Morphological and histochemical differentiation of intrafusal fibres in the posterior latissimus dorsi muscle of the developing chick

Sensory regulation of spontaneous limb movements in the midstage embryonic chick

It is becoming increasingly apparent that somatosensation plays an important role in regulating prenatal movement and developmental plasticity. Numerous studies performed on embryonic chicks and perinatal rats are beginning to implicate proprioception to be particularly important in modulating motility very soon after afferent connections are made in the spinal cord. In this report, we demonstrate new approaches in the chick embryo to explore the role of sensation in modulating embryonic movement. Force recordings from the legs of chick embryos on E9 and E11, during spontaneous motility, demonstrate changes in sensory regulation consistent with the concept that sensory regulation is functioning one day after sensory synapse formation and that the complexity of this regulation increases by E11. Additionally, we present new video data showing activation of embryonic motility on E5 and E9 in embryos expressing channelrhodopsin in the spinal cord as a novel way to approach the issues of sensorimotor development.

Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron microscopy to determine whether morphological or quantitative differences existed between these sensory receptors. Differences were found in the density, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsular components. The slow ALD possessed muscle spindles which were evenly distributed throughout the muscle, whereas in the fast PLD they were mainly concentrated around the single nerve entry point into the muscle. The muscle spindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5+/-2.0 vs 55.4+/-2.0 respectively, n = 6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters varied from 5.0 to 16.0 microm and 4.5 to 18.5 microm, respectively. Large diameter intrafusal fibres were more frequently encountered in spindles of the PLD. Unique to the ALD was the presence of monofibre muscle spindles (12.7% of total spindles observed in ALD) which contained a solitary intrafusal fibre. In muscle spindles of both the ALD and PLD, sensory nerve endings terminated in a spiral fashion on the intrafusal fibres in their equatorial regions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive in polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity with respect to the number of intrafusal fibres per spindle, range of intrafusal fibre diameters and development of their inner capsules. It is postulated that the differences in muscle spindle density and structure observed in this study reflect the function of the muscles in which they reside.

Distribution of muscle spindles in a simply structured muscle: integrated total sensory representation

BACKGROUND

The distribution of muscle spindles (Sps) in a small muscle of simple architecture, the capsularis at the cat's hip joint, was quantified to reveal the patterns of proprioceptive representation in the transverse and sagittal planes as well as to model the effect a local disturbance in muscle length would have on total Sp discharge.

METHODS

Locations in serial cross-sections of the 32 and 38 Sps in 2 muscles, 1 perfused with the hip joint flexed and the other extended, were plotted, and their patterns of integrated sensitivity calculated assuming that (1) the discharge rate of a Sp afferent varies linearly with change in length along the Sp's axis, and (2) that within a local disturbance produced by contraction of a motor unit (MU), lengths decrease either linearly or as the square of the distance from its center.

RESULTS

The isomeric pattern of "integrated, total Sp representation on cross-section" showed two peaks of sensitivity in the half of the muscle that had been next to the joint capsule, offset by low representation in a small, central area and along the extensive zone bordering the laterally facing "superficial surface." The equivalent radius of an idealized symmetrical MU territory was estimated from distributions of the few fast, oxidative-glycolytic fibers found in two muscles, and the effect of a MU's contraction on net Sp discharge predicted when the unit was positioned at distinctive sites within the pattern. As an index of Ia and II afferent representation in the sagittal plane, the distribution of the nucleated regions of Sps and the summed lengths of segments of Sp axial bundles and capsules, respectively, within successive 1-mm segments of the muscle were graphed.

CONCLUSIONS

The longitudinal representation and structure of the muscle are not suited for reflex adjustment of differences in length along the muscle. The isomeric pattern of high relief in the transverse plane suggests that in this approximately 0.2 g muscle, the localization of myotatic reflexes might be accommodated but the need for adjustment in activation of MUs seems minimal. This is because the muscle is not compartmentalized, its fibers extend between the muscle's origin and insertion, their angle of pinnation is low, and greater than 90% are of slow type. The distribution of Sps is consistent with gauging length of the entire muscle and hence angulation at the hip joint.

Basal lamina development in chicken muscle spindles

The development of basal laminas was examined in immunohistochemical sections of chicken leg muscle spindles from embryonic day (E) 13 to 8 weeks postnatal. Fragments of basal laminas as seen with immunostaining for isoforms of laminin were already observed in E6 muscles. When clusters of intrafusal myotubes were first recognized at E13-14, they were surrounded by basal laminas which were incomplete both in terms of coverage and molecular composition. More mature basal lamina tubes individually enclosed young myofibers at E18. After afferents made contact with myotubes, synaptic portions of basal laminas at myosensory junctions reacted strongly with antibodies against s-laminin and chondroitin sulfate proteoglycan, while extrasynaptic portions were negative or reacted only weakly. At synaptic basal laminas of neuromuscular junctions heparin sulfate proteoglycan and s-laminin became prominent after E16. Contrary to the early presence of basal lamina proteins around intrafusal fibers, initial deposition of basal lamina proteins in the outer spindle capsule was not recognized until E17-18, and significant amounts were not detected until postnatal week 1. Unlike intrafusal basal laminas, capsular basal laminas developed no distinct specialized regions; however, molecular compositions of intrafusal and capsular basal laminas were similar.

Development of chicken intrafusal muscle fibers

The first sign of developing intrafusal fibers in chicken leg muscles appeared on embryonic day (E) 13 when sensory axons contacted undifferentiated myotubes. In sections incubated with monoclonal antibodies against myosin heavy chains (MHC) diverse immunostaining was observed within the developing intrafusal fiber bundle. Large primary intrafusal myotubes immunostained moderately to strongly for embryonic and neonatal MHC, but they were unreactive or reacted only weakly with antibodies against slow MHC. Smaller, secondary intrafusal myotubes reacted only weakly to moderately for embryonic and neonatal MHC, but 1-2 days after their formation they reacted strongly for slow and slow-tonic MHC. In contrast to mammals, slow-tonic MHC was also observed in extrafusal fibers. Intrafusal fibers derived from primary myotubes acquired fast MHC and retained at least a moderate level of embryonic MHC. On the other hand, intrafusal fibers developing from secondary myotubes lost the embryonic and neonatal isoforms prior to hatching and became slow. Based on relative amounts of embryonic, neonatal and slow MHC future fast and slow intrafusal fibers could be first identified at E14. At the polar regions of intrafusal fibers positions of nerve endings and acetylcholinesterase activity were seen to match as early as E16. Approximately equal numbers of slow and fast intrafusal fibers formed prenatally; however, in postnatal muscle spindles fast fibers were usually in the majority, suggesting that some fibers transformed from slow to fast.

Transient expression of a ventricular myosin heavy chain isoform in developing chicken intrafusal muscle fibers

Sections of chicken tibialis anterior and extensor digitorium longus muscles were incubated with monoclonal antibodies against myosin heavy chains (MHC). Ventricular myosin was present in developing secondary intrafusal myotubes when they were first recognized at embryonic days (E) 13-14, and in developing extrafusal fibers prior to that date. The reaction in intrafusal fibers began to fade at E17, and in 2-week-old postnatal and older muscles the isoform was no longer recognized. Only those intrafusal fibers which also reacted with a monoclonal antibody against atrial and slow myosin contained ventricular MHC. Intrafusal myotubes which developed into fast fibers did not express the isoform. Hence, based on the presence or absence of ventricular MHC, two lineages of intrafusal fiber are evident early in development. Strong immunostaining for ventricular MHC was observed in primary extrafusal myotubes at E10, but the isoform was already downregulated at E14, when secondary intrafusal myotubes were still forming and expressed ventricular MHC. Only light to moderate and transient immunostaining was observed in coexisting secondary extrafusal myotubes, most of which developed into fast fibers. Thus at the time when nascent muscle spindles are first recognized, differences in MHC profiles already exist between prospective intrafusal and extrafusal fibers. If intrafusal fibers stem from a pool of primordial muscle cells, which is common to intrafusal and extrafusal myotubes, they diverged from it some time prior to E13.

The avian muscle spindle

The literature on the morphology and physiology of the avian muscle spindle is reviewed, with emphasis placed on the period from 1960 to 1991. Traits similar to or different from mammalian spindles are recognized. Apart from receptors with low intrafusal fiber counts, bird spindles contain two or three types of intrafusal fiber. Unlike that of mammals, the equatorial fiber structure in birds does not lend itself to classification into nuclear bag and nuclear chain types. Avian intrafusal fibers are separable into types based on differences in myosin heavy chain composition and motor innervation, but apportionment of these fiber types to individual spindles is more variable in birds than in mammals. There is morphological evidence in birds for the existence of both gamma and beta innervation; however, confirmation of these systems by physiological experiments is at best sketchy. A general lack of physiological data is currently the greatest drawback to a better understanding of how the avian receptor works, and what role it plays in sensorimotor integration.

Contours and distribution of sites that react with antiacetylcholinesterase in chicken intrafusal fibers

Serial cross and longitudinal sections of intrafusal fibers from the intracapsular portions of chicken tibialis anterior muscle spindles were incubated with a monoclonal antibody specific for chicken acetylcholinesterase (AchE) and examined by immunofluorescence for the presence of the enzyme on presynaptic and postsynaptic membranes of neuromuscular junctions. The midequatorial sensory region which lacks organized sarcomeres was negative, but immediately distal to it faintly staining regions of AchE localization were observed on intrafusal fibers. In cross sections at the juxtaequator, the outlines of areas that were positive for AchE were either thin and crescentlike or thick and compact. The distribution of both types of localization continued into the polar region. Toward the more distal polar region, the intensity of sites on the postsynaptic membrane that reacted with the anti-AchE progressively increased. In longitudinal sections, AchE localization was largely limited to two configurations. One was elongate, while the other was more round or oval and often also smaller. Both types might occur on the same, or on different, intrafusal fibers. Examination of silver-impregnated sections revealed the presence of platelike and of traillike axon terminals. The variety of shapes observed on presynaptic and postsynaptic membranes warrants further study to determine whether chicken muscle spindles are innervated by more than one type of motor neuron.

Noncoordinate expression of M band proteins in slow and fast embryonic chick muscles

The expression of the myosin-associated M band proteins myomesin and M protein in differentiating muscle fibers in the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles during embryonic chicken development was examined by immunocytochemistry using monoclonal antibodies. Early in the embryonic development of both muscles, both myomesin and M protein are expressed in primary and secondary myotubes. However, beginning at 10 days in ovo, M protein is gradually suppressed first in primary, then in secondary, presumptive slow-tonic type 3 fibers. M protein is transiently suppressed in presumptive fast-twitch type 2 fibers derived from primary myotubes but continuously expressed in those derived from secondary myotubes. Thus, initially all myotubes have a common intrinsic M band composition with respect to myomesin and M protein, whereas at later stages the expression of M protein is fiber-type specific. Intrafusal spindle fibers, which are segregated from extrafusal fibers around 14 days in ovo, have a heterogeneous M band composition atypical of extrafusal fibers.

Differentiation of fiber types in wing muscles during embryonic development: effect of neural tube removal

The embryonic precursors of the avian slow (type I and III) and fast (type II) fibers can be distinguished from each other early in muscle formation (stage 28, V. Hamburger and H. L. Hamilton, J. Morphol, 88, 49-92, 1951) on the basis of the differential sensitivity of their myosin ATPases. To test the neural dependence of fiber type differentiation, the source of motor innervation was eliminated by excision of the brachial neural tube at stages 16-18 before muscles are innervated. Removal of the brachial neural tube did not affect the number of primary myotubes in a sample muscle of the forelimb (ulnimetacarpalis dorsalis, UMD) up until stage 36. Myosin ATPase staining at a variety of pHs revealed the typical patterns of fiber types in muscles of neural-tube free embryos in stages 35-37. These muscles included the anterior latissimus dorsi, brachialis, and UMD which showed presumptive type III staining (type IIIEMB), the pronator superficialis and flexor carpi ulnaris which showed embryonic type II staining (type IIEMB), and the triceps brachii muscles which showed characteristic arrangements of both type IEMB and type IIEMB fibers. The normal patterns of type IEMB and type IIEMB myotubes were also seen in muscles containing a heterogeneous mixture of fiber types such as the biceps brachii, extensor metacarpi radialis, and adductor indicis muscles, although the intensity of acid-stable ATPase staining of the type IEMB myotubes in these muscles was lower than in innervated muscles. It is concluded that the earliest differentiation of muscle fiber types is independent of the nervous system.

Differences in muscle spindle structure between pigeon muscles used in aerial and terrestrial locomotion

Muscle spindle density (number of spindles per gram of muscle) of all 29 muscles of the forearm and leg of the domestic pigeon was evaluated by counting receptors in van Gieson-stained serial cross sections. Extra- and intra-fusal fiber-type profiles were determined from histochemical preparations. Muscles of the leg had on the average significantly more avian slow-twitch oxidative extrafusal fibers (22.5 vs. 0.8%) and slower contraction times than muscles of the forearm, but fiber-type profiles and gross actions of muscles showed no consistent relation to the relative abundance of receptors. Differences in intrafusal fiber-type composition among spindles were sought because of their potential effect on the quality of the afferent discharge. The number of intrafusal fibers per spindle was on the average significantly less (4.57 vs 5.99) in the muscles of the leg than in those of the forearm; and of spindles with the same number of intrafusal fibers, those in the leg had smaller periaxial spaces. Distribution of intrafusal fiber types identified with the myofibrillar adenosine triphosphatase reaction differed among spindles of varying sizes. An acid- and alkali-labile type occurred most frequently (P = 0.05) in spindles with one to three intrafusal fibers, and an acid-labile and alkali-stable type was most often seen (P = 0.05) in spindles with 4 to 7 intrafusal fibers. The smaller receptors were more abundant in the leg, while the larger ones were about equally distributed between the two extremities. Muscle fibers with dimensions that sometimes approached small extrafusal fibers were present in about 3% of the axial bundles examined, most of them in the forearm. The selective morphological variation of avian muscle spindles may represent the structural basis for qualitatively different afferent discharges that relate to the characteristic types of locomotion served by the two extremities.

Innervation of avian latissimus dorsi muscles and axonal outgrowth pattern in the posterior latissimus dorsi motor nerve during embryonic development

The distribution of the innervation to the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of the chicken are described on the day of hatching and 6 weeks later using electron microscopy. In the ALD muscle, there are 5,000 muscle fibres and 374,000 endplates supplied by about 169 skeletomotor axons; in the PLD muscle, there are 12,000 focally innervated muscle fibers supplied by about 20 skeletomotor axons. On the cell surface of the muscle fibers the mean total subsynaptic area contacted by each motor axon is comparable in the ALD and PLD muscles. The growth pattern of the axons in the PLD motor nerve was described from the ninth day in ovo up to 6 weeks after hatching. The axons arrive in the PLD muscle in two successive waves: first, the large somatic axons which are already present before the ninth day in ovo and second, the small autonomic axons which continue to accumulate until hatching. The total number of somatic axons decreases from the ninth day until the hatching day when it reaches its definitive value. This decrease takes place during a period when the numbers of myofibers and of endplates dramatically increase, and it coincides with the axonal segregation by the Schwann cells. The myelination of the axons starts on the 15th day in ovo and is essentially complete upon hatching. Despite the decreasing number of somatic axons in the PLD nerve, the decrease in number of nerve endings per PLD endplate and the increasing number of PLD endplates per PLD muscle, it was found that between the 16th day in ovo and 6 weeks after hatching the mean number of axonal branches per PLD motor axon does not decrease.

Quantitative and histochemical aspects of the differentiation of muscle spindles in the anterior latissimus dorsi of the developing chick

The differentiation of muscle spindles has been investigated both quantitatively and histochemically in the slow anterior latissimus dorsi (ALD) muscle during embryonic and post-hatching development of the chick. The first spindles are detected by the 13th day in ovo, and a rapid increase in number takes place until the 15th of embryonic life. Two histochemical fibre types of intrafusal fibres are distinguished as early as the 13th day of embryonic development when myofibrillar ATPase activity is demonstrated after acid preincubation. In ALD muscle from post-hatched animals, two intrafusal fibre types are also distinguished by histochemistry: one fibre type is characterized by an acid and alkali-stable myofibrillar ATPase activity while this activity is acid-labile and alkali-stable in other type. As far as the properties of myofibrillar ATPase are concerned, such types of intrafusal fibres resemble the extrafusal beta and alpha fibre types defined recently in ALD muscle. The development of spindles in the slow-tonic ALD muscle is compared to the differentiation of spindles in the fast-twitch posterior latissimus dorsi (PLD) muscle of the chick which has been previously described (Toutant et al. 1981).