ch 7

Overview of Bone Tissue and Osteology

• Osteology: The scientific study of bone tissue.

• Osseous: A term pertaining to bone.

• Osseous Tissue: A connective tissue (C.T.) that serves as the primary component of bone; its matrix is hardened by minerals, mainly calcium phosphate.

• Mineralization/Calcification: The process by which the bone matrix is hardened by the deposition of minerals.

• Bone (The Organ): While "bone" refers to osseous tissue, individual bones function as organs composed of multiple tissue types, including:   - Osseous tissue.   - Bone marrow.   - Blood.   - Cartilage.   - Nervous tissue.

• Clinical Relevance: Understanding bone tissue involves studying "Deeper Insight" topics such as Osteoporosis and completing the section-ending "Before You Go On" questions and end-of-chapter "Testing Your Recall" questions (1–17).

Functions of the Skeletal System

• Support: Provides a structural framework for the body.

• Protection: Encloses and protects soft internal organs.

• Movement: Acts as levers that are moved by the action of muscles.

• Electrolyte Balance: Stores and releases calcium and phosphate ions into the blood.

• Acid-Base Balance: Buffers blood against excessive pH changes by absorbing or releasing alkaline salts.

• Blood Formation: The red bone marrow is the primary site of hemopoiesis (blood cell production).

Classification of Bone Shapes

1. Long Bones: Characterized by being significantly longer than they are wide. They serve as levers for muscle action.    - Example: The humerus.

2. Short Bones: Nearly equal in length and width, essentially cube-shaped.    - Example: Carpal (wrist) and tarsal (ankle) bones.

3. Flat Bones: Thin, often curved plates that protect soft organs and provide broad surfaces for muscle attachment.    - Example: The scapula and the cranium (skull).

4. Irregular Bones: Bones with complex shapes that do not fit into other categories.    - Example: The vertebrae and certain skull bones.

Gross Anatomy of Bone Structures

The Long Bone

• Diaphysis: The shaft of the long bone; provides leverage.

• Epiphyses: The expanded heads at each end of the long bone; functions to strengthen joints and provide added surface area for the attachment of tendons and ligaments.

• Articular Cartilage: A layer of hyaline cartilage covering the joint surface where one bone meets another. This minimizes friction and absorbs shock.

• Epiphyseal Plate: Present in children; a zone of hyaline cartilage that allows for bone elongation. In adults, it is replaced by the Epiphyseal Line after growth has ceased.

• Medullary Cavity: The central hollow space in the diaphysis containing yellow bone marrow in adults.

• Periosteum: A specialized connective tissue (C.T.) sheath that wraps around the exterior of the bone.

• Endosteum: The internal lining of the bone, covering the surfaces of the marrow cavity and the canal system.

• Nutrient Foramen: Small holes in the bone surface that allow nutrient vessels (arteries and veins) to enter and exit, servicing the bone's internal requirements.

The Flat Bone (Cranium)

• Sandwich Structure: Flat bones of the skull consist of two layers of compact bone enclosing a middle layer of spongy bone.

• Diploe: The specific name for the middle layer of spongy bone in the cranium.

• Trabeculae: A lattice-like arrangement of slender rods and spines within the spongy bone.

• Protective Mechanism: A skull fracture may break the outer compact bone layer without harming the inner layer, thereby protecting the brain.

Histology of Osseous Tissue

Four Types of Bone Cells

1. Osteogenic Cells: Stem cells found in the endosteum, periosteum, and central canals. They undergo mitosis to produce new osteoblasts.

2. Osteoblasts: Known as "bone-forming cells." They synthesize and deposit the organic bone matrix. They are non-mitotic.

3. Osteocytes: Former osteoblasts that have become trapped in the matrix they formed. They reside in tiny cavities called lacunae.    - Functions:      - Maintain bone density.      - Regulate blood concentrations of calcium and phosphate.      - Act as strain sensors, detecting mechanical stress and signaling for bone remodeling.

4. Osteoclasts: Bone-dissolving cells found on the bone surface. They develop in the bone marrow from the fusion of $3 \times 10^0$ to $5 \times 10^1$ (3 to 50) stem cells.    - Resorption Bays: Pits etched into the bone surface where osteoclasts reside.    - Bone Remodeling: The combined action of osteoblasts (depositing) and osteoclasts (dissolving).

The Bone Matrix

• Composition by Dry Weight: Approximately $2/3$ inorganic matter and $1/3$ organic matter.

• Inorganic Matter: Provides the structural strength to support weight.   - 85% Hydroxyapatite: A crystallized calcium phosphate salt.   - 10% Calcium Carbonate.   - 5% Other Minerals.

• Organic Matter: High in collagen and protein-carbohydrate complexes; provides flexibility and prevents brittleness.

Specialized Bone Disorders

Mineral Deficiencies

• Rickets: A childhood disease resulting from mineral deficiency (calcium/vitamin D), leading to bone softening and deformity (e.g., bowed legs).

• Osteomalacia: The adult version of rickets.

Osteogenesis Imperfecta (Brittle Bone Disease)

• Definition: A genetic disorder resulting from a defect in collagen deposition.

• Symptoms:   - Brittle bones that fracture easily.   - Triangular-shaped face with a broad forehead.   - Blue, purple, or gray tint to the whites of the eyes.   - Brittle teeth and hearing loss (often beginning in 20s or 30s).   - Barrel-shaped rib cage and curved spine (Kyphosis/Hunchback).   - Short, small body with deformed bones.

• Treatment: No cure; management involves pain medication, physical therapy, assistive tools (braces, wheelchairs), good diet, and exercise. Avoidable risks include smoking, alcohol, and caffeine.

Microscopic Anatomy of Bone

Compact Bone

• Osteon (Haversian System): The basic structural unit of compact bone.

• Central Canal (Vascular/Haversian Canal): Contains blood vessels and nerves; runs longitudinally through the osteon.

• Perforating Canals (Volkmann's Canals): Transverse passages connecting central canals.

• Concentric Lamellae: Layers of matrix arranged around a central canal. Collagen fibers within a lamella are arranged in right- and left-handed helices to resist stress.

• Circumferential Lamellae: Matrix layers that run parallel to the bone surface rather than forming osteons.

• Interstitial Lamellae: Fragments of old osteons that fill the space between newer, complete osteons.

Spongy Bone

• Anatomy: Formed by plates and rods called trabeculae.

• Function: Provides significant strength with minimal weight; trabeculae develop along the bone's lines of stress.

• Histological Difference: Contains few osteons and few central canals because no osteocyte is far from the blood supply in the marrow spaces.

Bone Marrow

• Definition: Soft tissue occupying the medullary cavity of long bones, the spaces in spongy bone, and larger central canals.

• Red Bone Marrow:   - Appearance: Thick blood.   - Composition: Reticular fibers, immature blood cells, and adipocytes.   - Function: Hemopoietic (produces blood cells).

• Yellow Bone Marrow:   - Transition: Develops from red bone marrow as an individual ages.   - Function: Fatty storage; no longer produces blood.

Questions & Discussion

• Question: Where would be the most accessible places to draw red bone marrow from an adult?

• Answer: Based on adult distribution (as seen in Figure 7.7), red marrow is primarily found in the flat bones of the skull, the vertebrae, the ribs, the sternum, part of the pelvic girdle, and the proximal heads of the humerus and femur.

Physiology of Osseous Tissue: Mineral Exchange

Mineral Deposition

• Crystallization: A process in which calcium, phosphate, and other ions are taken from the blood and deposited in bone tissue.

• Timeline: Begins in fetal life and continues throughout life.

• Ectopic Ossification: Abnormal calcification of non-bone tissue. Examples include arteriosclerosis (hardening of the arteries) and calculus (a "little stone" or calcified mass in an organ).

Mineral Resorption

• Process: The dissolving of bone to release minerals into the blood.

• Key Cells: Performed by osteoclasts.

• Mechanism:   - Osteoclasts utilize calcium receptors, hydrogen pumps, and chloride ion channels.   - Hydrochloric Acid ($HCl$): Used to dissolve the inorganic bone minerals.   - Acid Phosphatase: An enzyme used to digest the organic collagen component of the matrix.

• Application: Orthodontic appliances (braces) use mineral resorption and deposition to reposition teeth within the alveolar bone.

Calcium Homeostasis

• Physiological Importance of Calcium: Critical for neuron action potentials, muscle contraction, blood clotting, exocytosis, and second messenger systems.

• Blood Calcium Forms: Exists in an ionized form ($Ca^{+2}$) or bound to solutes like proteins.

• States of Imbalance:   - Hypocalcemia: Deficiency of calcium in the blood.   - Hypercalcemia: Excess calcium in the blood.

Hormonal Regulation

1. Calcitriol (The Most Active Form of Vitamin D):    - Synthesis:      1. Keratinocytes use UV radiation to convert $7$-dehydrocholesterol to Vitamin $D_3$.      2. The liver converts Vitamin $D_3$ to calcidiol.      3. The kidney converts calcidiol to calcitriol.    - Function: Raises blood calcium levels.    - Mechanism: Increases calcium and phosphate absorption in the small intestine, promotes bone resorption (calcium release), and increases renal reabsorption of calcium.

2. Calcitonin:    - Synthesis: Secreted by C-cells (clear cells) of the thyroid gland when blood calcium is too high.    - Function: Lowers blood calcium levels.    - Mechanism:      - Osteoclast Inhibition: Reduces osteoclast activity within minutes.      - Osteoblast Stimulation: Promotes calcium deposition into bone.

3. Parathyroid Hormone (PTH):    - Synthesis: Secreted by the parathyroid glands (located on the posterior thyroid) when blood calcium is low.    - Function: Raises blood calcium levels.    - Mechanism:      - Osteoclast Stimulation: Increases osteoclast count and bone resorption.      - Osteoblast Inhibition: Decreases the rate of bone deposition.      - Renal Effects: Promotes calcium reabsorption by the kidneys and stimulates the final step of calcitriol synthesis.

Osteoporosis

• Definition: A degenerative bone disease characterized by a loss of bone mass and increased brittleness.

• Symptoms: Bones lose density and become increasingly porous.

• Causes: Aging and excessive bone resorption vs. deposition.

• Results: Severe damage to the lumbar vertebrae, fractures, and kyphosis (spinal curvature).

• Diagnosis: Measured via X-rays to assess bone density.

• Treatments:   - Estrogen replacement therapy (currently out of favor).   - Bisphosphonates.

• Prevention: Optimal prevention occurs between ages $2.5 \times 10^1$ and $4.0 \times 10^1$ (25–40); requires weight-bearing exercise and a calcium-rich, bone-building diet.