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Organ system development
Development of the muscular system
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The muscular system starts taking shape when the embryo is just a flat little pancake made up of two layers: the epiblast on the dorsal, or back, side, and the hypoblast on the ventral, or front, side.
A line called the primitive streak appears on the epiblast “back” of this two-layered creature.
During week 3, the embryo transitions from a flat organism to a more tubular creature by folding along its longitudinal and lateral axes.
At the same time, a solid rod of mesoderm called the notochord forms on the midline of the embryo.
Above the notochord, the ectoderm invaginates to form the neural tube, an early precursor of the central nervous system.
This is the embryo’s first symmetry axis, and the mesoderm on either side of the neural tube differentiates into three distinct portions: immediately flanking the neural tube there’s the paraxial mesoderm; next, there’s the intermediate mesoderm; and finally, the lateral plate mesoderm.
Development of the muscular system starts at around week d of gestation. The muscular system begins with the formation of muscle cells called myoblasts. Myoblasts originate from the mesoderm and fuse together to form long and multinucleated fibers called muscle fibers. Muscle fibers are attached by collagenous connective tissues, and the entire muscle is enclosed in a fibrous capsule. All skeletal and cardiac muscles and most smooth muscles arise from mesoderm cells, except pupillary muscles and the sweat and mammary glands, which arise from ectoderm.
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