7.4 & 7.5

7.4 The Thoracic Cage

  • Learning Objectives:

    • Discuss the components that make up the thoracic cage

    • Identify the parts of the sternum and define the sternal angle

    • Discuss the parts of a rib and rib classifications

  • Definition:

    • The thoracic cage, also known as the rib cage, forms the thorax (chest) portion of the body. It consists of:

    • 12 pairs of ribs with their costal cartilages

    • The sternum

    • The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12).

    • The thoracic cage protects vital organs such as the heart and lungs.

  • Components of the Thoracic Cage:

    • Figure Reference: Figure 7.32 shows the thoracic cage including the sternum and ribs.

    • Ribs Classification:

    • Ribs are classified into:

      • True ribs (1–7) - Attaching directly to the sternum

      • False ribs (8–12) - Do not attach directly to the sternum.

      • Floating ribs (11–12) - Last two pairs of false ribs that do not attach to the sternum at all.

Sternum

  • Definition:

    • The sternum is the elongated bony structure that anchors the anterior thoracic cage. It has three parts:

    • Manubrium - The wider, superior portion of the sternum.

      • Jugular notch - A shallow, U-shaped border at the top of the manubrium, felt at the anterior base of the neck.

      • Clavicular notch - Shallow depression on either side of the superior-lateral margins; site of the sternoclavicular joint between the sternum and clavicle.

      • The first ribs attach to the manubrium.

    • Body - The elongated central portion of the sternum.

      • Sternal angle - The joint between the manubrium and body, forming a slight bend where the second rib also attaches; vital for identifying and counting ribs as the second rib is the highest palpable rib.

      • Ribs 3–7 attach to the body.

    • Xiphoid process - The inferior tip of the sternum; cartilaginous early in life, ossifying during middle age.

Ribs

  • Definition of Ribs:

    • Each rib is a curved, flattened bone contributing to the wall of the thorax.

  • Attachment:

    • Ribs articulate posteriorly with thoracic vertebrae T1–T12, and most attach anteriorly via costal cartilages to the sternum.

  • Parts of a Typical Rib:

    • Head of the rib - Posterior end articulating with costal facets of corresponding thoracic vertebra.

    • Neck of the rib - Narrowed portion lateral to the head.

    • Tubercle of the rib - A small bump on the posterior surface of the rib, articulating with the facet on the transverse process of the corresponding vertebra.

    • Body/Shaft of the rib - Extends from the tubercle to the anterior end.

    • Angle of the rib - The point where the rib has the greatest curvature, aligning with the medial border of the scapula in anatomical position.

    • Costal groove - Shallow groove along the inferior margin of each rib for passage of blood vessels and nerves.

Rib Classifications

  • Classification based on Relationship to the Sternum:

    • True ribs (vertebrosternal ribs) 1–7:

    • Costal cartilage attaches directly to the sternum.

    • False ribs (vertebrochondral ribs) 8–12:

    • Do not attach directly to the sternum; connections via cartilage.

      • Ribs 8–10 attach to rib 7's cartilage indirectly.

    • Floating ribs (vertebral ribs) 11–12:

    • Short ribs that do not connect to the sternum or any other rib; terminate in the abdominal wall musculature.

7.5 Embryonic Development of the Axial Skeleton

  • Learning Objectives:

    • Discuss the two types of embryonic bone development within the skull

    • Describe the development of the vertebral column and thoracic cage

  • Development Overview:

    • The axial skeleton begins to form during early embryonic development, with growth, remodeling, and ossification continuing for several decades post-birth.

    • Understanding these processes is vital for recognizing abnormalities in skeletal structures.

Development of the Skull

  • Notochord Formation:

    • During the third week of embryonic development, a rod-like structure known as the notochord develops dorsal along the embryo's length.

    • Tissue over the notochord enlarges to form the neural tube, giving rise to the brain and spinal cord.

  • Somite Formation:

    • By the fourth week, mesoderm tissue thickens and separates into somites (block-like tissue structures); the most medial part is called the sclerotome.

    • Sclerotomes consist of mesenchyme, forming fibrous connective tissues, cartilages, and bones.

  • Skull Bone Development Mechanisms:

    • First Mechanism:

    • Forms the top and sides of the braincase via intramembranous ossification.

      • Mesenchymal cells localize, differentiate into bone-producing cells, forming skull bones.

      • Areas between growing braincase bones are called fontanelles (soft spots) that allow shape alteration during birth and continued skull growth post-birth.

    • The largest fontanelle is at the junction of the frontal and parietal bones, reducing in size to disappear by age 2.

    • Second Mechanism:

    • Develops facial bones and braincase floor through endochondral ossification.

      • Mesenchymal cells differentiate into cartilage cells, forming cartilage models that gradually convert to bone.

      • The skull is disproportionately larger at birth compared to the jaws and lower face due to the underdevelopment of the maxilla and mandible, and small paranasal sinuses.

      • Growth occurs during early childhood with significant enlargement of the face and jaws as teeth appear.

Development of the Vertebral Column and Thoracic Cage

  • Vertebrae Development:

    • Begins with mesenchyme cells from the sclerotome around the notochord differentiating into a hyaline cartilage model for each vertebra which ossifies via endochondral ossification.

    • The notochord largely disappears during vertebra growth, leaving remnants for intervertebral discs.

  • Ribs and Sternum Development:

    • Ribs develop as part of the cartilaginous model of vertebrae, separating from vertebrae by the eighth week.

    • Rib cartilage model ossifies except for the anterior portion, which remains as costal cartilage.

    • The sternum forms as paired hyaline cartilage models along the midline beginning in the fifth week, which fuse and ossify into bone.

    • The manubrium and body ossify first, with the xiphoid process remaining cartilage until late life.

Craniosynostosis

  • Definition:

    • The premature closure of a suture line in the skull, leading to abnormal skull growth and head deformation.

    • Caused by ossification defects or improper brain enlargement.

    • Affects about 1 in 2000 births, more common in males.

  • Types:

    • Primary craniosynostosis: Early fusion of one suture, restricting subsequent growth in that area.

    • Complex craniosynostosis: Involves premature fusion of multiple sutures, leading to more significant deformity and potentially impacting brain development.

  • Characteristics:

    • Compensatory growth in non-affected areas leads to abnormal head shapes (e.g., scaphocephaly: long, narrow wedge-shaped head).

    • Brain growth typically remains adequate despite skull deformities.

  • Treatment:

    • Usually surgical; involves opening the skull along the fused suture to allow for growth.

    • Some cases may require removal of skull parts and replacement with artificial plates.

    • Earlier surgery correlates with better outcomes, with most children developing normally thereafter without neurological issues.