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Organ system development
Development of the cardiovascular system
Development of the ear
Development of the eye
Development of the face and palate
Pharyngeal arches, pouches, and clefts
Development of the digestive system and body cavities
Development of the gastrointestinal system
Development of the teeth
Development of the tongue
Development of the integumentary system
Development of the axial skeleton
Development of the limbs
Development of the muscular system
Development of the nervous system
Development of the renal system
Development of the reproductive system
Development of the respiratory 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.
Cells migrate along the primitive streak during gastrulation, leading to a now three-layered embryo pancake, with each layer containing germ cells that form organs and tissues of the body.
The ventral, or bottom, germ layer is called endoderm, the dorsal, or top, germ layer is called ectoderm, and the layer in between these two is called mesoderm.
Collectively, these germ cells produce all of the organs and tissues in the body.
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.
Between the cells of the lateral plate mesoderm, small gaps appear and coalesce to form the intraembryonic coelom, a cavity inside the embryo’s body.
This cavity separates the lateral plate mesoderm into two layers: a parietal layer that’s in contact with the ectoderm, and a visceral layer that’s in contact with the endoderm.
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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