Skeletal System

Mesodermal cells are responsible for the formation of bones and cartilage. In the regions of the head and neck, neural crest cells also participate in the formation of bones and connective tissues.

The development of the skeletal system can be described based on the region of bone development as follows:

  1. Axial bones including the following:

    • Ribs - these are formed from sclerotomes

    • Sternum - this is formed from somatic mesoderm

    • Cranium which can be further catergorised as:

      • Neurocranium

        • The ethmoid, sphenoid, occipital, and the petrous part of the temporal bone form through endochondral ossification from cells of the occipital somites.

        • The parietal and frontal bones form through intramembranous ossification.

      • Viscerocranium

        • These form the facial skeleton and the cells that take part in their development originate from the pharyngeal arches. The cartilaginous viscerocranium forms the bones of the middle ear (malleus, incus, stapes), the hyoid bone, and laryngeal cartilages. The membranous viscerocranium forms the maxilla, mandible, zygomatic bones, and the squamous portion of the temporal bone

    • Vertebrae

      • Such bones form from sclerotomes that surround the notochord

  2. Appendicular bones

    • Such bones are formed through endochondral ossification with the diaphysis being the primary center of ossification and the epiphyses the secondary centers of ossification. A cartilaginous growth plate persists between the ossified diaphysis and epiphysis and this remains as an area of continued endochondral ossification where bone lengthening occurs. This growth plate eventually ossifies, marking the end of bone growth.

Ossification

Ossification is the process of bone formation and this may occur in the following ways:

  1. Endochondral Ossification

  2. Intramembranous Ossification

Endochondral Ossification

The steps describing endochondral ossification can be summarized as:

  • The cells from the mesenchymal model that will give rise to future bones will begin differentiating into chondrocytes (cartilage).

  • This cartilage model will begin ossifying at it’s primary ossification center (diaphysis)

  • The chondrocytes hypertrophy and begin to calcify their surrounding extracellular matrix (before dying)

  • The perichondrium surrounding the cartilage model becomes periosteum as the cells begin differentiating into osteoblasts

  • Bone begins to form at the diaphysis to become compact bone

  • Blood vessels forming in the diaphysis will allow for the migration of progenitor cells to this region

  • These progenitor cells will form osteoblasts

  • Bone matrix is deposited by osteoblasts onto the previously calcified cartilage

  • Osteoclasts resorb and remodel new spongey bones

  • The secondary ossification center (epiphyses) remain cartilaginous until the chondrocytes hypertrophy and calcify the matrix in the region

  • Once again, blood vessels allow for the migration of progenitor cells to this region that will differentiate into osteoblasts

  • While the entire epiphyses ossify, bands of cartilage (epiphyseal growth plate) remain between the epiphysis and diaphysis until about 21 years of age when they are also replaced by bone

Intramembranous Ossification

The steps describing intramembranous ossification can be summarized as:

  • Flat mesenchymal cells set down the template of bones

  • These templates contain cells that will condense to form osteoblasts and blood capillaries

  • The osteoblasts will secrete matrix that binds with calcium phosphate to calcify

  • Spicules of bones will extend from earlier sites of ossification and new mesenchymal cells will surround them to give rise to the periosteum

  • Compact bone forms at the periphery and spongey bone forms in between

  • Osteoclasts resorb and remodel the bone

Joint Formation

The steps describing various types of joint formation can be summarized as:

  • Joints between flat bones of the skull: the mesenchyme between bones differentiate to form fibrous tissue

  • Joints between the ribs and sternum: cells may differentiate into chondrocytes to form a hyaline cartilage

  • Joints in the midline (pubic symphysis): forms as a fibrocartilaginous joint

  • Synovial joints: mesenchymal cells between the cartilage condensations of bones that are developing will differentiate into fibroblast cells. These cells will differentiate to form layers of articular cartilage and a central area of connective tissue. The cells lining the periphery of the central area of connective tissue will differentiate into synovial cells that line the joint cavity before the central area of connective tissue degenerates to form a synovial joint cavity that will later be filled by synovial fluid.

Clinical Notes:

  1. Craniosynostosis - this occurs when the sutures of the cranium close earlier than normal which results in an abnormally shaped head.

  2. Spina bifida - this occurs when there is inadequate closure of the lumbosacral regions of the vertebra.