Development of the muscular system

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.

The paraxial and lateral plate mesoderm will become the skeletal muscles in our body.

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, which is a soupy fetal tissue containing pluripotent cells.

Around day 20 of development, somites begin to form in the occipital region of the embryo, which is at the base of the head.

Somites continue to form cranio-caudally, or from head-to-tail end of the embryo, with about three pairs forming each day.

Up to 40 somite pairs form by the end of week 5. Some degenerate, while the rest go on to form bone and muscle structures.

Each somite undergoes a split, with cells from the ventral portion forming sclerotome, creating 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) to form a new, mixed tissue called dermomyotome.

Dermomyotome cells 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.

One way to categorize the muscles is according to their innervation.