Acetylcholine receptor formation in mouse-chick chimera

Axial and limb muscle development: dialogue with the neighbourhood

Skeletal muscles are part of the musculoskeletal system which also includes nerves, tendons, connective tissue, bones and blood vessels. Here we review the development of axial and limb muscles in amniotes within the context of their surrounding tissues in vivo. We highlight the reciprocal dialogue mediated by signalling factors between cells of these adjacent tissues and developing muscles and also demonstrate its importance from the onset of muscle cell differentiation well into foetal development. Early embryonic tissues secrete factors which are important regulators of myogenesis. However, later muscle development relies on other tissue collaborators, such as developing nerves and connective tissue, which are in turn influenced by the developing muscles themselves. We conclude that skeletal muscle development in vivo is a compelling example of the importance of reciprocal interactions between developing tissues for the complete and coordinated development of a functional system.

Chick embryos can form teratomas from microinjected mouse embryonic stem cells

We examined whether chick embryos are a suitable experimental model for the evaluation of pluripotency of stem cells. Mouse embryonic stem cells (mESCs) expressing the reporter gene, LacZ or GFP were injected into the subgerminal cavity of blastoderms (freshly oviposited) or the marginal vein of chick embryos (2 days of incubation). Injected mESCs were efficiently incorporated into the body and extra-embryonic tissues of chick embryos and formed small clusters. Increased donor cell numbers injected were positively associated with the efficiency of chimera production, but with lower viability. A single mESC injected into the blastoderm proliferated into 34.7 ± 3.8 cells in 3 days, implying that the chick embryo provides an optimal environment for the growth of xenogenic cells. In the embryo body, mESCs were interspersed as small clustered chimeras in various tissues. Teratomas were observed in the yolk sac and the brain with three germ layers. In the yolk sac, clusters of mESCs gradually increased in volume and exhibited varied morphology such as a water balloon-like or dark-red solid mass. However, mESCs in the brain developed into a large soft tissue mass of whitish color and showed a tendency to differentiate into ectodermal lineage cells, including primitive neural ectodermal and neuronal cells expressing the neurofilament protein. These results indicate that chick embryos are useful for the teratoma formation assays of mESCs and have a broad-range potential as an experimental host model.

Developmental fate of the mammalian myotome

The myotome is a segmented paraxial muscle present in all early vertebrate embryos, which in amniotes disappears in mid-embryogenesis, and is replaced by complex epaxial and hypaxial musculature. Little is known about how this transition occurs. Here, we describe the detailed morphogenesis of the epaxial muscles from the epaxial myotome, in rodent embryos. The results show there is no apoptosis of myotomal fibres during the transition, and that the epaxial muscles arise by translocation, re-orientation, and elongation of the myotomal myocytes followed by cleavage of the myotomal masses. Myotomal myocytes transit from a mononucleated to a multinucleated state just before onset of this transformation. Each newly-formed epaxial muscle anlagen includes populations of Pax3- and Pax7-positive muscle progenitors, with different distributions. Using transgenic mouse embryos bearing a GFP marker for Scleraxis, we show that tendon progenitors are tightly associated with the sides and ends of myotomal myocytes as they re-orient and elongate.

Regulation of myosin expression during myotome formation

The first skeletal muscle fibers to form in vertebrate embryos appear in the somitic myotome. PCR analysis and in situ hybridization with isoform-specific probes reveal differences in the temporal appearance and spatial distribution of fast and slow myosin heavy chain mRNA transcripts within myotomal fibers. Embryonic fast myosin heavy chain was the first isoform expressed, followed rapidly by slow myosin heavy chains 1 and 3, with slow myosin heavy chain 2 appearing several hours later. Neonatal fast myosin heavy chain is not expressed in myotomal fibers. Although transcripts of embryonic fast myosin heavy chain were always distributed throughout the length of myotomal fibers, the mRNA for each slow myosin heavy chain isoform was initially restricted to the centrally located myotomal fiber nuclei. As development proceeded, slow myosin heavy chain transcripts spread throughout the length of myotomal fibers in order of their appearance. Explants of segments from embryos containing neural tube, notochord and somites 7-10, when incubated overnight, become innervated by motor neurons from the neural tube and express all four myosin heavy chain genes. Removal of the neural tube and/or notochord from explants prior to incubation or addition of d-tubocurare to intact explants prevented expression of slow myosin chain 2 but expression of genes encoding the other myosin heavy chain isoforms was unaffected. Thus, expression of slow myosin heavy chain 2 is dependent on functional innervation, whereas expression of embryonic fast and slow myosin heavy chain 1 and 3 are innervation independent. Implantation of sonic-hedgehog-soaked beads in vivo increased the accumulation of both fast and slow myosin heavy chain transcripts, as well as overall myotome size and individual fiber size, but had no effect on myotomal fiber phenotype. Transcripts encoding embryonic fast myosin heavy chain first appear ventrolaterally in the myotome, whereas slow myosin heavy chain transcripts first appear in fibers positioned midway between the ventrolateral and dorsomedial lips of the myotome. Therefore, models of epaxial myotome formation must account for the positioning of the oldest fibers in the more ventral-lateral region of the myotome and the youngest fibers in the dorsomedial region.

Mouse-chick chimera: an experimental system for study of somite development

As the mammalian embryo is implanted in the uterus and not readily accessible to direct observation or manipulation, much of our understanding of mammalian somite development is based on findings in lower vertebrates. One means of overcoming the difficulties raised by intrauterine development is to engraft mouse tissue in ovo. The experiments described in this chapter relate to the unilateral replacement of somites in chick embryo with those from mouse fetus. Mouse somites differentiate in ovo in dermis, cartilage, and skeletal muscle and are able to migrate into chick host limb. A LacZ transgenic mouse strain was used to ascertain the role of the implanted somites in forming epaxial and hypaxial muscle in the chick embryo. Myogenesis occurred normally in in ovo developing mouse somites, and muscle cells from mouse myotome formed neuromuscular contacts with chick motor axons. After fragments of fetal mouse neural primordium were transplanted into chick embryo, mouse neural tube contributed to the mechanism maintaining myogenesis in the somites of the host embryo. A recently developed double-grafting procedure involving neural tube and somites from knockout mouse strains should elucidate the molecular events involved in early somitogenesis.

Use of a repetitive mouse B2 element to identify transplanted mouse cells in mouse-chick chimeras

Monitoring the migrations of cells during embryonic development requires a system in which cells can be identified in situ during locomotion. One promising system involves the generation of chimeras by transplanting mouse cells into chick embryos in ovo to exploit the wealth of mouse genetic variants. The success of this technique relies on the ability to detect individual mouse cells in a chick environment with high specificity. The murine B2 family of short interspersed elements is present in the mouse genome at copy numbers in excess of 10(5), whereas this sequence is absent in the chick genome based on hybridization techniques. This differential of five orders of magnitude produces signals in mouse cells that are easily identified, even in an environment that is predominantly chick. Thus, the B2 repeat probe is highly effective for the purpose of identifying mouse cells in mouse-chick chimeras.

BEN/SC1/DM-GRASP expression during neuromuscular development: a cell adhesion molecule regulated by innervation

BEN/SC1/DM-GRASP is a cell adhesion molecule belonging to the Ig superfamily that is transiently expressed during avian embryogenesis in a variety of cell types, including the motoneurons of the spinal cord. We have investigated the pattern of BEN expression during neuromuscular development of the chick. We show that both motoneurons and their target myoblasts express BEN during early embryonic development and that the protein becomes restricted at neuromuscular contacts as soon as postsynaptic acetylcholine receptor clusters are observed in muscle fibers. Muscle cells grown in vitro express and maintain BEN expression even when they fuse and give rise to mature myotubes. When embryos are deprived of innervation by neural tube ablation, BEN expression is observed in muscle fibers, whereas, in control, the protein is already restricted at neuromuscular synaptic sites. These results demonstrate that all myogenic cells intrinsically express BEN and maintain the protein in the absence of innervation. Conversely, when neurons are added to myogenic cultures, BEN is rapidly downregulated in muscle cells, demonstrating that innervation controls the restricted pattern of BEN expression seen in innervated muscles. After nerve section in postnatal muscles, BEN protein becomes again widely spread over muscle fibers. When denervated muscles are allowed to be reinnervated, the protein is reexpressed in regenerating motor axons, and reinnervation of synaptic sites leads to the concentration of BEN at neuromuscular junctions. Our results suggest that BEN cell adhesion molecule acts both in the formation of neuromuscular contacts during development and in the events leading to muscle reinnervation.