Chapter 6

Module 6.1 – Introduction to Bones

Functions of Bones

Bones are essential organs in the human body, providing a variety of structural and physiological functions necessary for survival and movement.

• Protection: Bones protect vital organs such as the brain (skull), heart (sternum), and lungs (ribs).

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals such as calcium (Ca2+) and phosphate (PO43−), which are necessary for electrolyte and acid-base balance.

• Blood Cell Formation: Red bone marrow, a specialized connective tissue, is the site of blood cell formation (hematopoiesis), which affects skeletal and cardiovascular systems.

• Fat Storage: Yellow bone marrow stores triglycerides, which can be broken down into fatty acids/lipids, serving as a great source of energy.

• Movement: Muscles produce body movement via their attachment to bones.

• Support: The skeleton supports the weight of the body and provides a structural framework.

Bone Structure

Bones are classified by their shape and internal structure, which relate to their function and location in the body.

• Compact Bone: Relatively solid, dense outer region; forms a structural/protective layer. Also known as cortical bone; more in diaphyses (shafts of long bones).

• Spongy Bone: Inside cortical bone; honeycomb-like framework of bony struts (trabeculae). Allows long bones to resist forces from many directions and provides a cavity for bone marrow. More in epiphyses (ends of long bones); lines walls of medullary cavity.

Bone Coverings

• Periosteum: Dense connective tissue membrane covering the outer surface of bones. Contains blood vessels and nerves; muscle tendons attach here. Anchored by perforating fibers (Sharpey's fibers) made of collagen, which penetrate deep into bone matrix.

• Endosteum: Thin connective tissue lining the marrow cavity (inside) and covering trabeculae of spongy bone.

Structure of a Long Bone

• Diaphysis: Shaft of a long bone.

• Epiphysis: Ends of a long bone, covered with thin layer of hyaline cartilage (articular cartilage).

• Epiphyseal Plate (Growth Plate): Hyaline cartilage plate in growing bones; separates epiphysis from diaphysis. Remnant in adults is the epiphyseal line.

• Medullary Cavity: Central space within diaphysis containing either red or yellow bone marrow, depending on age and bone type. Also contains spongy bone.

• Blood Supply: Long bones receive blood from nutrient arteries that enter via nutrient foramina and pass into the medullary cavity.

Structure of Short, Flat, and Irregular Bones

• No diaphysis or epiphysis; medullary cavity or epiphyseal lines/plates are absent.

• "Bread" - outer layer of compact bone (2 layers); periosteum on outside with perforating fibers.

• "Filling" - spongy bone (in middle); houses bone marrow.

Module 6.2 – Microscopic Structure

Bone (Osseous) Tissue

Bone tissue is a specialized connective tissue composed mostly of extracellular matrix with a small population of cells scattered throughout.

Extracellular Matrix

• Inorganic Materials: Minerals make up about 65% of the bone matrix by weight. The main minerals are calcium salts, mainly calcium phosphate, which form hydroxyapatite crystals and give bone its hardness.

  • Bones without minerals become overly flexible and unable to resist compression.

• Organic Materials (Osteoid): The other 1/3 of the bone matrix (35%).

  • Collagen fibers: Strong, but flexible and bendable.

  • Other organic molecules that make up the ground substance.

  • Bones without osteoid become brittle and shatter easily.

Bone Cells

• Osteogenic Cells: Stem cells; undergo mitosis, divide, and differentiate into osteoblasts. Found in endosteum and inner layer of periosteum.

• Osteoblasts: Bone-forming cells that secrete organic matrix (osteoid) and initiate calcification. Once trapped in the matrix, they become osteocytes. Responsible for bone deposition.

• Osteocytes: Mature bone cells stuck inside the matrix; maintain the bone ECM. Reside in small cavities called lacunae.

• Osteoclasts: Large, multinucleated cells that break down (resorb) bone tissue. Secrete hydrogen ions and enzymes that degrade the ECM, releasing calcium into the blood. Responsible for bone resorption.

Histology of Bone Tissue

Structure of Compact Bone

• Osteon (Haversian System): Basic functional unit; circular structure; all aligned parallel with each other; run the length of a long bone; tightly packed together.

• Concentric Lamellae: Rings of matrix; collagen fibers of adjacent lamellae go in different directions to allow twisting and bending forces in multiple directions.

• Central Canal (Haversian Canal): Dark spot in center of osteon where blood vessels are located; lined by endosteum.

• Lacunae: Little dark spots between concentric rings of lamellae; small cavities filled with extracellular fluid; contain osteocytes.

• Canaliculi: Fine/little canals within the osteon that connect the central canal to the lacuna and lacuna to each other so that nutrients and waste can move back and forth between main blood vessel and the cell (osteocyte).

• Circumferential Lamellae: A couple of layers of matrix on the inner and outer perimeter of the compact bone (deep to periosteum and superficial to spongy bone).

• Perforating Canal (Volkmann's Canal): Blood vessels coming in from the outside of the bone and periosteum that connect central canals of neighboring osteons together; run perpendicular to the osteon.

Structure of Spongy Bone

• Endosteum: The outer layer that surrounds the spongy bone (on the inside of the bone).

• Trabeculae: Equivalent to an osteon in compact bone; project into marrow cavity; covered with endosteum; contains concentric rings/lamellae, lacunae with osteocytes, and canaliculi.

• No central canal or perforating canal because blood vessels are around and weave through the trabeculae.

Bone Development and Growth

General Vocabulary

• Ossification (Osteogenesis): The process of bone formation that begins during embryonic development and continues through childhood (until epiphyseal plates close).

• Primary Bone: Irregularly arranged collagen bundles, osteocytes, and little inorganic matrix.

• Secondary Bone: Fully formed lamellae with regularly arranged collagen bundles running parallel to provide strength; increase in organic matrix (calcification starts to take place for strengthening).

• Primary Ossification Center: Where primary bone is laid down and turns into secondary bone (location in long bone = diaphysis).

• Secondary Ossification Center: Located in the epiphysis of long bones where ossification starts later in the development process (not present in flat bones/intramembranous ossification).

Intramembranous Ossification

Flat bones (such as those of the skull) develop in the dermis of the skin and then fall a little deeper (relatively superficial).

1. Osteoblasts develop in the primary ossification center from mesenchymal cells (stem cells in embryonic connective tissue). These differentiate into osteogenic cells, then into osteoblasts, all occurring in the primary ossification center.

2. Osteoblasts secrete organic matrix (osteoid) which calcifies, and trapped osteoblasts develop to become osteocytes.

3. Formation of trabeculae and periosteum: Osteoblasts around perimeter continue to secrete more matrix, causing bone tissue to grow and bone to thicken outward. Osteoblasts lay down compact bone on the outside (spongy bone still in center); periosteum forms on the outside early as compact bone; more calcified than spongy bone.

Additional info: This process is sometimes described as making a sandwich backwards: create filling (spongy bone) in the middle, then bread (compact bone) on the outside.

Endochondral Ossification

Long bones (and all other bones below the head besides clavicles) develop from a cartilage model. This process takes a couple of months as the baby develops in utero; baby's bones are still soft and developing for birth.

1. Chondroblasts differentiate into osteoblasts (starting on the outer perimeter of the diaphysis because no blood supply on the inside; hyaline cartilage is avascular). The chondroblasts in the perichondrium (outer layer of hyaline cartilage) differentiate into osteogenic cells and then osteoblasts.

2. Formation of bone collar: Osteoblasts build the bone collar on the bone's external surface as the bone begins to ossify from the outside through secretion of organic ECM deep to periosteum.

3. Chondrocytes die and disintegrate to produce cavities in the shaft (simultaneously with step 2, calcification of bone collar). Inside of the diaphysis is still all cartilage; cartilage starts to break down (due to lack of blood supply to chondrocytes from calcification) and holes/cavities form from chondrocyte death. Blood vessels can penetrate into the bone now (from the collar made by osteoblasts) to create the first perforating canal to penetrate into the bone and create blood supply to inside of future bone.

4. Development of primary ossification center: Where bone development really starts = area on the inside of the diaphysis.

5. Diaphysis enlarges and medullary cavity forms. Diaphysis enlarges as spongy bone is laid from inside out and other osteoblasts increase size of bone collar. Medullary cavity develops as cavities enlarge and combine.

6. Development of secondary ossification center. Develops on the epiphysis; blood supply comes into medullary cavity and starts to create spongy bone tissue by replacing the remaining cartilage.

7. After completion of secondary ossification, hyaline cartilage remains as articular cartilage at ends of epiphysis and between epiphysis and diaphysis at the epiphyseal plates.

Bone Growth

• Longitudinal Growth: Occurs at the epiphyseal plate (growth plate) in long bones. Chondrocytes divide and produce new cartilage, which is then replaced by bone tissue, allowing the bone to lengthen.

• Appositional Growth: Growth in width; occurs by osteoblasts in the periosteum laying down new bone matrix on the bone surface.

• Growth continues throughout childhood and adolescence until the epiphyseal plates close at puberty due to hormonal changes (e.g., sex hormones such as estrogen and testosterone).

Bone Remodeling and Repair

Bone Remodeling

Bone remodeling is a continuous process of bone deposition and resorption that keeps bones healthy and strong. It is necessary for growth, repair, and calcium homeostasis.

• Bone Deposition: Osteoblasts lay down new bone matrix, especially in the periosteum and endosteum.

• Bone Resorption: Osteoclasts break down bone ECM, releasing calcium and phosphate into the blood.

Factors Affecting Bone Remodeling

• Mechanical Stress: Muscle tension and gravitational forces stimulate bone deposition, making bones stronger.

• Calcium and Vitamin D: Necessary for calcium absorption and bone mineralization.

• Vitamin C and Protein: Required for collagen fiber formation and organic matrix synthesis.

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclast activity and bone resorption.

• Calcitonin: Decreases blood calcium by stimulating osteoblast activity and bone deposition.

Negative Feedback: Both PTH and calcitonin operate via negative feedback mechanisms to maintain calcium homeostasis.

Bone Fractures and Repair

• Simple vs. Compound Fractures: Simple fractures do not break the skin; compound fractures do.

• Types of Fractures: Include compression, comminuted, avulsion, greenstick, and epiphyseal plate fractures.

• Stages of Bone Repair:

  1. Hematoma formation: Blood clot forms between the bone ends.

  2. Soft callus formation: Fibroblasts create new collagen; chondroblasts form cartilage.

  3. Hard callus formation: Osteoblasts replace soft callus with bone.

  4. Bone remodeling: Bone is remodeled and primary bone is replaced with secondary bone, restoring the bone's original structure.

Summary Table: Bone Cell Types and Functions

Cell Type	Location	Function
Osteogenic Cell	Endosteum, periosteum	Stem cell; divides to form osteoblasts
Osteoblast	Bone surface	Secretes bone matrix; bone formation (deposition)
Osteocyte	Lacunae within bone matrix	Maintains bone tissue
Osteoclast	Bone surface	Breaks down bone matrix; bone resorption