Embryonic growth and innervation of rat skeletal muscles. II. Neural regulation of muscle cholinesterase

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

Neurite growth patterns leading to functional synapses in an identified embryonic neuron

We explored the relationship between neurite outgrowth and the onset of synaptic activity in the central neuropil of the leech embryo in vivo. To follow changes in early morphology and the onset of synaptic activity in the same identified neuron, we obtained whole-cell patch-clamp recordings and fluorescent dye fills from dorsal pressure-sensitive (P) cells, the first neurons that could be reliably identified in the early embryo. We followed the development of the P cell from the first extension of neurites to the elaboration of an adult-like arbor. After the growth of primary neurites, we observed a profuse outgrowth of transient neurites within the neuropil. Retraction of the transient neurites left the primary branches studded with spurs. After a dormant period, stable secondary branches grew apparently from the spurs and became tipped with terminals. At this time, neurites of the Retzius (R) cell, a known presynaptic partner in the adult, were observed to apparently contact the terminals. Although voltage-dependent currents were seen in the P cell at the earliest stage, spontaneous synaptic activity was only observed when terminals had formed. Spontaneous release was observed before evoked release could be detected from the R cell. Our results suggest that transient neurites are formed during an exploratory phase of development, whereas the more precisely timed outgrowth of stable neurites from the spurs signals functional differentiation during synaptogenesis. Because spurs have also been observed in neurons of the mammalian brain, they may constitute a primordial synaptic organizer.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

Congruity of acetylcholine receptor, acetylcholinesterase, and Dolichos biflorus lectin binding glycoprotein in postsynaptic-like sarcolemmal specializations in noninnervated regenerating rat muscles

Noninnervated regenerating muscles are able to form focal postsynaptic-like sarcolemmal specializations either in places of the former motor endplates ("junctional" specializations) or elsewhere along the muscle fibers (extrajunctional specializations). The triple labeling histochemical method was introduced to analyse the congruity of focalization in such specializations of 3 synaptic components: acetylcholinesterase (AChE), acetylcholine receptor (AChR), and a specific synaptic glycoprotein which binds Dolichos biflorus lectin (DBAR). Noninnervated regenerating soleus and extensor digitorum longus (EDL) muscles of the rat were examined and compared with denervated muscles of neonatal and adult rats. All junctional sarcolemmal specializations in noninnervated regenerating muscles accumulated AChE and AChR. Localization of the 2 components was identical within the limits of resolution of the method. DBAR could not be demonstrated in junctional specializations in 17-day-old regenerating muscles. It seems that an agrin-like inducing substance in the former junctional basal lamina invariably triggers the accumulation of both AChE and AChR in the underlying sarcolemma of the regenerating muscle fiber. However, accumulation of DBAR would probably require the presence of the motor nerve. In most of the extrajunctional sarcolemmal specializations in 8-day-old regenerating soleus and EDL muscles, both AChE and AChR accumulated. However, about 10 percent of AChE accumulations lacked AChR and about 35% of AChR accumulations lacked AChE. Even greater variability was observed in 17-day-old regenerating muscles. The presence of DBAR in the extrajunctional postsynaptic-like sarcolemmal specializations could not be demonstrated. Similar extrajunctional sarcolemmal specializations were observed in denervated postnatal rat muscles. About 70% contained both AChE and AChR, and 30% contained only AChR, but none contained DBAR. In denervated mature muscles, sparse extrajunctional AChR accumulations did not contain detectable amounts of AChE. The ability to form complex postsynaptic-like sarcolemmal specializations in the absence of nerve, which is probably inherent to noninnervated immature muscle fibers, may be reduced with muscle maturation. Variable accumulation of individual components in the postsynaptic-like specializations indicates that different triggering factors may be involved in their accumulation or, at least, the mechanisms of their accumulation can function relatively independently.

Early stages of myogenesis in a large mammal: formation of successive generations of myotubes in sheep tibialis cranialis muscle

The generation of myotubes was studied in the tibialis cranialis muscle in the sheep hindlimb from the earliest stage of primary myotube formation until a stage shortly before muscle fascicles began to segregate. Primary myotubes were first seen on embryonic day 32 (E32) and reached their maximum number by E38. Small numbers of secondary myotubes were first identified at E38, and secondary myotube numbers continued to increase during the period of study. The ratio of adult muscle fibre to primary myotube numbers was approximately 70:1, making it seem unlikely that every later generation myotube used a primary myotube as scaffold for its formation, as described in small mammals. By E62, some secondary myotubes were supporting the formation of a third generation of myotubes. Experiments with diffusible dye markers showed that primary myotubes extended from tendon to tendon of the muscle, whereas most adult fibres ran for only part of the muscle length, terminating with myo-myonal attachments to other muscle fibres in a series arrangement. Acetylcholinesterase (AChE) and acetylcholine receptor (AChR) aggregations appeared in multiple bands across the muscle shortly after formation of the primary generation of myotubes was complete. The number of bands and their pattern of distribution across the muscle as they were first formed was the same as in the adult. Primary myotubes teased from early muscles had multiple focal AChE and AChR deposits regularly spaced along their lengths. We suggest that the secondary generation of myotubes forms at endplate sites in a series arrangement along the length of single primary myotubes, and that tertiary and possibly later generations of myotubes in their turn use the earlier generation myofibres as a scaffold. Although the fundamental cellular mechanisms appear to be similar, the process of muscle fibre generation in large mammalian muscles is more complex than that described from previous studies in small laboratory rodents.

Diisopropylfluorophosphate inhibits acetylcholinesterase activity and disrupts somitogenesis in the zebrafish

Acetylcholinesterase (AChE) activity, localized histochemically, appeared in the nuclei of presumptive somitic mesodermal cells prior to the onset of somitogenesis. AChE activity appeared in a rostro-caudal sequence, in cells located the equivalent of five somite lengths caudal to the last formed somite. To investigate whether AChE activity was required for somitogenesis, several inhibitors of AChE activity were tested for their ability to block somitogenesis. Diisopropylfluorophosphate (DFP), a broad spectrum inhibitor of serine proteases and related enzymes, was the only AChE inhibitor tested that disrupted somitogenesis. Gastrulae at 50% epiboly exposed continuously to DFP at concentrations between 40 microM and 90 microM completed epiboly, but exhibited a dose-dependent decrease in the number of somites formed, and a parallel decrease in the caudal extent of somite innervation, by 24 hours post-fertilization (h). Fifteen somite (15h) embryos exposed to DFP at the ED50 of 70 microM for 3 hours, followed by recovery to 24h, developed abnormal somites. Approximately five normal somites formed after drug treatment before the first abnormal somite formed. The abnormal somites corresponded in location to that area of the presumptive somitic mesoderm that would have initiated AChE activity while the DFP was present. While exposed to 70 microM DFP, presumptive somites formed and motoneurons extended processes that had initiated AChE activity at the time of treatment with DFP, although at a slower than normal rate. However, embryos exposed to 1 mM DFP for 30 minutes at both the 5 and 15 somite stages, followed by recovery to 24h, developed the normal number of somites but were reduced in the caudal extent of somite innervation, and occasionally developed abnormal primary motoneurons. As with the abnormal somites, the abnormal motoneurons would have initiated AChE activity while the DFP was present. Presumptive somitic mesoderm unable to initiate AChE activity due to inhibition by DFP developed abnormally. While the effects of DFP are not limited to inhibiting AChE, the data support the "clock and wavefront" model proposed for somite formation, and support the hypothesis that AChE activity has a role in somitogenesis in zebrafish.

Branching patterns of the rat phrenic nerve during development and reinnervation

Previous studies have shown that the phrenic motor nucleus in the rat projects onto the diaphragm muscle, forming an orderly topographic map. Moreover, this topography is partially restored upon reinnervation. This orderly map is expressed prior to birth, suggesting that early contacts between nerve and muscle are topographically appropriate. The phrenic divides during embryonic development into rostral and caudal branches, and motor axons preferentially enter the appropriate branch. In an effort to understand the mechanisms that underlie the choices growing phrenic neurons make in selecting their appropriate muscle targets, we examined the patterns of branching displayed by the phrenic nerve during development and reinnervation. In all muscles studied the phrenic nerve splits into three primary branches, rostral, caudal, and crural. At a coarse level the pattern of branching of the phrenic is remarkably consistent from animal to animal and at all ages of development. At a finer level of resolution, however, there is an asymmetry between right and left hemidiaphragms. Moreover, the precise emergence of any particular branch is unpredictable, resulting in an overall incongruence in branching architecture from animal to animal. The hemidiaphragm muscle grows unevenly, particularly on the right side, resulting in greater muscle fiber elongation medially. Upon reinnervation, the same coarse pattern of branching is reestablished, but the higher order pattern is much simpler and muscle growth is slower than in controls. These results suggest that very early in development primary branches of the phrenic funnel axons into three well-defined zones in the muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental modulation of physicochemical variants of the tailed asymmetric (16S) acetylcholinesterase by neuromuscular activity and innervation in the mouse embryo

We have studied the physicochemical properties of acetylcholinesterase (AChE) during embryonic development of normal and functionally impaired mouse skeletal muscle, focusing on the tailed asymmetric (16S) form of the enzyme. The muscle-specific 16S AChE exists in two different variants. One is associated with extracellular matrix and is high-salt soluble (HSS, also termed hydrophilic AChE), whereas the other form is anchored to cell membranes and is detergent extractable (DE, or hydrophobic AChE). Before innervation during normal embryonic development, both hydrophilic and hydrophobic 16S AChE exist in equal amounts. After muscle innervation, there was an increase (amounting three-fold on E18) in the levels of hydrophilic vs. hydrophobic 16S AChE. This alteration of the relative proportions of the two variants of 16S AChE did not occur in chronically inactive muscles either from the mouse mutant, muscular dysgenesis, or from tetrodotoxin-treated mouse embryos. Taken together with previous reports, the present results suggest that postsynaptic membrane depolarization-induced Ca2+ fluxes are important in modulating not only the synthesis of 16S AChE, but also the relative proportions of both physicochemical variants of this molecular form of AChE.

Synaptic dynamics at the neuromuscular junction: mechanisms and models

During development, the neuromuscular junction passes through a stage of extensive polyinnervation followed by a period of wholesale synapse elimination. In this report we discuss mechanisms and interactions that could mediate many of the key aspects of these important developmental events. Our emphasis is on (1) establishing an overall conceptual framework within which the role of many distinct cellular interactions and molecular factors can be evaluated, and (2) generating computer simulations that systematically test the adequacy of different models in accounting for a wide range of biological data. Our analysis indicates that several relatively simple mechanisms are each capable of explaining a variety of experimental observations. On the other hand, no one mechanism can account for the full spectrum of experimental results. Thus, it is important to consider models that are based on interactions among multiple mechanisms. A potentially powerful combination is one based on (1) a scaffold within the basal lamina or in the postsynaptic membrane which is induced by nerve terminals and which serves to stabilize terminals by a positive feedback mechanism; (2) a sprouting factor whose release by muscle fibers is down-regulated by activity and perhaps other factors; and (3) an intrinsic tendency of motor neurons to withdraw some connections while allowing others to grow.

Two types of focal accumulations of acetylcholinesterase appear in noninnervated regenerating skeletal muscles of the rat

Muscle fibers in the soleus muscle of the rat, injured by bupivacaine and free autografting, were allowed to regenerate within their old basal laminae. Histochemical and cytochemical analysis of newly synthesized acetylcholinesterase (AChE) revealed that two kinds of focal accumulations of AChE appeared in regenerating myotubes. First, AChE gets concentrated at the sites of the former motor endplates. Accumulation of AChE starts in places where a tight contact between the remnants of the old junctional basal lamina and the budding surface of the myotube engulf the extracellular material. Appearance of these AChE accumulations can be prevented by papain treatment of the soleus muscle before autografting but not by predenervating it for 1 month. Focalization of AChE is probably induced by a component of the junctional basal lamina, possibly a protein, the existence of which is not dependent upon continuous presence of the motor nerve and may be produced by the muscle. This view is corroborated by the fact that an additional kind of AChE accumulation appeared in regenerating muscles in regions remote from the sites where motor endplates were located in the muscles of origin. Although differing in localization, size, and appearance, both kinds of AChE accumulations ultrastructurally resemble the postsynaptic specialization of the motor endplate: they consist of tubelike sarcolemmal invaginations containing AChE. The extrajunctional AChE accumulations seem to arise spontaneously and are usually located more than 750 micron away from the junctional ones as if some local inhibitory mechanism prevents their formation in the immediate vicinity.

Development of postsynaptic-like specializations of the neuromuscular synapse in the absence of motor nerve

It was previously reported that the acetylcholine receptor clusters and acetylcholinesterase appear on embryonic superior oblique muscle cells developing in vivo without motor nerve contacts. The objective of this study was to examine whether some other components of neuromuscular junction also form on muscle cells developing in vivo in the absence of motor neurons. In the present study, postsynaptic specializations such as junctional folds, postsynaptic density and basal lamina were studied in normal and aneural muscles. The superior oblique muscle of duck embryos was made aneural by permanent destruction of trochlear motor neurons by cauterizing midbrain on embryonic day 7; 3 days before the motor neurons normally project their axons into the muscle. Normal and aneural muscles from embryonic days 10 to 25 were processed for electron microscopy. The results indicate that morphological specializations such as junction-like folds, postsynaptic-like density, and basal lamina also develop in the absence of motor neuron contacts. Whether the differentiation of specialized synaptic basal lamina is dependent on the presence of motor neurons was examined by utilizing a monoclonal antibody against heparan sulfate proteoglycan. Immunohistochemical studies indicate that specialized synaptic basal lamina differentiates in the absence of motor neurons. Thus, the mechanism of development of postsynaptic components of neuromuscular junction in this muscle is not dependent on motor neuron contacts. These results also suggest that the postsynaptic cell plays a more active role in synapse formation than previously realized. The results are discussed in relation to the control of synapse numbers by the postsynaptic cell.

Molecular forms and localization of acetylcholinesterase and nonspecific cholinesterase in regenerating skeletal muscles

Molecular forms and histochemical localization of acetylcholinesterase and nonspecific cholinesterase were analysed in muscle regenerates obtained from rat EDL and soleus muscles after ischaemic-toxic degeneration and irreversible inhibition of preexistent enzymes. Regenerating myotubes and myofibres produce the 16S AChE form in the absence of innervation. The 10S AChE form prevails over 4S form with maturation into striated fibres. Although the patterns of AChE molecular forms in normal EDL and soleus muscles differ significantly no such differences were observed in noninnervated regenerates from both muscles. Two types of focal accumulation of AChE appear on the sarcolemma of regenerating muscles: first, in places of former motor endplates and, second, in extra-junctional regions. The 4S form of nonspecific cholinesterase is prevailing in regenerating myotubes whereas its asymmetric forms or focal accumulations could not be identified reliably. The satellite cells which survive after muscle degeneration probably originate from some type of late myoblasts and transmit the information concerning the ability to synthesize the asymmetric AChE forms and to focally accumulate AChE to regenerating muscle cells. Synaptic basal lamina from former motor endplates may locally induce AChE accumulations in regenerating muscles.

Neonatal and adult myosin heavy chain isoforms in a nerve-muscle culture system

When adult mouse muscle fibers are co-cultured with embryonic mouse spinal cord, the muscle regenerates to form myotubes that develop cross-striations and contractions. We have investigated the myosin heavy chain (MHC) isoforms present in these cultures using polyclonal antibodies to the neonatal, adult fast, and slow MHC isoforms of rat (all of which were shown to react specifically with the analogous mouse isoforms) in an immunocytochemical assay. The adult fast MHC was absent in newly formed myotubes but was found at later times, although it was absent when the myotubes myotubes were cultured without spinal cord tissue. When nerve-induced muscle contractions were blocked by the continuous presence of alpha-bungarotoxin, there was no decrease in the proportion of fibers that contained adult fast MHC. Neonatal and slow MHC were found at all times in culture, even in the absence of the spinal cord, and so their expression was not thought to be nerve-dependent. Thus, in this culture system, the expression of adult fast MHC required the presence of the spinal cord, but was probably not dependent upon nerve-induced contractile activity in the muscle fibers.

Increased motor neuron projection during development does not increase the number of neuromuscular synapses

We investigated in developing embryos whether the total number of neuromuscular synapses is determined by the muscle or by the number of innervating motor neurons. The superior oblique muscle of duck embryos was hyperinnervated by preventing the naturally occurring death of trochlear motor neurons using immunoglobulin G from patients with acquired myasthenia gravis. In spite of a significant increase in the number of motor neurons innervating the muscle, a corresponding increase in the number of neuromuscular synapses did not occur. These results suggest that the total number of synapses in a muscle is independent of the number of innervating motor neurons and that it is determined intrinsically by the muscle itself.

Aggregating factor from Torpedo electric organ induces patches containing acetylcholine receptors, acetylcholinesterase, and butyrylcholinesterase on cultured myotubes

A factor in extracts of the electric organ of Torpedo californica causes the formation of clusters of acetylcholine receptors (AChRs) and aggregates of acetylcholinesterase (AChE) on myotubes in culture. In vivo, AChRs and AChE accumulate at the same locations on myofibers, as components of the postsynaptic apparatus at neuromuscular junctions. The aim of this study was to compare the distribution of AChRs, AChE, and butyrylcholinesterase (BuChE), a third component of the postsynaptic apparatus, on control and extract-treated myotubes. Electric organ extracts induced the formation of patches that contained high concentrations of all three molecules. The extract-induced aggregation of AChRs, AChE, and BuChE occurred in defined medium, and these components accumulated in patches simultaneously. Three lines of evidence indicate that a single factor in the extracts induced the aggregation of all three components: the dose dependence for the formation of patches of AChRs was the same as that for patches of AChE and BuChE; the AChE- and BuChE-aggregating activities co-purified with the AChR-aggregating activity; and all three aggregating activities were immunoprecipitated at the same titer by a monoclonal antibody against the AChR-aggregating factor. We have shown previously that this monoclonal antibody binds to molecules concentrated in the synaptic cleft at neuromuscular junctions. Taken together, these results suggest that during development and regeneration of myofibers in vivo, the accumulation at synaptic sites of at least three components of the postsynaptic apparatus, AChRs, AChE, and BuChE, are all triggered by the same molecule, a molecule similar if not identical to the electric organ aggregating factor.

Development of the trochlear nucleus in quail and comparative study of the trochlear nucleus, nerve, and innervation of the superior oblique muscle in quail, chick, and duck

The present study was undertaken to examine the development of the trochlear nucleus in quail and to compare the mature trochlear nucleus, nerve, and their sole target of innervation, the superior oblique muscle, in quail, chick, and duck. Study of the trochlear nucleus in quail from embryonic day 5 through hatching shows a maximum of 1,248 neurons on embryonic day 10 followed by spontaneous degeneration of 40% of the neurons between days 10 and 16. Previous studies have shown that although the initial and final number of neurons is different in chick and duck, the magnitude of trochlear cell loss in both species is about 40%. This study shows the average number of neurons in the nucleus of quail, chick, and duck, 2 weeks post-hatching, to be 658, 743 and 1,459, respectively. Fiber counts in the trochlear nerve from electron micrograph montages at the same period indicated a ratio of about 1:1 between neurons and axons. While a majority of the fibers in these nerves are myelinated, an average of 3-6% of the fibers are unmyelinated. The nucleus in the quail not only contains the smallest number of neurons but it also innervates the smallest muscle in terms of total number of muscle cells and endplates. However, the opposite relationship does not hold true. The nucleus in duck contains the largest number of neurons, yet the largest number of muscle cells and endplates were found in the chick. The ratios between the neurons and muscle cells as well as between neurons and endplates are about the same in quail and duck. These ratios are much higher in the chick, reflecting the relatively small neuron pool destined for a relatively large target. In spite of variations in the number of neurons, muscle fibers, and endplates the average number of endplates per muscle fiber is relatively constant among the three species.

Development of synaptic currents in immobilized muscle of Xenopus laevis

The effect of chronic immobilization on the development of synaptic currents was studied in myotomal muscle of Xenopus laevis. Embryos and tadpoles were immobilized by rearing them in the presence of tetrodotoxin (TTX) after removal of the egg membranes. Immobilization did not affect the developmental change in duration of miniature end-plate currents (m.e.p.c.s). Rise times decreased from about 3 to 0.6 ms in both immobilized and control muscle, and decay constants decreased from about 7 to 1 - 2 ms in both conditions. M.e.p.c.s with double exponential decays were recorded in both immobilized and control muscle at intermediate and late developmental stages. The fast and slow decay constants were 0.7 ms and slightly less than 3 ms in older muscle of both groups. These values are comparable to the apparent open times of fast and slow ACh receptors present on Xenopus muscle. Application of an anticholinesterase (methanesulphonyl fluoride) lengthened the duration of m.e.p.c.s comparably in immobilized and control muscle. These data indicate that the deposition of junctional acetylcholinesterase and the reduction in open time of acetylcholine receptor channels in developing Xenopus myotomal muscle are independent of contractile activity of muscle and TTX-blockable action potentials in muscle or motoneurones.

Acetylcholinesterase is functional in embryonic rat muscle before its accumulation at the sites of nerve-muscle contact

We have examined the expression, the location, and the physiological activity of acetylcholinesterase (AChE) in developing intercostal muscles in the rat. Although focal accumulations of AChE at developing end plates do not appear until Embryonic Day (ED) 16-17, 16 S AChE is present at ED 14. Experiments with permeable and impermeable inhibitors established that prior to focal accumulation most of the 16 S enzyme is on the surface of muscle fibers, where it constitutes the major species. Intracellular recording from developing muscle fibers showed that as early as ED 14, AChE inhibitors prolonged evoked end-plate potentials. We conclude that prior to its focal accumulation, AChE is present on the surface of muscle fibers and is physiologically active. Histochemical staining of the focally accumulated enzyme demonstrated that the enzyme is concentrated both intracellularly and extracellularly at the sites of developing nerve-muscle contacts.

Development of basal lamina in synaptic and extrasynaptic portions of embryonic rat muscle

Each vertebrate skeletal muscle fiber is ensheathed by a basal lamina (BL) which passes through the synaptic cleft of the neuromuscular junction. In the adult, the synaptic portion of the BL is both functionally and chemically specialized. We have used an immunofluorescence method to compare the development of synaptic and extrasynaptic portions of BL in embryonic rat intercostal muscles. Immunohistochemical staining of adult muscle fibers with monoclonal and serum antibodies defines "synaptic" antigens (including acetylcholinesterase) that are concentrated in synaptic BL, "extrasynaptic" antigens that are concentrated in extrasynaptic regions, and "shared" antigens (including collagen IV, fibronectin, laminin, and a heparan sulfate proteoglycan) that are present in both synaptic and extrasynaptic BL ( Sanes and Chiu , 1983). Synapses appear on newly formed myotubes on embryonic Day 14 (E14; birth is on E22 ). Patches of BL that contain shared and extrasynaptic antigens are present on myotube surfaces by E15, and BL forms a continuous sheath by E17. Shared antigens are present at but not confined to synaptic areas by E15. Two synaptic antigens appear in synaptic areas a day later, and are not detectable extrasynaptically . At least one extrasynaptic antigen is present at immature synapses, and lost or masked by E19 . Thus synaptic BL is not assembled as a unit; rather, components are added, lost, or modified as synaptogenesis proceeds.

In vivo development of cholinesterase at a neuromuscular junction in the absence of motor activity in Xenopus laevis

Embryos of Xenopus laevis were selected prior to the onset of innervation and were raised for 2 days in the anaesthetic tricaine methanesulphonate (200 micrograms/ml). The gross development of these tricaine-reared animals appeared normal despite the absence of spontaneous motor activity and the lack of motor responses to prodding with a pin. Motor activity quickly appeared when the anaesthetic was withdrawn. Intracellular recording from the myotomes of intact, tricaine-maintained animals failed to reveal any spontaneous muscle action potentials. Synaptic potentials increased in frequency and amplitude upon withdrawing tricaine, but resting potentials remained unchanged. Cholinesterase activity, detected histochemically, was observed at the ends of the myotomes, the main site of innervation. The intensity of the histochemical reaction product at these sites appeared to be about as great in the myotomes of tricaine-reared animals as in control myotomes. Miniature end-plate currents (m.e.p.c.s), examined by focal external recording, declined with a time constant of 2.9 +/- 0.2 ms (mean +/- S.E. of mean) in the myotomes of tricaine-reared animals (stages 40-41). The time constants in the myotomes of control animals were 1.8 +/- 0.1 ms at stages 40-41 and 8.7 +/- 0.7 ms at stages 24-26 (shortly after the onset of innervation). The anticholinesterase neostigmine doubled m.e.p.c. time constants in the myotomes of tricaine-reared animals as well as in control myotomes at stages 40-41. It is concluded that motor activity is not required for the in vivo development of physiological levels of synaptic cholinesterase in Xenopus myotomal muscle.

Developmental changes in the distribution of acetylcholine receptors in the myotomes of Xenopus laevis

The acquisition and distribution of nerve fibres and of acetylcholine (ACh) receptors were examined in the myotomes of Xenopus laevis during normal development. This muscle is well-suited for investigating temporal relationships during neuromuscular synaptogenesis because the age of the Xenopus embryo at the onset of innervation can be assessed with an accuracy of about one hour. Myotomal nerve fibres were visualized after staining them with nitroblue tetrazolium and ACh receptors were examined after exposure to alpha-bungarotoxin labelled with 125I or fluorescent dye. Nerve fibres were seen in the myotomes of some embryos as early as stage 19 (20 . 75 hr) and in virtually all embryos by stage 24 (26 . 25 hr). From the outset they were located mainly at the ends of the myotomes, but some myotomes also exhibited nerve fibres in more central regions. ACh receptors were already present in myotomes by stage 19 (20 . 75 hr) and initially had a widespread, uniform distribution. The density of extrajunctional ACh receptors increased until stage 36 (50 hr) and then declined less than 3-fold over the next 10 days of development. Discrete patches of high ACh receptor density began to appear at the ends of the myotomes at stage 22 (24 hr) and were seen in almost all embryos by stage 26 (29 . 5 hr). ACh receptor patches were also seen in central regions of some myotomes and these were usually aligned in patterns which resembled the course of nerve fibres. The present findings suggest that myotomal muscle cells in Xenopus embryos begin to acquire ACh receptors shortly before the arrival of nerve fibres and that discrete patches of ACh receptors begin to form at presumptive synaptic sites on the average about 3 hr after the arrival of the nerve fibres. The latter delay is considerably shorter than that in developing rat muscle. The temporal and spatial relationships between nerve fibres and the development of ACh receptor patches in Xenopus myotomes in vivo are consistent with findings in Xenopus cell cultures which indicate that nerve fibres can rapidly induce ACh receptor localization at sites of nerve--muscle contact.

Embryonic growth and innervation of rat skeletal muscles. III. Neural regulation of junctional and extra-junctional acetylcholine receptor clusters

The number and distribution of acetylcholine (ACh) receptors on muscle cells was studied during development of normal, paralysed and aneural embryonic rat diaphragm muscles. (i) ACh receptors initially are dispersed over the surface of rat embryo myotubes. At day 15| of gestation junctional receptor clusters (‘J-clusters’) form in a well ordered band across the midline of the diaphragm muscle; these also form in denervated and paralysed muscles. At about day 18 of gestation additional ‘EJ-clusters’ develop to either side of the midpoint of treated muscles. (ii) If a nerve terminal is present, J-clusters increase in length with time. The time course of generation of new endplates calculated from frequency distributions of J-cluster lengths accurately predicts the muscle growth curve established from muscle fibre counts. (iii) The mean length of J-clusters in paralysed muscles was greater than in controls, due to small new-formed clusters failing to appear. In muscles allowed to recover from paralysis the mean length was less, due to a preponderance of small, new-formed clusters. These observations show that development of new endplates, which is thought to reflect the development of new muscle cells, is halted in paralysed muscles, and recovery from paralysis is associated with the generation of many new endplates. (iv) J-clusters appeared, but failed to grow, in aneural muscles. In muscles denervated during the later stages of gestation, analysis of the distribution of J-cluster lengths shows that new clusters failed to appear, and existing clusters showed little or no increase in length after the time of removal of the nerve. (v) EJ-clusters form by aggregation of dispersed receptors, and their mean length increases with time. They do not appear to be stable entities, and are removed within 2 d of recovery from paralysis. In paralysed muscles, with both J-clusters and EJclusters present, only J-clusters attract nerve sprouts or become innervated. (vi) A curve is derived showing development of the total number of synaptic terminals in a muscle. This number increases during days 13-18 of gestation, reaching a peak of about 170 % of the adult value during dl8 and d l9 of gestation. There are two episodes of terminal elimination, one during days 19-21 of gestation, and another about 2 weeks postnatally. During the first postnatal week the number of terminals remains constant at about 140% of the adult number, while the average number of inputs per fibre goes down and the number of muscle fibres increases. (vii) Innervation is essential for muscle development. Motoneurons cannot regulate the number of muscle fibres by requiring a simple one-to-one relation between nerve terminal and muscle fibre, and if their role is regulatory as well as supportive of muscle development then some more complex relationship between nerve terminals and developing myotubes must be postulated.