Chapter 7 Skeletal System Components Lecture

steps of endochondral ossification

1. Starts in the hyaline cartilage with chondroblasts secreting a matrix. The chondroblasts become chondrocytes in lacunae, and perichondrium surrounds the cartilage. 2. The chondrocytes begin to hypertrophy in the future diaphysis of the future bone. They resorb some of the original matrix which makes large holes. 3. The cartilage matrix begins to calcify and the condrocytes die because blood supply to the matrix is cut off. 4. Blood vessels grow toward and into the cartilage. Stem cells in the perichondrium create osteoblasts. 5. Perichondrium becomes periosteum. 6. The osteoblasts lay down osteoid matrix in the calcified cartilage. 7. The osteoid hardens and forms a periosteal bone collar around the shaft. 8. Primary ossification center happens 9. secondary ossification center happens.

Steps of intramembranous ossification

1. This process starts in the mesenchyme with osteoprogenitor cells. Some of these osteoprogenitor cells become osteoblasts and begin to secrete osteoid to create a matrix. 2. Salts and minerals come in during calcification, mineralization, and deposit on the osteoid. When the osteoblasts get trapped in the lacuna in their own matrix, they become osteocytes. 3. Woven bone is formed (primary bone) and replaced by secondary bone (lamellar bone). 4. The mesenchyme surrounding the woven bone thickens to form the periosteum that surrounds a bone. Blood vessels continue to branch through the region. 5. Calcified trabeculae and intertrabecular spaces are made of spongy bone. 6. Lamellar bone replaces trabeculae of woven bone. Osteoblasts fill in the spaces of trabeculae on the outside to make compact bone.

scurvy

A Vitamin C deficiency. Vitamin C is needed to make collagen proteins. This entails open wounds that won't heal. The fibers can't be made that are needed during tissue repair. There is not enough organic portion of the bone, so bones are brittle.

rickets

A Vitamin D deficiency in children. There are not enough minerals to make the inorganic parts of bone. There will be more organic than inorganic tissue, and this makes the bones soft. This happens because kids can't go outside because of the weather.

hydroxyapatite

A combination of calcium phosphate and calcium hydroxide found in inorganic composition of bone matrix.

ossification

Also called osteogenesis. This refers to the formation and development of bone connective tissue. It beings in the embryo and continues as the skeleton grows during childhood and teenage years.

woven bone

Also called primary bone - this is a type of newly formed immature bone that is formed first during the process of intramembranous ossification.

lamellar bone

Also called secondary bone. It replaces woven bone during intramembranous ossification.

Differences between appositional growth of cartilage and interstitial growth

Appositional growth of cartilage is growth in width. it happens in the perichondrium and you start with stem cells differentiating into chondroblasts. Interstitial growth is growth in length of cartilage. You start with chondrocytes that differentiate into chondroblasts.

How is epiphyseal line formed?

At a certain point, the rate of bone growth overcomes the rate of cartilage growth, and bone takes over.

Articular and epiphyseal cartilage

By late bone development during endochondral ossification, all hyaline cartilage is replaced by bone except for the articular surfaces and the epiphyseal plates.

Appositional growth in bone

Growth in bone width. It happens in the periosteum of the bone. Osteoblasts along the inner cellular layer of periosteum deposit bone matrix. This forms circumferential lamellae. As these increase, the bone gets thicker. At the same time, osteoclasts in the medullary cavity resorb bone matrix and expand the medullary cavity.

interstitial growth of cartilage

Growth in length of cartilage. This occurs in the matrix of the tissue - happens internal regions of cartilage. Starts with chondrocytes in lacunae. They undergo mitosis. Now, there is one chondroblast and one chondrocyte in a lacuna. The chondroblast begins to secrete cartilage matrix - this pushes the two cells apart. They each have their own lacuna. Now, they are called chondrocytes. Cartilage will continue to grow.

Bone remodeling

Happens throughout a person's life. The constant, dynamic process of continual addition of new bone tissue (bone deposition) and bone resoprtion. It happens at the periosteal and endosteal surfaces of a bone.

epiphyseal line

Interstitial bone growth continues until puberty when this replaces the epiphyseal plates. This happens between ages 10 and 25.

cartilage growth

Some types of bone growth rely on this. It begins during embryologic development. It can grow two ways: interstitial growth and appositional growth.

Fracture healing

Step 1: A fracture hematoma forms - bleeding inside the bone. Step 2: A soft callus forms - increased osteoblasts production in periosteum and endosteum. Chondroblasts form cartilage in the bone. Step 3: Osteoprogenitor cells create osteoblasts and produce trabeculae of primary bone. Step 4: Bone is remodeled.

hyaline cartilage

The chondrocytes make up the matrix of the bone - this helps to maintain this type of cartilage. This is surrounded by the perichondrium which has stems cells in it. The stem cells differentiate into chrondroblasts - these release proteins that will make up the matrix of cartilage. The chondroblasts make the chondrocytes as they create matrix.

zone of hypertrophic cartilage

The chondrocytes stop dividing and begin to hypertrophy (get bigger). The lacunae walls thin on - the chondrocytes resorb matrix.

endochondral ossification

The process of turning cartilage into bone. It builds all the bones of the body that intramembranous ossification does not. It happens in the 8-12th week of embryonic development. It starts with hyaline cartilage.

zone of ossification

The walls break down in this final step of interstitial growth of bone in the epiphyseal plates. Osteoprogenitor cells and capillaries arrive from the medullary cavity. New bone matrix is deposited on the calcified cartilage.

zone of calcified cartilage

There are 2-3 layers of chondrocytes in this zone. The minerals are deposited in the matrix between the lacunae columns. The chondrocytes are destroyed and the matrix looks opaque.

calcium homeostasis

There are two hormones involved in this. Parathyroid hormone and calcitonin.

How do osteoclasts resorb bone?

There are two ways osteoclasts do this: the first is to release hydrogen ions which makes the solution more acidic and this breaks down the bone matrix. The second way is to release proteolytic enzymes - these can break down proteins.

circumferential lamellae

These are formed as the osteoblasts in the inner cellular layer of the periosteum produce and deposit bone matrix within layers parallel to the surface of the bone. They look like tree rings.

osteoblasts

These are formed from osteoprogenitor cells in the endosteum and periosteum of bone. They have a cuboidal shape and lots of endoplasmic reticulum and Golgi Apparatus. They synthesize and secrete osteoid. This will later calcify as a result of salt crystal deposition. These become trapped in the matrix and later become osteocytes.

osteoclasts

These are large, multinucleated phagocytic cells. They come from fused bone marrow cells/ precursors in bone marrow. Do not come from osteoprogenitor cells. It is located within or next to a pit on the bone surface called a resorption lacuna. Their function is to resorb bone/break down.

osteocytes

These are mature bone cells that come from osteoblasts. They have lost their bone-forming ability because they have been enveloped by calcified osteoid. They work to maintain the bone matrix and detect mechanical stress on a bone. If there is stress, they contact osteoblasts who may deposit new bone matrix. They keep the bone matrix healthy.

osteoprogenitor cells

These are stem cells derived from mesenchyme. They divide through mitosis. They differentiate into osteoblasts. These are located in the periosteum and endosteum.

When do appositional and interstitial growth of cartilage typically occur?

These happen during embryological development simultaneously. Interstitial growth declines as the cartilage becomes semirigid. Later growth is appositional because it can only happen at the periphery of the tissue.

gucocorticoids

These hormones can impede growth. An example would be if a child suffers from asthma and has too much cortisone. The body already makes this in the adrenal gland.

sex hormones

These include testosterone and estrogen. They start at puberty and will cause an increase in the rate of bone growth on the diaphysal end of the epiphyseal plate.

Parathyroid hormone

This comes from the parathyroid glands . When there is decreased Ca2+ levels, the calcitriol is activated and acts on the bone and kidneys. Osteoclast activity increases in the bone which breaks them down and releases calcium. In the kidneys, calcium is released in urine. Calcitriol by itself will increase calcium absorption in small intestine.

primary ossification center

This forms in the diaphysis during endochondral ossification. Capillaries and osteoblasts extend from the periosteum into the core of the cartilage shaft and the osteoblasts begin to produce osteoid. This is the first major center of bone formation. Bone extends toward epiphyses. Healthy bone connective tissue displaces calcified cartilage. This forms by 12th week of development.

secondary ossification center

This forms in the epiphysis during endochondral ossification. This occurs at the time of birth. The hyaline cartilage in the epiphysis calcifies and degenerates. Epiphyseal blood vessels and osteoprogeniter cells enter the epiphysis. This term forms as bone displaces the calcified cartilage. At the same time, osteoclasts in the diaphysis resorb some bone matrix and create the medullary cavity.

intramembranous ossification

This happens in the 8th to 12th week of development of the embryo. This is bone growth within a membrane. It starts with mesenchyme and we end up with bone. This type produces the flat bones of the skull, some of the facial bones, the mandible, and the central part of the clavicle.

serotonin

This hormone can also impede osteoblastic activity. It can stop stem cells from differentiating into osteoblasts.

Interstitial growth of bone

This is bone growth in the epiphyseal plate. It is dependent on growth of cartilage in the epiphyseal plate. There are five distinct microscopic zones. The first zone is closest to the epiphysis and the last zone is closest to the diaphysis.

appositional growth of cartilage

This is growth in width of cartilage. This occurs in the perichondrium. This starts with a stem cell. These cells differentiate into chondroblasts. They are located at the periphery of old cartilage. They begin to secrete new matrix. They push apart and become chondrocytes. The cartilage grows at the periphery.

mechanical stress

This is one of the facts that affects the activities of bone remodeling osteoblasts, osteocytes, and osteoclasts. This happens in the form of weigh-bearing movement and exercise. It is required for normal remodeling. Osteocytes detect stress and tell osteoblasts. Osteoblasts increase synthesis of osteoid, and then mineralization happens. More mechanical stress (exercise) strengthens bones. Lack of, weakens bones.

zone of proliferating cartilage

This is the second zone. Making more chondrocytes. The chondrocytes begin to undergo mitotic division, enlarge, and line up like coin stacks of flattened lacunae. In the picture, it's second to the top.

resorption lacuna

This is where osteoclasts are located on a bone.

organic composition of bone matrix

This makes up 35% of the bone matrix. This is made of osteoid, which is made from osteoblasts. It's made of collagen protein plus a semisolid ground substance called proteoglycans and glycoproteins. These support the collagen fibers. The organic bone gives bone more flexibility.

inorganic composition of bone matrix

This makes up 65% of the bone matrix. This is made up of salt crystals - minerals stored in bone- like calcium phosphate and calcium hydroxide. These create hydroxyapatite. The crystals harden the matrix and create bone strength.

zone of resting cartilage

This zone is closest to the epiphysis and furthest from the diaphysis. It secures the epiphysis to the epiphyseal plate. It looks like normal hyaline cartilage. It is made of small chondrocytes. (Zone 1)

bone formation

When osteoblasts secrete an initial semisolid organic form of bone matrix called osteoid. Then, calcification, or mineralization, happens when hydroxyapatite crystals deposit in bone matrix. This process requires Vitamin D, Vitamin C, calcium, and phosphate.

compound fracture

bone breaks and comes through skin

osseus

bone connective tissue. The primary component of bone. It is composed of cells and cextracellular matrix.

bmu

bone multicellular unit - group of osteoblast a nd osteoclasts that are formed to remodel bone. It is a temporary formation that lasts for about 6 months. They break down old bone and replace with new. We get a new skeleton every 10 years. 20% of the adult human skeleton is replaced each year. Compact bone takes a longer time to replace than spongy bone.

pathologic fracture

break as a result from a disease or disorder (like cancer). Ewing's sarcoma and osteosarcoma.

Simple fracture

break without breaking through skin

osteomalacia

rickets in adults - a Vitamin D deficiency. This happens because of a digestive system disorder that won't allow adults to absorb calcium.

Growth hormone

stimulates osteoblasts in bone to stimulate bone growth

osteoid

the initial semisolid organic form of bone matrix that osteoblasts produce.

bone resorption

the process by which bone matrix is destroyed by substances released from osteoclasts into extracellular space adjacent to bone. This may occur when blood calcium levels are low because calcium and phosphate ions are released and enter the blood as the bone breaks down.

calcitonin

this comes from the thyroid gland. the stimulus is if there are increased calcium levels. In bones, osteoclast activity is decreased. In the kidneys, there is an increase in the calcium released in urine.

thyroid hormone

this stimulates bone growth by stimulating the osteoblasts. It has a big affect on interstitial bone growth