Development of Musculoskeletal System

Axial Skeleton

Includes the skull, vertebral column, ribs, and sternum

paraxial and lateral plate (parietal layer) mesoderm and from neural crest

skeletal system develops from

Paraxial mesoderm

somitomeres in the head region and somites from the occipital region caudally

Somites

ventromedial part, (the sclerotome) and dorsolateral part, (the dermomyotome)

Sclerotome

• become polymorphous and form loosely organized tissue, called mesenchyme, or embryonic connective tissue

• migrate and to differentiate in many ways (fibroblasts, chondroblasts, or osteoblasts (bone- forming cells)

• bone-forming capacity of mesenchyme is not restricted to cells of the sclerotome but occurs also in the parietal layer of the lateral plate mesoderm of the body wall → forms bones of the pelvic and shoulder girdles, limbs, and sternum

Neural crest cells in the head region

also differentiate into mesenchyme and participate in formation of bones of the face and skull

occipital somites and somitomeres

remainder of the skull is derived from

True

in some bones, such as the flat bones of the skull, mesenchyme in the dermis differentiates directly into bone, a process known as intramembranous ossification

True

In most bones, however, including the base of the skull and the limbs, mesenchymal cells first give rise to hyaline cartilage models, which in turn become ossified by endochondral ossification

Neurocranium

Viscerocranium

skull is divided into two part

Neurocranium

protective case around the brain

• membranous part

• cartilaginous part or chondrocranium

Viscerocranium

Forms the skeleton of the face

0. formed mainly from the first two pharyngeal arches

   • first arch

   • Ventral portion

   • dorsal tip of the mandibular process, along with that of the second pharyngeal arch

• Ossification of the three ossicles begins in the fourth month (the first bones to become fully ossified)

membranous part

Neurocranium

• consisting of flat bones (surround the brain as a vault)

cartilaginous part or chondrocranium

Neurocranium

• forms bones of the base of the skull

first arch

0. gives rise to a dorsal portion, the maxillary process, which extends forward beneath the region of the eye and gives rise to the maxilla, the zygomatic bone, and part of the temporal bone

Ventral portion

the mandibular process, contains the Meckel cartilage

dorsal tip of the mandibular process, along with that of the second pharyngeal arch

later gives rise to the incus, the malleus, and the stapes

Skull

• At first, the face is small in comparison with the neurocranium caused by:

  (1)virtual absence of the paranasal air sinuses
  (2)the small size of the bones, particularly the jaws.

• With the appearance of teeth and development of the air sinuses, the face loses its babyish characteristics

Vertebrae form from the sclerotome portions of the somites

derived from paraxial mesoderm

sclerotome cells migrate around the spinal cord and notochord to merge with cells from the opposing somite on the other side of the neural tube

During the 4th week

True

sclerotome portion of each somite also undergoes a process called resegmentation

Resegmentation

• occurs when the caudal half of each sclerotome grows into and fuses with the cephalic

  half of each subjacent sclerotome

• each vertebra is formed from the combination of the caudal half of one somite and the cranial half of its neighbor

• muscles derived from the myotome region of each somite become attached to two adjacent somites across the intervertebral discs and can therefore move the vertebral column

• Patterning of the shapes of the different vertebrae is regulated by HOX genes

vertebral arch and foramen (through which the spinal cord passes), a body, transverse processes, and usually a spinous process

Typical vertebra consists of

True

• Resegmentation of sclerotomes into definitive vertebrae causes the myotomes to bridge the intervertebral discs, and this alteration gives them the capacity to move the spine

cervical curvature, lumbar curvature

2 secondary curvatures are established

cervical curvature

child learns to hold up his/her head

lumbar curvature

child learns to walk

thoracic and sacral curvatures

• As the vertebrae form, two primary curves of the spine are established:

Bony portion of each rib

from sclerotome cells that remain in the paraxial mesoderm and that grow out from the costal processes of thoracic vertebrae

Costal cartilages

from sclerotome cells that migrate across the lateral somitic frontier into the adjacent lateral plate mesoderm

Sternum

develops independently in the parietal layer of lateral plate mesoderm in the ventral body wall

• Two sternal bands are formed in the parietal (somatic) layer of lateral plate mesoderm on either side of the midline, and these later fuse to form cartilaginous models of the manubrium, sternebrae, and Xiphoid process.

Muscular system

develops from the mesodermal germ layer and consists of

skeletal, smooth, and cardiac muscle

Skeletal system

is derived from paraxial mesoderm - forms somites from the

occipital to the sacral regions and somitomeres in the head

Smooth muscle

differentiates from Visceral (splanchnic) mesoderm surrounding the gut and its derivatives and from ectoderm (pupillary, mammary gland, and sweat gland muscles).

Cardiac muscle

is derived from Visceral (splanchnic) mesoderm surrounding the heart tube.