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 (diploë) 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 diploë 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