Myogenesis

This graphic depicts normal myoblast (early muscle cells with a single nucleus) fusing together to form muscle fibers (multinucleated muscle cells) during myogenesis
Patient with Waardenburg syndrome III (Waardenburg Klein Syndrome) with wide-set eyes.
MyoD1 (MYF3).
Muscular Dystrophy Histopathology.
Depiction of man exhibiting the Gowers's sign: common symptom of centronuclear myopathy that results from the weakness of lower limb muscles.
Basic helix–loop–helix.

Formation of skeletal muscular tissue, particularly during embryonic development.

- Myogenesis

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Mesoderm

Middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals.

Tissues derived from mesoderm.

Mesoderm forms the muscles in a process known as myogenesis, septa (cross-wise partitions) and mesenteries (length-wise partitions); and forms part of the gonads (the rest being the gametes).

Cellular differentiation

Process in which a stem cell alters from one type to a differentiated one Usually, the cell changes to a more specialized type.

Stem cell differentiation into various tissue types.
Micrograph of a liposarcoma with some dedifferentiation, that is not identifiable as a liposarcoma, (left edge of image) and a differentiated component (with lipoblasts and increased vascularity (right of image)). Fully differentiated (morphologically benign) adipose tissue (center of the image) has few blood vessels. H&E stain.
Mechanisms of cellular differentiation.
An overview of major signal transduction pathways.

A small molecule dubbed reversine, a purine analog, has been discovered that has proven to induce dedifferentiation in myotubes.

Cell fusion

Important cellular process in which several uninucleate cells combine to form a multinucleate cell, known as a syncytium.

a Cells of the same lineage fuse to form a cell with multiple nuclei, known as a syncytium. The fused cell can have an altered phenotype and new functions such as barrier formation. b  Cells of different lineage fuse to form a cell with multiple nuclei, known as a heterokaryon. The fused cells might have undergone a reversion of phenotype or show transdifferentiation. c  Cells of different lineage or the same lineage fuse to form a cell with a single nucleus, known as a synkaryon. New functions of the fused cell can include a reversion of phenotype, transdifferentiation and proliferation. If nuclear fusion occurs, the fused nucleus initially contains the complete chromosomal content of both fusion partners (4N), but ultimately chromosomes are lost and/or re-sorted (see arrows). If nuclear fusion does not occur, a heterokaryon (or syncytium) can become a synkaryon by shedding an entire nucleus.
BTX ECM 2001 Electrofusion generator cell fusion applications manufactured by BTX Harvard Apparatus, Holliston MA USA

Cell fusion occurs during differentiation of myoblasts, osteoclasts and trophoblasts, during embryogenesis, and morphogenesis.

Metalloproteinase

Any protease enzyme whose catalytic mechanism involves a metal.

The structure of a protease (TEV protease) complexed with its peptide substrate in black with catalytic residues in red.

An example is ADAM12 which plays a significant role in the fusion of muscle cells during embryo development, in a process known as myogenesis.

Skeletal muscle

Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton.

A top-down view of skeletal muscle
3D rendering of a skeletal muscle fiber
Muscle types by fiber arrangement
Types of pennate muscle. A – unipennate; B – bipennate; 
C – multipennate
ATPase staining of a muscle cross section. Type II fibers are dark, due to the alkaline pH of the preparation. In this example, the size of the type II fibers is considerably less than the type I fibers due to denervation atrophy.
Structure of muscle fibre showing a sarcomere under electron microscope with schematic explanation.
Diagram of sarcoplasmic reticulum with terminal cisternae and T-tubules.
Human embryo showing somites labelled as primitive segments.
When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap.
Contraction in more detail
(a) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. (b) Each glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria.
Exercise-induced signaling pathways in skeletal muscle that determine specialized characteristics of slow- and fast-twitch muscle fibers
Jogging is one form of aerobic exercise.
In muscular dystrophy, the affected tissues become disorganized and the concentration of dystrophin (green) is greatly reduced.
Prisoner of war exhibiting muscle loss as a result of malnutrition.

Muscle fibers are formed from the fusion of developmental myoblasts in a process known as myogenesis resulting in long multinucleated cells.

Receptor tyrosine kinase

Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones.

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An example of a vital signal transduction pathway involves the tyrosine kinase receptor, c-met, which is required for the survival and proliferation of migrating myoblasts during myogenesis.

Hepatocyte growth factor

Paracrine cellular growth, motility and morphogenic factor.

Overview of signal transduction pathways.

It has been shown to have a major role in embryonic organ development, specifically in myogenesis, in adult organ regeneration, and in wound healing.

MyoD

Evidence suggests that Setdb1 inhibits a repressor of MyoD and this is the mechanism through which MyoD expression is retained in differentiated myoblasts.
MyoD works with a transient placeholder protein that functions to prevent other transcription factors from binding to the DNA and also retains an inactive conformation for the DNA. Once the placeholder is removed (or possibly deactivated) the necessary transcription factors are free to bind and initiate recruitment of RNA Polymerase II and initiate active RNA transcription.

MyoD, also known as myoblast determination protein 1, is a protein in animals that plays a major role in regulating muscle differentiation.

Somite

The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals.

Transverse section of half of a chick embryo of forty-five hours' incubation. The dorsal (back) surface of the embryo is towards the top of this page, while the ventral (front) surface is towards the bottom.
Chick embryo of thirty-three hours’ incubation, viewed from the dorsal aspect. X 30
Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. Dermatome
Scheme showing how each vertebral centrum is developed from portions of two adjacent segments. (Myotome labelled in upper left.)
Human embryo at the end of week 4 with somite development.

The myoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls.

Myosatellite cell

Myosatellite cells, also known as satellite cells, muscle stem cells or MuSCs, are small multipotent cells with very little cytoplasm found in mature muscle.

Schematic of myosatellite cell transition into myofiber.

More specifically, upon activation, satellite cells can re-enter the cell cycle to proliferate and differentiate into myoblasts.