Bones and Osseous Tissue (Chapter 6)
Functions of Bones
Five primary functions:
Support
Storage of minerals (calcium) and lipids (yellow bone marrow)
Blood cell production (red bone marrow)
Protection
Leverage (force of motion)
Osteology: The study of bone structure and treatment of bone disorders.
The skeletal system includes:
Bones of the skeleton.
Cartilages, ligaments, and connective tissues that stabilize or interconnect the bones.
Classification of Bones (Based on Shape)
Long bones (e.g., humerus, femur).
Irregular bones (e.g., vertebra, sphenoid bone).
Flat bones (e.g., sternum, parietal bone).
Short bones (e.g., carpal, talus).
Bone Structure
Long Bone (Example: Femur):
Diaphysis:
The shaft.
A heavy wall of compact bone (dense bone).
A central space called the medullary cavity: contains yellow bone marrow (yellow marrow).
Epiphysis:
Wide part at each end.
Articulates with other bones.
Mostly spongy bone (cancellous or trabecular bone).
Covered with compact bone: layer called cortical bone or cortex.
May contain red bone marrow (red marrow) or yellow bone marrow (yellow marrow).
Flat Bone (Example: Parietal Bone):
Resembles a sandwich of spongy bone between two layers of compact bone (cortical bone or cortex).
Within the cranium, the layer of spongy bone is called diploë.
Osseous Tissue
A type of supporting connective tissue.
Contains specialized cells, ground substance, and extracellular protein fibers, which form the matrix of bone tissue.
Consider:
Matrix of Bone.
Bone Cells.
Structure of Compact and Spongy Bone.
Periosteum and Endosteum.
Matrix of Bone
Organic Matrix:
Collagen fibers (provide tensile strength and flexibility, but offer little resistance to compression forces: 1/3 of bone weight).
Bone cells (2% of bone mass).
Inorganic Matrix:
Gives rigidity to bone and provides compression strength.
Calcium salts:
Calcium phosphate (2/3 of bone weight).
Calcium hydroxide.
Calcium carbonate.
Minerals:
Sodium.
Magnesium.
Fluoride.
Phosphorus (phosphate).
Bone Cells
Osteocytes:
Mature bone cells.
Account for most of the bone cell population.
Amitotic.
Confined within a lacuna.
Two major functions:
Maintain and monitor protein and mineral content of the surrounding bony matrix.
Participate in the repair of damaged bone.
Osteoblasts:
Produce new bone (bone forming cells): a process called Osteogenesis or Bone Deposition.
Make and release proteins and other organic components of the organic matrix of bone.
Promote the deposition of calcium salts (calcification) in the organic matrix.
Mineralization: Addition of calcium and other minerals to bone.
Osteoprogenitor (Osteogenic Cells):
Arise from mesenchymal cells.
Maintain the population of osteoblasts.
Important in the repair of fractures and bone remodeling.
Found in the:
Inner, cellular layer (osteogenic layer) of the periosteum.
Endosteum.
Osteoclasts:
Destroy or erode bone (bone destroying cells): a process called Osteolysis or Bone Resorption.
Giant cells with 50+ nuclei.
Derived from stem cells that produce white blood cells and not from the osteoprogenitor cell.
Secrete acids and digestive enzymes that dissolve the bony matrix and release the stored minerals into blood (important in the regulation of calcium and phosphate concentrations in blood).
Structure of Compact and Spongy Bone
Compact Bone:
Osteon (Haversian System) is the basic unit.
Osteocytes are arranged in concentric lamellae around a Central Canal (Haversian Canal) containing blood vessels & nerve.
Perforating Canals (Volkmann’s Canals):
Perpendicular to the central canal.
Carry blood vessels & nerves into bone.
Types of Lamellae:
Concentric Lamellae:
Form the osteons.
Contains a central canal.
Circumferential Lamellae:
Found at the outer & inner bone surfaces.
Covered by the periosteum or endosteum.
Interstitial Lamellae:
Fill in spaces between osteons.
Remnants of almost completely recycled osteons.
Spongy Bone (Trabecular Bone or Cancellous Bone):
Do not have osteons.
The bony matrix forms a meshwork of supporting thin columns (bony struts) called trabeculae which are covered or lined with the endosteum.
Trabeculae do not have central canals or perforating canals.
Spongy bone is located where bones are not heavily stressed, but where stresses come from many directions.
Lighter than compact bone.
The spaces between trabeculae are filled with:
Red Bone Marrow (Red Marrow):
Filled with blood.
Site of Blood Cell production (process called hemopoiesis / hematopoiesis).
Yellow Bone Marrow (Yellow Marrow):
In some bones, spongy bone holds yellow bone marrow.
Yellow because it stores fat.
Periosteum and Endosteum
Periosteum: external membrane
Covers all external bone surfaces except parts enclosed in joint capsules and at articular cartilages.
Made up of an outer, fibrous layer and an inner, cellular layer (osteogenic layer).
Functions of Periosteum:
Isolates bone from surrounding tissues.
Provides a route for blood vessels and nerves to enter and leave.
Participates in bone growth, fracture repair, and bone remodeling.
Serves as attachment point for ligaments and tendons.
Perforating fibers (Sharpey’s Fiber): thick bundles of collagen fibers of the periosteum.
Secures the periosteum to the underlying bone.
Endosteum: internal membrane
A thin membrane composed of a single layer of endosteal cells:
Osteoblast.
Osteoprogenitor.
Osteoclast.
Lines the medullary cavity.
Covers trabeculae of spongy bone.
Lines inner surface of the central canals and perforating canals (canals that pass through compact bone).
Function of Endosteum:
Participates in bone growth, fracture repair, and bone remodeling.
Bone Development and Bone Growth
Bony skeleton begins to form about 6 weeks after fertilization.
Fetal skeleton completed after 3 months and is made primarily of cartilage, then ossification and bone growth occurs.
Osteogenesis:
Process of replacing tissue with bone = Ossification.
Process of calcium salt and other mineral deposition on organic matrix = Mineralization (Calcification).
Bone Formation occurs in 4 situations:
Initial bone formation in an embryo and fetus.
Bone growth during infancy, childhood and adolescence.
Bone remodeling (replacing old bone with new bone tissue).
Repair of fractures.
Initial Bone Formation in Embryo and Fetus
Two Major Forms of Ossification:
Intramembranous Ossification (Dermal Ossification): bone forms directly within mesenchyme.
Simpler type of ossification.
Occurs in the flat bones of the cranium (skull bones), mandible, clavicle.
Endochondral Ossification: bone forms within hyaline cartilage.
Process begins in the second month of development.
Occurs in essentially all bones of the skeleton.
Intramembranous Ossification:
Development of ossification center: osteoblasts secrete organic extracellular matrix.
Calcification: calcium and other mineral salts are deposited and extracellular matrix calcifies (hardens).
Formation of trabeculae: extracellular matrix develops into trabeculae that fuse to form spongy bone.
Development of the periosteum: mesenchyme at the periphery of the bone develops into the periosteum.
Endochondral Ossification:
Development and Growth of Cartilage Model.
Endochondral Ossification (Long Bone):
Chondroblasts produce a cartilage model that is surrounded by perichondrium, except where joints will form.
The perichondrium of the diaphysis becomes the periosteum, and a bone collar is produced. Internally, the chondrocytes hypertrophy, and calcified cartilage forms.
A primary ossification center forms as blood vessels and osteoblasts invade the calcified cartilage. The osteoblasts lay down bone matrix, forming spongy bone.
The process of bone collar formation, cartilage calcification, and spongy bone production continues. Calcified cartilage begins to form in the epiphyses. A medullary cavity begins to form in the center of the diaphysis.
Secondary ossification centers form in the epiphyses of long bones.
The original cartilage model is almost completely ossified. Unossified cartilage becomes the epiphyseal plate and the articular cartilage.
In a mature bone, the epiphyseal plate has become the epiphyseal line, and all the cartilage in the epiphysis, except the articular cartilage, has become bone.
Bone Growth During Infancy, Childhood and Adolescence
Interstitial Growth:
Increase in the length of bones (longitudinal or vertical growth).
Occurs at the epiphyseal growth plate (growth plate or epiphyseal plate) of long bones (e.g., femur, humerus).
5 Zones:
Zone of Resting Cartilage (Resting Zone): Non-dividing chondrocytes.
Zone of Proliferating Cartilage (Proliferating Zone): Chondrocytes actively divide and produce new cartilage.
Zone of Hypertrophic Cartilage (Hypertrophic Zone): Chondrocytes hypertrophy.
Zone of Calcified Cartilage (Calcification Zone): Calcified cartilage form, chondrocytes die and leave an open space.
Zone of Ossification (Ossification Zone): Osteoblast fill in open space and begin the process of osteogenesis.
Appositional Growth:
Increase in the diameter and thickness of bone.
Osteoblasts beneath the periosteum lay down bone to form ridges separated by grooves. Blood vessels of the periosteum lie in the grooves.
The groove is transformed into a tunnel when the bone built on adjacent ridges meets. The periosteum of the groove becomes the endosteum of the tunnel.
Appositional growth by osteoblasts from the endosteum results in the formation of a new concentric lamella.
The production of additional concentric lamellae fills in the tunnel and completes the formation of the osteon.
Bone Remodeling
Childhood → Adulthood → Throughout Life
Involves the continuous recycling and the renewal of the organic and inorganic (mineral) components of the bony matrix.
This turnover and recycling of minerals gives each bone the ability to adapt to environmental stresses (ex: lifting weights, exercise or lack thereof).
Involves the osteocytes, osteoblasts, and osteoclasts.
Bone continually remodels (bone remodeling): bone resorption & bone deposition.
Bone deposition and bone resorption must balance.
More breakdown than building, bones become weak.
Exercise, particularly weight-bearing exercise, stimulates osteoblasts to build bone.
Goes on throughout life as part of the normal bone maintenance.
Osteolysis or Bone Resorption: Removal of minerals and collagen fibers by osteoclasts.
Osteogenesis or Bone Deposition: Addition of minerals and collagen fibers by osteoblasts.
Exercise, Hormones and Nutrition on Bone Development and Bone Health
Normal Bone Growth and Maintenance Depend on Nutritional and Hormonal Factors:
Growth hormone and thyroxine stimulate osteoblasts.
Estrogens and androgens (testosterone) stimulate osteoblasts.
Calcitonin and parathyroid hormone regulate calcium and phosphate levels.
Vitamin C: Required for collagen synthesis and stimulation of osteoblast differentiation.
Vitamin A: Stimulates osteoblast activity.
Vitamins K and B12: Needed for synthesis of bone proteins.
The hormone calcitriol
Active form of vitamin D3.
Helps absorb calcium and phosphorus from the digestive tract.
A dietary source of calcium and phosphate salts
Plus, small amounts of magnesium, fluoride, iron, and manganese.
Effects of Exercise on Bone: bone resorption < bone deposition
Bones adapt to stress: heavily stressed bones become thicker and stronger.
Effects of Lack of Exercise on Bone: bone resorption > bone deposition
Bone degenerates quickly.
Up to one third of bone mass can be lost in a few weeks of inactivity.
Calcium Homeostasis
Calcium (Ca^{2+}):
Most abundant mineral in the body (represents 40% of all the minerals present in the body = 2.5 lbs).
> 99% of calcium is stored in bone tissue (as structural components).
As calcium circulates in blood, it supplies the calcium needs of body cells (~1% found in blood and soft tissues).
Blood calcium levels are monitored very carefully, and the body responds quickly to restore homeostasis.
Normal Range of Blood Calcium: 8.5 mg/dL - 10.8 mg/dL (8 mg/dL – 11 mg/dL).
Important Primary Functions of Calcium:
Bone Development and Maintenance:
The most well-known function of calcium is to build and strengthen bones and teeth.
Calcium deficiency in the diet leads to low BMD (Bone Mineral Density) → bone becomes brittle and weak.
Nerve Impulse Transmission:
Calcium allows for neurons to release neurotransmitters (chemical messenger) allowing a neuron to communicate with another cell.
Muscle Contraction:
Action potential along the plasma membrane of the muscle cell causes the release of calcium ions into the cytoplasm of the muscle cell.
This initiates a series of events that eventually leads to muscle contraction.
Blood Clotting:
Calcium ions participate in several reactions that leads to the formation of a blood clot.
Blood Calcium (Ca^{2+}) Regulation:
Calcium ions in blood must be closely regulated.
Homeostasis is maintained by: calcitonin and parathyroid hormone (PTH): control storage, absorption, and excretion of calcium.
Parathyroid Hormone (PTH):
Produced by parathyroid glands.
Increases blood calcium ion levels by:
Stimulate osteoclasts.
Increase intestinal absorption of Ca^{2+}.
Decrease calcium excretion at kidneys.
Increase calcitriol secretion by kidneys.
Calcitonin:
Produced by thyroid gland (parafollicular cells).
Decreases blood calcium ion levels by:
Inhibit osteoclast activity.
Decrease intestinal absorption of Ca^{2+}.
Increase calcium excretion at kidneys.
Decrease calcitriol secretion by kidneys.
Factors That Increase Blood Calcium Levels
These responses are triggered when plasma calcium ion concentrations fall below 8.5 mg/dL.
Factors That Decrease Blood Calcium Levels
These responses are triggered when plasma calcium ion concentrations rise above 11 mg/dL.
Fracture
A crack or break in bone.
Repair Process may take several months, 4 Steps:
Fracture Hematoma (a mass of clotted blood):
Forms within 6-8 hours.
After a fracture, blood escapes from ruptured blood vessels and forms a fracture hematoma.
Fibrocartilaginous callus:
Tissue repair begins.
A fibrocartilaginous callus fills the space between the ends of the broken bone for about 3 weeks.
Bony callus:
Osteoblasts produce trabeculae of spongy bone and convert the fibrocartilaginous callus to a bony callus joining the broken bones together.
Lasts about 3-4 months.
Remodeling:
Osteoblasts build new compact bone reforming the bone collar.
Osteoclasts absorb the spongy bone creating a new medullary cavity.
General Descriptions of Fractures
Position of the bone ends after fracture:
Nondisplaced Fracture: retain their normal position.
Displaced Fracture: bone ends are out of alignment.
Completeness of fracture:
Complete Fracture: bone is broken through (completely broken).
Incomplete/Partial Fracture: bone is not broken through (partially broken).
Orientation of break relative to the long axis of bone:
Linear Fracture: break is parallel to the long axis.
Transverse Fracture: break is perpendicular to the long axis.
Oblique Fracture: break is angular to the long axis.
Penetration of skin:
Open/Compound Fracture: broken bone has penetrated skin.
Closed/Simple Fracture: broken bone has not penetrated skin.
Classification of Bone Fractures
Avulsion: Complete severing of a body part (typically a toe or finger).
Colles: Fracture of the distal end of the lateral forearm bone (radius); produces a "dinner fork" deformity.
Comminuted: Bone is splintered into several small pieces between the main parts.
Complete: Bone is broken into two or more pieces.
Compound (open): Broken ends of the bone protrude through the skin.
Compression: Bone is squashed (may occur in a vertebra during a fall).
Depressed: Broken part of the bone forms a concavity (as in skull fracture).
Displaced: Fractured bone parts are out of anatomic alignment.
Epiphyseal: Epiphysis is separated from the diaphysis at the epiphyseal plate.
Greenstick: Partial fracture; one side of bone breaks-the other side is bent.
Hairline: Fine crack in which sections of bone remain aligned (common in skull).
Impacted: One fragment of bone is firmly driven into the other.
Incomplete: Partial fracture extends only partway across the bone.
Linear: Fracture is parallel to the long axis of the bone.
Oblique: Diagonal fracture is at an angle.
Pathologic: Weakening of a bone caused by disease process (e.g., cancer).
Pott: Fracture is at the distal ends of the tibia and fibula.
Simple (closed): Bone does not break through the skin.
Spiral: Fracture spirals around axis of long bone; results from twisting stress.
Stress: Thin fractures due to repeated, stressful impact such as running.
Transverse: Fracture is at right angles to the long axis of the bone.
Some Common Fractures
Open (Compound): The broken ends of the bone protrude through the skin. Conversely, a closed (simple) fracture does not break the skin.
Comminuted: The bone is splintered, crushed, or broken into pieces at the site of impact, and smaller bone fragments lie between the two main fragments.
Greenstick: A partial fracture in which one side of the bone is broken and the other side bends; similar to the way a green twig breaks on one side while the other side stays whole but bends; occurs only in children, whose bones are not fully ossified and contain more organic material than inorganic material.
Impacted: One end of the fractured bone is forcefully driven into the interior of the other.
Pott: Fracture of the distal end of the lateral leg bone (fibula), with serious injury of the distal tibial articulation.
Colles: Fracture of the distal end of the lateral forearm bone (radius) in which the distal fragment is displaced posteriorly.
Effects of Aging
Bones become thinner and weaker with age called osteopenia
Begins between ages 30 and 40.
Women lose 8% of bone mass per decade, men lose 3% of bone per decade.
Epiphyses, vertebrae, and jaws are most affected: resulting in fragile limbs, reduction in height and tooth loss.
Osteoporosis: reduction of bone mass is sufficient to compromise normal function
Severe bone loss.
Affects normal function.
Over age 45, occurs in:
29% of women.
18% of men.
Bone Mineral Density (BMD)
Total amount of bone mass in a defined area
For a specific bone:
Total amount of mineral content (calcified bone tissue in grams) per unit area of bone (cm^{2}).
BMD measurement can help predict the risk of bone fracture (indicator of overall bone strength and health):
The higher the BMD, the stronger the bone.
The lower the BMD, the weaker the bone.
Methods to Measure BMD:
Dual Energy X-ray Absorptiometry (DEXA):
Most common technique to determine the BMD.
Focuses on the hip joint and the vertebrae.
A DEXA scanner passes 2 X-ray beams through the body – the very small amount of X-ray energy that passes through bone is measured:
The total amount of energy that penetrates the bone depends upon the thickness of the bone
Using this information and a defined area of bone, the amount of calcified tissue in grams per unit area (g/cm^{2}) is measured (BMD).
Can measure changes in BMD as little as 1%.
Used in osteopenia and osteoporosis patients.
Bone Disorders, Disease and Abnormalities
Acromegaly:
Overproduction of growth hormones after closure of the epiphyseal plate.
Bones get thicker, especially in the face, jaw, and hands.
Gigantism:
Overproduction of growth hormones before closure of the epiphyseal plate.
Fibrodysplasia Ossificans Progressiva (FOP):
Rare genetic disease.
Muscle tissue replaced by bone.
Osteosarcoma (Osteogenic Sarcoma):
A type of cancer that starts in bone.
Tibia, femur, and humerus of males between the ages of 10 – 25 are most often affected.
Osteogenesis Imperfecta (Brittle Bone Disease):
Genetic disorder of bone.
Characterized by extreme fragility of the bones due a defect in collagen deposition.
Achondroplasia:
Genetic disorder of bone growth.
Cartilage being abnormally converted to bone.
Osteitis Deformans (Paget Disease):
Chronic disorder that results in enlarged, weak, deformed bones due to abnormal bone destruction by excessive proliferation of osteoclasts and regrowth by osteoblasts.
Osteomyelitis:
A bone infection usually caused by Staphylococcus aureus.
Site of infection can begin at the bone due to an open fracture or after surgery or can be from an infected area of the body that has spread to the bone (example: a foot ulcer caused by diabetes that has spread to the bone).
Rickets:
A disease that affects the bones of children and adolescents.
Caused by the failure of the osteoid to calcify resulting in soft and weak bones due to prolong and extreme vitamin D deficiency.
Failure of osteoid to calcify in adults is called osteomalacia.