Bones and Skeletal Tissue

Cartilage

Features between dense CT & bone → tough but flexible

• avascular, devoid of nerve fibers

• all types are made up of cells encased in small cavities (lacunae) within jelly-like extracellular matrix

  • ground subsatnce contains lots of the glycosaminioglycans (GAGs): chondroitin sulfate & hyaluronic acid - also chondronectin (adhesive protein)

• Collagen fibers (can have some elastic fibers)

• up to 80% H2O

Perichondrium

Layer of dense connective tissue surrounding cartilage like a girdle

• Helps cartilage resist outward expansion

• Contains blood vessels for nutrient delivery to cartilage

• In damaged areas, perichondrium can form scar tissue because poorly vascularized cartilage repairs badly

Chondroblasts

immature cartilage cells - actively form cartilage

Chondrocytes

mature cartilage cells - maintain cartilage

Lacunae

Localized clusters of chondrocytes in cartilage

Hyaline Cartilage

• Most abundant

• Firm support + pliability

• lots of collagen

• appears glassy blue-white

• chondrocytes - only 1-10% of volume

Elastic Cartilage

• Like hyaline cartilage, but more elastic fibers

• external ear, epiglottis

Fibrocartilage

rows of chondrocytes alternating with rows of thick collagen fibers; great tensile strength

Appositional Growth (Growth of Cartilage)

• New matrix laid down on surface of cartilage

• Cartilage - forming cells in perichondrium secrete matrix against external face of existing cartilage

• Cartilage increases in width

Interstitial growth ( Growth of Cartilage)

• new matrix made within cartilage

• Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within

• cartilage increases in length

• Cartilage growth ends during adolescence

Bone Tissue

Bone is a living dynamic tissue which respond to its environment:

1. Bone reacts to amount of force applied by increasing both the density & amount of roughening on bone or decreasing density when force is reduced or eliminated (eg. paralysis) (deposition vs. resorption)

2. Bone stores calcium - resorbed & transferred to bloodstream when needed

Functions of Bones

• Support

• Protection

• Anchorage & Movement

• Mineral storage

• Blood cells formation

• fat storage

• Hormone production (osteocalcin)

Classification of Bones

206 named bones in the human skeleton

Two main groups, divided by location

Axial Skeleton:

• Long axis of body

• Skull, vertebral column, rib cage

Appendicular Skeleton

• Bones of upper and lower limbs

• Girdles attaching limbs to axial skeleton

Lots of variation in size/shapes of bones (e.g. pisiform bone vs. femur)

Unique shape of each bone fulfils a particular need

e.g. femur - maximum strength with minimum weight - achieves this with hollow cylindrical design

Classified by their SHAPE and not SIZE

Long Bones

• much longer than wide

• shaft + 2 expanded ends

• mostly compact bone with marrow cavity; spongy bone near joint ends

Irregular Bones

• leftovers

• complicated shapes: primarily spongy bone + thin covering layer of compact bone

• Ex. vertebrae & hip bones

Flat Bones

• Thin, flattened & sometimes curved

• include ribs, sternum & scapula & most cranial bones

Short Bones

• roughly cube-shaped; e.g. wrist, ankle

• primarily spongy bone + thin outer layer of compact bone

• Sesamoid bones form within tendons - Ex. Patella

Bone Structure

• Bones are organs

• Bone (osseous) tissue dominates

• Also contain nervous tissue, cartilage, dense connective tissue, muscle cells and epithelial cells in its blood vessels

• three levels of structure: gross / microscopic / chemical

Compact bone

dense outer layer on every bone that appears smooth and soild

Spongy bone

made up of a honeycomb of small, needle-like or flat pieces of bone called trabeculae

open spaces between trabeculae are filled with red or yellow bone marrow

Structure of short, irregular and flat bones

• All have similar structure

• thin plates of spongy bone (dipole) covered by compact bone

• Compact bone sandwiched between connective tissue membranes

• Periosteum covers outside of compact bone and endosteum covers inside portion of compact bone + spongy bone

• Bone marrow scattered throughout the spongy bone/ no defined marrow cavity

• Hyaline cartilage covers articular surfaces

Diaphysis (Structure of a Typical Long Bone)

• tubular shaft of a long bone = long axis of the bone

• collar of compact bone surrounding marrow cavity (medullary cavity)

• in adults, medullary cavity contains fat (yellow marrow) and is called the yellow bone marrow cavity

Epiphyses ( Structure of a Typical Long Bone)

• extremities of a long bone; expanded for articulation with other bones

• compact bone externally; interior filled with spongy bone

• thin layer of hyaline (articular) cartilage on the outer surface

Epiphyseal Line (structure of a typical long bone)

• between diaphysis & each epiphysis

• remnant of epiphyseal (growth) plate

Periosteum (structure of a typical long bone)

Covers external surface, 2 layers

Outer fibrous layer

dense irregular connective tissue with Sharpey’s fibers that secure to bone matrix

Inner osteogenic layer

contains primitive osteogenic stem cells that gives rise to most all bone cells

Nutrient foramen

nerve fibers and blood vessels to the shaft

Endosteum

• Delicate connective tissue covering trabeculae of spongy bone & lines canals of compact bone

• Like periosteum, contains osteogenic cells that can differentiate into other bone cells

Bone Markings

• Sites of muscle, ligament and tendon attachment on external surfaces

• Areas involved in joint formation or conduits for blood vessels and nerves

  Three types of markings:

• Projection: outward bulge of bone

• depressions and openings

• surfaces

Microscopic Anatomy of Bone

Calcium salts give hardness & strength for support/protection of softer tissues; cavities for fat storage & synthesis of blood cells

Cells of bone tissue

five major cell types, each is a specialized form of the same basic cell types:

1. Osteoprogenitor (osteogenic) cells

2. Osteoblasts

3. Osteocytes

4. Bone-lining cells

5. Osteoclasts

Microscopic anatomy of compact bone

• Also called lamellar bone

• Consists of:

  • Osteon (haversian system)

  • Canals and canaliculi

  • Interstitial and circumferential lamellae

Osteon (Haversian System)

• An osteon is the structural unit of compact bone

• an elongated cylinder that runs parallel to long axis of bone / acts as tiny weight-bearing pillars

• An osteon cylinder consists of several rings of bone matrix called lamellae

Canals and Canaliculi

• central (Haversian) canal runs through core of osteon

  • Contains blood vessels and nerve fibers

• Perforating (Volkmann’s) canals: canals lined with endosteum that occur at right angles to central canal

  • Connect blood vessels and nerve of periosteum, medullary cavity and central canal

• Lacunae: small cavities that contain osteocytes

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

  • Enables communication between all osteocytes of osteon and permit nutrients and wastes to be relayed from one cell to another

Interstitial Lamellae

Some fill gaps between forming osteons; other are remnants of osteons destroyed by bone remodeling

Circumferential Lamellae

Sheets of bone located just deep to periosteum; extend around entire circumference of shaft / Help long bone to resist twisting

Microscopic Anatomy of Spongy Bone

• contains trabeculae, lamellarly arranged osteocytes & canaliculi

• Trabeculae arranged along lines of stress; hips bone to resist stress

  • Trabeculae, like cables on a suspension bridge, confer strength to bone

  • Trabeculae onl a few cell layers thick; contain irregularly arranged lamellae & osteocytes interconnected by canaliculi

  • there are no osteons

  • nutrients (fromcapillaries in the endosteum) diffuse through canaliculi from the maroow spaces between the trabeculae to reach the osteocytes

Organic Components (Chemical Composition of Bone)

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts and osteoid

  • Osteoid, which makes up one-third of organic bone matrix, is secreted by osteoblasts

    • Consists of ground substance and collagen fibers, which contribute to high tensile strength and flexibility of bone

Inorganic Components (Chemical Composition of Bone)

• Hydroxypatites (mineral salts)

• Makeup 65% of bone by mass

• Consist mainly of tiny calcium phosphate crystals in and around collagen fibers

• responsible for hardness and resistance to compression

• Bone is half as strong as steel in resisting compression and as strong as steel in resisting tension

Bone Development

Osteogenesis or ossification is the process of bone tissue formation includes:

• Formation of bony skeleton in embryos

• postnatal bone growth during childhood & adolescence

• Bone remodeling and repair throughout life

Endochondral Ossification

• Bone forms by replacing hyaline cartilage model

• Bones are called cartilage (endochondral) bones

• Form most of skeleton / all bones below the skull (except the clavicles)

• Begins in 2nd month

• more complex

Intramembranous Ossification

• Bone develops from fibrous CT membrane containing mesenchymal cells

• Begins about 8 weeks of development & bones are called membrane bones

• Cranial bones of the skull and clavicles - flat bones

• NB: in short bones, only the primary ossification centre is formed; most irregular bones are formed using several distinct ossification centres

When secondary ossification is complete, hyaline cartilage remains:

1. on the epiphyseal surfaces as the articular cartilages

2. at the junctions of diaphysis and epiphyses where it forms the epiphyseal plates

Intramembranous ossification: begins within fibrous connective tissue membranes formed by mesenchymal cells

Four major steps are involved

1. Ossification centers are formed when mesenchymal cells cluster and become osteoblasts

2. Osteoid is secreted, then calcified

3. Woven bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae

4. Lamellar bone replaces woven bone, and red bone marrow appears

Postnatal Bone Growth

• During infancy & youth, long bones lengthen entirely by interstitial growth of the epiphyseal plates

• All bones grow in thickness by appositional growth

• Most bones stop growing during adolescence or in early adulthood - some facial bones (eg. nose & lower jaw) continue to grow (almost imperceptibly) throughout life

Growth in Length of Long Bones

• Epiphyseal plate stays ~ same size throughout childhood & adolescence

• Near the end of adolescence, chondroblast divide less often (cartilage cells in zone 2 multiply more & more slowly)

• Epiphyseal plate becomes thinner longitudinal growth ends when bone of the epiphysis & diaphysis fuses = epiphyseal plate closure

  • About age 18 in females and age 21 in males

Growth in Width

• growth in width = appositional growth

• Bones thicken in response to increased stress from muscle activity or added weight

• layers of bone are laid down on top of one another

1. primarily osteoblasts on periosteal side secreting bone matrix

2. primarily osteoclasts on the endosteal side remove bone matrix

Long Bone Growth and Remodeling During Youth

• As the long bone lengthens, the shape of the ends must be altered (remodeling)

• Remember that the epiphyseal plates are located in the wider parts of long bones

• Bone has to be reshaped to be incorporated into the diaphysis, but the diaphysis also has to be get thicker and stronger as the bone lengthens

• In summary, bone is destroyed by osteoclasts and laid down by osteoblasts on both the inner and outer surfaces of a growing long bone

Growth Hormone (Hormonal Regulation of Bone Growth)

Most important hormone in stimulating epiphyseal plate activity in infancy and childhood

Thyroid Hormone (Hormonal Regulation of Bone Growth)

Modulates activity of growth hormone, ensuring proper proportions

Testosterone (males) and Estrogens (females)

• At puberty promote adolescent growth spurts

• End growth by inducing epiphyseal plate closure

• Excesses (eg. acromegaly from too much GH) or deficits (pituitary dwarfism from GH insufficiency) of any hormones cause abnormal skeletal growth

Bone Modeling / Remodeling

• Bone remodeling consists of both bone deposit and bone resorption

• Occurs at surfaces of both periosteum and endosteum

• Remodeling units: packets of adjacent osteoblasts and osteoclasts coordinate remodeling process

  • About 5-7 % of bone mass is recycled each week

    • Spongy bone replaced ~ every 3-4 years

    • Compact bone replaced ~ every 10 years

• Resorption is function of osteoclasts

  • Dig depressions or grooves as they break down matrix

  • Secrete lysosomal enzymes and protons (H+) that digest matrix

  • Acidity converts calcium salts to soluble forms

  • Osteoclasts activation involved PTH (parathyroid hormone)

• New bone matrix is deposited by osteoblasts

Osteoid seam

band of unmineralized bone matrix that marks area of new matrix

Calcification Front

Abrupt transition zone between osteoid seam and older mineralized bone

Control of Remodelling

Remodelling is regulated by two control loops

Maintaining Ca2+ homeostasis

• negative feedback loop involving Parathyroid hormone (PTH) and Ca2+ in the blood

• Keep bones strong: Mechanical and gravitational forces acting on bone drive remodeling to keep bone strong

Calcitonin

• Released from parafollicular cells of thyroid gland in response to high levels of blood calcium levels

• Effects are negligible, but at abnormally high doses it can lower blood Ca2+ levels temporarily

Bone Repair

Fractures are breaks

• During youth, most fractures result from trauma

• In old age, most result from weakness of bone due to bone thinning

Repair involved four major stages:

1. Hematoma Formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Key Events in Fracture Repair

1. Formation of a hematoma - local bone cells are deprived of oxygen and die; inflammation causes pain

2. Formation of a fibrocartilaginous callus (soft) - invaded by blood vessels that also bring macrophages to clean up the area; osteoclats also resorb damaged bone; fibroblasts, chondroblasts, osteoblasts get busy laying down collagen fibers and tissue components to span the break

3. Conversion to bony callus - cartilage converted to trabecular bone - complete in ~2 months

4. Bone remodelling - any extra bony material is removed; outer bone of shaft walls converted to compact bone and bone regains original shape

• Final structure resembles original structure

• Responds to same mechanical stressors

Osteoporosis

Group of diseases in which bone reorption → bone formation → bone becomes porous

some areas if skeleton especially vulnerable: spine, neck of femur

Risk Facts

• Age

  • post menopausal women

  • estrogen promotes bone health by restraining osteoclast activity and promoting deposition of new bone

• insufficient exercise

• diet poor in calcium & protein

• smoking (reduces estrogen levels)

• Genetics

• Dibaetes mellitus