LECTURE OUTLINE CH6

Bones and Skeletal Tissues

Skeletal Cartilage

§    Contains no blood vessels or nerves

§    Surrounded by the perichondrium (dense irregular connective tissue) 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 larynx, 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

§    Calcification of cartilage occurs

§     During normal bone growth

§     During old age

 

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)

§    Short bones

§     Cube-shaped bones of the wrist and ankle

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

§    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

 

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

Gross Anatomy of Bones: Bone Textures

§    Compact bone – dense outer layer

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

Bone Markings

 

Structure of Long Bone

§    Long bones consist of a diaphysis and an epiphysis

§    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 connective tissue

§     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 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 connective tissue 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, 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 the:

§     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 maintains calcium homeostasis in the blood

§     Mechanical and gravitational forces acting on 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

 

Homeostatic Imbalances

§    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

§     Bones become so fragile that sneezing or stepping off a curb can cause fractures

Osteoporosis: Treatment

§    Calcium and vitamin D supplements

§    Increased weight-bearing exercise

§    Hormone (estrogen) replacement therapy (HRT) slows bone loss

§    Natural progesterone cream prompts new bone growth

§    Statins increase bone mineral density