Chapter 6: The Skeletal System - Bone Tissue

Functions of Bone and the Skeletal System

• Bone is an organ composed of various tissues (bone, cartilage, dense connective tissue, adipose, and nervous tissue).

• The skeletal system includes bones and cartilages.

Functions of the Skeletal System

1. Support:

   • Provides the body's framework.

   • Supports soft tissues.

   • Provides attachment points for skeletal muscles.

2. Protection:

   • Offers mechanical protection to internal organs, reducing injury risk.

   • Examples: Cranial bones protect the brain, vertebrae protect the spinal cord, and the ribcage protects the heart and lungs.

3. Assisting in Movement:

   • Skeletal muscles attach to bones; muscle contraction causes bone movement.

4. Mineral Homeostasis:

   • Bone tissues store minerals such as calcium (Ca) and phosphorus (P).

   • Bone releases minerals into the blood to balance mineral levels in the body.

5. Blood Cell Production (Hemopoiesis):

   • Occurs in the red bone marrow inside some larger bones.

6. Storage of Chemical Energy:

   • With age, some red bone marrow converts to yellow bone marrow.

   • Yellow bone marrow consists mainly of adipose cells and some blood cells, serving as an energy reserve.

Skeletal System: Made of Connective Tissue

• Types of connective tissue include bone, ligaments, tendons, and cartilage.

Types of Bones: Shape

• Bones are Classified into 5 main types based on shape:

  • Long

  • Short

  • Flat

  • Irregular

  • Sesamoid

Structure of Bone

• A long bone consists of:

  • Diaphysis (bone shaft)

  • 2 epiphyses (both ends of the bone at the joints)

  • 2 metaphyses (region between diaphysis and epiphysis)

  • Articular cartilage covering both epiphyses

  • Periosteum (connective tissue surrounding the diaphysis)

  • Medullary cavity (hollow space within diaphysis)

  • Endosteum (thin membrane lining the medullary cavity)

• Hyaline Cartilage - makes up most skeletal cartiliage, found in nose, the ribs, the larynx

  • Matrix containing ground substance and collagen

    • Contains condrocytes , perichodrium

    • permits flexibility and support, and surface for joint movement

• Elastic cartilage: provides support while allowing for greater flexibility, commonly found in structures such as the ear and the epiglottis.

• Fibro cartilage

  • Stronger, collagen fibers in matrix

    • Chondrocytes are the primary cells found in cartilage, responsible for maintaining the cartilage matrix and aiding in its repair.

• Dense connective tissue

  • Collagen bundles, arranged in paralleled rows

    • Fibroblast, collagen bundles arranged inot parral rows

    • forms tendons, connecting muscle to bone and liagaments and connecting bones to bones

• Spongy bone tissue (Cancellous)

• Compact bone

  • structural unit of a compact bone

Bone Membranes: Periosteum

• Periosteum forms the outer surface of bone; endosteum lines the medullary cavity.

  • Wraps the superficial layer of compact bone

  • Two layers:

  1. Fibrous outer layer

  2. Cellular inner layer

  • Functions:

  1. Isolates bone from surrounding tissues

  2. Route for blood and nervous supply

  3. Actively participates in bone growth and repair

  • Perforating fibers:

  • Collagen fibers from surrounding tendons, ligaments, and joint capsules are cemented into circumferential lamellae.

  • Osteoblasts from the cellular layer of the periosteum involved in the process.

  • Very strong attachment; excessive force will break the bone before snapping these fibers.

    • Perforating fibers act like anchors, securing the periosteum—and any attached tendons or ligaments—firmly to the bone.

    • This connection is so strong that under great stress, the bone itself may break before the fiber attachments give way.

Bone Membranes: Endosteum

• Incomplete cellular layer lining the medullary cavity.

  • Active during bone growth, repair, and remodeling.

  • Covers spongy bone and lines central canals.

  • Consists of a simple layer of osteogenic cells.

  • Where the layer is incomplete, the exposed matrix is remodeled by osteoclasts and osteoblasts.

  • Osteoclasts are in shallow depressions called osteoclastic crypts.

Fl collagen

• Contains condrocytes , perichodrium

• A cross-section of a flat bone shows the spongy bone (diploe) lined on either side by a layer of compact bone.

Histology of Bone (Part two)

• Chemical Composition of Bone

  • Bone Tissue

  • Composition

  • Osteo= bone

    • 70% mineral (Ca^{2+}) and (PO_4) as hydroxyapatite

    • 22% protein (95% Type I collagen + 5% proteoglycans and other materials)

    • 8% water

  • Two major types:

    • Compact (cortical, i.e., long bones):

    • Mechanical and protective functions

    • Spongy (spongy, i.e., vertebrae):

    • Metabolic regulation of calcium

  • Four types of bone cells:

    • Osteoblasts

    • Osteoclasts

    • Osteocytes

    • Bone lining cells

Bone Cells

• Bone contains 4 types of cells:

  • Osteoblasts (bone-building cells that secrete matrix; initiate calcification)

  • Osteocytes (mature bone cells)

  • Osteoclasts (remodel bones and cause them to release calcium; bone resorption)

  • Osteoprogenitor cells (bone stem cells able to differentiate into the other types of cells)

Bone Cells Function

• Osteoblasts: build new bone, synthesize collagen, and control mineralization.

• Osteocytes: direct bone to form in the places where it is most needed. They lie within mineralized bone and they may detect mechanical deformation and mediate the response of the osteoblasts.

• Osteoclasts: Specialized cells that resorb bone. They seal off an area of bone surface then, when activated, they pump out hydrogen ions to produce a very acid environment, which dissolves the hydroxyapatite. Used enzymes to break down bones (resorption)

Bone Cell Types

Histology of Bone

• Bone contains an abundant extracellular matrix that surrounds widely separated cells

  • The extracellular matrix is about 15% water, 30% collagen, and 55% crystalized mineral salts

  • Mineral salts calcium phosphate and calcium hydroxide combine to form crystals called hydroxyapatite

• Compact bone provides protection and support; the strongest

• Spongy bone is lightweight and provides tissue support; also called trabecular or cancellous bone

Classification of Bone

• Compact Bone

• Spongy (Cancellous) Bone

Blood and Nerve Supply of Bone

• Periosteal arteries (accompanied by nerves) enter the diaphysis through Volkmann’s canals. They are accompanied by periosteal veins.

• A nutrient artery enters the center of the diaphysis through a nutrient foramen. Nutrient veins exit via the same canal.

• The metaphyses and epiphyses also have their own arteries and veins.

Bone Formation (part 3)

• Ossification (osteogenesis) is the process of bone formation. Bones form in 4 situations:

  1. During embryological and fetal development

  2. When bones grow before adulthood

  3. When bones remodel

  4. When fractures heal

• Types of ossification:

  1. Intramembranous

  2. Endochondral

Bone Development

• Intramembranous ossification – bone develops from a fibrous membrane

  • Formation of most of the flat bones of the skull and the clavicles

• Endochondral ossification – bone forms by replacing hyaline cartilage

  • Uses hyaline cartilage “bones” as models for bone construction

  • Requires breakdown of hyaline cartilage prior to ossification

Intramembranous Ossification

• Intramembranous ossification follows four steps:

  • (a) Mesenchymal cells group into clusters, and ossification centers form.

  • (b) Secreted osteoid traps osteoblasts, which then become osteocytes.

  • (c) Trabecular matrix and periosteum form.

  • (d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow.

Endochondral Ossification

• Endochondral ossification follows five steps:

  • (a) Mesenchymal cells differentiate into chondrocytes forming the bone model.

  • (b) Cavitation of hyaline shaft. The cartilage model of the future bony skeleton and the perichondrium form.

  • (c) Invasion of internal cavities. Capillaries penetrate cartilage. Perichondrium transforms into periosteum. Periosteal collar develops. Primary ossification center develops.

  • (d) Formation medullary cavity. Cartilage and chondrocytes continue to grow at ends of the bone.

  • (e) Secondary ossification centers develop at epiphysis.

  • (f) Cartilage remains at epiphyseal (growth) plate and at joint surface as articular cartilage.

Stages of Endochondral Ossification

1. Formation of bone collar around hyaline cartilage model.

2. Cavitation of the hyaline cartilage within the cartilage model.

3. Invasion of internal cavities by the periosteal bud and spongy bone formation.

4. Formation of the medullary cavity as ossification continues; the appearance of secondary ossification centers in the epiphyses in preparation for stage 5.

5. Ossification of the epiphyses; when completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages.

Longitudinal Bone Growth

• The epiphyseal plate is responsible for longitudinal bone growth.

Longitudinal Growth

Progression from Epiphyseal Plate to Epiphyseal Line

• As a bone matures, the epiphyseal plate progresses to an epiphyseal line.

  • (a) Epiphyseal plates are visible in a growing bone.

  • (b) Epiphyseal lines are the remnants of epiphyseal plates in a mature bone.

Appositional Bone Growth

• Increases diameter of existing bones

• Does not form original bones

• Osteogenic cells differentiate into osteoblasts that add bone matrix under periosteum
  Adds successive layers of circumferential lamellae

• Trapped osteoblasts become osteocytes

• Deeper lamellae are recycled and replaced by osteons

• Osteoclasts remove matrix at inner surface to enlarge medullary cavity

• Bone modeling is the process in which matrix is resorbed on one surface of a bone and deposited on another

Bone Remodeling

• Bone Remodeling in adult life, bone undergoes remodeling, in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed. Injury, exercise, and other activities lead to remodeling

Functional Zones in Long Bone

• Growth zone:

  • Cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis.

• Transformation zone:

  • Older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate.

• Osteogenic zone:

  • New bone formation occurs.

Hormonal Regulation of Bone Growth During Youth

• During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone.

• During puberty, testosterone and estrogens

  • Initially promote adolescent growth spurts

  • Cause masculinization and feminization of specific parts of the skeleton

  • Later induce epiphyseal plate closure, ending longitudinal bone growth

Mature Bone Remodeling and Repair

• Changes in shape, size, strength

  • Dependent on diet, exercise, age

• Bone cells are regulated by hormones

  • Parathyroid hormone (PTH): removes calcium from bone

  • Calcitonin: adds calcium to bone

• Repair hematoma and callus formation

Bone Resorption

• Accomplished by osteoclasts

• Resorption involves osteoclast secretion of

  • Lysosomal enzymes that digest organic matrix

  • Acids that convert calcium salts into soluble forms

Control of Remodeling

• Two hormonal control loops regulate bone remodeling

  • Hormonal mechanism maintains calcium homeostasis in the blood

  • Mechanical and gravitational forces acting on the skeleton

  • Rising blood (Ca^{2+}) levels trigger the thyroid to release calcitonin

  • Calcitonin stimulates calcium salt deposit in bone

  • Falling blood (Ca^{2+}) levels signal the parathyroid glands to release parathyroid hormone (PTH)

  • PTH signals osteoclasts to degrade bone matrix and release (Ca^{2+}) into the blood

Bone Formation

• Bones thicken with the cooperative action of osteoblasts and osteoclasts

• As osteoblasts deposit bone on the outer surface, osteoclasts widen the medullary cavity from within

Factors Affecting Bone Growth and Remodeling

• Minerals: calcium, phosphorus, magnesium, fluoride, manganese

• Vitamins: Vitamins A, C, D, K, and B12

• Hormones: IGFs (stimulated by GH), T3 and T4, sex hormones (estrogen in females, testosterone in males)

Fracture and Repair of Bone

• A fracture is a break in a bone. The healing process involves 3 different phases in 4 steps.

  • The reactive phase is an early inflammatory phase.

  • The reparative phase includes the formation of a fibrocartilaginous callus first and a bony callus second.

  • The bone remodeling phase is the last step as the bony callus is remodeled.

Bone’s Role in Calcium Homeostasis

• Bones store 99% of the body’s calcium.

• The parathyroid gland secretes parathyroid hormone (PTH) when calcium levels drop.

• Osteoclasts are stimulated to increase bone resorption, and calcium is released.

• PTH also stimulates the production of calcitriol by the kidneys to increase calcium absorption in the intestines.

Exercise and Aging in Bone Tissue

• From birth through adolescence, more bone is produced than is lost during remodeling. In adults, the rates are the same.

• Older individuals, especially post-menopausal women, experience a decrease in bone mass when resorption outpaces deposition.

Summary of Factors That Affect Bone Growth: Minerals

Summary of Factors That Affect Bone Growth: Vitamins

Summary of Factors That Affect Bone Growth: Hormones

Summary of Factors That Affect Bone Growth: Hormones

Summary of Factors That Affect Bone Growth: Exercise and Aging

Disorders: Homeostatic Imbalances

• Osteoporosis – bone resorption outpaces formation; 80% of those affected are women.

• Rickets and Osteomalacia – inadequate calcification of extracellular bone matrix. Rickets affects children and leads to bowed legs and deformations in skull, rib cage, or pelvis. Osteomalacia affects adults and causes painful/tender bones and fractures with minor trauma.

Medical Terminology

• Osteoarthritis – degeneration of articular cartilage, leads to friction of bone against bone.

• Osteomyelitis – infection of bone often caused by Staphylococcus aureus.

• Osteopenia – reduced bone mass below normal.

• Osteosarcoma – bone cancer that primarily affects osteoblasts.