The axial skeleton's embryonic development is a fascinating journey from simple cells to complex structures. It involves two main processes: intramembranous ossification for flat skull bones and endochondral ossification for the skull base and vertebral column.

The formation of the vertebral column, ribs, and sternum is a intricate dance of cellular differentiation and migration. Fontanelles, the soft spots on a baby's head, play a crucial role in birth and early brain growth, gradually closing as the skull develops.

Embryonic Development of the Axial Skeleton

Types of skull bone development

• Intramembranous ossification
  • Mesenchymal cells differentiate directly into osteoblasts, which are the cells responsible for bone formation
  • Osteoblasts secrete osteoid matrix, a protein-rich substance that calcifies to form bone tissue (frontal and parietal bones)
  • Occurs in the flat bones of the skull, such as the frontal, parietal, and parts of the occipital and temporal bones, resulting in the formation of the skull vault
• Endochondral ossification
  • Mesenchymal cells differentiate into chondrocytes, which are cells that produce cartilage, forming a cartilage model of the future bone (ethmoid bone)
  • Chondrocytes hypertrophy (enlarge) and die, leaving cavities in the cartilage matrix
  • Blood vessels and osteoblasts invade the cavities, depositing bone matrix and replacing the cartilage with bone tissue
  • Occurs in the base of the skull, including the ethmoid, sphenoid, and parts of the occipital and temporal bones, forming the skull base

Formation of axial skeleton

• Vertebral column
  • Somites, which are blocks of paraxial mesoderm, differentiate into sclerotomes, which are the embryonic precursors of the vertebrae
  • Sclerotomes migrate and surround the notochord, a flexible rod-like structure that provides support to the embryo
  • Sclerotomes undergo endochondral ossification to form the vertebral bodies and arches, creating the segmented structure of the spine
  • Notochord degenerates, leaving the nucleus pulposus, the gelatinous center of the intervertebral discs, which provide cushioning between the vertebrae
• Ribs
  • Costal processes, which are embryonic rib precursors, develop from the thoracic (chest) sclerotomes
  • Costal processes elongate and undergo endochondral ossification to form the ribs, which protect the thoracic organs (heart and lungs)
• Sternum
  • Sternal bars, which are two parallel cartilaginous structures, develop from the ventrolateral (front and side) body wall mesenchyme
  • Sternal bars fuse in the midline to form the cartilaginous sternum, the breastbone
  • Sternum undergoes endochondral ossification, with multiple ossification centers, to form the bony sternum, which serves as an attachment point for the ribs

Fontanelles in infant skulls

• Fontanelles are fibrous membrane-covered gaps between the incompletely ossified bones of the infant skull, allowing for flexibility and growth
• Allow for skull deformation during birth, facilitating passage through the birth canal, which is essential for successful delivery
• Provide space for rapid brain growth during the first year of life, accommodating the increasing size of the brain
• Anterior fontanelle
  • Located at the junction of the frontal and parietal bones, at the top front of the skull
  • Normally closes between 12-18 months of age, indicating proper skull development
• Posterior fontanelle
  • Located at the junction of the parietal and occipital bones, at the back of the skull
  • Normally closes between 2-3 months of age, earlier than the anterior fontanelle
• Sphenoidal and mastoid fontanelles
  • Located at the junctions of the temporal, parietal, and sphenoid/occipital bones, on the sides of the skull
  • Close within the first few months of life, contributing to the stability of the skull
• Delayed closure or abnormal enlargement of fontanelles may indicate underlying developmental disorders (craniosynostosis) or increased intracranial pressure (hydrocephalus), requiring medical attention
• As fontanelles close, cranial sutures form, allowing for continued skull growth and flexibility

Developmental processes in axial skeleton formation

• Chondrification: The process by which cartilage models of future bones are formed
• Neural crest cells contribute to the formation of the skull, particularly the facial bones and parts of the skull vault
• Ossification centers appear within the cartilage models, initiating the process of bone formation and growth