Development of the muscular system

Definitions & Key takeaways

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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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, giving rise 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 give rise to 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 for the central nervous system.
This is the embryo’s first symmetry axis, and the mesoderm on either side of the neural tube differentiate in 3 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 in two layers: a parietal layer in contact with the ectoderm, and a visceral layer in contact with the endoderm.
The paraxial and lateral plate mesoderm give rise to the skeletal muscles in our body. But before the mesoderm cells develop into skeletal muscle, they first organize into cell blocks called somites.
Somites arise in pairs from a combination of paraxial mesoderm cells and mesenchyme - a soupy fetal tissue containing pluripotent cells.
Around day 20 of development, somites begin to form in the occipital region of the embryo, at the base of the head. Somites continue to form craniocaudal - going from head-to-tail end of the embryo - with about three pairs forming each day.
So the number of somite pairs can be used to determine the age of the embryo - like rings on a tree trunk. Up to 40 pairs of somites form by the end of week 5 - some of these degenerate, while the rest go on to form bone and muscle structures.
Alright, zooming back into the somites, they split and cells from the ventral portion form the sclerotome, which gives rise to the vertebrae and the ribs.
Cells from the dorsomedial lip of the somite - the top right layer of cubes here - mix with some cells from the ventrolateral lip in the opposite corner of the cube - the bottom left - and form a new, mixed tissue called the dermomyotome.
Cells of the dermomyotome further differentiate into dermatome and myotome cells, which turn into the dermis layer of the skin and into muscles, respectively.
Now, fast forwarding a bit, the muscles of the myotome start to develop, and one way to categorize the muscles is according to their innervation.
Each myotome develops along with a spinal nerve that emerges from the spinal cord and branches off into a ventral and a dorsal division.
This divides the myotome into two muscle-forming regions: the first is the epimere, which gives rise to the muscles of the back, is innervated by dorsal rami, or back branch, of spinal nerves.
The second is the hypomere, which gives rise to the muscles of the limbs and body wall, and is innervated by the ventral rami, or front branch, of the corresponding spinal nerve.
Another way to categorize muscles is according to their origin. Lateral to the somite, there’s an area called the lateral somitic frontier that separates the somites from the parietal mesoderm flanking them.
Like a border between two countries, this line splits embryonic mesoderm into two distinct territories: the primaxial domain and the abaxial domain.
The primaxial domain originates from somites around the neural tube, so it’s mainly composed of paraxial mesoderm. Primaxial mesoderm cells receive signals from the neural tube and the notochord, and differentiate into back, shoulder, and intercostal muscles.
The abaxial domain originates from parietal mesoderm and migrating somite cells that cross over the lateral somitic frontier.
Abaxial mesoderm cells receive signals from the parietal mesoderm and differentiate into abdominal wall muscles, limb muscles and one muscle in the neck - the infrahyoid muscle.
Finally, the last little guy we didn’t mention is the syndetome, which arises between the sclerotome and myotome, and the syndetome gives rise to the tendons, which are flexible collagen cords that attach Skeletal muscles to bones.
Early muscle cells are called myoblasts, and they merge into long spindles of muscle tissue called myofibrils. These myofibrils bundle together and fuse to form cylindrical skeletal muscle fibers.
As an analogy, if you held a bunch of straws, the individual straws would be the myofibrils, and the whole bunch would represent one muscle fiber.
Cardiac muscle is closely related to skeletal muscle in structure, but differs a bit in that it can contract and relax continuously and automatically.
Cardiac muscle forms when visceral mesoderm cells wrap around the primitive heart, and differentiate into cardiac myoblasts, which form cardiac muscle.
Finally, there’s the smooth muscle. Smooth muscle lining the gut tube develops from visceral mesoderm cells, whereas smooth muscle cells that line the arteries derive from visceral mesoderm cells as well as neural crest cells.
Some smooth muscles like the ciliary muscles that allow the pupils that to expand and contract, and the smooth muscle of the mammary and sweat glands derive from ectoderm.
To remember these ectoderm exceptions, think about a woman named Vectoria, who is sweating - sweat glands - with pupils dilated - pupillary muscles, as she runs into battlefield with a giant breast plate - mammary glands.
All right, as a quick recap… during development, mesoderm cells differentiate into the majority of the body’s muscular tissues, forming the skeletal and cardiac muscle, and most of the smooth muscle.
The only muscles that don’t arise from mesoderm are the smooth muscles of the pupils, and the sweat and mammary glands, which instead arise from ectoderm.