Bone Homeostasis and Axial Skeleton

Bone and Calcium Homeostasis

Bone serves as a major storage site for calcium. Movement of calcium in and out of bone helps determine blood levels of calcium:

  • Osteoblasts: Build new bone, moving calcium into bone.
  • Osteoclasts: Break down bone, moving calcium out of bone.

Calcium Homeostasis Diagram

Hormonal Regulation:

  • Parathyroid Hormone (PTH): Stimulated by decreased blood Ca2+\text{Ca}^{2+} levels and secreted from parathyroid glands.
    • Stimulates osteoclasts to break down bone and release Ca2+\text{Ca}^{2+} into the blood.
    • Increases Ca2+\text{Ca}^{2+} reabsorption from urine in the kidneys.
    • Stimulates active Vitamin D formation, which promotes Ca2+\text{Ca}^{2+} absorption from the small intestine into blood.
  • Calcitonin: Secreted from the thyroid gland when blood Ca2+\text{Ca}^{2+} is increased.
    • Inhibits osteoclasts, allowing enhanced osteoblast uptake of Ca2+\text{Ca}^{2+} from blood to deposit into bone.

Bone Anatomical Terms

  • Foramen: Hole (e.g., foramen magnum).
  • Fossa: Depression (e.g., glenoid fossa).
  • Process: Projection (e.g., mastoid process).
  • Condyle: Smooth, rounded end (e.g., occipital condyle).
  • Meatus: Canal-like passageway (e.g., external auditory meatus).
  • Tubercle: Lump of bone (e.g., greater tubercle).

Axial Skeleton Overview

The axial skeleton is composed of the skull, the vertebral column, and the thoracic cage.

  • Skull: Consists of 22 total bones divided into braincase and face.
  • Auditory Ossicles: 3 in each middle ear (6 total).

Cranial Bones

The braincase encloses the cranial cavity and consists of 8 bones that surround and protect the brain:

  • Frontal bone: Anterior part of cranium.
  • Parietal bones: Sides and roof of cranium.
  • Occipital bones: Posterior portion and floor of cranium.
  • Temporal bones: Inferior to parietal bones on each side of the cranium; contains temporomandibular joint.
  • Sphenoid bone: Forms part of cranium floor, lateral posterior portions of eye orbits, and lateral portions of cranium anterior to temporal bones; features the sella turcica.
  • Ethmoid bone: Anterior portion of cranium, including medial surface of eye orbit and roof of nasal cavity; features nasal conchae.

Facial Bones

The bony structure of the face has 14 facial bones. Thirteen of these are solidly connected to form the bulk of the face, while the mandible forms a freely movable joint:

  • Maxillae: Form upper jaw, anterior portion of hard palate, part of lateral walls of nasal cavity, and floors of eye orbits; contain maxillary sinus.
  • Palatine bones: Form posterior portion of hard palate and lateral wall of nasal cavity.
  • Vomer: Located in midline of nasal cavity; forms nasal septum with the ethmoid bone.
  • Inferior nasal conchae: Attached to lateral walls of nasal cavity.
  • Mandible: Lower jawbone; the only movable skull bone.