Development of the Musculoskeletal System

Overview


The major components of the musculoskeletal system are derivatives of the mesoderm. They include bones, ligaments, tendons and joints and the skeletal muscles that move them.
The skeleton is divided into axial and appendicular parts originating in the mesoderm of somite and lateral plate and neural crest ectoderm.
 Muscle tissue is subdivided into three functional types, skeletal, smooth and cardiac. All skeletal muscles are derived from mesoderm.

In the fourth week of development, limb buds appear around the same time, paraxial mesoderm begins to from somites.
It is from the mesoderm of these primordia, along with the somatic layer of lateral plate mesoderm that the major musculoskeletal elements develop.
The two exceptions are:

    Development of the skull and eye will be discussed in a later module in the unit on head and neck.

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    Skeleton

    Each somite becomes subdivided into functionally distinct masses of mesoderm.
    The     ventromedial part of each somite is induced to become a sclerotome by factors secreted by the ventral neural tube and notochord. The sclerotomes will form the vertebrae and intervertebral discs.

    Factors secreted by the dorsal neural tube induce the     dorsomedial part of the somite to become the epimere which will develop into the epaxial musculature, ie. deep muscles of the back (erector spinae).

    Factors produced by the     surface ectoderm and the somatic layer of the lateral plate mesoderm induce the formation of the hypomere from the lateral ventral part of the myotome.

    The dorsal part of the neural tube induces the formation of the     dermatome to form the dermis of the skin in each specific body segment.

    Muscle

    In order to add confusion to the process, embryologists have now come up with new designations for the origins of muscles.

    • The primaxial domain refers to mesoderm derived exclusively from the somite mesoderm, the myotome. Muscles derived from the primaxial domain include the epaxial muscles or erector spinae and related deep back muscles innervated by the dorsal primary rami of spinal nerve and also some of the muscles of the shoulder girdle and intercostal muscles, innervated by ventral primary rami of spinal nerves.

    • The abaxial domain refers to mesoderm of both somite and lateral plate origin, forming the infrahyoid muscles, abdominal wall muscles and the limb muscles. These muscles are all innervated by the ventral primary rami of spinal nerves.