Distinct modes of vertebrate hypaxial muscle formation contribute to the teleost body wall musculature

Biomechanics and neural circuits for vestibular-induced fine postural control in larval zebrafish

Land-walking vertebrates maintain a desirable posture by finely controlling muscles. It is unclear whether fish also finely control posture in the water. Here, we showed that larval zebrafish have fine posture control. When roll-tilted, fish recovered their upright posture using a reflex behavior, which was a slight body bend near the swim bladder. The vestibular-induced body bend produces a misalignment between gravity and buoyancy, generating a moment of force that recovers the upright posture. We identified the neural circuits for the reflex, including the vestibular nucleus (tangential nucleus) through reticulospinal neurons (neurons in the nucleus of the medial longitudinal fasciculus) to the spinal cord, and finally to the posterior hypaxial muscles, a special class of muscles near the swim bladder. These results suggest that fish maintain a dorsal-up posture by frequently performing the body bend reflex and demonstrate that the reticulospinal pathway plays a critical role in fine postural control.

Development of fin-innervating motor neurons after peripheral target removal in medaka fish

Peripheral targets regulate the development and survival of the nerve centers that serve them, because the elimination of the target normally results in massive death of the developing neurons that innervate it. This widely accepted theory appears to be well supported by developing limbs and their innervation in tetrapods, but it is unclear whether this concept applies to primitive vertebrates that have paired appendages. In this study, we examined the development of spinal motor neurons following pectoral fin bud removal (FBR) in medaka fish. After FBR, motor axons initially extended to the plexus region in a morphologically normal pattern. During the period of fin innervation, motor axons in the FBR-medaka failed to form the normal brachial plexus and elongated ventrally toward the abdominal region. In the ventral horn that would normally innervate the pectoral fin, however, neurons did not undergo cell death following FBR. There were no differences in the numbers of axons in the ventral roots between the FBR and control sides. Motor neuron markers, RALDH2 and FOXP1, that are expressed in limb-innervating motor neurons in the lateral motor column in tetrapods, were also expressed in the ventral horns of both the control and FBR sides in medaka fish. These results suggest that, although both tetrapod and medaka motor neurons share the same molecular characteristics for innervating paired appendages, the fates of neurons differ following the removal of their peripheral target. Therefore, the relationship between the peripheral target and its nerve center may be altered among vertebrates.

Anterior trunk muscle shows mix of axial and appendicular developmental patterns

BACKGROUND

Skeletal muscle in the trunk derives from the somites, paired segments of paraxial mesoderm. Whereas axial musculature develops within the somite, appendicular muscle develops following migration of muscle precursors into lateral plate mesoderm. The development of muscles bridging axial and appendicular systems appears mixed.

RESULTS

We examine development of three migratory muscle precursor-derived muscles in zebrafish: the sternohyoideus (SH), pectoral fin (PF), and posterior hypaxial (PHM) muscles. We show there is an anterior to posterior gradient to the developmental gene expression and maturation of these three muscles. SH muscle precursors exhibit a long delay between migration and differentiation, PF muscle precursors exhibit a moderate delay in differentiation, and PHM muscle precursors show virtually no delay between migration and differentiation. Using lineage tracing, we show that lateral plate contribution to the PHM muscle is minor, unlike its known extensive contribution to the PF muscle and absence in the ventral extension of axial musculature.

CONCLUSIONS

We propose that PHM development is intermediate between a migratory muscle mode and an axial muscle mode of development, wherein the PHM differentiates after a very short migration of its precursors and becomes more anterior primarily by elongation of differentiated muscle fibers.

The zebrafish HGF receptor met controls migration of myogenic progenitor cells in appendicular development

The hepatocyte growth factor receptor C-met plays an important role in cellular migration, which is crucial for many developmental processes as well as for cancer cell metastasis. C-met has been linked to the development of mammalian appendicular muscle, which are derived from migrating muscle progenitor cells (MMPs) from within the somite. Mammalian limbs are homologous to the teleost pectoral and pelvic fins. In this study we used Crispr/Cas9 to mutate the zebrafish met gene and found that the MMP derived musculature of the paired appendages was severely affected. The mutation resulted in a reduced muscle fibre number, in particular in the pectoral abductor, and in a disturbed pectoral fin function. Other MMP derived muscles, such as the sternohyoid muscle and posterior hypaxial muscle were also affected in met mutants. This indicates that the role of met in MMP function and appendicular myogenesis is conserved within vertebrates.

Muscle precursor cell movements in zebrafish are dynamic and require Six family genes

Muscle precursors need to be correctly positioned during embryonic development for proper body movement. In zebrafish, a subset of hypaxial muscle precursors from the anterior somites undergo long-range migration, moving away from the trunk in three streams to form muscles in distal locations such as the fin. We mapped long-distance muscle precursor migrations with unprecedented resolution using live imaging. We identified conserved genes necessary for normal precursor motility (six1a, six1b, six4a, six4b and met). These genes are required for movement away from somites and later to partition two muscles within the fin bud. During normal development, the middle muscle precursor stream initially populates the fin bud, then the remainder of this stream contributes to the posterior hypaxial muscle. When we block fin bud development by impairing retinoic acid synthesis or Fgfr function, the entire stream contributes to the posterior hypaxial muscle indicating that muscle precursors are not committed to the fin during migration. Our findings demonstrate a conserved muscle precursor motility pathway, identify dynamic cell movements that generate posterior hypaxial and fin muscles, and demonstrate flexibility in muscle precursor fates.

Developmental mechanisms of migratory muscle precursors in medaka pectoral fin formation

Limb muscles are formed from migratory muscle precursor cells (MMPs) that delaminate from the ventral region of dermomyotomes and migrate into the limb bud. MMPs remain undifferentiated during migration, commencing differentiation into skeletal muscle after arrival in the limb. However, it is still unclear whether the developmental mechanisms of MMPs are conserved in teleost fishes. Here, we investigate the development of pectoral fin muscles in the teleost medaka Oryzias latipes. Expression of the MMP marker lbx1 is first observed in several somites prior to the appearance of fin buds. lbx1-positive cells subsequently move anteriorly and localize in the prospective fin bud region to differentiate into skeletal muscle cells. To address the developmental mechanisms underlying fin muscle formation, we knocked down tbx5, a gene that is required for fin bud formation. tbx5 morphants showed loss of fin buds, whereas lbx1 expression initiated normally in anterior somites. Unlike in normal embryos, expression of lbx1 was not maintained in migrating fin MMPs or within the fin buds. We suggest that fin MMPs appear to undergo two phases in their development, with an initial specification of MMPs occurring independent of fin buds and a second fin bud-dependent phase of MMP migration and proliferation. Our results showed that medaka fin muscle is composed of MMPs. It is suggested that the developmental mechanism of fin muscle formation is conserved in teleost fishes including medaka. Through this study, we also propose new insights into the developmental mechanisms of MMPs in fin bud formation.

Reorganization of mammalian body wall patterning with cloacal septation

Septation of the cloaca is a unique mammalian adaptation that required a novel reorganization of the perineum–the caudal portion of the trunk body wall not associated with the hindlimb. Fish, the basal vertebrates, separate ventrolateral body wall musculature of the trunk into two discrete layers, while most tetrapods expand this pattern in the thorax and abdomen into four. Mammals, the only vertebrate group to divide the cloaca into urogenital and anorectal portions, exhibit complex muscle morphology in the perineum. Here we describe how perineal morphology in a broad sample of mammals fits into patterning of trunk musculature as an extension of the four-layer ventrolateral muscular patterning of the thorax and abdomen. We show that each perineal muscle layer has a specific function related to structures formed by cloacal septation. From superficial to deep, there is the subcutaneous layer, which regulates orifice closure, the external layer, which supplements both erectile and micturition function, the internal layer, which provides primary micturition and defecation regulation, and the transversus layer, which provides structural support for pelvic organs. We elucidate how the four-layer body wall pattern, restricted to the non-mammal tetrapod thorax and abdomen, is observed in the mammalian perineum to regulate function of unique perineal structures derived from cloacal septation.

Hypaxial muscle: controversial classification and controversial data?

Hypaxial muscle is the anatomical term commonly used when referring to all the ventrally located musculature in the body of vertebrates, including muscles of the body wall and the limbs. Yet these muscles had very humble beginnings when vertebrates evolved from their chordate ancestors, and complex anatomical changes and changes in underlying gene regulatory networks occurred. This review summarises the current knowledge and controversies regarding the development and evolution of hypaxial muscles.

Body wall development in lamprey and a new perspective on the origin of vertebrate paired fins

Classical hypotheses regarding the evolutionary origin of paired appendages propose transformation of precursor structures (gill arches and lateral fin folds) into paired fins. During development, gnathostome paired appendages form as outgrowths of body wall somatopleure, a tissue composed of somatic lateral plate mesoderm (LPM) and overlying ectoderm. In amniotes, LPM contributes connective tissue to abaxial musculature and forms ventrolateral dermis of the interlimb body wall. The phylogenetic distribution of this character is uncertain because lineage analyses of LPM have not been generated in anamniotes. We focus on the evolutionary history of the somatopleure to gain insight into the tissue context in which paired fins first appeared. Lampreys diverged from other vertebrates before the acquisition of paired fins and provide a model for investigating the preappendicular condition. We present vital dye fate maps that suggest the somatopleure is eliminated in lamprey as the LPM is separated from the ectoderm and sequestered to the coelomic linings during myotome extension. We also examine the distribution of postcranial mesoderm in catshark and axolotl. In contrast to lamprey, our findings support an LPM contribution to the trunk body wall of these taxa, which is similar to published data for amniotes. Collectively, these data lead us to hypothesize that a persistent somatopleure in the lateral body wall is a gnathostome synapomorphy, and the redistribution of LPM was a key step in generating the novel developmental module that ultimately produced paired fins. These embryological criteria can refocus arguments on paired fin origins and generate hypotheses testable by comparative studies on the source, sequence, and extent of genetic redeployment.