Chapter 7: Bone Tissue

Skeletal system?

Composed of bones, cartilages, and ligaments

Cartilage?

Covers many joint surfaces of mature bone

Ligaments?

Hold bones together at joints

Tendons?

Attach muscle to bone

Functions of the Skeletal System?

Support

Protection

Movement

Electrolyte balance

Acid-Base balance

Blood formation

Support?

Our bones act as the body’s structural framework and as a point of attachment

Protection?

Internal organs are protected from injury

Ex. Skull and ribs

Movement?

Most muscles of the skeletal system are attached to bone

When muscles contract, bones are pulled to produce → movement

Electrolyte balance?

Bones store calcium (99%) and phosphorous

Give bones → strength

Bones release minerals to be distributed throughout the body and maintain → homeostasis

Acid-Base balance?

Bone tissue can buffer the blood and prevent major pH changes

Absorbs or releases phosphate and carbonate salts

Blood formation?

Red bone marrow – produces all major blood cells and the cells of the immune system

Bone (osseous tissue)?

A connective tissue with a hardened matrix of calcium phosphate and other ions

Hardened during mineralization or calcification

Most of the long bone outer shell structure is composed of?

Compact (dense) bone

Shell encloses the?

Medullary cavity (marrow cavity), which contains → bone marrow

At the ends of the bones the central space contains?

Spongy (cancellous) bone tissue that is loosely organized

Diaphysis?

Shaft or body of the long bone

Epiphyses?

The proximal and distal ends of the long bone

Strengthen joints and anchor ligaments and tendons

Mature bones have an?

Epiphyseal line, which contains denser spongy bone between epiphysis and diaphysis

Contains the epiphyseal → plate (growth plate)

Present until bones → stop growing (ages 14-24)

Articular cartilage?

Thin layer of hyaline cartilage that covers the → epiphyses

Absorbs shock and reduces friction for free moving joints

No blood vessels (avascular), slow repair time

Periosteum?

Tough cover of connective tissue covering the → diaphysis

Associated with the blood supply surrounding the bone surface

Offers protection, assists in fracture repair, bone tissue nourishment, and as attachment for tendons and ligaments

Medullary cavity (marrow cavity)?

Hallow cylindrical space found in the → diaphysis

Contains yellow bone marrow and → blood vessels

Endosteum?

Thin membrane that lines the → marrow cavity

Osteogenic cells?

Are stem cells that can become the other bone cell types

Found in the endosteum and the inner periosteum

Multiple continually into more stem cells or become (differentiate into) → osteoblasts

Osteoblast?

Cells that → build bone

Line up under the endosteum and periosteum

They do not undergo mitosis, only made from osteogenic cells

They secrete bone extracellular matrix, which is hardened by mineral deposits

Osteocytes?

Are former osteoblasts that had gotten stuck in the bone extracellular matrix (ECM)

They are found in cavities called lacunae, which connect through channels called canaliculi

Contribute to bone maintenance and sense bone strain

Osteoclasts?

Bone-carver, dissolve bone on the surface

Develop from bone marrow → stem cells

Perform bone resorption, which is key in bone remodeling

This is normal and part of bone development and growth (osteolysis)

Osteoblasts?

Build bone

Osteoclasts?

Carve out bone

Compact bone tissue?

Is the strongest form of bone with few spaces

Found beneath the periosteum in all bones and makes up most of the diaphysis of long bones

Provides protection, support, and stress → resistance

Osteon?

Is a structural unit of compact bone and consists of:

Repeating circular units of concentric lamellae, which is layers of the bone hardened bone matrix

The layers of lamellae surround a?

Central canal - which contains blood vessels and nerves

The vessels in the central canal are fed by → perforating canals

Between layers of lamellae are?

Tiny cavities called lacunae (contain osteocytes), which are all interconnected by canaliculi

Circumferential lamellae?

Are found in the inner and outer boundaries and run parallel to osteons and the bone surface for → support

Old osteons that have been broken down for → regrowth

Osteons are aligned and parallel with?

The diaphysis → length

Allow for resistance to bending or fracturing

Spongy bone tissue?

Does not have osteons

Consist of trabeculae – lamellae arranged in an irregular pattern

Located in the interior of the bone and is protected by compact bone → covering

What is Bone Marrow?

A soft tissue that is found in the marrow cavity and between the trabeculae of → spongy bone

What are the two types of bone marrow?

Red bone marrow(myeloid tissue)

Yellow bone marrow

Red bone marrow(myeloid tissue)?

Produces blood cells (hemopoiesis)

Found in almost every bone of a child

In adults, the skull, vertebrae, ribs, sternum, pelvic girdle, and the proximal epiphyses of the humerus and femur

Yellow bone marrow?

Fatty marrow that was red bone marrow but no longer produces blood cells

Bone is formed through?

Ossification (osteogenesis)

The embryo skeleton is initially in the what?

General shape of bones and is made of → mesenchyme (embryonic tissue)

Cartilage formation and ossification start in the 6th week of development

Intramembranous ossification?

Bone forms within mesenchyme in sheet-like layers

Forms flat bones in the skull, facial bones, mandible, and part of the clavicle

Also forms the “soft spots” (fontanel) that help the

fetus pass through the birth canal

Mesenchyme is a what?

An embryonic tissue which develops into connective and skeletal tissues, including blood and lymph

Endochondral ossification?

Bone forms within → hyaline cartilage

This also starts at 6 weeks of development and continues into your 20s (stops sometime between ages 14-24...)

Cartilage forms what?

A bone-like structure, called a → cartilage model

Bone will initially form in the what?

In the diaphysis then → hallow out

Forms the primary ossification center and primary marrow cavity

Bone will then develop in what?

In proximal epiphysis then → hallow out

Forms the secondary ossification center and secondary marrow cavity

Epiphyseal plate?

Cartilage left over from endochondral ossification is → turned to bone

Functions as growth zone where bone → elongates

Has hyaline cartilage in the middle with transition zones on each side where cartilage is replaced by bone

Metaphysis is zone of transition facing the marrow cavity

Interstitial growth?

Growth from within

Bone elongation is a result of cartilage growth within the epiphyseal plate

Epiphyses close when cartilage is gone, epiphyseal line of spongy bone marks site of former epiphyseal plate

Lengthwise growth is → finished

Occurs at different ages in different bones (14-24)

Appositional growth?

Occurs at bone → surface

Continual growth in diameter and thickness

Similar to intramembranous ossification

Osteoblasts (bone builders) of inner periosteum deposit?

Osteoid tissue (prebone)

Become trapped as tissue calcifies and become osteocytes

Osteocytes lay down what?

Matrix in layers parallel to surface

Forms circumferential lamellae

Osteoclasts (bone carvers) of endosteum enlarge marrow cavity

Ossification continues throughout life with what?

The growth and remodeling of bones

Bone remodeling?

Replacement of old bone with new bone tissue

Includes bone reabsorption – minerals and collagen fibers are removed by → osteoclasts

Followed by bone deposition – the addition of minerals and collagen fibers by → osteoblasts

Mineral deposition (mineralization)?

Occurs when calcium, phosphate, and other ions are taken from blood and deposited in → bone

Osteoblasts produce collagen fibers that spiral the length of the osteon

Fibers become encrusted with minerals

Mineral resorption?

Process of dissolving bone and releasing minerals into blood

Performed by → osteoclasts

Dissolve bone and digest collagen

Calcium homeostasis depends on what?

A balance between dietary intake, urinary and fecal losses, and exchanges between osseous tissue

Calcium homeostasis is regulated by three hormones:

Calcitriol, calcitonin, and parathyroid hormone

What is Calcitriol?

A hormone that raises blood calcium level

Is also necessary for bone resorption, helping provide calcium and phosphate

Calcitonin?

Secreted by the thyroid gland when blood calcium levels rise too → high

Lowers blood calcium concentration in two ways:

Inhibits osteoclasts thereby reducing bone resorption

Stimulates osteoblasts to deposit calcium into bone

Parathyroid hormone (PTH)?

Secreted by parathyroid glands

PTH released when calcium levels are low in blood

Acts to raise blood calcium levels

Causes bone → resorption

Osteoporosis?

Porous bone condition

Bone resorption (breakdown) out does bone deposition (formation)

Mostly caused by calcium depletion in the body

Fracture?

Is a break in the bone

Formation of hematoma and granulation tissue?

Blood vessels rupture across the fracture line and stimulate → inflammation

A fracture hematoma – blood clots in a mass (6-8 hours)

Clot is converted to granulation tissue, a soft fibrous mass

Bone cells die around the fracture due to lack of circulation

Causes more inflammation and → swelling

Phagocytes (part of our immune cells) and osteoclasts remove dead or damaged tissue around the hematoma (several weeks)

Formation of a soft callus?

Soft callus - Osteogenic cells become chondroblasts (cartilage builder) that produce patch of fibrocartilage and fibroblasts depositing collagen in granulation tissue

Conversion to hard callus?

Osteogenic cells differentiate into osteoblasts, which produces a bony collar called the hard callus around the fracture

Acts as a temporary splint around the fracture to join the two broken ends together

Takes 4-6 weeks to form

Remodeling?

Hard callus lasts for 3-4 → months

Osteoclasts dissolve small fragments of bone

Osteoblasts deposit spongy bone between the broken ends

Spongy bone gradually fills to become compact bone (like intermembranous ossification)