Biomedical subjects

L Landmesser

Publications and source records attributed to L Landmesser.

At least 37 records · Page 2Linked to original sources

PubMed1986

The development of sensory projection patterns in embryonic chick hindlimb under experimental conditions.

In the chick, sensory neurons grow to their segmentally appropriate target sites in the hindlimb from the outset during normal development. To elucidate the underlying mechanisms, we performed various manipulations of the neural tube, including the neural crest, or of the hindlimb, before axonal outgrowth and assessed the resulting sensory projections using retrograde and anterograde HRP labeling and electrophysiological techniques. Previous experiments had shown that motoneurons are specified to project to their appropriate target muscles prior to axon outgrowth and that they respond to cues in the limb in order to grow to those targets (C. Lance-Jones and L. Landmesser, 1980, J. Physiol. (London) 302, 559-602; C. Lance-Jones and L. Landmesser, 1981, Proc. R. Soc. London, B 214, 19-52). When several segments of neural tube and neural crest were deleted, sensory neurons in the remaining segments still projected along their correct pathways, as did motoneurons. In situations in which motoneurons grew to their correct targets from altered positions with respect to the limb (e.g., small neural tube reversals), sensory neurons also tended to project along the segmentally appropriate pathways both to skin and to muscle. In situations in which motoneurons were displaced greater distances from their normal point of entry into the limb and made wrong connections (e.g., large neural tube reversals, anterior-posterior limb reversals), sensory neurons also projected incorrectly. The patterns of sensory projections to muscles were, in each situation, generally similar to the motoneuron projections. These results are consistent with the possibility that sensory neurons, like motoneurons, are specified with respect to their peripheral connectivity. Alternatively, the results suggest that motoneurons may play a role in the process of pathway selection by sensory neurons.

Afferent Pathways↗

PubMed1986

The distribution of NCAM in the chick hindlimb during axon outgrowth and synaptogenesis.

We have determined the distribution and form of the neural cell adhesion molecule (NCAM) in the chick hindlimb from initial axon outgrowth (stage 17 1/2) until 3 days posthatching by immunohistological staining and sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblots. Axons stained intensely for NCAM at all ages, whereas nonneuronal limb components exhibited dynamic changes in staining. Mesenchymal cells in the sclerotome adjacent to the neural tube developed NCAM immunoreactivity in an anterior-posterior sequence which correlated with the sequence of axonal outgrowth. Low to moderate amounts of NCAM were detected within and surrounding presumptive nerve pathways, consistent with a permissive role for NCAM in axon extension, but not with a precise delineation of pathway boundaries. On myotubes immunoreactivity for NCAM remained low from stage 26 to 30 when it increased dramatically in both aneural and control limbs, indicating that its appearance is not triggered by nerve-dependent activity or trophic interactions. The increase was temporally associated with muscle cleavage and may encourage subsequent axon ramification as well as synaptogenesis. Staining remained high on muscle fibers during secondary myotube formation and only declined during the week before hatching when polyneuronal innervation is withdrawn and the mature synaptic pattern becomes stabilized. This loss of muscle NCAM occurred first on fast and then on slow muscle fibers. Together these results suggest that the timing of innervation may be controlled by the muscle, through NCAM expression, but that the subsequent suppression of muscle NCAM may occur as a result of nerve-mediated activity.

Animals↗

PubMed1981

Pathway selection by chick lumbosacral motoneurons during normal development.

Pathways taken by motoneuron axons from the lumbosacral lateral motor column to individual hindlimb muscles have been characterized throughout the normal period of outgrowth and the establishment of specific functional connections in the chick embryo. Axon pathways from individual cord segments were identified after injections of horseradish peroxidase (HRP) directly into the cord. Labelled motoneuron axons were then traced through the plexus and major nerve trunks to termination sites within the limb. At stages 23-24 labelled axons within spinal nerves have just reached the base of the limb and have begun to converge and form the crural and the ischiadic plexus. Even at this early stage, before periods of muscle cleavage, motoneuron cell death and muscle nerve formation, axons show no evidence of widespread random distribution within the limb. Rather, they generally maintain their anterior-posterior position as far as the base of the limb. At stages 27-30, although axons to individual muscles were found to course in discrete tracts within the plexus and nerve trunks they also changed their topographical position with respect to other axons. Axon pathways to single muscles were characterized by tracing retrogradely labelled axons back to the cord after injections of HRP into specific muscle nerves. Axons destined for a single muscle are intermingled with other axons in the spinal nerves and proximal plexus but by the distal plexus have converged to form a discrete tract which then diverges as an individual muscle nerve at more distal levels. These observations exclude models for the establishment of specific connections in which there is widespread testing of the environment with removal of projection errors by cell death and/or axon retraction. They also exclude models that require axons to maintain their topographical position with respect to each other throughout their course.

Animals↗

PubMed1981

Pathway selection by embryonic chick motoneurons in an experimentally altered environment.

To characterize cues used by motoneuron axons to reach their appropriate targets, connectivity patterns within the embryonic chick hindlimb have been analysed after early experimental manipulations of the limb or spinal cord. The manipulations altered the anterior-posterior (a.-p.) relationship between motoneurons within the lumbosacral motor column and their specific targets in the limb. Primary emphasis was placed on analysing the pathways taken by embryonic motoneuron axons at stages 23-36 which had been orthogradely labelled by horseradish peroxidase (HRP) injection into the motor column. Motoneuron pool topography and functional patterns of connectivity were also identified by retrograde HRP labelling and spinal cord stimulation coupled with electromyographic recording. With small shifts in position, as in two or three segment a.-p. cord reversals or a.-p. limb shifts, motoneuron axons frequently entered the appropriate plexus but in an inappropriate spinal nerve sequence. Despite this, axons altered their course to innervate specifically and consistently their current target. When motoneuron axons entered an inappropriate plexus as the result of a greater positional shift (i.e. more extensive cord reversal or limb shift) or in experiments where posterior cord segments were replaced with anterior cord segments and supernumerary limbs were added, they behaved in one of two ways. They either formed inappropriate and largely unpatterned or unordered connections or they took totally aberrant paths within the limb to reach their appropriate target. We conclude that axons are capable of responding in a precise and specific manner to environmental cues when displaced up to a certain distance from their target or normal point of entry into the limb. Their failure to form patterned connections at more extreme distances suggests that the cues to which they are responding may be local, or that an axon's ability to respond to them is restricted to subclasses of the motoneuron population.

Animals↗

PubMed1980

Motoneurone projection patterns in embryonic chick limbs following partial deletions of the spinal cord.

1. Several anterior segments of the lumbosacral spinal cord were deleted in a series of St. 15-16 chick embryos, which is prior to the birthdate of the motoneurones and to limb bud formation. Deleted segments did not regenerate and therefore some muscles were deprived of their normal source of innervation. 2. The projection pattern of motoneurones in remaining lumbosacral segments was assessed electrophsiologically (St. 30-36) and by orthograde and retrograde labelling of motoneurones with horseradish peroxidase (St. 24-36) from the time that axons first enter the limb until after the normal motoneurone cell death period. 3. The projection pattern of remaining segments was not altered by the deletion. At no time did inappropriate segments innervate muscles or regions of the primary muscle mass which had had their enter innervation source removed by the deletion, although they often innervated immediately adjacent regions. 4. The pathways taken by remaining motoneurones both in the plexus and muscle nerves were not different from their normal control pattern. From the time that axons first reached the base of the limb, there was no evidence that they compensated for missing spinal nerves by projecting down inappropriate pathways. 5. With respect to pathway selection, we can conclude that axons are normally not excluded from certain pathways by interactions with other axons, nor do they interact amongst each other in distributing themselves to all available pathways. 6. We can also conclude that once axons enter a muscle or muscle mass, they are not kept within appropriate regions by competition with axons that normally occupy adjacent regions. In summary we can exclude both the process of competition and a timed outgrowth mechanism as playing a major role in the development of specific motor connexions in the chick hind limb. 7. Muscles totally deprived of their innervation by the deletion underwent cleavage, became contractile and appeared to differentiate normally until St. 20. However by St. 35 they had become extremely atrophic. 8. In some cases following the deletion of part of a motoneurone pool, the number of motoneurones per unit of cord in the remaining pool at St. 35-36 was significantly greater than control values. Since these motoneurones were located in the position (both rostro-caudal and transverse) of the control motoneurone pool, we conclude that we were able to rescue some of the neurones that would normally have died during the motoneurone cell death period (St. 30-35).

Animals↗

PubMed1980

Motoneurone projection patterns in the chick hind limb following early partial reversals of the spinal cord.

1. The development of motoneurone projection patterns in the chick hind limb from reversed spinal cord segments was studied from the onset of axonal outgrowth (St. 24) to the establishment of mature connectivity patterns (St. 36). Approximately the first three lumbosacral cord segments were reversed along the anterior-posterior axis at St. 15-16. 2. Projection patterns from reversed cord segments were assessed electrophysiologically by direct spinal cord and spinal nerve stimulation and anatomically by retrograde horseradish peroxidase (HRP) labelling of motoneurones in St. 30-36 embryos. In younger embryos, paths taken by reversed axons were characterized by orthograde HRP labelling of motoneurones in specific reversed cord segments. 3. Lumbosacral motoneurones formed appropriate functional connexions with individual limb muscles in spite of anterior-posterior shifts in their spinal cord position aned consequent shifts in their spinal nerve entry point into the limb bud. Reversed motoneurones supplying individual hind limb muscles formed discrete nuclei in the transverse plane of the cord. Each nucleus and the lateral motor column as a whole showed reversed topographical characteristics when compared to control embryos. These observations were made before (St. 30) and after (St. 35-36) the major period of motoneurone cell death. 4. Correct connectivity resulted from specific alterations in axonal pathways within the plexus or major nerve trunks proximal to the branching of individual muscle nerves. Further such directed outgrowth was present from the earliest times that axons could be traced into the limb which is before the onset of motoneurone cell death and muscle cleavage. 5. It is concluded that motoneurones are specified to project to individual muscles or to follow particular pathways prior to motoneurone birthdays and limb bud formation. The establishment of specific motoneurone connectivity can not be accounted for by passive or mechanical guidance models alone. Rather, motoneurones must also actively respond to cues within the limb or interact among themselves on the basis of an early central specification.

Animals↗

PubMed1980

Competition for survival among developing ciliary ganglion cells.

1. Functionally different subgroups, each innervating a different part of the peripheral target, were defined within the ciliary population of the avian ciliary ganglion by electrical stimulation of the various ciliary nerve branches. 2. Although neurons innervating defined parts of the peripheral target consistently sent their axons through certain nerves, the technique of retrograde horseradish peroxidase (HRP) transport showed that the ganglion cell bodies were not spatially grouped but distributed throughout the ganglion, both before and after the period of naturally occurring cell death. However, such neurons tended to be clustered into groups of two or greater. 3. Ciliary and choroid populations, however, were found to be for the most spatially separate and recognizable by location and soma size before the period of cell death. Choroid cells did not project out the ciliary nerves even prior to the cell death period, confirming previous observations of selective axon outgrowth in the two populations. 4. Competition for survival was demonstrated within the ciliary population by experimentally removing approximately two-thirds of the neurons by axotomy-induced cell death at stage 32-34 just prior to the normal cell death period. This reduction in the number of competing neurons resulted in rescue of approximately 40% of the neurons that would have died, as assessed both by the number of axon profiles in the remaining intact nerve branch, as well as the number of somata that could be retrogradely labeled from this nerve. 5. It was concluded that many of the neurons that are normally removed during the cell death period are not destined to die, but can be rescued by reducing the number of neurons competing for a limited supply of some aspect of the peripheral target. Further, the postulated interaction with the target was shown to occur relatively late, just prior to the onset of cell death. 6. At the time of the peripheral interaction, the target was found to consist primarily of myoepithelial cells, which had migrated into the target region following the arrival of the ciliary axons. The target per se, therefore, cannot be involved in the selective growth of ciliary axons to the appropriate region. Well-defined synapses were rare, although many axonal endings were observed in close contact with both myoepithelial cells and the sparser differentiated muscle fibers, which increased to account for 60% of the target by the end of the cell death period. 7. Competition was also found to retard the rate of neuronal maturation because intact axons in the partially axotomized ganglion developed more rapidly than control axons, as assessed by axon diameter, conduction velocity, and degree of glial ensheathment. 8. Finally, at least some of the neurons in the partially axotomized ganglion expanded to innervate the peripheral territory of the axotomized branches, suggesting that competition between neurons is involved in the establishment of the observed peripheral innervation pattern.

Animals↗

PubMed1978

Interactions between neurons and their targets during in vivo synaptogenesis.

In vivo synaptogenesis is described in a simple vertebrate system, the chick ciliary ganglion, a parasympathetic autonomic ganglion. An attempt is made to integrate anatomical, physiological and biochemical observations during synapse formation in the ganglion and in the peripheral target structures; the iris, ciliary muscle, and smooth muscle of the choroid coat. The relationship between synaptogenesis and neuron survival is explored, and it is shown that a critically timed interaction between the neuron and target organ is necessary for full neuronal maturation and survival. The existence of an active competition between neurons for survival is documented, and the possible relationship between neuronal cell death and specificity of connections is discussed.

Animals↗

PubMed1978

The distribution of motoneurones supplying chick hind limb muscles.

1. The motor nuclei supplying many of the hind limb muscles were localized in late chick embryos (stage 36-37; 10-11 days) by utilizing the technique of retrograde transport of horseradish peroxidase. 2. Each nucleus was found to be localized in a characteristic position in both the rostro-caudal and transverse plane of the spinal cord with only slight individual variation. 3. Each motor nucleus consisted of an elongate, coherent cluster of labelled cells, with few cells occurring outside the cluster. Thus, there did not appear to be extensive overlap of nuclei nor extensive intermingling of motoneurones projecting to different muscles. 4. The position of a motor nucleus in the transverse plane was not correlated with whether its muscle was used as an extensor or flexor; nor were adjacent nuclei necessarily co-activated during normal unrestrained walking movements as deduced from e.m.g. recordings. The position of a motor nucleus also was not correlated in a topographical manner with the adult position in the limb of the muscle to which it projected. 5. Further, while no correlation was found between the rostrocaudal position of a motor nucleus and the embryonic muscle mass from which its muscle was derived, such a relationship existed for the medio-lateral position; all muscles arising from the dorsal muscle mass, regardless of their function or adult position, were innervated by laterally situated motoneurones, all muscles arising from the ventral muscle mass by medially situated motoneurones. 6. It is concluded that motoneurone position is most closely correlated with ontogenetic events presumaeriphery. It can also be inferred that the central connexions onto motoneurones, responsible for their proper activation, cannot be achieved by a simple mechanism based largely on the position of the motoneurone soma.

Action Potentials↗

PubMed1978

The development of motor projection patterns in the chick hind limb.

1. Retrograde transport of horseradish peroxidase was used to map the initial projection patterns of lumbosacral motoneurones to the embryonic chick hind limb. 2. The stage 28 segmental projection pattern to each of the four primary muscle masses was characteristic and indistinguishable from the stage 36 projection pattern to the sum of the muscles derived from that mass. In addition, the adductor motoneurone pool was found to be similar in position (both rostro-caudal and mediolateral) at stages 29, 30, 32, 33 1/2 and 36. 3. Therefore axons from lumbosacral motoneurones project for the most part only to appropriate regions from early times shortly after they grow into the limb bud. Furthermore, the attainment of the segmental projection pattern occurs prior to the normal time of, and therefore without the aid of, cell death. This conclusion was supported by electrophysiological recordings made from muscle nerves. 4. A regionalization of the projection patterns within a single muscle mass could be shown both anatomically and physiologically prior to the cleavage of the mass into individual muscles and the projections were in a general way appropriate for the muscles derived from those regions. 5. Therefore the process of muscle cleavage does not in itself create the specific projection patterns observed, and motoneurone axons appear to grow to and to ramify and make synapses only within regions which correspond to their adult muscles. 6. Finally, the termination site of each motoneurone axon in the early limb was found to be tightly correlated in a somatotopic fashion with the position occupied by its soma in the cord. This suggests that some feature of the motoneurone related to its position may be of importance in achieving the specific projection patterns observed.

Action Potentials↗

PubMed1976

Ultrastructural differences during embryonic cell death in normal and peripherally deprived ciliary ganglia.

Normally occurring neuron death and that brought about by prior removal of the peripheral target organ was studied ultrastructurally in embryonic chick ciliary ganglion in order to better understand the mechanism of cell death in this system. Before the period of cell death, all neurons in the normal ganglion developed a well-organized rough endoplasmic reticulum (RER) which coincided with peripheral synapse formation. None of the peripherally deprived neurons underwent this change, suggesting that some interaction with the periphery, possibly synapse formation, triggered them into the secretory state. Cell death in peripherally deprived neurons was signalled by nuclear changes followed by freeing of ribosomes from polysomes and RER and presumably cessation of protein synthesis. In contrast, normal cell death was brought about by dilation of the RER with eventual cytoplasmic disruption, nuclear changes appearing only secondarily. It is suggested that failure to form or maintain peripheral synapses could result in the accumulation of transmission-related proteins with consequent cisternal dilation, and eventual cell death.

Animals↗

PubMed1976

Fate of ganglionic synapses and ganglion cell axons during normal and induced cell death.

In order to understand the significance of cell death in the formation of neural circuits, it is necessary to determine whether before cell death neurons have (a) sent axons to the periphery; (b) reached the proper target organs; and (c) have established synaptic connections with them. Axon counts demonstrated that, after sending out initial axons, ciliary cells sprouted numerous collaterals at the time of peripheral synapse formation. Subsequently, large numbers of axons were lost from the nerves, slightly later than the onset of ganglion cell death. A secondary loss of collaterals later occurred unaccompanied by cell death. Measurements of conduction velocity and axon diameters indicated that all ganglion cell axons grew down the proper pathways from the start, but it was not possible to determine whether all axons had actually formed proper synapses. This was ascertained, however, in the ganglion itself where preganglionic fibres were shown to synapse selectively with all ganglion cells before cell death. During this period, degenerating preganglionic synapses were observed on normal cells. It can therefore be inferred that at least some preganglionics established proper synapses before dying and that a single synapse is not sufficient to prevent cell death. In this system neither preganglionic nor ganglionic cell death seems designed to remove improper connections but rather to remove cells that have not competed effectively for a sufficient number of synapses, resulting in a quantitative matching up of neuron numbers.

Animals↗

PubMed1975

Size and shape of transverse tubule openings in frog twitch muscle fibers.

The openings of transverse tubules in frog twitch fibers are described. The tubules open to the extracellular space by a narrow neck, with an inner diameter of 20 nm. The most peripheral portion of the tubules is tortuous and has a variable diameter. The similarity in size of the openings of T tubules and caveolae and the meandering path of the tubules are sufficient to account for the paucity of observed openings.

Animals↗

PubMed1975

The development of functional innervation in the hind limb of the chick embryo.

1. The development of functional motor innervation was studied in the hind limb of chick embryos from Stages 25 to 43 by observing contraction of individual muscles and by recording the resultant tension when individual spinal nerves were electrically stimulated. 2. At later developmental stages (35-43) a given muscle always received functional innervation from specific spinal nerves. This pattern, with respect to the craniocaudal position of motoneurones, was similar to those described for amphibians and mammals. 3. The observed pattern was similar throughout development from the time that movement could first be elicited at Stages 27-28. There was no indication that motoneurones form initial synapses with inappropriate muscles. 4. Recordings from muscle nerves during excitation of individual spinal nerves gave results similar to the tension recordings, showing that even at early developmental stages muscle nerves did not contain substantial numbers of inappropriate axons. 5. Most limb muscles or primitive muscle masses became functionally innervated at the same time with no clearly defined proximo-distal sequence of limb innervation. 6. It appears that chick motoneurones are initially specified with respect to their peripheral destination and grow out selectively to synapse with appropriate muscles from the outset.

Action Potentials↗

PubMed1974

Synapse formation during embryogenesis on ganglion cells lacking a periphery.

1. The development of transmission was studied in chick ciliary ganglia that had been deprived of their periphery during early embryonic development.2. Peripherally deprived neurones in the ganglion differentiate in normal numbers and send functional axons into the post-ganglionic nerve.3. Ganglion cells lacking a periphery follow the normal developmental sequence sending out transient dendrites at the time ganglion cell synapses are formed, and later retracting them when calyces appear.4. Synapses, which appear functionally and ultrastructurally normal, form on all ganglion cells at the normal time and transmission is normal until Stage 34. Therefore information from the periphery is not required for ganglion cell synapse formation per se.5. From Stages 35 to 38 most cells die, so that only 8% of the original number of cells remain in the operated ganglion. Transmission fails in many cells during this same time, but precedes cell loss by only a short time, so that deafferentation probably does not contribute substantially to cell death.6. Both ciliary and choroid cells achieve full cytologic differentiation and are distinct from each other, indicating that the periphery is not required for the elaboration of the distinctive characteristics of these cells. Presynaptic fibres also differentiate into typical bouton as well as calyciform endings. Therefore, the type of preganglionic ending does not depend on ganglion cells establishing proper peripheral contacts.7. It has not been possible to ascertain whether ganglion cell specificity is affected by the periphery.8. Peripheral removal affects ganglion cell migration, so that two ganglia are formed. Approximately half of the cells migrate into the remnant optic cup forming a second misplaced ganglion. Ciliary and choroid cells occur in both ganglia and these cells go through the typical sequence of events described above.

Action Potentials↗

PubMed1974

Synaptic transmission and cell death during normal ganglionic development.

1. During normal embryonic development of the chick ciliary ganglion, cell death over a 4-day period (Stages 35-39) reduces the number of ganglion cells by half, from 6500 to 3200. Both ciliary and choroid populations are affected by approximately the same amount.2. Previous to cell death, preganglionic fibres form functional synapses on all ganglion cells, indicating that synapses form on cells which are destined to die.3. Shortly before the period of cell death, there is a failure of transmission in approximately half the cells. Some evidence suggests that transmission failure in at least some of the cells is of preganglionic origin.4. Cell death is nearly synchronous with the establishment of peripheral connexions by ganglion cells, at least with respect to the ciliary population which forms functional synapses with iris muscle. This implies that those cells which die do so because they have failed to form adequate peripheral connexions.5. It is suggested that many of the cells in which transmission has failed die, bringing transmission through the ganglion back to 100%. However, transmission failure appears to be a transitory phenomenon in other cells which survive and probably results from death of their preganglionic elements. Restoration of transmission would then be brought about by the formation of new or more effective synapses by surviving preganglionic fibres.

Action Potentials↗

PubMed1972

Axotomy mimicked by localized colchicine application.

Comparable depression of synaptic transmission in the avian ciliary ganglion resulted from either section or localized colchicine treatment of the ciliary nerves. Both colchicine treatment and axotomny produced similar changes in RNA distribution in the cell bodies as well. Colchicine did not directly affect transmission, and action potential propagation along the ciliary nerves was normal. Interference with axoplasmic transport of material in both cases is postulated to signal the observed chromatolytic changes.

Action Potentials↗

PubMed1972

Pharmacological properties, cholinesterase activity and anatomy of nerve-muscle junctions in vagus-innervated frog sartorius.

1. The pharmacological properties of the nerve-muscle junctions of vagus-innervated frog sartorius muscles were investigated. Vagus-evoked junctional potentials were ten times more sensitive to hexamethonium than were control end-plate potentials from muscles re-innervated with sartorius nerve. Hexamethonium did not affect passive electrical muscle membrane properties or quantal content. The sensitivity of vagus junctions to D-tubocurarine (dTC) did not differ from the controls.2. The amplitude and time course of vagus-evoked junctional potentials were not affected by eserine or neostigmine. Furthermore, histologically detectable acetylcholinesterase activity was not demonstrable at vagus-muscle junctions.3. The ultrastructure of vagus-innervated muscle fibres was normal in appearance indicating that the vagus was capable of structurally maintaining the muscle fibres.4. Junctional contacts were made by varicosities of unmyelinated vagal fibres. Although such varicose vagal fibres ran along the muscle fibres for long distances, they seemed to make synaptic contacts primarily at old end-plate regions, as deduced from persisting junctional folds.5. It is concluded that at least several ;trophic' factors are probably involved in the different effects of the vagus nerve on the sartorius muscle.

Acetylcholinesterase↗