Bones and Bone Structure
Skeletal System
Axial and Appendicular Divisions
The skeletal system consists of approximately 206 bones and is divided into two main divisions:
Axial Skeleton (80 bones):
Includes bones of the skull, thorax, and vertebral column.
Forms the longitudinal axis of the body.
Appendicular Skeleton (126 bones):
Includes bones of the limbs and girdles that attach them to the axial skeleton.
Also includes associated cartilages, ligaments, and other connective tissues.
Functions of the Skeletal System
The skeletal system performs several critical functions:
Support: Provides structural support for the body.
Storage: Stores minerals (e.g., calcium) and lipids (energy reserves).
Blood Cell Production: Produces blood cells in the red bone marrow.
Protection: Protects underlying soft tissues and organs.
Leverage: Provides a system of levers for skeletal muscle action, enabling movement.
Bone Classification
Bones are classified into six categories based on their shape and structure:
Flat Bones:
Thin, roughly parallel surfaces.
Examples: cranial bones, sternum, ribs, scapulae.
Function: protect underlying soft tissues and provide surface area for muscle attachment.
Sutural Bones (Wormian Bones):
Irregular bones formed between cranial bones.
Vary in number, size, and shape.
Long Bones:
Relatively long and slender.
Examples: bones of the limbs.
Irregular Bones:
Complex shapes with short, flat, notched, or ridged surfaces.
Examples: vertebrae, bones of the pelvis, facial bones.
Sesamoid Bones:
Small, flat, and shaped like sesame seeds.
Develop inside tendons of the knee, hands, and feet.
Exhibit individual variation in location and number; the patella (kneecap) is an example.
Short Bones:
Small and boxy.
Examples: bones of the wrist (carpals) and ankles (tarsals).
Bone Markings
Bone markings, also known as surface features, are related to specific functions:
Elevations/Projections: Sites for muscle, tendon, and ligament attachment; also at joints where adjacent bones articulate.
Depressions/Grooves/Tunnels: Sites for blood vessels or nerves to lie alongside or penetrate bone.
Long Bone Features
Long bones have distinct features:
Epiphysis:
Expanded area at each end of the bone.
Consists largely of spongy bone (trabecular bone).
Outer covering of compact bone (cortical bone) provides strength.
Articular cartilage covers portions of the epiphysis that form articulations.
Metaphysis:
Connects the epiphysis to the shaft (diaphysis).
Diaphysis:
Shaft of the long bone.
Contains the medullary cavity (marrow cavity), which is filled with:
Red bone marrow (involved in red blood cell production).
Yellow bone marrow (adipose tissue; an important energy reserve).
Blood Supply
Growth and maintenance of bone require an extensive blood supply:
Vascular Features:
Nutrient artery and nutrient vein (usually one of each per bone).
Nutrient foramen: A tunnel providing access to the marrow cavity.
Metaphyseal artery and metaphyseal vein:
Carry blood to/from the metaphysis.
Connect to epiphyseal arteries/veins.
Bone Tissue
Bone tissue is maintained and altered by four types of cells:
Osteogenic Cells (Osteoprogenitor Cells):
Mesenchymal (stem) cells that produce cells which differentiate into osteoblasts.
Important in fracture repair.
Locations: inner lining of periosteum, lining endosteum in medullary cavity, lining passageways containing blood vessels.
Osteoblasts:
Produce new bony matrix (osteogenesis or ossification).
Produce unmineralized matrix (osteoid) and then assist in depositing calcium salts to convert osteoid to bone.
Become osteocytes once surrounded by bony matrix.
Osteocytes:
Mature bone cells that cannot divide.
Maintain protein and mineral content of the surrounding matrix.
Occupy lacunae (pockets) separated by layers of matrix (lamellae) and interconnected by canaliculi.
Osteoclasts:
Remove and remodel bone matrix.
Release acids and proteolytic enzymes to dissolve the matrix and release stored minerals in a process called osteolysis.
Bone Matrix
The bone matrix is composed of:
Collagen Fibers:
Account for approximately 1/3 of bone weight, providing flexibility.
Calcium Phosphate:
(Ca3(PO4)_2) accounts for approximately 2/3 of bone weight.
Interacts with calcium hydroxide (Ca(OH)2) to form crystals of hydroxyapatite (Ca{10}(PO4)6(OH)_2) salts.
Incorporates other salts (calcium carbonate, CaCO_3) and ions (Na^+, Mg^{2+}, F^−), providing strength.
Compact Bone
Compact bone consists of parallel osteons:
Osteon (Haversian System): The functional unit of compact bone.
Organized concentric lamellae around a central canal.
Osteocytes (in lacunae) lie between lamellae.
Central canal contains small blood vessels.
Canaliculi connect lacunae with each other and the central canal.
Strong along its length.
Compact and Spongy Bone Structure
Long bone organization includes:
Periosteum: Outermost layer.
Compact Bone: Outer bone tissue layer.
Circumferential lamellae at outer and inner surfaces.
Interstitial lamellae fill spaces between osteons.
Osteons connected by perforating canals (perpendicular to the surface).
Spongy Bone: Innermost layer.
Spongy Bone
Lamellae form struts and plates (trabeculae) creating an open network.
No blood vessels in the matrix; nutrients reach osteocytes through canaliculi open to trabeculae surfaces.
Red bone marrow is found between trabeculae.
Appositional Bone Growth
Increases bone diameter of existing bones:
Osteogenic cells differentiate into osteoblasts that add bone matrix under the periosteum.
Adds successive layers of circumferential lamellae.
Trapped osteoblasts become osteocytes.
Deeper lamellae are recycled and replaced by osteons.
Osteoclasts remove matrix at the inner surface to enlarge the medullary cavity.
Periosteum
Wraps the superficial layer of compact bone.
Two layers:
Fibrous outer layer.
Cellular inner layer.
Functions:
Isolates bone from surrounding tissues.
Provides a route for blood and nervous supply.
Actively participates in bone growth and repair.
Perforating fibers allow for strong attachment.
Endosteum
Incomplete cellular layer lining the medullary cavity.
Active during bone growth, repair, and remodeling.
Covers spongy bone and lines central canals.
Where the layer is incomplete, exposed matrix is remodeled by osteoclasts and osteoblasts.
Osteoclasts reside in shallow depressions called osteoclastic crypts (Howship’s lacunae).
Endochondral Ossification
Replaces a cartilage model with bone:
The initial skeleton of the embryo is formed of hyaline cartilage.
Cartilage is gradually replaced by bone through endochondral ossification.
Uses cartilage as a small model.
Bone grows in diameter and length. Diameter growth involves appositional bone deposition.
Steps in Endochondral Ossification
Cartilage Model Enlarges:
Chondrocytes near the center of the shaft enlarge.
Enlarged chondrocytes die and disintegrate, leaving cavities within the cartilage.
Blood Vessels Grow Around the Edge of the Cartilage Model:
Cells of the perichondrium convert to osteoblasts.
Osteoblasts form a superficial layer of bone along the shaft.
Blood Vessels Penetrate Cartilage and Enter the Central Region:
Entering fibroblasts differentiate into osteoblasts.
Begin spongy bone production at the primary ossification center.
Bone formation spreads along the shaft toward both ends.
Growth Continues Along with Remodeling:
The medullary cavity is created.
Osseous tissue of the shaft thickens.
Cartilage near the epiphyses is replaced by shafts of bone.
Bone grows in length and diameter.
Capillaries and Osteoblasts Migrate into the Epiphyses:
Create secondary ossification centers.
Epiphyses Fill with Spongy Bone:
Articular cartilage remains exposed to the joint cavity.
Epiphyseal cartilage (epiphyseal plate) separates the epiphysis from the diaphysis.
Bone Grows in Length at the Epiphyseal Cartilage:
Chondrocytes actively produce more cartilage on the epiphyseal side.
Osteoblasts actively replace cartilage with bone on the diaphyseal side.
Epiphyses are pushed away by continued production of new cartilage.
Bone Growth
At puberty, hormones stimulate increased bone growth, and epiphyseal cartilage is replaced.
Osteoblasts produce bone faster than chondrocytes produce cartilage.
Epiphyseal cartilage narrows until it disappears – a process called epiphyseal closure resulting in the epiphyseal line in adults.
Intramembranous Ossification
Forms bone without a prior cartilage model:
Begins when mesenchymal (stem) cells differentiate into osteoblasts within embryonic or fibrous connective tissue.
Normally occurs in deeper layers of the dermis.
Bones formed are called dermal bones or membrane bones.
Examples: roofing bones of the skull, lower jaw, collarbone, sesamoid bones (patella).
Steps of Intramembranous Ossification
Mesenchymal Cells Cluster:
Differentiate into osteoblasts, which secrete osteoid matrix.
Osteoid matrix becomes mineralized, forming bone matrix.
The location in the tissue where ossification begins is called the ossification center.
Bone Grows Out in Small Struts (Spicules):
Osteoblasts become trapped in pockets and mature into osteocytes.
Mesenchymal cells produce more osteoblasts.
Blood Vessels Enter the Area:
Bone spicules meet and fuse.
Blood vessels are trapped in developing bone.
Continued Deposition of Bone by Osteoblasts:
Results in spongy bone with interwoven blood vessels.
Remodeling Around Blood Vessels:
Produces osteons of compact bone.
Connective tissue around bone organizes into the fibrous layer of the periosteum.
Osteoblasts near the bone surface remain as the cellular layer of the periosteum.
Intramembranous Ossification in Development
Begins during the eighth week of embryonic development.
Ossification centers and progressing bone formation are visible at 10 weeks.
At 16 weeks, most bones of the adult skeleton can be identified.
Abnormalities of Bone Growth and Development
Disorders can cause shortened or lengthened bones.
Disorders Causing Shortened Bones
Pituitary Growth Failure:
Inadequate growth hormone production.
Reduced epiphyseal cartilage activity; abnormally short bones.
Rare due to treatment with synthetic growth hormone.
Achondroplasia:
Epiphyseal cartilage of long bones grows slowly and is replaced by bone early in life.
Results in short, stocky limbs, while the trunk is normal size.
No effects on sexual or mental development.
Disorders Causing Lengthened Bones
Marfan Syndrome:
Inherited metabolic condition.
Excessive cartilage formation at epiphyseal cartilages.
Results in a very tall person with long, slender limbs.
Affects other connective tissues throughout the body, commonly causing cardiovascular problems.
Gigantism:
Overproduction of growth hormone before puberty.
Can reach heights of over 2.7 m (8 ft. 11 in.).
Puberty is often delayed.
The most common cause is a pituitary tumor, treated by surgery, radiation, or medications suppressing growth hormone release.
Acromegaly:
Overproduction of growth hormone after epiphyseal plates close.
Bones get thicker, not longer, especially in the face, jaw, and hands.
Alterations in soft-tissue structure change physical features.
Bones as Mineral Reservoirs
Minerals are inorganic ions that contribute to the osmotic balance of body fluids and are vital in many physiological processes.
Importance of Calcium
The most abundant mineral in the body.
$1–2$ kg ($2.2–4.4$ lb).
Approximately 99 percent is deposited in the skeleton.
Variety of physiological functions (muscle contraction, blood coagulation, nerve impulse generation).
Concentration variation greater than 30–35 percent affects neuron and muscle function.
Normal daily fluctuations are less than 10 percent.
Maintaining Calcium Levels
Controlled by activities of:
Intestines:
Absorb calcium and phosphate under hormonal control.
Bones:
Osteoclasts erode matrix and release calcium.
Osteoblasts use calcium to deposit new matrix.
Kidneys:
Varying levels of calcium and phosphate loss in urine under hormonal control.
Hormones Regulating Calcium Ion Metabolism
The primary hormones are parathyroid hormone, calcitriol, and calcitonin.
Factors that Increase Blood Calcium Levels
Parathyroid Hormone (PTH):
Secreted from parathyroid glands.
Responses:
In bones: Osteoclasts are stimulated to erode matrix, releasing stored calcium.
In intestines: Calcitriol effects are enhanced, and calcium absorption is increased.
In kidneys: Increased release of hormone calcitriol, stimulating calcium reabsorption in kidneys.
Calcium and the Skeleton
As a calcium reserve, the skeleton has a primary role in calcium homeostasis.
Has a direct effect on the shape and strength of bones.
Release of calcium into the blood weakens bones.
Deposition of calcium salts strengthens bones.
Bone Fractures
A fracture is a crack or break due to extreme mechanical stress.
Most heal as long as the blood supply and cellular parts of the periosteum and endosteum survive.
Repair involves four steps.
Steps in Fracture Repair
Fracture Hematoma Formation:
A large clot closes injured vessels.
Develops within several hours.
Callus Formation:
Internal callus: A network of spongy bone that unites the inner edges of the fracture.
External callus: Composed of cartilage and bone that stabilizes the outer edges of the fracture.
Spongy Bone Formation:
Cartilage of the external callus is replaced by spongy bone.
Bone fragments and dead bone are removed and replaced.
Ends of the fracture are held firmly in place.
Compact Bone Formation:
Spongy bone is replaced by compact bone.
Remodeling over time eliminates evidence of the fracture.
General Categories of Fractures
Closed or Simple:
Completely internal (no break in the skin).
Only seen on x-rays.
Open or Compound:
Project through the skin.
More dangerous due to infection and uncontrolled bleeding.
Specific Types of Fractures
Transverse Fractures:
Break the shaft across its long axis.
Spiral Fractures:
Produced by twisting stresses and spread along the length of the bone.
Displaced Fractures:
Produce new and abnormal bone arrangements; while nondisplaced fractures retain normal alignment.
Compression Fractures:
Occur in vertebrae subjected to extreme stresses, often associated with osteoporosis.
Greenstick Fractures:
One side of the shaft is broken, and the other side is bent.
Generally occurs in children because long bones have yet to fully ossify.
Comminuted Fractures:
Shatter the affected area, producing fragments.
Epiphyseal Fractures:
Occur where bone matrix is calcifying.
A clean transverse fracture of this type heals well, but if not monitored, breaks between the epiphyseal plate and cartilage can stop growth at the site.
Pott’s (Bimalleolar) Fracture:
Occurs at the ankle and affects both the medial malleolus and the lateral malleolus.
Colles Fracture:
A break in the distal radius.