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Bones and Skeletal Tissue

 

Skeletal Cartilage

•      Contains no blood vessels or nerves

•      Surrounded by the perichondrium (dense irregular CT) that resists outward expansion

•      Three types – hyaline, elastic, and fibrocartilage

Hyaline Cartilage

•      Provides support, flexibility, and resilience

•      Is the most abundant skeletal cartilage

•      Is present in these cartilages:

•    Articular – covers the ends of long bones

•    Costal – connects the ribs to the sternum

•    Respiratory – makes up the larynx and reinforces air passages

•    Nasal – supports the nose

Elastic Cartilage

•      Similar to hyaline cartilage but contains elastic fibers

•      Found in the external ear and the epiglottis

Fibrocartilage

•      Highly compressed with great tensile strength

•      Contains collagen fibers

•      Found in menisci of the knee and in intervertebral discs

Growth of Cartilage

•      Appositional – cells in the perichondrium secrete matrix against the external face of existing cartilage

•      Interstitial – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within

Bones and Cartilages of the Human Body

Classification of Bones

•      Axial skeleton – bones of the skull, vertebral column, and rib cage

•      Appendicular skeleton – bones of the upper and lower limbs, shoulder, and hip

Classification of Bones: By Shape

•      Long bones – longer than they are wide
(e.g., humerus)

Classification of Bones: By Shape

•      Short bones

•    Cube-shaped bones of the wrist and ankle

•    Bones that form within tendons (e.g., patella)

Classification of Bones: By Shape

•      Flat bones – thin, flattened, and a bit curved (e.g., sternum, and most skull bones)

•      Irregular bones – bones with complicated shapes (e.g., vertebrae and hip bones)

Function of Bones

•      Support – form the framework that supports the body and cradles soft organs

•      Protection – provide a protective case for the brain, spinal cord, and vital organs

•      Movement – provide levers for muscles

•      Mineral storage – reservoir for minerals, especially calcium and phosphorus

•      Blood cell formation – hematopoiesis occurs within the marrow cavities of bones

Gross Anatomy of Bones

•      Compact bone – dense outer layer

•      Spongy bone – honeycomb of trabeculae filled with yellow bone marrow

Structure of Long Bone

•      Diaphysis

•    Tubular shaft that forms the axis of long bones

•    Composed of compact bone that surrounds the medullary cavity

•    Yellow bone marrow (fat) is contained in the medullary cavity

•      Epiphyses

•    Expanded ends of long bones

•    Exterior is compact bone, and the interior is spongy bone

•    Joint surface is covered with articular (hyaline) cartilage

•    Epiphyseal line separates the diaphysis from the epiphyses

Bone Membranes

•      Periosteum – double-layered protective membrane

•    Outer fibrous layer is dense regular CT

•    Inner osteogenic layer is composed of osteoblasts and osteoclasts

•    Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina

•    Secured to underlying bone by Sharpey’s fibers

•      Endosteum – delicate membrane covering internal surfaces of bone

Structure of Short, Irregular, and Flat Bones

•      Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploλ) on the inside

•      Have no diaphysis or epiphyses

•      Contain bone marrow between the trabeculae

Location of Hematopoietic Tissue (Red Marrow)

•      In infants

•    Found in the medullary cavity and all areas of spongy bone

•      In adults

•    Found in the diploλ of flat bones, and the head of the femur and humerus

Microscopic Structure of Bone: Compact Bone

•      Haversian system, or osteon – the structural unit of compact bone

•    Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

•    Haversian, or central canal – central channel containing blood vessels and nerves

•    Volkmann’s canals – channels lying at right angles to the central canal, connecting blood and nerve supply of the periosteum to that of the Haversian canal

•    Osteocytes – mature bone cells

•    Lacunae – small cavities in bone that contain osteocytes

•    Canaliculi – hairlike canals that connect lacunae to each other and the central canal

Chemical Composition of Bone: Organic

•      Osteoblasts – bone-forming cells

•      Osteocytes – mature bone cells

•      Osteoclasts – large cells that resorb or break down bone matrix

•      Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen

Chemical Composition of Bone: Inorganic

•      Hydroxyapatites, or mineral salts

•    Sixty-five percent of bone by mass

•    Mainly calcium phosphates

•    Responsible for bone hardness and its resistance to compression

Bone Markings

•      Bulges, depressions, and holes that serve as:

•    Sites of attachment for muscles, ligaments, and tendons

•    Joint surfaces

•    Conduits for blood vessels and nerves

Bone Markings: Projections – Sites of Muscle and Ligament Attachment

•      Tuberosity – rounded projection

•      Crest – narrow, prominent ridge of bone

•      Trochanter – large, blunt, irregular surface

•      Line – narrow ridge of bone

Bone Markings: Projections – Sites of Muscle and Ligament Attachment

•      Tubercle – small rounded projection

•      Epicondyle – raised area above a condyle

•      Spine – sharp, slender projection

•      Process – any bony prominence

Bone Markings: Projections That Help to Form Joints

•      Head – bony expansion carried on a narrow neck

•      Facet – smooth, nearly flat articular surface

•      Condyle – rounded articular projection

•      Ramus – armlike bar of bone

Bone Markings: Depressions and Openings

•      Meatus – canal-like passageway

•      Sinus – cavity within a bone

•      Fossa – shallow, basinlike depression

•      Groove – furrow

•      Fissure – narrow, slitlike opening

•      Foramen – round or oval opening through a bone

Bone Development

•      Osteogenesis and ossification – the process of bone tissue formation, which leads to:

•    The formation of the bony skeleton in embryos

•    Bone growth until early adulthood

•    Bone thickness, remodeling, and repair

Formation of the Bony Skeleton

•      Begins at week 8 of embryo development

•      Intramembranous ossification – bone develops from a fibrous membrane

•      Endochondral ossification – bone forms by replacing hyaline cartilage

Intramembranous Ossification

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

•      Fibrous connective tissue membranes are formed by mesenchymal cells

Stages of Intramembranous Ossification

•      An ossification center appears in the fibrous CT membrane

•      Bone matrix is secreted within the fibrous membrane

•      Woven bone and periosteum form

•      Bone collar of compact bone forms, and red marrow appears

Endochondral Ossification

•      Begins in the second month of development

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

•      Requires breakdown of hyaline cartilage prior to ossification

Stages of Endochondral Ossification

•      Formation of bone collar

•      Cavitation of the hyaline cartilage

•      Invasion of internal cavities by the periosteal bud, and spongy bone formation

•      Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses

•      Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Postnatal Bone Growth

•      Growth in length of long bones

•    Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive

•    Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth

•    Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic

Functional Zones in Long Bone Growth

•      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

Long Bone Growth and Remodeling

•      Growth in length – cartilage continually grows and is replaced by bone as shown

•      Remodeling – bone is resorbed and added by appositional growth as shown

Hormonal Regulation of Bone Growth During Youth

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

•      During puberty, by 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 

Bone Remodeling

•      Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

Bone Deposition

•      Occurs where bone is injured or added strength is needed

•      Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese

•      Alkaline phosphatase is essential for mineralization of bone

•      Sites of new matrix deposition are revealed by:

•    Osteoid seam – unmineralized band of bone matrix

•    Calcification front – abrupt transition zone between the osteoid seam and the older mineralized bone

Bone Resorption

•      Accomplished by osteoclasts

•      Resorption bays – grooves formed by osteoclasts as they break down bone matrix

•      Resorption involves osteoclast secretion of:

•    Lysosomal enzymes that digest organic matrix

•    Acids that convert calcium salts into soluble forms

•      Dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood

Importance of Ionic Calcium in the Body

•      Calcium is necessary for:

•    Transmission of nerve impulses

•    Muscle contraction

•    Blood coagulation

•    Secretion by glands and nerve cells

•    Cell division

Control of Remodeling

•      Two control loops regulate bone remodeling

•    Hormonal mechanism that maintains calcium homeostasis in the blood

•    Mechanical and gravitational forces acting to the skeleton

Hormonal Mechanism

•      Rising blood Ca2+ levels trigger the thyroid to release calcitonin

•      Calcitonin stimulates calcium salt deposit in bone

•      Falling blood Ca2+ levels signal the parathyroid glands to release PTH

•      PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood

Response to Mechanical Stress

•      Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it

•      Observations supporting Wolff’s law include:

•    Long bones are thickest midway along the shaft (where bending stress is greatest)

•    Curved bones are thickest where they are most likely to buckle

•    Trabeculae form along lines of stress

•    Large, bony projections occur where heavy, active muscles attach

Bone Fractures (Breaks)

•      Bone fractures are classified by:

•    The position of the bone ends after fracture

•    Completeness of the break

•    The orientation of the bone to the long axis

•    Whether or not the bones ends penetrate the skin

Types of Bone Fractures

•      Nondisplaced – bone ends retain their normal position

•      Displaced – bone ends are out of normal alignment

•      Complete – bone in broken all the way through

•      Incomplete – bone is not broken all the way through

•      Linear – the fracture is parallel to the long axis of the bone

•      Transverse – the fracture is perpendicular to the long axis of the bone

•      Compound (open) – bone ends penetrate the skin

•      Simple (closed) – bone ends do not penetrate the skin

Common Types of Fractures

•      Comminuted – bone fragments into three or more pieces; common in the elderly

•      Spiral – ragged break when bone is excessively twisted; common sports injury

•      Depressed – broken bone portion pressed inward; typical skull fracture

•      Compression – bone is crushed; common in porous bones

•      Epiphyseal – epiphysis separates from diaphysis along epiphyseal line; occurs where cartilage cells are dying

•    Greenstick –  incomplete fracture – one side of the bone breaks and the other side bends; common in children

Stages in the Healing of a Bone Fracture

•      Hematoma formation

•    Torn blood vessels hemorrhage

•    A mass of clotted blood (hematoma) forms at the
fracture site

•    Site becomes swollen, painful, and inflamed

•      Fibrocartilaginous callus forms

•      Granulation tissue (soft callus) forms a few days after the fracture

•      Capillaries grow into the tissue and phagocytic cells begin cleaning debris

•      The fibrocartilaginous callus forms when:

•    Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the bone

•    Fibroblasts secret collagen fibers that connect broken bone ends

•    Osteoblasts begin forming spongy bone

•    Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

•      Bony callus formation

•    New bone trabeculae appear in the fibrocartilaginous callus

•    Fibrocartilaginous callus converts into a bony (hard) callus

•    Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later

•      Bone remodeling

•    Excess material on the bone shaft exterior and in the medullary canal is removed

•    Compact bone is laid down to reconstruct shaft walls

Homeostatic Imbalances

•      Osteomalacia

•    Bones are inadequately mineralized causing softened, weakened bones

•    Main symptom is pain when weight is put on the affected bone

•    Caused by insufficient calcium in the diet, or by vitamin D deficiency

•      Rickets

•    Bones of children are inadequately mineralized causing softened, weakened bones

•    Bowed legs and deformities of the pelvis, skull, and rib cage are common

•    Caused by insufficient calcium in the diet, or by vitamin D deficiency

•      Osteoporosis

•    Group of diseases in which bone reabsorption outpaces bone deposit

•    Spongy bone of the spine is most vulnerable

•    Occurs most often in postmenopausal women

•      Treatment

•    Calcium and vitamin D supplements

•    Increased weight bearing exercise

•    Hormone (estrogen) replacement therapy (HRT)

•      Prevented or delayed by sufficient calcium intake and weight-bearing exercise

Paget’s Disease

•      Characterized by excessive bone formation and breakdown

•      Pagetic bone with an excessively high ratio of woven to compact bone is formed

•      Pagetic bone, along with reduced mineralization, causes spotty weakening of bone

•      Osteoclast activity wanes, but osteoblast activity continues to work

•      Usually localized in the spine, pelvis, femur, and skull

•      Unknown cause (possibly viral)

•      Treatment includes the drugs Didronate and Fosamax

Developmental Aspects of Bones

•      Mesoderm gives rise to embryonic mesenchymal cells, which produce membranes and cartilages that form the embryonic skeleton

•      The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms

•      At birth, most long bones are well ossified (except for their epiphyses)

•      By age 25, nearly all bones are completely ossified

•      In old age, bone resorption predominates

•      A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life