7.5 Embryonic Development of the Axial Skeleton
3 min read•june 18, 2024
The axial skeleton's embryonic development is a fascinating journey from simple cells to complex structures. It involves two main processes: for flat skull bones and for the and vertebral column.
The formation of the vertebral column, , and is a intricate dance of cellular differentiation and migration. , 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
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- differentiate directly into , 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
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- Mesenchymal cells differentiate into , which are cells that produce cartilage, forming a cartilage model of the future bone ()
- 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
- , which are blocks of , differentiate into , which are the embryonic precursors of the
- Sclerotomes migrate and surround the , 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
- degenerates, leaving the , the gelatinous center of the , which provide cushioning between the vertebrae
- Ribs
- , 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)
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- , 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 , to form the bony sternum, which serves as an attachment point for the ribs
Fontanelles in infant skulls
- 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
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- 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
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- 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
- 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 () or increased intracranial pressure (), requiring medical attention
- As fontanelles close, form, allowing for continued skull growth and flexibility
Developmental processes in axial skeleton formation
- : The process by which cartilage models of future bones are formed
- 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
Key Terms to Review (51)
Anterior Fontanelle: The anterior fontanelle is a soft spot on the skull of a newborn infant, formed by the junction of the frontal, parietal, and sphenoid bones. It is an important landmark in the embryonic development of the axial skeleton.
Chondrification: Chondrification is the process by which cartilage is formed from mesenchymal cells during embryonic development. It is a crucial step in the formation of the skeletal system, as cartilage serves as a template for future bone formation.
Chondrocytes: Chondrocytes are specialized cells found within cartilage that are responsible for maintaining the cartilaginous matrix, which includes producing and maintaining the extracellular components such as collagen and proteoglycans. These cells derive from chondroblasts that become embedded in the matrix they produce.
Chondrocytes: Chondrocytes are the specialized cells found within cartilage that are responsible for the production, maintenance, and repair of the cartilaginous extracellular matrix. These cells play a crucial role in the embryonic development of the axial and appendicular skeletons, as well as the formation and development of cartilaginous joints.
Costal Processes: Costal processes are bony projections that extend laterally from the vertebral bodies, providing attachment points for the ribs. They are an important structural component of the axial skeleton, particularly in the development and organization of the thoracic region.
Cranial Sutures: Cranial sutures are the fibrous joints that connect the bones of the skull. They allow the skull to expand during development and accommodate the growing brain, while also providing flexibility and protection to the delicate structures within.
Craniosynostosis: Craniosynostosis is a congenital condition characterized by the premature fusion of one or more of the cranial sutures, which are the fibrous joints between the bones of the skull. This early fusion can lead to an abnormal head shape and potential complications in brain development and function.
Endochondral ossification: Endochondral ossification is the process by which bone tissue forms from cartilage, replacing it with hard bone over time. It is crucial for the development and growth of most bones in the body, including long bones like the femur.
Endochondral Ossification: Endochondral ossification is the process by which most of the bones in the body, except for the flat bones of the skull, form and develop. It involves the replacement of cartilage with bone tissue, allowing for the growth and shaping of the skeletal system.
Ethmoid Bone: The ethmoid bone is a delicate, sieve-like bone located at the anterior base of the cranium. It is a crucial component of the nasal cavity and plays a vital role in the structure and function of the skull.
Fontanelle: A fontanelle is a soft spot on a baby's skull where the bones have not yet fused together. These gaps allow for growth of the brain and skull during an infant's first year.
Fontanelles: Fontanelles are soft spots on a baby's skull where the bones have not yet fused together, allowing for flexibility during childbirth and growth of the brain. These gaps are covered by tough membranes to protect the underlying soft tissues and brain.
Fontanelles: Fontanelles are the soft, membranous gaps between the bones of an infant's skull that allow the skull to change shape during childbirth and early development. They are an important feature of the developing axial skeleton.
Frontal bone: The frontal bone is a large bone at the front of the skull that forms the forehead, the roofs of the orbits (eye sockets), and most of the anterior part of the cranial floor. It also contains sinuses known as frontal sinuses, which are air-filled cavities that reduce the weight of the skull and improve voice resonance.
Frontal Bone: The frontal bone is one of the eight cranial bones that form the anterior portion of the skull. It is responsible for the formation of the forehead and the upper part of the orbital cavities, which house the eyes.
Greater wings of sphenoid bone: The greater wings of the sphenoid bone are large, wing-shaped extensions on either side of the sphenoid bone, contributing to the floors of the middle cranial fossa and the orbits. They play a crucial role in forming part of the skull's base and sides, providing structural support for the brain and protection for several critical nerves and blood vessels.
Hydrocephalus: Hydrocephalus is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid (CSF) within the ventricles of the brain, leading to an increase in intracranial pressure. This condition is particularly relevant in the context of the embryonic development of the axial skeleton, as the brain and spinal cord are integral components of the axial skeleton.
Intervertebral Discs: Intervertebral discs are flexible, cushion-like structures located between the vertebrae of the spine. They serve to connect and support the vertebrae, absorb shock, and facilitate spinal flexibility and movement.
Intramembranous ossification: Intramembranous ossification is a process by which certain bones, such as some of the skull bones and clavicle, are formed directly from mesenchymal (loosely organized embryonic) tissue without first being cartilage. This process contributes to the formation of the flat bones of the skull, mandible, and clavicle.
Intramembranous Ossification: Intramembranous ossification is a process of bone formation where bone develops directly from mesenchymal cells without a cartilage precursor. This type of ossification is responsible for the formation of flat bones, such as the skull, and certain parts of the axial and appendicular skeletons.
Mastoid fontanelle: The mastoid fontanelle is a soft spot located on an infant's skull, situated between the parietal and occipital bones, and bordered by the mastoid process of the temporal bone. It plays a crucial role in allowing for the flexibility and growth of the skull during early development, especially as the brain expands and matures. This fontanelle typically closes as the child grows, usually around the age of 6 months to 1 year.
Mesenchymal cells: Mesenchymal cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts, chondrocytes, and adipocytes. These cells play a critical role in tissue repair and embryonic development by providing the foundation for the formation of various tissues, including bone and cartilage.
Neural Crest Cells: Neural crest cells are a transient, multipotent population of cells that originate from the dorsal region of the neural tube during embryonic development. These cells migrate throughout the body and differentiate into a diverse array of cell types, playing a crucial role in the formation of the axial skeleton.
Notochord: The notochord is a flexible rod made out of a material similar to cartilage that provides support in the embryonic stages of all chordates, including humans. It plays a crucial role in the development of the spinal column and the central nervous system.
Notochord: The notochord is a flexible, rod-like structure that forms the main skeletal support of the body in the earliest stages of embryonic development. It is a key feature in the development of the axial skeleton and plays a crucial role in fetal development.
Nucleus pulposus: The nucleus pulposus is the inner core of the vertebral disc, composed of a jelly-like material that provides the disc with its elasticity and ability to absorb shock. It is crucial for the normal function and flexibility of the vertebral column.
Nucleus Pulposus: The nucleus pulposus is a gelatinous, cushion-like structure located at the center of the intervertebral discs in the vertebral column. It plays a crucial role in the structure and function of the vertebral column, as well as the embryonic development of the axial skeleton and the formation of cartilaginous joints.
Occipital bone: The occipital bone is a cranial dermal bone located at the posterior part of the skull and is crucial in encasing and protecting the brain. It articulates with the first cervical vertebra (atlas) and contains the foramen magnum, through which the spinal cord connects to the brain.
Occipital Bone: The occipital bone is a flat, curved bone that forms the posterior and inferior portions of the skull. It is one of the eight cranial bones and plays a crucial role in the structure and function of the skull.
Ossification Centers: Ossification centers are the specific locations within developing bones where the process of bone formation, or ossification, begins. These centers serve as the starting points for the growth and development of the skeletal system during embryonic and fetal development.
Osteoblasts: Osteoblasts are specialized bone cells responsible for the formation and mineralization of bone tissue. They play a crucial role in the growth, maintenance, and repair of the skeletal system, as well as in calcium homeostasis throughout the body.
Paraxial mesoderm: Paraxial mesoderm is a specialized layer of mesoderm located adjacent to the developing notochord in the embryo, playing a crucial role in the formation of somites. These somites give rise to various structures, including vertebrae, skeletal muscles, and dermis of the skin, thus contributing significantly to the development of the axial skeleton.
Parietal Bone: The parietal bones are two large, curved bones located on the top and sides of the skull, forming the majority of the cranium. These bones play a crucial role in protecting the brain and providing structural support to the skull, connecting with several other bones to form the braincase.
Perpendicular plate of the ethmoid bone: The perpendicular plate of the ethmoid bone is a thin, vertical bony structure that forms the upper part of the nasal septum, dividing the nasal cavity into two chambers. It extends downward from the cribriform plate of the ethmoid bone in the skull.
Posterior fontanelle: The posterior fontanelle is a soft spot located on the back of an infant's skull, formed by the intersection of the parietal and occipital bones. This anatomical feature plays a crucial role in allowing flexibility during childbirth and contributes to the overall growth and development of the cranial structure as the brain expands.
Ribs: Ribs are curved, bony structures that form the thoracic cage, which protects the vital organs in the chest cavity. They are an essential component of the axial skeleton and play a crucial role in various functions of the skeletal system.
Sclerotome: A sclerotome is a group of mesodermal cells in a vertebrate embryo that differentiates into bones, cartilage, and connective tissues of the axial skeleton. It originates from the segmental division of the somites during embryonic development.
Sclerotomes: Sclerotomes are segmented blocks of mesoderm tissue that form the vertebral column and associated structures during embryonic development. They are a key component in the embryonic development of the axial skeleton.
Skull Base: The skull base is the inferior, or lower, portion of the cranium that forms the foundation of the skull. It is a complex and intricate structure that supports the brain and houses several important foramina and openings for the passage of nerves, blood vessels, and other structures.
Skull Vault: The skull vault, also known as the cranial vault or calvaria, refers to the upper, dome-like portion of the skull that encloses and protects the brain. It is a key component of the axial skeleton that develops during embryonic growth and formation.
Somite: Somites are segmented blocks of mesoderm found on either side of the neural tube in the developing embryo, which give rise to skeletal muscles, vertebrae, and dermis of the skin. They play a crucial role in organizing the segmented structure of the vertebrate body.
Somites: Somites are blocks of mesodermal cells located on either side of the neural tube in a developing embryo. These cells differentiate into structures such as vertebrae, ribs, and skeletal muscles.
Somites: Somites are segmented blocks of mesoderm that form along the embryonic body axis, giving rise to the axial skeleton, muscles, and other structures. They are a crucial component in the embryonic development of the body's main systems.
Sphenoid Bone: The sphenoid bone is a complex, centrally located bone in the skull that plays a crucial role in the structure and function of the cranium. It is one of the seven bones that make up the neurocranium and is intimately involved in the divisions of the skeletal system, the overall structure of the skull, and the embryonic development of the axial skeleton.
Sphenoidal Fontanelle: The sphenoidal fontanelle is a small gap or opening that forms in the fetal skull during embryonic development, located between the sphenoid and frontal bones. It is one of the six fontanelles that allow the skull to change shape and pass through the birth canal during childbirth.
Sternal Bars: Sternal bars are a pair of cartilaginous bars that develop on either side of the midline of the embryonic chest wall, forming the foundation for the sternum. These structures play a crucial role in the embryonic development of the axial skeleton.
Sternum: The sternum, or breastbone, is a long, flat bone located in the center of the chest. It connects to the rib bones via cartilage, forming the front part of the rib cage which protects vital organs such as the heart and lungs.
Sternum: The sternum is a flat, elongated bone located in the center of the chest that serves as the anterior portion of the thoracic cage. It plays a crucial role in the structure and function of the skeletal system.
Temporal Bone: The temporal bone is one of the eight bones that make up the skull. It is located on the side and base of the cranium and plays a crucial role in the structure and function of the auditory and vestibular systems.
Vertebrae: Vertebrae are the individual bones that make up the vertebral column, also known as the spine or backbone. They are the central structural components of the axial skeleton and play a crucial role in the functions of the skeletal system.
Zygomatic process of the temporal bone: The zygomatic process of the temporal bone is a bony projection that extends from the temporal bone of the skull to articulate with the zygomatic bone, forming part of the cheekbone or zygoma. This connection helps form the prominence of the cheek and contributes to the facial structure and articulation with the jaw.