the bulk of all synapses counted). Thereafter a decline in number set in. Chronic exposure of these dissociated cerebral cortex cultures to 50 microgram/ml xylocaine (a concentration adequate to block all measurable bioelectrical activity) did not prevent the formation of functional synaptic networks having normal synaptic ultrastructure. These results are in agreement with previous studies on fetal cerebral explants in culture. However, in some groups of our cultures, xylocaine led to a retardation in neurite outgrowth and in numerical synapse formation. Since these xylocaine effects were dose-related at a concentration double that required to silence the cultures, the growth retardation was probably not due to selective suppression of bioelectric activity, but rather to some general cytotoxic effect of the drug. In one of the xylocaine-treated groups (50 microgram/ml) which showed an exceptionally rapid neuronal outgrowth by the use of horse serum obtained from a different source than in the other groups, no deficit was noted in the number of synapses formed. Extracellular recording in this latter drug-treated group of cultures during the third week revealed (after return to control medium) spontaneous isolated action potentials, burst patterns and slow waves which were indistinguishable from the bioelectric activity seen in the control cultures. It is concluded that bioelectric activity in dissociated cortex cultures is not a prerequisite for the formation of apparently normal, functional synaptic networks.

Transneuronal vestibular afferent influence on the nodular molecular layer synaptogenesis

The effect of vestibular afferent deprivation on the synaptogenesis of the nodular molecular layer has been studied quantitatively. No detectable effect on the time sequence of the development of the molecular layer and the external granular layer was found. Only around hatching a significantly reduced synaptic profile density was found in otocyst-deprived chickens on both halves of the nodulus. This effect can most easily be explained by the assumption of an anterograde transient transneuronal influence of vestibular afferents on the ability of parallel fibers to form synapses.

Neuronal maturation in mammalian cell culture is dependent on spontaneous electrical activity

Fetal mouse spinal cord (SC) and dorsal root ganglion (DRG) neurons undergo a process of maturation in cell culture lasting a month or more. We have investigated the role of electrical activity in this maturational process with the use of tetrodotoxin (TTX), the specific blocker of the voltage-sensitive sodium channel responsible for action potential generation. This agent completely eliminates the spikes and related synaptic activity which occur abundantly in untreated cultures. Such blockade of electrical activity in the cultures, when begun early (day 1 or day 8 in vitro), results in a 85-95% reduction in the number of large SC neurons, without affecting DRG neuron numbers. TTX treatment initiated when cultures are mature (day 70) has no significant effect on either DRG or SC neurons. Intermediate effects are obtained when treatment is initiated at day 35 in vitro. The activity of the nerve-specific enzyme choline acetyltransferase, is significantly decreased by early TTX treatment, while DNA and protein content of the cultures (primarily contributed by glial and fibroblastic cells) is not affected.

Effects of anesthetic treatment on motor neuron death in xenopus

Xenopus tadpoles were introduced into anesthetic (chlorbutanol) solutions during a developmental period in which naturally occurring loss of lumbar spinal motor neurons takes place. Animals that developed in these solutions were sacrificed after various lengths of time and the number of lumbar motor neurons were counted. No significant difference was detected between experimental and control motor neuron counts throughout the period in which most of the cells are normally lost. A similar finding is reported when one or more lumbar spinal nerves were blocked locally with implants of anesthetic (procaine) impregnated silastic chips.

Synaptogenesis in the ventromedial hypothalamus of the prenatal mouse

Mouse embryos from day 13 to day 19 of gestation (E13 through E19) were removed by Caesarean section and their brains were prepared for electron microscopy. Coronal sections were examined in three planes through the ventromedial hypothalamus: anterior, the level at which the optic tracts pass posterolaterally to become partially enclosed in the major brain mass; middle, the level at which the floor of the third ventricle begins to widen and flatten; posterior, the level at which the most anterior infundibulum appears. Three types of junctional complexes were examined. Close junctions are identified as straight, parallel areas of two apposed membranes which appear more electron-dense than immediately adjacent regions. The membranes are separated by a clearly visible cleft. Unlike synapses, no clear synaptic vesicles are found in either of the adjacent profiles unless randomly distributed and accompanied by ribosomes or glycogen. Close junctions are seen most frequently on day E15, then decrease in number through E19. Their participation in synaptogenesis is discussed from temporal, morphological, distributional and quantitative perspectives and is provisionally rejected. Immature synapses show only the minimal membrane specialization found in close junctions, but vesicles are present, ribosomes absent in at least one of the adjacent cytoplasms. Their appearance peaks on E17-E18, paralleling and slightly preceding that of the mature synapses. Their evolution from close junctions has only weak temporal support. Mature synapses display the cytoplasmic densities which immature synapses lack. They attain their greatest prenatal numbers on E18, then decrease in number on E19. The conclusion is advanced that synaptogenesis does--or at least can--occur without the prior appearance of avesicular regions of increased membrane density.

Use of the local anesthetic lidocaine for cell harvesting and subcultivation

Cell suspensions from monolayer cultures of 3T3, SV-3T3, BHK, BS-C-1, or macrophages, were prepared by brief exposure of the cultures to the amide anesthetic lidocaine. Cells were subcultivated six times without marked impairment of plating efficiency. The method should be applicable to physiological, biochemical, or immunological studies in which exposure of cells to proteolytic enzymes is to be avoided.

Differentiation of sympathicoblasts in cultures of chick ganglia: light and electron microscopic, fluorescence and enzyme histochemical observations

Immature sympathetic ganglia prepared from 5 1/2-or 6-day-old chick embryos were cultured up to one month. The in vitro development was followed by phase microscopy, electron microscopy and using histochemistry for catecholamines, monoamine oxidase and cholinesterases. During the first week of culture extensive plexuses of nerve fibres were formed between and around the clusters of nerve cells. Mature-looking neurons were observed in the cultures by phase microscopy after three weeks, at which age the mean diameter of the perikarya was more than doubled. Varying catecholamine fluorescence was observed in the perikarya during the entire culture period. The nerve fibres showed usually only weak fluorescence, but, in the older cultures, bright varicosities were regularly found in the fibres. Monoamine oxidase activity was demonstrated already at three days of culture and the reaction was maintained positive. Weak or moderate acetyl-cholinesterase activity was demonstrated in the sympathicoblasts and young sympathetic neurons and their processes. The axolemma showed acetylcholinesterase activity also around the nerve terminals containing small dense cored vesicles. Reactions for the non-specific cholinesterases were negative. Electron microscopy of the 30-day-old cultures revealed that the clusters of nerve cells consisted of mature sympathetic neurons, which contained large (60-200 nm) and small (35-60 nm) granular catecholamine-storing vesicles. Glial cells were almost totally lacking. Large numbers of nerve terminals containing both large and small granular vesicles were observed in the clusters, often in synaptic contact with the sympathetic neurons. It is concluded that the primitive sympathicoblasts are, in favourable conditions, capable of differentiation in culture up to mature sympathetic neurons.