Bones and Skeletal Tissue: Anatomy and Physiology Chapter 6

BIOL251 Binghamton University

Chondr-

“cartilage”

Osteo-

“bone”

What are the functions of bones?

Support, protection, attachment point, storage, blood cell formation, and hormone production

How do bones support the body?

They hold up the body and cradles organs

How do bones protect the body?

They protect the central nervous system and visceral organs

How do bones attach to one another?

Skeletal muscle attaches to them via tendons

What is the purpose of tendons?

They are a cord-like band that attaches bones together with skeletal muscle tissue and allows them to move.

They are necessary for movement.

What is stored in bones?

Minerals, fat, and yellow marrow (in adults)

Hematopoiesis

Formation of blood cells in red bone marrow

(Blood cell formation in bones)

Osteocalcin

Regulates insulin release, glucose homeostasis, and energy expenditure/use

(Hormone production in bones)

What is the embryonic skeleton made of before the osseous skeleton forms?

Cartilage and connective tissue fibers

Characteristics of all cartilage types

Strong and resilient

Surrounded externally by perichondrium

Perichondrium

Fibrous connective tissue containing blood vessels for cartilage tissue that resists excessive outward expansion of cartilage

What are the three types of cartilage?

Hyaline cartilage

Elastic cartilage

Fibrocartilage

Hyaline cartilage

Most abundant cartilage type

Contains collagen fibers

Elastic cartilage

Cartilage type containing more elastic fibers

Fibrocartilage

Cartilage type containing rows of chondrocytes alternating with thick collagen bands (most of any type), making it very compressible and hard to do any damage to

What are the types of cartilage growth?

Appositional growth

Interstitial growth

Appositional growth

Laying down new cartilage on top of old cartilage, occurring at the surface of cartilage tissue causing it to increase in width/thickness

Interstitial growth

Cells divide and secrete matrix within pre-existing cartilage, occurring deeper in the cartilage tissue causing it to increase in length

Axial skeleton

Long axis of body

Which bones are located in the axial skeleton?

Skull, vertebral column, and ribs

Appendicular skeleton

Limbs (aka appendages) and the girdles

What bones are located in the appendicular skeleton?

Pectoral and pelvic girdles, arm and leg bones

What are the different shapes of bones?

Long bones, short bones, flat bones, irregular bones, and sesamoid bones

Long bones

Longer than they are wide

Short bones

cube-shaped

Sesamoid bones

bones that form in a tendon

ex: patella

Flat bones

thin, flat, curved

ex: ribs

Irregular bones

anything that does not fit in any of the other bone shape types

ex: vertebrae, os coxa

Compact bone

Outer layer of a bone that looks smooth and solid

Spongy bone

Inner layer of a bone that has open spaces with needle-like pieces of bone called trabeculae

Open space is filled with red or yellow marrow

Where is trabeculae mostly found?

Along lines of stress in spongy bone to help transmit pressure to resist mechanical stress and pushing to prevent bones from breaking easily

Structure of flat, irregular, and short bone

Thin plate of spongy bone covered by compact bone

No large cavities for bone marrow (all bone marrow found around trabeculae)

What features make up the structure of long bones?

Diaphysis

Epiphysis

Membranes

Vascularization and innervation

Diaphysis

bone shaft making up most of the length of the bones composed of compact bone “collar” with internal medullary cavity containing bone marrow

Epiphysis

Bone ends that form joints and are filled with spongy bone and covered with hyaline/articular cartilage. They are composed of compact bone externally and spongy bone internally.

What is the function of the epiphysis?

Location where one bone articulates with another bone at a joint and/or allows for attachment of ligaments and tendons

What membranes make up the structure of long bone?

Periosteum and endosteum

Periosteum in long bone

Double layered membrane covering external bone surface except at epiphysis

Well vascularized

Endosteum in long bone

Covers internal bone surfaces

How is the diaphysis vascularized?

The nutrient artery and nutrient vein

How is the epiphysis vascularized?

The epiphyseal artery and epiphyseal vein

How is long bone innervated?

Nerves travel with blood vessels

Red bone marrow

Produces blood cells

Hematopoietic tissue

Where is red bone marrow found in the skeleton of an infant?

Everywhere to support rapid growth and high blood cell production

Where is red bone marrow found in the skeleton of adults?

skull, ribs, hips, sternum, clavicle, scapula, vertebrae, heads of femur, and humerus

Yellow marrow

Is located in the medullary cavity of long bones in adults and contains more fat and less blood supply than red marrow

When is yellow marrow converted back to red marrow?

When a person is short on red blood cells (low on blood supply and/or oxygen)

Organic chemical composition of bone

Cells and osteoid

Inorganic chemical composition of bone

Mineral salts → mostly calcium phosphate salts packed around collagen fibers

How do mineral salts affect bone tissue?

Gives hardness and resistance to breakdown

What are the 4 primary cell types responsible for bone growth?

Osteoprogenitor (osteogenic) cells

Osteoblasts

Osteocytes

Osteoclasts

Osteoprogenitor (osteogenic) cells

Mitotically active cells that can remain as osteogenic cells or differentiate to form osteoblasts

What makes osteoprogenitor cells important throughout life?

These stems cells are foundational for making sure bone is continuously replenished

Osteoblasts

Bone-forming cells that secrete unmineralized extracellular matrix (osteoid) that forms bone tissue and secrete matrix until surrounded

Unmineralized

Organic bone tissue that has not yet had mineral crystals deposited into it

What happens to an osteoblast once it is surrounded by matrix?

It is converted to an osteocyte

Osteocytes

Mature bone cell that monitors and maintains the bone matrix

What do osteocytes respond to?

Mechanical stress on bone and chemical signals

Osteoclasts

Secrete collagenase to break down collagen in the bone matrix. They are bone-degrading cells that maintain, repair, and remodel bones and play an important role in blood calcium homeostasis.

What structures make up the microscopic anatomy of compact bone?

osteons than contain a central canal and circumferential lamellae that attach osteons together

Osteon

structural unit of compact bone to help bone withstand pressure and stress

Lamella

concentric rings inside a single osteon packed closely together with calcium salts found on and in between fibers

Collagen fibers in a single lamella

Run in one direction

Collagen fibers in adjacent lamella

Run in opposite directions

Lamella can twist against each other

Central canals

Open space that runs through the center of each osteon containing nerve and blood vessels

Perforating canals…

Extending from central canal and connect neighboring osteons and medullary cavity

Circumferential lamellae

Wrap around osteons to form outermost layer of bone by holding the osteons together

Endochondral ossification

Bone formation within cartilage

Uses hyaline cartilage as a “blueprint” to form ossified bone by breaking down cartilage and replacing it with ossified bone

What is the early embryonic skeleton made up of?

Cartilage

Where does endochondral ossification happen?

In most bones below the skull

Step 1 of Endochondral Ossification

Formation of a POC and bone collar

Osteoblasts lay down bone against cartilage diaphysis to form a collar

Formation of primary ossification center in the diaphysis center

POC is rigid and tough on the outside

Step 2 of Endochondral Ossification

Cavity forms in diaphysis center

Cartilage cells inside primary ossification center die off because cartilage tissue inside POC breaks down

Cartilage outside cavity continues to grow → elongates bone

Step 3 of Endochondral Ossification

Formation of initial spongy bone in diaphysis

Periosteal bud invades cavity

Osteoblasts secrete bone matrix inside cavity

(initial formation of spongy bone is bone-covered cartilage trabeculae)

Periosteal bud

Contains nutrient artery/vein, nerve fibers, red marrow elements, osteoprogenitor cells than differentiate to form osteoblasts and lay down bone, osteoclasts

Step 4 of Endochondral Ossification

Formation of medullary cavity and elongation of diaphysis

Spongy bone from step 3 broken down by osteoclasts to form the medullary cabity

Cartilage in remaining diaphysis is calcified, broken down, and replaced with bone

Secondary ossification center forms in epiphyses

Step 5 of Endochondral Ossification

Secondary ossification continues in epiphyses

Similar to primary ossification except spongy bone is retained in bone ends → no medullary cavity formed

There are still some areas with cartilage, but most of the skeleton is ossified by birth

Bone growth in length

accomplished by interstitial growth

Interstitial growth

Occurs at epiphyseal plate found between the epiphysis and diaphysis of a bone

Overall process of growth in length

1. New cartilage laid down in epiphyseal plate

2. Cartilage is calcified at center of plate

3. Calcified cartilage is broken down by osteoclasts and osteoblasts lay down new bone tissue

Growth in width

accomplished by appositional growth

Appositional growth

Osteoblasts secrete new matrix at the periosteum and osteoclasts break down bone tissue at the endosteum

What difference in activity rate between the cells must there be for growth in width to occur?

Need to lay down more bone tissue than is being broken down

Bone growth continues from _________ through ___________.

infancy, adolescence

At the end of adolescence cartilage production in epiphyseal plate _____ and _____.

slows, stops

A person officially stops growing when…

the remaining cartilage in plate is calcified and replaced by bone and the bone of diaphysis fuses with bone of epiphysis

Growth hormone

controls activity at the epiphyseal plate to stimulate growth

What effect does growth hormone have on the epiphyseal plate?

Makes it lay down new cartilage

Sex hormones

Estrogen and testosterone

Estrogen

Causes growth spurt at puberty and “feminization” of certain parts of skeleton

In high levels, like it adults, it induces epiphyseal plate closure

What is the typical age at which a female stops growing?

17-18

Testosterone

Causes “masculinization” of certain parts of skeleton

What is the typical age at which a male stops growing?

21

Bone remodeling

Bone deposition and resorption to maintain Ca²⁺ homeostasis and bone health

Ca²⁺ is essential for excitability of body cells

How does bone remodeling function to improve bone health?

Mechanical and gravitational forces acting on bone tissue drive remodeling → strengthens bone exactly where it is needed

Deposition

Lay down new bone tissue

Resorption

Getting rid of old bone tissue

What are the factors involved in the control of deposition and resorption?

Release of parathyroid hormone and mechanical stress

When is the parathyroid hormone (PTH) released?

In response to decreasing blood Ca²⁺ levels

The hormone increases the number of osteoclasts at the bone and osteoclasts become more active in bone tissue

Once blood Ca²⁺ returns to normal, hormone release decreases along with the osteoclast number and rate of activity