Chapter 6: Bones and Bone Tissue

Module 6.1: Introduction to Bones as Organs

The skeletal system is composed of bones, joints, and their associated supporting tissues. Bones are complex organs made up of osseous tissue, connective tissues, and bone marrow. They perform several vital functions in the body.

• Protection: Bones such as the skull, sternum, ribs, and pelvis protect underlying organs.

• Mineral Storage and Acid-Base Homeostasis: Bones store calcium, phosphorus, and magnesium salts, which are essential for electrolyte and acid-base balance.

• Blood Cell Formation: Red bone marrow produces blood cells through hematopoiesis.

• Fat Storage: Yellow bone marrow stores triglycerides, which can be used as an energy source.

• Movement: Bones serve as attachment sites for muscles, enabling movement at joints.

• Support: The skeleton provides structural support for the body.

Classification of Bones by Shape

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped, as long as they are wide (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., ribs, sternum, skull bones).

• Irregular Bones: Irregular shapes (e.g., vertebrae, certain skull bones).

• Sesamoid Bones: Small, oval-shaped bones within tendons (e.g., patella).

Structure of a Long Bone

• Periosteum: Dense irregular connective tissue covering the bone's outer surface, rich in blood vessels and nerves.

• Perforating Fibers: Collagen fibers anchoring the periosteum to bone.

• Diaphysis: The shaft of the bone, containing the marrow cavity.

• Epiphyses: The ends of the bone, covered with articular (hyaline) cartilage.

• Compact Bone: Dense outer layer providing strength.

• Spongy Bone (Cancellous Bone): Inner honeycomb-like structure housing bone marrow.

• Endosteum: Thin membrane lining internal bone surfaces.

• Epiphyseal Lines: Remnants of growth plates separating epiphyses from diaphysis.

Structure of Short, Flat, Irregular, and Sesamoid Bones

• No diaphyses, epiphyses, or medullary cavities.

• Covered by periosteum; internal structure is two layers of compact bone with a middle layer of spongy bone (diploe).

• Some skull bones contain sinuses (air-filled spaces).

Blood and Nerve Supply

• Bones are highly vascularized and innervated.

Bone Marrow

• Red Bone Marrow: Loose connective tissue supporting hematopoietic cells. In adults, found in pelvis, proximal femur/humerus, vertebrae, ribs, sternum, clavicles, scapulae, and some skull bones.

• Yellow Bone Marrow: Composed of triglycerides, blood vessels, and adipocytes.

Module 6.2: Microscopic Structure of Bone Tissue

Bone (osseous tissue) consists of an extracellular matrix (ECM) and a small population of cells. The ECM is divided into inorganic and organic components.

Bone Matrix

• Inorganic Matrix (65%): Mainly calcium and phosphorus salts as hydroxyapatite crystals (), providing hardness and resistance to compression. Also contains bicarbonate, potassium, magnesium, and sodium.

• Organic Matrix (Osteoid, 35%): Contains collagen fibers (resist torsion and tensile forces), proteoglycans, glycosaminoglycans (draw water in for compression resistance), glycoproteins (bind components), and bone-specific proteins.

Bone Cells

• Osteoblasts: Build bone by secreting organic matrix and assisting in inorganic matrix formation. Derived from osteogenic cells.

• Osteocytes: Mature bone cells maintaining bone matrix, residing in lacunae.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown), derived from bone marrow cell fusion.

Histology of Compact and Spongy Bone

• Compact Bone: Organized into osteons (Haversian systems) with concentric lamellae, central canals (blood vessels/nerves), lacunae (osteocytes), and canaliculi (cell communication).

• Other features: Interstitial lamellae (remnants of old osteons), circumferential lamellae (outer/inner layers), and perforating (Volkmann's) canals (connect osteons).

• Spongy Bone: Network of trabeculae (struts) with concentric lamellae, lacunae, and canaliculi. No central/perforating canals; blood supply comes from bone marrow vessels.

Module 6.3: Bone Formation (Ossification)

Ossification (osteogenesis) is the process of bone formation, beginning in the embryonic period and continuing through childhood. Two mechanisms exist: intramembranous and endochondral ossification.

General Features

• First bone formed is primary (woven) bone: irregular collagen, sparse inorganic matrix.

• Primary bone is replaced by secondary (lamellar) bone: more inorganic matrix, greater strength.

Intramembranous Ossification

• Forms many flat bones (e.g., skull, clavicles).

• Begins in a mesenchymal membrane (embryonic connective tissue).

• Steps:

  1. Mesenchymal cells differentiate into osteogenic cells, then osteoblasts at the primary ossification center.

  2. Osteoblasts secrete organic matrix; calcification occurs as calcium salts are deposited.

  3. Early spongy bone forms as trabeculae develop.

  4. Some mesenchymal cells form periosteum; vascular tissue becomes bone marrow.

  5. Early compact bone forms as spongy bone deep to periosteum becomes calcified and reorganized.

• Larger bones may have multiple ossification centers (e.g., fontanels in newborn skulls).

Endochondral Ossification

• Forms all bones below the head except clavicles.

• Begins with a hyaline cartilage model surrounded by perichondrium.

• Steps:

  1. Chondroblasts in perichondrium become osteogenic cells, then osteoblasts; periosteum forms.

  2. Osteoblasts build a bone collar on the external surface.

  3. Internal cartilage calcifies; chondrocytes die, leaving cavities.

  4. Osteoblasts replace calcified cartilage with early spongy bone at the primary ossification center; secondary centers form in epiphyses.

  5. Medullary cavity enlarges; remaining cartilage is replaced by bone.

  6. Cartilage persists at epiphyseal plates and articular surfaces.

  7. Epiphyseal plates close when bone growth in length ceases.

Module 6.4: Bone Growth in Length and Width

Bone growth occurs by two processes: longitudinal (length) and appositional (width) growth.

Longitudinal Growth

• Occurs at the epiphyseal plate (hyaline cartilage) via chondrocyte division.

• Five zones in the epiphyseal plate; all but the reserve zone are active in growth.

• Sequence:

  1. Chondrocytes divide in the zone of proliferation.

  2. Chondrocytes enlarge and mature in the next zone.

  3. Chondrocytes die and matrix calcifies in the zone of calcification.

  4. Osteoblasts invade and lay down bone in the zone of ossification.

  5. Osteoclasts resorb calcified cartilage and primary bone, replaced by mature bone.

• Growth continues as long as mitosis occurs in the proliferation zone; epiphyseal plate closes (becomes epiphyseal line) when growth ceases (ages 18–21).

Appositional Growth

• Osteoblasts between periosteum and bone surface lay down new bone, increasing width.

• Forms new circumferential lamellae; older lamellae may be removed or restructured into osteons.

• Can continue after longitudinal growth stops.

Hormonal Regulation of Bone Growth

• Growth Hormone: Increases chondrocyte division, osteogenic cell activity, and osteoblast activity (stimulates both longitudinal and appositional growth).

• Testosterone: Increases appositional growth, mitosis in epiphyseal plate (growth spurts), and accelerates plate closure.

• Estrogen: Increases longitudinal growth, inhibits osteoclasts, causes growth spurts, and accelerates plate closure faster than testosterone (explains average height differences).

Module 6.5: Bone Remodeling and Repair

Bone is constantly remodeled through deposition (formation) and resorption (breakdown) to maintain homeostasis, repair damage, and adapt to stress.

Bone Remodeling

• Bone Deposition: Osteoblasts build new bone matrix.

• Bone Resorption: Osteoclasts secrete hydrogen ions (dissolve hydroxyapatite) and enzymes (degrade organic matrix), releasing minerals for reuse.

• Remodeling is influenced by mechanical stress (compression leads to deposition; tension leads to deposition; pressure leads to resorption), hormones (testosterone, estrogen), age, dietary calcium, vitamins C/D/K, and protein intake.

Calcium Ion Homeostasis

• Maintained by a negative feedback loop involving parathyroid hormone (PTH) and calcitonin.

• Vitamin D is essential for calcium absorption from the gut.

Bone Repair

• Fractures: Simple (skin intact) or compound (skin/tissue damaged).

• Healing Process:

  1. Hematoma (blood clot) forms at the fracture site.

  2. Fibroblasts and chondroblasts form a soft callus (mixture of hyaline cartilage and collagenous tissue).

  3. Osteoblasts build a hard (bone) callus of primary bone.

  4. Bone callus is remodeled and replaced with secondary bone.

Summary Table: Bone Fracture Types

Additional info: The above notes expand on the provided outline with definitions, examples, and explanations suitable for college-level Anatomy & Physiology students. All key terms are defined, and processes are described in logical, stepwise fashion for clarity.