Variations in the distribution of motor end-plates in the avian pectoralis

Most avian muscles consist of serially arranged, overlapping fibers that do not extend the length of the muscle. This condition appears to be plesiomorphic with respect to diapsid reptiles. The presence of this serialfibered architecture is evidenced by bands of stained motor end-plates (meps) perpendicular to the columns of fibers and dividing each column into a series of “segments.” The avian pectoralis was chosen for a study of variation in the distribution of meps within a single muscle. We report the interspecific variation for 158 specimens in 63 species. We also use additional specimens to examine intraspecific variation. Setting aside hummingbirds, which have an unique and clearly derived condition, the number of mep bands along a column of fibers near the shoulder falls within a remarkably small range. The number of segments is not obviously related to phylogenetic relatedness or to any characteristic of flight or ecology and is only slightly related to size. The largest specimens do average more segments per column, but there are no trends among small to medium-sized species, suggesting that there is an upper limit to fiber length. However, the shape of the sternum and pattern of connective tissue in the pectoralis alleviate the need for additional fibers in many large birds. These findings suggest that the architecture of the avian pectoralis is subject to some as yet unexplained selection that stabilizes the number of myofibers and/or motor neurons. The findings provide few clues as to whether the significant factors are phylogenetic, functional, ontogenetic, or some combination of these. © 1993 Wiley-Liss, Inc.     

Histochemical and myosin composition of vampire bat (Desmodus rotundus) pectoralis muscle targets a unique locomotory niche

The vampire bat pectoralis muscle contains at least four fiber types distributed in a nonhomogeneous pattern. One of these fiber types, here termed IIe, can be elucidated only by adenosine triphosphatase (ATPase) histochemistry combined with reactions against antifast and antislow myosin antibodies. The histochemical and immunohistochemical observations indicate a well-developed specialization of function within specific regions of the muscle. In parallel, analyses of native myosin isoforms and myosin heavy chain isoforms indicate two points. First, the histochemical “type IIe” fiber is predominant in cranial portions of the muscle, and myosin extracted from these regions exhibits a unique electrophoretic mobility not observed in the myosin isoforms of more traditional laboratory mammals. Second, the type I fibers are confined to the pectoralis abdominalis muscle and a small adjacent region of the caudal part of the pectoralis. This pattern of type I fiber distribution is considered a derived character state compared to muscle histochemical phenotype and isoform composition in the pectoralis muscles of other phyllostomids we have studied (Artibeus jamaicensis, Artibeus lituratus, Carollia perspicillata). We relate this to the unique locomotory needs of the common vampire bat, Desmodus rotundus. © 1993 Wiley-Liss, Inc.     

Distribution and innervation of short, interdigitated muscle fibers in parallel-fibered muscles of the cat hindlimb

The cat hindlimb contains several long, biarticular strap muscles composed of parallel muscle fascicles that attach to short tendons. Three of these muscles--sartorius, tenuissimus, and semitendinosus--were studied by dissecting individual gold-stained fibers and determining the surface distribution of acetylcholinesterase-stained end-plate zones. In each muscle, fascicles were composed of muscle fibers that ran only part of the fascicle length and tapered to end as fine strands that interdigitated with other tapering fibers within the muscle mass. Most muscle fibers measured 2-3 cm in length. Fascicles of muscle fibers were crossed by short transverse bands of endplates (1 mm wide by 1-5 mm long) that were spaced at fairly regular intervals from the origin to the insertion of the muscle. The endplate pattern suggested that the fiber fascicles were organized into multiple longitudinal strips. In the sartorius, the temporospatial distribution of electromyographic (EMG) activity evoked by stimulating fine, longitudinal branches of the parent nerve confirmed that each strip was selectively innervated by a small subset of the motor axons. These axons appeared to distribute their endings throughout the entire length of the fascicles, providing for synchronous activation of their in-series fibers.     

A histochemical study of twitch and tonus fibers

The distribution of glycogen, lipids and succinic dehydrogenase (SDH) in twitch and tonus fibers of several amphibians and birds is described, and the correlation of histochemical properties with fiber structure and function is discussed. Twitch and tonus fibers were identified histologically by the presence of Fibrillenstruktur and Felderstruktur respectively. The rectus abdominis, sartorius and semitendinosus were studied in Rana pipiens, Xenopus laevis and Necturus maculosus; the pectoralis major, pectoralis minor, anterior latissimus dorsi and posterior latissimus dorsi were investigated in Gallus gallus and Passer domesticus. Periodic acid-Schiff was used to stain for glycogen, Sudan Black B for lipids and Nitro BT for localization of SDH activity. In amphibian muscles, fibers with Fibrillenstruktur and Felderstruktur constitute the rectus abdominis. Except in one case, only Fibrillenstruktur fibers were seen in the sartorius and semitendinosus. In the avian muscles, fibers with Fibrillenstruktur comprise the pectoralis major, pectoralis minor and posterior latissimus dorsi, while fibers with Felderstruktur constitute the anterior latissimus dorsi. These types of muscle fibers showed no consistent pattern in the distribution of glycogen, lipids and SDH. The evidence precludes the use of such data alone for distinguishing twitch (Fibrillenstruktur) and tonus (Felderstruktur) fibers.     

Histochemical and biochemical properties of flight muscle fibers in the little brown bat,Myotis lucifugus

Summary The purpose of this investigation was (1) to determine the fiber composition of pectoralis muscle of the little brown bat,Myotis lucifugus; (2) to compare the fiber composition of this muscle with two of the animal's accessory flight muscles; and (3) to study the effect of hibernation on pectoralis muscle fiber composition. Bat skeletal muscle fibers were also compared with those of white laboratory rats (Rattus norvegicus). Bat pectoralis muscles possessed exceptionally high oxidative capacities as indicated by their succinate dehydrogenase activities, but relatively low glycolytic potentials (phosphofructokinase activities). Muscle histochemistry demonstrated that fiber composition of bat pectorlis muscle was homogeneous; all fibers possessed high aerobic and low glycolytic potentials, and high myofibrillar ATPase activities indicating fast contractile properties. In contrast, accessory flight muscles possessed three distinguishable fiber types. During hibernation there was a significant decline in oxidative potential, no change in glycolytic potential, and no alteration in basic fiber composition of bat pectoralis muscle. The findings of this study suggest that pectoralis muscles ofM. lucifugus may approach the ultimate adaptation of a mammalian locomotory muscle for aerobic generation of muscular power.Abbreviations FG fast-twich glycolytic - FOG fast-twitch-oxydative-glycolytic - -GPDH -glycerophosphate dehydrogenase - LDH lactate dehydrogenase - NADH-D reduced nicotinamide adenine dinucleotide diaphorase - PFK phosphofructokinase - SDH succinate dehydrogenase - SO slowtwich-oxidative     

Ontogeny of the Pectoralis muscle in the little brown bat,Myotis lucifugus

The ontogeny of a primary flight muscle, the pectoralis, in the little brown bat (Myotis lucifugus: Vespertilionidae) was studied using histochemical, immunocytochemical, and electrophoretic techniques. In fetal and early neonatal (postnatal age 1–6 days) Myotis, histochemical techniques for myofibrillar ATPase (mATPase) and antibodies for slow and fast myosins demonstrated the presence of two fiber types, here called types I and IIa. These data correlated with multiple transitional myosin heavy chain isoforms and native myosin isoforms demonstrated with SDS-PAGE and 4% pyrophosphate PAGE. There was a decrease in the distribution and number of type I fibers with increasing postnatal age. At postnatal age 8–9 days, the adult phenotype was observed with regard to muscle fiber type (100% type IIa fibers) and myosin isoform profile (single adult MHC and native myosin isoforms). This “adult” fiber type profile and myosin isoform composition preceeded adult function by about 2 weeks. For example, little brown bats were incapable of sustained flight until approximately postnatal day 24, and myofiber size did not achieve adult size until approximately postnatal day 25. Although Myotis pectoralis is unique in being composed of 100% type IIa fibers, transitional fiber types and isoforms were present. These transitional forms had been observed previously in other mammals bearing mixed adult muscle fibers and which undergo transitional stages in muscle ontogeny. However, in Myotis pectoralis, this transition transpires relatively early in development. © 1994 Wiley-Liss, Inc.     

Muscle Fiber Types in the Pectoralis of the White Pelican,a Soaring Bird

The pectoralis muscle (M. pectoralis) of many premier soaring birds contains a smaller, accessory, deep belly in addition to the much larger superficial belly found in all flying birds. Here we describe the muscle fiber types in both the superficial and deep bellies of the pectoralis of one such adept soaring species, the white pelican (Pelecanus erythrorhynchos).Histochemical techniques are used to demonstrate both nicotinamide adenine dinucleotide (reduced) and myofibrillar adenosine triphosphatase activities within the muscle fibers. Immunocytochemical methods employing several monoclonal antibodies, each directed against a different myosin heavy chain epitope of the chicken, are also used to characterize the fibers. While the superficial belly of the muscle consists entirely of fast-twitch oxidative-glycolytic fibers, the deep belly is composed exclusively of slow fibers. These slow fibers are labelled by two different antibodies specific for chicken slow myosin. We suggest that the fibers of the superficial belly are best suited to flapping flight, and that the fibers of the deep belly would be recruited only during soaring flight. Furthermore, we hypothesize that the deep belly found in the pectoralis of soaring species probably evolved from a deep neuromuscular compartment of the superficial belly.     

Elimination of multiple innervation in the developing avian superior oblique muscle (1)

Motor endplates in the developing avian superior oblique muscle first appear on day 18 of incubation. Most of the endplates from this time through hatching (day 27) are innervated by multiple fibers. Each endplate in the post-hatching period is innervated by only one fiber. Time of elimination of multineuronal innervation does not correlate with the time of trochlear neuron loss; the former occurs much later in development. Removal of multiple innervation is therefore, not the cause of the naturally occurring neuron loss.     

Regeneration of specific innervation in Xenopus pectoralis muscle

We investigated the motor unit organization and precision of reinnervation in the Xenopus pectoralis muscle following different manipulations, including crush or section of the posterior pectoralis nerve, foreign nerve innervation, and crush coupled with activity modulation or block. Most fibers have two neuromuscular junctions, and multielectrode recordings were used to identify the axonal origin of all inputs to both junctions on most or all fibers covering about 25% of the muscle surface. Following simple nerve crush, a highly organized innervation pattern was restored, indistinguishable from the normal pattern, including selective innervation of fibers of similar input resistance (R_in), compact motor unit organization, and high incidence of exclusive innervation of both end plates on each fiber by the same axon (distributed mononeuronal innervation, or a/a pattern). Initial reinnervation was equally precise when nerve conduction in the regenerating nerve was blocked by tetrodotoxin. More distant or repeated nerve crush or nerve section delayed and reduced the precision of reinnervation, but the majority of fibers still received input to both end plates by the same axon, often in combination with others. A foreign nerve, the pectoralis sternalis, which in its own muscle forms only single end plates, showed less precise reinnervation, but still had an incidence of a/a innervation far above chance. These data imply the expression and recognition of remarkably precise chemospecific cues even in mature animals, superimposed on which is a further refinement by synapse elimination, probably based on an activity-dependent process. © 1996 John Wiley & Sons, Inc.     

Histochemical Characterization and Distribution of Fiber Types in the Pectoralis Muscle of the Ostrich (Struthio camelus) and Emu (Dromaius novaehollandiae)

The muscle fibers of the pectoralis (M. pectoralis pars thoracicus) of a male and a female ostrich (Struthio camelus) and a male and a female emu (Dromaius novaehollandiae) were studied histochemically for succinate dehydrogenase and myofibrillar adenosine triphosphatase. Slow-tonic (ST), fast-twitch oxidative-glycolytic (FOG) and fast-twitch glycolytic (FG) fibers were approximately equal in number and distribution in the emu pectoralis examined. In the ostriches, both predominantly FG and approximately equal areas, were present. ST fibers were significantly (P ≤ 0.05) larger than the similarly (P ≥ 0.05) sized FG and FOG fibers in the female ostrich and emus. In the male ostrich ST fibers were smaller (P ≤ 0.05) than FG fibers, neither of which were significantly (P ≥ 0.05) different from FOG fibers. The ratites have the greatest percentage and widest distribution of ST fibers found in any avian pectoralis studied to date. This could represent the ancestoral avian pectoralis, neoteny or an effect of flightlessness. ST fibers are used in the maintenance of posture, which is probably the main role of the pectoralis in the emu. The predominantly FG areas of the ostrich are indicative of an additional function, namely, behavioural display. Sexual dimorphism in the ostrich pectoralis is strongly suggested.     

Anatomy and histochemistry of spread-wing posture in birds. 2. Gliding flight in the California Gull,Larus californicus: A paradox of fast fibers and posture

Gliding flight is a postural activity which requires the wings to be held in a horizontal position to support the weight of the body. Postural behaviors typically utilize isometric contractions in which no change in length takes place. Due to longer actin-myosin interactions, slow contracting muscle fibers represent an economical means for this type of contraction. In specialized soaring birds, such as vultures and pelicans, a deep layer of the pectoralis muscle, composed entirely of slow fibers, is believed to perform this function. Muscles involved in gliding posture were examined in California gulls (Larus californicus) and tested for the presence of slow fibers using myosin ATPase histochemistry and antibodies. Surprisingly small numbers of slow fibers were found in the M. extensor metacarpi radialis, M. coracobrachialis cranialis, and M. coracobrachialis caudalis, which function in wrist extension, wing protraction, and body support, respectively. The low number of slow fibers in these muscles and the absence of slow fibers in muscles associated with wing extension and primary body support suggest that gulls do not require slow fibers for their postural behaviors. Gulls also lack the deep belly to the pectoralis found in other gliding birds. Since bird muscle is highly oxidative, we hypothesize that fast muscle fibers may function to maintain wing position during gliding flight in California gulls. J. Morphol. 233:237–247, 1997. © 1997 Wiley-Liss, Inc.     

Capillarization,mitochondrial densities,oxygen diffusion distances and innervation of red and white muscle of the lizard Dipsosaurus dorsalis

Summary The white and red regions of the iliofibularis muscle of the lizard Dipsosaurus dorsalis were analyzed using histologic and morphometric analysis. These regions are composed of fast glycolytic (FG) and both fast oxidative, glycolytic (FOG) and tonic fibers, respectively. Endplate morphology and number of endplates per fiber were estimated from fibers from both areas. Capillary volume densities of the red and white regions were quantified from transverse sections. Mitochondrial volume of fibers from the red and white regions were estimated from electron micrographs.All fibers from the white region of the iliofibularis possessed a single, well defined endplate, as did most red region fibers. The remaining red fibers (28±5%) possessed an average of 14.7±3 endplates each, distributed along the entire length of the fiber at intervals of approximately 1124 m.Red fibers possessed twice the mitochondrial volume of white fibers (7.6±0.4%, red; 3.8±0.3%, white). Mitochondria were distributed uniformly through the fibers from both regions. Capillary anisotropy was low ( = 1.018) in both regions. Capillary densities of the red region (629±35 mm^-2) were much greater than those of the corresponding White region (73±8 mm^-2).The data indicate that capillary densities, mitochondrial volumes and theoretical diffusion distances correlate well with the oxidative capacity of lizard muscle fibers. Tonic fibrs of this species appear oxidative and therefore metabolically capable of functioning during locomotion. The similar mitochondrial volumes and capillary densities of reptilian and mammalian muscles suggest that the greater oxidative capacity of mammalian muscle is due in part to possession of more oxidatively active mitochondria rather than to possession of more mitochondria per se.     

Ontogeny of flight in the little brown bat,Myotis lucifugus: behavior,morphology, and muscle histochemistry

Summary Postnatal changes in wing morphology, flight ability, muscle morphology, and histochemistry were investigated in the little brown bat, Myotis lucifugus. The pectoralis major, acromiodeltoideus, and quadriceps femoris muscles were examined using stains for myofibrillar ATPase, succinate dehydrogenase (SDH), and mitochondrial -glycerophosphate dehydrogenase (-GPDH) enzyme reactions. Bats first exhibited spontaneous, drop-evoked flapping behavior at 10 days, short horizontal flight at 17 days, and sustained flight at 24 days of age. Wing loading decreased and aspect ratio increased during postnatal development, each reaching adult range before the onset of sustained flight. Histochemically, fibers from the three muscles were undifferentiated at birth and had lower oxidative and glycolytic capacities compared to other age groups. Cross-sectional areas of fibers from the pectoralis and acromiodeltoideus muscles increased significantly at an age when dropevoked flapping behavior was first observed, suggesting that the neuromuscular mechanism controlling flapping did not develop until this time. Throughout the postnatal growth period, pectoralis and acromiodeltoideus muscle mass and fiber cross-sectional area increased significantly. By day 17 the pectoralis muscle had become differentiated in glycolytic capacity, as indicated by the mosaic staining pattern for -GPDH. By contrast, the quadriceps fibers were relatively large at birth and slowly increased in size during the postnatal period. Fiber differentiation was evident at the time young bats began to fly, as indicated by a mosaic pattern of staining for myosin ATPase. These results indicate that flight muscles (pectoralis and acromiodeltoideus) are less well developed at birth and undergo rapid development just before the onset of flight. By contrast the quadriceps femoris muscle, which is required for postural control, is more developed at birth than the flight muscles and grows more slowly during subsequent development.     

Ultrastructural and cytological changes in the muscle fibers of the pectoralis of the giant Canada goose (Branta canadensis maxima) in disuse atrophy during molt

Summary Adult male Branta canadensis maxima were collected from a nonmigratory feral population during their premolt, molt and postmolt phases. Lean dry weight of the pectoralis muscle decreased significantly (p0.0001) during molt, as a result of disuse atrophy. Histochemical analysis revealed that the region of the pectoralis muscle sampled consisted of Red (fast-twitch oxidative-glycolytic) and White (fast-twitch glycolytic) muscle fiber types, in an approximate ratio of 9 to 1. There was no significant (p= 0.1238) difference in the relative percentages of the two fiber types during the three periods of study. There was, however, a significant decrease in mean cross-sectional area of both Red (p0.0194) and White (p0.0001) fibers during molt. Red and White fiber areas were strongly correlated with each other during molt (r ²=0.76, p=0.0010) and postmolt (r ²=0.70, p=0.0052), but not during premolt (r ²=0.02, p=0.7626). The latter finding may be related to fiber-type specific hypertrophy in premolt breeding males. Analysis of ultrastructure revealed that there was a significant (p=0.0003) decrease in the mean myofibrillar crosssectional area, and a significant increase in both the density (p=0.0227) and total number (p=0.0058) of myofibrils within the muscle fibers of the molting birds. These results indicate that the myofibrils split longitudinally during moltassociated disuse atrophy. A significant (p=0.0375) reduction in the amount of non-myofibrillar material (mitochondria) was also observed in the periphery of the muscle fibers of the molting birds. The changes observed during disuse atrophy are neither as pathological nor as extreme as those induced by experimental models of avian muscle atrophy.     

Sequence of Age-Associated Changes to the Mouse Neuromuscular Junction and the Protective Effects of Voluntary Exercise

Loss of connections between motor neurons and skeletal muscle fibers contribute to motor impairment in old age, but the sequence of age-associated changes that precede loss of the neuromuscular synapse remains uncertain. Here we determine changes in the size of neuromuscular synapses within the tibialis anterior muscle across the life span of C57BL/6J mice. Immunofluorescence, confocal microscopy and morphometry were used to measure the area occupied by nerve terminal synaptophysin staining and postsynaptic acetylcholine receptors at motor endplates of 2, 14, 19, 22, 25 and 28month old mice. The key findings were: 1) At middle age (14-months) endplate acetylcholine receptors occupied 238±11 µm² and nerve terminal synaptophysin 168±14 µm² (mean ± SEM). 2) Between 14-months and 19-months (onset of old age) the area occupied by postsynaptic acetylcholine receptors declined 30%. At many endplates the large acetylcholine receptor plaque became fragmented into multiple smaller acetylcholine receptor clusters. 3) Between 19- and 25-months, the fraction of endplate acetylcholine receptors covered by synaptophysin fell 21%. By 28-months, half of the endplates imaged retained ≤50 µm² area of synaptophysin staining. 4) Within aged muscles, the degree to which an endplate remained covered by synaptophysin did not depend upon the total area of acetylcholine receptors, nor upon the number of discrete receptor clusters. 5) Voluntary wheel-running exercise, beginning late in middle-age, prevented much of the age-associated loss of nerve terminal synaptophysin. In summary, a decline in the area of endplate acetylcholine receptor clusters at the onset of old age was followed by loss of nerve terminal synaptophysin from the endplate. Voluntary running exercise, begun late in middle age, substantially inhibited the loss of nerve terminal from aging motor endplates.     

Histochemical patterns in normal and splaylegged piglet muscle fibers

The histochemical pattern of muscle fiber types of the longissimus dorsi and biceps femoris muscles was investigated in normal and splaylegged piglets at birth and seven days later. Only slight differences between the muscle fibers at birth were found using histochemical reactions for alkaline adenosine triphosphatase (ATPase), succinate dehydrogenase (SDH), phosphorylase (PH) activities, and for the periodic acid-Schiff (PAS) reaction. With the method for acid-preincubated ATPase activity, high activity was observed in Type I muscle fibers and low activity in Type II muscle fibers in animals of both groups investigated. However, a higher number of Type I fibers was found in muscles of normal piglets, suggesting a faster and more advanced process of transformation of Type II into Type I muscle fibers in unaffected animals. Thus the histochemical conversion appears to be retarded in muscles of splaylegged animals, which have a histochemical pattern similar to that of normal prenatal animals. Cholinesterase activity in motor endplates was well developed; its staining revealed smaller sized and irregularly arranged endplates in muscles of affected piglets. Fiber type differentiation in muscles of animals which recovered from splayleg becomes fully developed and comparable to normal piglets seven days after birth. The number of fibers which became converted from Type II to Type I was increased; the fiber types were differentiated with regard to the PAS reaction and to their ATPase, SDH and PH activities. Morphological features of motor endplates in muscles of normal and surviving splaylegged piglets are similar. Histochemical investigation of the fiber type differentiation thus suggests that full recovery occurs within the first week of postnatal life in muscles affected by pathological changes accompanying splayleg.     

The three-dimensional structure of motor endplates in different fiber types of rat intercostal muscle

Summary The three-dimensional organization of the motor end plates in the red, white and intermediate striated muscle fibers of the rat intercostal muscle was observed under a field-emission type scanning electron microscope after removal of connective tissue components by HCl hydrolysis.The motor endplate of the white fiber had terminal branches (or axon terminals), which were large, long and thin, and small but numerous nerve swellings (or terminal boutons). The motor endplate of the red fiber had terminal branches, which were small, short and thick, and had large but fewer nerve swellings. The motor endplate of the intermediate fiber was intermediate in size and structure between these two. In detached nerve-ending preparations, primary synaptic grooves with slit-like openings of the junctional folds appeared on the surface of the muscle fibers. The primary synaptic grooves were more developed in the white fiber than in the red fiber, and they were intermediate in the intermediate fiber. The numerical ratio of slit-like openings was 11.83.5 in the red, intermediate and white fiber, respectively.The Schwann cells and their processes were observed on the surface of the motor endplate, with the processes covering the upper orifices of the primary synaptic grooves and sealing the terminal branches. The number of Schwann cells was usually three in the white fiber, two in the intermediate fiber and one in the red fiber.     

Fiber type homogeneity of the flight musculature in small birds

Studies of medium- and large-bodied avian species have suggested that variation in flight muscle composition is related to differences in flight behavior. For example, slow-twitch or tonic fibers are generally found only in the flight muscles of non-volant or soaring/gliding birds. However, we know comparatively little about fiber composition of the muscles of the smallest birds. Here we describe the fiber composition of muscles from the wings, shoulders, and legs of two small avian species, which also display very high wingbeat frequencies: Anna's hummingbirds (Calypte anna) and zebra finches (Taeniopygia guttata). All flight muscles examined in both species contained exclusively fast oxidative glycolytic (FOG) fibers. These unique results suggest that fast oxidative fibers are both necessary and sufficient for the full range of flight behaviors in these small-bodied birds. Like all other studied birds, the zebra finch gastrocnemius, a tarsometatarsal extensor, contained a mixture of FOG (27.1%), slow oxidative (SO, 12.7%), and fast glycolytic (FG, 60.2%) fibers. By contrast, the hummingbird gastrocnemius lacked FG fibers (85.5% FOG, 14.5% SO), which may reflect the reduced role of the hindlimb during take-off. We further hypothesize that thermogenic requirements constrain fiber type heterogeneity in these small endothermic vertebrates.     

Histochemical patterns in normal and splaylegged piglet muscle fibers

Summary The histochemical pattern of muscle fiber types of the longissimus dorsi and biceps femoris muscles was investigated in normal and splaylegged piglets at birth and seven days later. Only slight differences between the muscle fibers at birth were found using histochemical reactions for alkaline adenosine triphosphatase (ATPase), succinate dehydrogenase (SDH), phosphorylase (PH) activities, and for the periodic acid-Schiff (PAS) reaction. With the method for acid-preincubated ATPase activity, high activity was observed in Type I muscle fibers and low activity in Type II muscle fibers in animals of both groups investigated. However, a higher number of Type I fibers was found in muscles of normal piglets, suggesting a faster and more advanced process of transformation of Type II into Type I muscle fibers in unaffected animals. Thus the histochemical conversion appears to be retarded in muscles of splaylegged animals, which have a histochemical pattern similar to that of normal prenatal animals. Cholinesterase activity in motor endplates was well developed; its staining revealed smaller sized and irregularly arranged endplates in muscles of affected piglets. Fiber type differentiation in muscles of animals which recovered from splayleg becomes fully developed and comparable to normal piglets seven days after birth. The number of fibers which became converted from Type II to Type I was increased; the fiber types were differentiated with regard to the PAS reaction and to their ATPase, SDH and PH activities. Morphological features of motor endplates in muscles of normal and surviving splaylegged piglets are similar.Histochemical investigation of the fiber type differentiation thus suggests that full recovery occurs within the first week of postnatal life in muscles affected by pathological changes accompanying splayleg.     

Spatial and temporal expression of acetylcholine receptor RNAs in innervated and denervated rat soleus muscle

In adult vertebrate skeletal muscle acetylcholine receptors are localized to the neuromuscular junction. Upon denervation, this distribution changes, with new receptors appearing in extrajunctional regions of the muscle fiber. The location of acetylcholine receptors in innervated or denervated muscle may result, in part, from the distribution of their RNAs. This was tested by assaying for receptor RNAs in junctional and extrajunctional regions of innervated and denervated rat soleus muscle using in situ hybridization and RNAase protection assays. These experiments showed alpha, beta, and delta subunit RNAs concentrated beneath the endplates of innervated muscle fibers. Following denervation, there was an unequal distribution of receptor RNAs along the muscle fiber, with highest levels occurring in extrajunctional regions near the endplate. These data are consistent with a nonuniform pattern of gene expression in adult skeletal muscle fibers.