Chapter 6: Skeletal System (Bone Physiology & Osseous Tissue)

Functions of Bones

• Support: Provides a framework for the body.
• Protection: Shields vital organs (e.g., the skull protects the brain).
• Movement: Serves as levers for muscles to act upon.
• Mineral Storage/Release: Stores minerals such as calcium and phosphorus.
• Blood Cell Production: Hematopoiesis occurs in the red marrow.
• Triglyceride Storage: Yellow marrow stores fat.

Bone Anatomy Terms

• Diaphysis: The central shaft of the bone.
• Epiphysis: The end part of a long bone, initially growing separately from the shaft.
• Metaphysis: The region between the diaphysis and epiphysis, where growth plates are found.
• Articular Cartilage: Cartilage that covers the ends of bones in synovial joints to reduce friction.
• Periosteum: A dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.
• Sharpey's Fibers: Perforating fibers that anchor the periosteum to the bone.
• Medullary Cavity: The central cavity within the diaphysis of long bones, containing red and yellow marrow.
• Endosteum: A thin membrane lining the medullary cavity that also contains bone-forming cells.

Histology of Osseous Tissue

• Extracellular Matrix Composition:
  • 15% water
  • 30% collagen fibers
  • 55% mineralized salts

Osteoblasts, Osteocytes, and Osteoclasts

1. Osteoblasts:

   • Definition: Immature bone cells that secrete organic matrix called "osteoid," which consists of collagen fibers and organic material.
   • Process: Once these cells become surrounded by the osteoid/matrix, they cease secretion and differentiate into osteocytes.

2. Osteocytes:

   • Definition: Mature bone cells that maintain the bone matrix and tissue metabolism.
   • Characteristics: They do not divide.

3. Osteoclasts:

   • Definition: Large cells derived from monocytes (not osteogenic cells).
   • Location: High concentrations found in the endosteum of bones.
   • Characteristics: Have ruffled borders and secrete enzymes and acids that digest and dissolve the bone matrix, a process known as "resorption." They also help regulate blood calcium homeostasis.

Compact Bone Histology

• Functional Unit: Osteon
  • Central (Haversian) Canal: Located at the center of each osteon, containing blood vessels and nerves.
  • Concentric Lamellae: Layered arrangement of bone matrix surrounding the central canal, organized in a circumferential pattern.
  • Lacunae: Small cavities that contain osteocytes.
  • Canaliculi: Tiny channels connecting the lacunae to each other and to the central canal, facilitating nutrient delivery.
  • Interstitial Lamellae: The bone matrix found between the osteons.
  • Perforating Canals: Run perpendicular to the central canal, allowing for the transfer of nutrients and waste.

Spongy Bone

• Composed of trabecular bone; blood and nerve supply to bone is obtained through nutrient arteries that penetrate through foramina.

Embryonic Bone Formation (Ossification)

• Two types of ossification:

  1. Intramembranous Ossification:
  • Involves the formation of bones from mesenchymal tissue directly. This process primarily forms the bones of the skull (except for the mandible) and the medial clavicles.
  • Recipe for Ossification: Involves the presence of osteoblasts (derived from osteogenic cells) and a sufficient blood supply to provide the necessary nutrients and minerals.
  1. Endochondral Ossification:
  • Detail about this process not provided, typically involves the replacement of a cartilage model with bone.

Definition of Key Terms:

• Calcification: The process of depositing calcium salts in the bone tissue, which is necessary for the hardness and strength of the bone.
• Ossification: The process of bone tissue formation that occurs both in early skeletal development and during the bone remodeling process throughout life.
• Collagen Fibers: Crucial components of the bone matrix, providing tensile strength and structure.

Notes on Bone Cell Types

1. Osteoblasts: Responsible for bone formation by secreting the osteoid matrix.
2. Osteocytes: Maintain the bone matrix and support cellular activity within the bone.
3. Osteoclasts: Facilitate bone resorption, releasing minerals back into the bloodstream and playing a pivotal role in calcium homeostasis.