Bones and Skeletal Tissues 1

Characteristics of Cartilage

• features between dense Connective tissue and bone

• avascular, devoid of nerve fibers

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

• ground substances contain lots of GAGs

• Collagen fibers

Perichondrium

layer of dense irregular connective tissue surrounding cartilage like a girdle

functions of perichondrium

• helps cartilage resist outward expansion

• contains blood vessels for nutrient delivery to cartilage

• it can also 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 that also contain osteocytes

What are the 3 kinds of cartilage

hyaline, elastic, Fibro cartilage

Hyaline cartilage (4)

- most abundant; firmsupport + pliability;
- lots of collagen; appears glassy blue-white;
- chondrocytes - only 1-10% of volume

occurs where your bones connect together( joints)

Location of hyaline cartilage (2)

embryonic skeleton, ends of long bones (epiphyseal plates in growing children)
- costal cartilages of ribs, cartilages of nose, trachea, larynx

Function of hyaline cartilage (2)

- supports & flexibility; reinforces; resilient
- cushioning & resists compressive stress

Elastic cartilage

like hyaline, but has more elastic fibres

Location of elastic cartilage

external ears, epiglottis

Fibrocartilage (2)

rows of chondrocytes alternating rows of thick collagen fibres
- great tensile strength( strongest and sturdiest)

Location of fibrocartilage

• intervertebral discs, pubic symphysis, discs of knee joints (where hyaline cartilage meets a ligament or a tendon)

Function of fibrocartilage

-tensile strength
- ability to absorb compressive shock

Appositional growth

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

• Cartilage increases in width or diameter

• bones also grow this way in thickness not width

Interstitial growth

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

• Cartilage increases in length

• bones cannot grow interstitially because the matrix is mineralized

Bone

living dynamic tissue that reacts to its environment

What are the two main functions of bones

1)reacts to amount of force applied by increasing density and amount of roughening on bone or decreasing density when force is reduced (deposition vs. resorption)

2) bone stores calcium- reabsorbed and transferred to bloodstream when needed

What are the 7 functions of bones

• support: Provide framework that supports body

• protection: fused bones skull => protects brain, Vertebrae surrounds spinal cord

• movement- muscles pull on them in order to move.

• mineral and fat storage: reservoir for minerals

• blood cell formation: happens in red marrow

• hormone production: Produce osteocalcin (regulate insulin secretion, glucose homeostasis, energy)

Classification of bones

Two main groups, divided by location
- Axial
- Appendicular

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

How are bones classified

classified by shape, NOT size

What are the four types of bones

long, short, flat, irregular

Long bones (4)

• much longer than wide( shaft + 2 expanded ends)

• mostly compact bone with marrow cavity

• spongy bone near joint ends

• i.e. radius, ulna, phalanges
  - all limbs except patella, wrist and ankle

short bones (4)

• roughly cube-shaped (i.e. wrist, ankles)

• primarly spongy + thin outer layer of compact bone

• sesamoid (independent) bones (bone that forms tendons)( ex tendons)

• vary in size and # in different individuals

Flat bones (3)

• thin, flattened, and sometimes curved

• include skull, ribs, sternum, & scapula and most cranial bones

• self-bracing and stronger bc of shape

Irregular bones (2)

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

• leftovers: e.g. vertebrae and hip bones

Why are bones considered organs

• organs have two or more tissues

• bones have bone tissue, nervous tissue, cartilage. dense connective tissue, muscle cells and epithelial cells in its blood vessels

Gross Anatomy of Bones (2)

Compact and spongy bones

Compact bone

dense outer layer on every bone that appears smooth and solid/ very organized structure

Spongy (cancellous) 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

2 types of lining

periosteum

endosteum

periosteum

- outer fibrous layer+ inner osteogenic layer

endosteum

covers inside portion of compact bone + spongy bone and lines canals of compact bone

Structure of short, irregular and flat bones

• All have similar structure

• thin plates of spongy bone covered by compact bone

• Compact bone covered with periosteum & spongy bone lined with endosteum

• Not cylindrical so no shaft, marrow cavity or epiphyses – but do contain bone marrow between trabeculae

Structure of Long bone

all long bone have a shaft( diaphysis), bone ends( epiphyses) and membranes

Diaphysis (3)

• tubular shaft of long bone

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

• in adults, medullary cavity contains fat called the yellow bone marrow cavity

Epiphyses (sing. = epiphysis)

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

• compact bone forms the thin outer layer; interior filled with spongy bone

• has a thin layer of hyaline (articular) cartilage to cushion the meeting of two bones

Epiphyseal line

lies between diaphysis & each epiphysis
- remnant of epiphyseal plate

Bone textures (2)

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

Osteogenic( osteoprogeniotor) cells

stem cells that differentiate into osteoblasts

Osteoblasts

- bone growth (immature, rapidly dividing)
- reabsorbing cells that release Ca and P

Osteocytes

mature bone cells that monitor and maintain minerals in the bone matrix

What does a compact bone( lamellar bone) consist of

• osteon

• canals and canaliculi

• interstitial and circumferential lamellae

Osteon

• this is the structural unit of compact bone (also called lamellar bone)

• it's an elongated cylinder oriented parallel to the long axis of bone and acts as tiny weight bearing pillars

• a single one is a group of hollow tubes of bone matrix

Haversian (central) canal of compact bone

- one in each osteon
- hold blood vessels, nerve fibres, etc.

Volkmann's (perforating) canal of compact bone

• canals lined with endosteum that occur at right angles to central canal

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

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; others 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

Trabeculae of spongy bone

• contains trabeculae, lamellarly arranged osteocytes & canaliculi

• arranged along lines of stress; helps bone to resist stress

• only a few cell layers thick

• there are no osteons

• nutrients diffuse through canaliculi from the marrow spaces between the trabeculae to reach the osteocytes

what is the chemical composition of bone( 2)

organic and inorganic components

Organic composition of bone

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, andosteoid

Osteoid

• is a fluid 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 composition of bone

• remaining 2/3 of bone

• Hydroxyapitites

Hydroxyapitites( 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

Mechanisms of bone formation

Osteogenesis or Ossification

Osteogenesis or Ossification

• is the process of bone tissue formation and includes formation of bony skeleton in embryos

• growth of bones during childhood & adolescence

• remodelling/repair of bones in adults

Intermembraneous Ossification

• bone develops from fibrous CT membrane containing mesenchymal cells

• cranial bones of the skull and the clavicles - these are flat bones

• Begins at about 8 weeks of development and are called membrane bones

What are the four major steps involved in intramembranous ossification

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

Endochondral Ossification

• bone development via the replacement of a hyaline cartilage model

• all bones below the skull (except the clavicles)

• more complex than intermembraneous

• begins in 2nd month

Endochondral ossification steps

①A bone collar forms around the diaphysis of the hyaline cartilage model

②Cartilage calcifies in the center of the diaphysis and then develops cavities.

③The periosteal bud invades the internal cavities and spongy bone forms

④The diaphysis elongates and a medullary cavity forms. Secondary ossification centers appear in the epiphyses

⑤The epiphyses ossify. When ossification in complete, hyaline cartilage remains only in the epiphyseal plates and articular cartilages

When secondary ossification is complete, hyaline cartilage remains on

- on the epiphyseal surfaces as the articular cartilages
- at the junctions of diaphysis and epiphyses where it forms the epiphyseal plates (this is where long bones continue to grow)

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 but some facial bones (e.g.nose & lower jaw) continue to grow throughout life

Epiphyseal plate

• stays same size throughout childhood & adolescence

• becomes thinner at the end of adolescence (cartilage cells in zone 1 multiply more & more slowly)

• Longitudinal growth ends when bone of the epiphysis & diaphysis fuses = closure of this

Bone Remodelling

- As the long bone lengthens, the shape of the ends must be altered
- Growing bones widen as they lengthen through appositional growth
- As the length increases, external surfaces of the ends made slimmer while the internal surface made thicker
- Bone is destroyed by osteoclasts and laid down by osteoblasts on both the inner and outer surfaces of a growing long bone

What are fractures

Fractures are breaks

– During youth, most fractures result from trauma

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

What are the four major stages for repair of bones

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; osteoclasts 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

Hormonal Regulation of Bone Growth in Childhood

- Growth hormone (GH) stimulates epiphyseal plate activity in infancy and childhood
- Thyroid hormone modulates activity of GH, ensuring proper proportions

Hormonal Regulation of Bone Growth at puberty: Testosterone and estrogens

End growth by inducing epiphyseal plate closure

Bone remodelling

• consists of both bone deposit and bone resorption

• occurs at surfaces of both periosteum and endosteum

Remodelling Units

packets of adjacent osteoblasts and osteoclasts coordinate remodelling process

Resorption is the function of what

osteoclasts

What do the osteoclasts do

• 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

• Osteoclast activation involves PTH (parathyroid hormone)

What is bone matrix 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

When is spongy bone replaced

every 3-4 years

When is compact bone replaced

every 10 years

Bone remodelling hormonal controls: PTH

• Produced by PT glands

• Removes calcium from bone regardless of bone integrity

• negative feedback loop

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 remodelling: Response to mechanical stress

Bones reflect stresses they encounter they are stressed when weight bears on them or muscles pull on them
- Wolfe's law

Wolfe's Law

• Stress is usually off center, so bones tend to bend

• Bending compresses one side, stretches other side

• Diaphysis is thickest where bending stresses are greatest

• Bone can be hollow because compression and tension cancel each other out in center of bone

Osteoporosis

• Bone resorption outpaces bone formation making bone becomes porous

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

Osteoporosis risk factor: age

estrogen & testosterone promote bone health by restraining osteoclast activity and promoting deposition of new bone

Osteoporosis contributing factors

Other contributing factors include

● insufficient exercise

● diet poor in calcium & protein

● smoking (reduces estrogen levels)

● Genetics

● Diabetes mellitus

Bone markings

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

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

What are the three types of markings

Projection: outward bulge of bone

● Depressions and openings

● Surfaces

Bone markings

study pictures