Bones and Bone Tissue

The Skeletal System: Overview

The skeletal system is a complex organ system that includes bones, joints, and supporting tissues. Bones are the main organs, with adults typically having 206 bones. Each bone contains osseous tissue, dense regular and irregular connective tissue, and bone marrow.

• Bones: Provide structure and support.

• Joints: Allow movement and flexibility.

• Bone Marrow: Site of blood cell formation and fat storage.

Functions of the Skeletal System

Bones serve several essential functions for the human body:

• Protection: Skull, sternum, and ribs protect vital organs.

• Mineral Storage & Acid-Base Homeostasis: Stores calcium, phosphorus, and magnesium salts, which are critical for electrolyte and acid-base balance.

• Blood Cell Formation: Red bone marrow is the site of hematopoiesis (formation of blood cells).

• Fat Storage: Yellow bone marrow stores triglycerides in adipocytes.

• Movement: Bones act as levers for muscle action.

• Support: Provides structural framework and supports body weight.

Bone Structure

Classification of Bones by Shape

Bones are classified based on their shape, which relates to their function and location:

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

• Short Bones: Cube-shaped (e.g., wrist, ankle bones).

• Flat Bones: Thin and broad (e.g., skull, pelvis).

• Irregular Bones: Complex shapes (e.g., vertebrae).

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

Structure of a Long Bone

Long bones have a distinct anatomy that supports their function:

• Periosteum: Dense irregular connective tissue membrane covering the bone, containing blood vessels and nerves.

• Perforating Fibers: Collagen anchors that attach periosteum to bone matrix.

• Diaphysis: Shaft of the bone, contains medullary (marrow) cavity lined by endosteum.

• Epiphyses: Ends of the bone, filled with red marrow and covered with articular cartilage (hyaline cartilage).

Structure of Short, Flat, Irregular, and Sesamoid Bones

These bones share similarities with long bones but have fewer structures. In flat bones, the spongy bone is called diploë, and some skull bones contain sinuses to reduce weight.

Blood and Nerve Supply to Bone

Bones are highly vascularized and innervated. Blood supply to short, flat, irregular, and sesamoid bones comes from vessels in the periosteum. Long bones receive blood from the periosteum and nutrient arteries entering through the nutrient foramen.

Red and Yellow Bone Marrow

Bone marrow changes with age:

• Red Bone Marrow: Hematopoietic tissue, abundant in infants and children.

• Yellow Bone Marrow: Contains adipocytes and blood vessels, increases with age.

• Adults retain red marrow in select bones (pelvis, proximal femur/humerus, vertebrae, ribs, sternum, clavicles, scapulae).

Bone Marrow Transplantation

Clinical Application: Bone Marrow Transplantation

Bone marrow transplantation is used to treat diseases like leukemia, sickle-cell anemia, and aplastic anemia. Donor marrow is harvested and transplanted after recipient marrow is destroyed. Peripheral blood stem cell donation is an alternative method.

The Extracellular Matrix of Bone

Composition of Bone Matrix

The bone matrix consists of inorganic and organic components:

• Inorganic Matrix: 65% of bone weight, mainly hydroxyapatite crystals (calcium and phosphate), provides strength and resistance to compression.

• Organic Matrix (Osteoid): 35% of bone weight, mainly collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and osteocalcin, provides flexibility and resistance to tension.

Importance of Organic and Inorganic Components

Both components are essential for bone function. Removing organic matrix makes bone brittle; removing inorganic matrix makes bone unable to resist compression.

Bone Cells

Types of Bone Cells

Bone is a dynamic tissue, constantly remodeled by three main cell types:

• Osteoblasts: Derived from osteogenic cells, build bone by secreting organic matrix and facilitating mineralization.

• Osteocytes: Mature osteoblasts trapped in lacunae, maintain bone matrix and signal areas under tension.

• Osteoclasts: Large, multinucleated cells that break down bone matrix by secreting acids and enzymes.

Bone Cell Functions and Relationships

Osteoblasts build bone, osteocytes maintain bone, and osteoclasts resorb bone. Osteogenic cells are stem cells that differentiate into osteoblasts.

Histology of Bone

Compact Bone

Compact bone is organized into osteons (Haversian systems):

• Lamellae: Concentric rings of bone matrix.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small cavities housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae for cell communication.

• Interstitial and Circumferential Lamellae: Strengthen bone.

• Perforating (Volkmann) Canals: Connect central canals and carry blood vessels.

Spongy Bone

Spongy bone consists of trabeculae, which house osteocytes and allow access to blood supply from bone marrow. It is not weight-bearing but provides a protective framework.

Bone Formation: Ossification

Ossification (Osteogenesis)

Ossification is the process of bone formation, which continues through childhood. Two types of bone are produced:

• Primary (Woven) Bone: Immature, irregular collagen bundles, abundant osteocytes, little inorganic matrix.

• Secondary (Lamellar) Bone: Mature, organized lamellae, parallel collagen bundles, higher inorganic matrix.

Intramembranous Ossification

Occurs in flat bones (skull, clavicles). Steps:

1. Osteoblasts develop in the primary ossification center from mesenchymal cells.

2. Osteoblasts secrete organic matrix, which calcifies; trapped osteoblasts become osteocytes.

3. Osteoblasts lay down trabeculae of early spongy bone; some mesenchyme becomes periosteum.

4. Osteoblasts in periosteum lay down early compact bone; matrix is remodeled to mature bone.

Endochondral Ossification

Occurs in long and short bones. Steps:

1. Chondroblasts in perichondrium differentiate into osteoblasts.

2. Osteoblasts build bone collar; internal cartilage calcifies and chondrocytes die.

3. Osteoblasts replace calcified cartilage with early spongy bone; medullary cavity and secondary ossification centers develop.

4. Remaining cartilage is replaced by bone; epiphyses finish ossifying; cartilage remains in epiphyseal plates and articular cartilage.

Comparison: Intramembranous vs. Endochondral Ossification

• Intramembranous: Flat bones, spongy bone forms first, no perichondrium initially.

• Endochondral: Long/short bones, compact bone forms first, perichondrium becomes periosteum.

Bone Growth

Longitudinal Growth

Occurs at the epiphyseal plate via endochondral ossification. The plate has five zones:

• Zone of Reserve Cartilage: Cells not directly involved in growth.

• Zone of Proliferation: Chondrocytes actively divide.

• Zone of Hypertrophy and Maturation: Mature chondrocytes.

• Zone of Calcification: Dead chondrocytes, calcified matrix.

• Zone of Ossification: Osteoblasts build bone, osteoclasts resorb calcified cartilage.

Appositional Growth

Growth in bone width occurs as osteoblasts lay down new circumferential lamellae under the periosteum, thickening the compact bone. Osteoclasts enlarge the medullary cavity.

The Role of Hormones in Bone Growth

• Growth Hormone: Increases mitosis of chondrocytes, stimulates osteoblasts.

• Testosterone: Increases appositional growth and mitosis, accelerates epiphyseal plate closure.

• Estrogen: Similar effects, but less pronounced; earlier plate closure.

Gigantism and Acromegaly

Excess growth hormone before plate closure causes gigantism; after closure, acromegaly. Both result in abnormal bone growth and are treated by removing hormone-secreting tumors.

Bone Remodeling

Bone Remodeling Process

Bone remodeling is the continual process of bone formation (deposition) and bone loss (resorption), maintaining calcium homeostasis, repairing bone, and adapting to stress.

• Bone Deposition: Osteoblasts secrete matrix and facilitate mineralization.

• Bone Resorption: Osteoclasts secrete acids and enzymes to break down matrix.

Factors Influencing Bone Remodeling

• Hormones: Testosterone promotes deposition; estrogen inhibits osteoclasts.

• Age: Hormone levels decline, reducing bone formation.

• Nutrient Intake: Calcium, vitamin D, K, C, and protein are essential for bone health.

Calcium Ion Homeostasis

Calcium ions are vital for muscle contraction, nerve transmission, and blood clotting. Blood calcium is regulated by:

• Parathyroid Hormone (PTH): Increases blood calcium.

• Calcitonin: Decreases blood calcium.

Bone Repair

Steps of Fracture Healing

1. Hematoma fills the gap between bone fragments.

2. Fibroblasts and chondroblasts form a soft callus.

3. Osteoblasts build a bone callus.

4. Bone callus is remodeled to secondary bone.

Classes and Types of Fractures

• Simple (Closed) Fractures: Skin and tissue remain intact.

• Compound (Open) Fractures: Surrounding tissue is damaged.

• Treatment: Stabilization and immobilization; closed or open reduction.

Table: Types of Fractures

Clinical Conditions

Osteopetrosis

Defective osteoclasts cause increased bone mass but weak, brittle bones. Infantile form is severe and can be fatal; adult form causes pain and fractures.

Osteoporosis

Inadequate inorganic matrix leads to brittle bones and increased fracture risk. Prevention includes adequate calcium, vitamin D, exercise, and medications.

Achondroplasia

Most common cause of dwarfism, due to abnormal growth factor receptor on cartilage, resulting in decreased endochondral ossification and shortened limbs.

Summary Table: Bone Components

Additional info: Academic context and table entries inferred for completeness.