DAT Bootcamp Biology 11.6 Skeletal System:

An exoskeleton is an...

external skeleton

Many

invertebrates and all arthropods possess

exoskeletons.

Vertebrates contain an endoskeleton on the

inside.

An endoskeleton can be divided into the

axial

skeleton (central bones: skull, spinal column, and

ribcage) and the appendicular skeleton

(appendages).

Types of bones in the endoskeleton:

1.

Long bones - made of cortical bone (compact)

and pockets of cancellous bone (spongy).

Important features include the epiphysis,

diaphysis, medullary cavity, metaphysis, and

epiphyseal plate.

Epiphysis -

end of a long bone that forms

joints with other bones and contains red bone

marrow for hematopoiesis (blood cell

synthesis).

Diaphysis -

long hollow shaft in center of

bone.

Medullary cavity -

located within the

diaphysis and contains red and yellow bone

marrow (area of fat storage).

Metaphysis -

similar to epiphyses and found

between the medullary cavity and epiphyseal

plates.

Epiphyseal plate - "growth plate" located

between epiphysis and metaphysis. Made out

of hyaline cartilage and works to lengthen

the diaphysis through growth and ossification.

Short bones -

as wide as they are long and

mainly provide support (eg. parts of the wrist).

3. Flat bones -

mainly provide protection (eg. skull).

4. Sesamoid bones -

found within tendons to help

muscles pull (eg. kneecap).

5. Irregular bones -

irregularly shaped (eg. pelvis).

Cortical bone is the dense outer layer of bone that

supports the

weight of our bodies. It is composed of

many microstructures:

Osteons -

cortical bone's functional unit,

composed of tiny multi-layered cylinders. Also

known as haversian systems because they

contain a haversian canal in their center.

Haversian canals -

'tubes' that contain blood

vessels for nutrient supply.

Lamellae -

layers of the osteon.

Lacunae -

small spaces between lamellae that

hold bone cells and interconnect through

canaliculi.

Canaliculi -

small channels that connect lacunae

and the haversian canal.

Volkmann's canals -

connect Haversian canals to

the periosteum, which provides nutrients.

Cancellous bone is the

spongy inner layer of bone

that soaks up red bone marrow via a web of

trabeculae (connective tissue that supports

cancellous bone).

Bone remodeling is the process of going back and

forth between

the processes of ossification (bone

formation) and resorption (bone loss).

Types of cells involved in bone remodeling:

Osteoprogenitors - immature precursor cells that

differentiate into osteoblasts.

Osteoblasts - build bone by secreting proteins

and utilizing blood calcium. They mature into

osteocytes after getting trapped inside the bone

matrix they create.

Osteocytes - live in lacunae in osteons to maintain

bone.

Osteoclasts - eat and resorb bone, releasing

calcium and phosphate back into the blood.

Derived from monocytes.

Osteoprogenitors -

immature precursor cells that

differentiate into osteoblasts.

Osteoblasts -

build bone by secreting proteins

and utilizing blood calcium. They mature into

osteocytes after getting trapped inside the bone

matrix they create.

Osteocytes -

live in lacunae in osteons to maintain

bone.

Osteoclasts -

eat and resorb bone, releasing

calcium and phosphate back into the blood.

Derived from monocytes.

Mechanisms involved in bone remodeling:

Parathyroid hormone - increases blood calcium

levels by stimulating osteoclasts and depressing

osteoblasts. Secreted by the parathyroid gland.

Vitamin D - increases blood calcium levels by

raising intestinal calcium absorption. Activated by

parathyroid hormone, but provides negative

feedback on PTH production.

Calcitonin - decreases blood calcium levels by

depressing osteoclasts, allowing osteoblasts to

build bone without competition. Secreted by

parafollicular cells (C cells) of the thyroid gland.

Parathyroid hormone -

increases blood calcium

levels by stimulating osteoclasts and depressing

osteoblasts. Secreted by the parathyroid gland.

Vitamin D -

increases blood calcium levels by

raising intestinal calcium absorption. Activated by

parathyroid hormone, but provides negative

feedback on PTH production.

Calcitonin -

decreases blood calcium levels by

depressing osteoclasts, allowing osteoblasts to

build bone without competition. Secreted by

parafollicular cells (C cells) of the thyroid gland.

Mnemonic: CalciTONin =

"Tone down calcium"

Osteoid is the

organic component of bone containing

many proteins such as collagen (gives bone tensile

strength).

Hydroxyapatite is the inorganic mineral component

of bone that gives the bone density and strength.

Two types of embryonic ossification:

1.

2. Endochondral ossification - bone is created

indirectly through a cartilage model, mainly for

long bones. The cartilage model calcifies during

fetal development, creating ossification centers

that help form the features of long bones.

Intramembranous ossification - bone is created

directly within fibrous membranes, mainly for flat

bones. Osteoblasts start by secreting osteoid,

which hardens and houses osteocytes. Eventually,

cortical bone is created.

Two types of embryonic ossification:

1.

2.

Endochondral ossification - bone is created

indirectly through a cartilage model, mainly for

long bones. The cartilage model calcifies during

fetal development, creating ossification centers

that help form the features of long bones.

Types of connective tissue:

1.

2. Cartilage is avascular (lacks blood vessels) and is

not innervated (as opposed to bone which is

highly vascular and innervated).

Fibrous connective tissue has a matrix made up

of fibers. It is maintained and repaired by

fibroblasts.

● Tendons - connect muscle to bone.

● Ligaments - connect bone to bone.

● Periosteum - membrane that covers cortical

bone with an outer fibrous layer

(vascularized) and an inner/cambium layer

(collagen for attachment to cortical bone)

● Endosteum - membrane located between cortical and cancellous bone.

Types of connective tissue:

1.

2.

Cartilage is avascular (lacks blood vessels) and is

not innervated (as opposed to bone which is

highly vascular and innervated).

Chondroblasts build cartilage by secreting

collagen and elastin.

Hyaline cartilage -

slightly flexible and

important in providing support and stability

to joints.

Fibrous cartilage -

high rigidity and resists

tension, found in intervertebral discs and knee

meniscus.

Elastic cartilage -

highly flexible and found in

ears and epiglottis.

3. Joints are

vascularized and innervated. They are

found between bones. Below are types of joints.

Synarthroses -

dense, fibrous joints that do

not move.

Amphiarthroses -

cartilaginous joints that

partially move.

Diarthroses -

synovial joints that fully move.

Typically contain hyaline cartilage.

What uses an exoskeleton?

invertebrate animals and arthropods

What are exoskeletons composed of?

two protein layers

What are long bones made of?

cortical bone and cancellous bone

In what bones does hematopoiesis occur in?

long bones

What is hematopoiesis?

blood cell production

Describe the anatomy of long bones

• epiphysis - end of a long bone that forms joints with other bones, has red bone marrow for hematopoesis

• epiphyseal plate - region of cartilage between epiphysis and metapysis that can lengthen diaphysis through ossification

• metaphysis - between medullary cavity and epiphyseal palte

• diaphysis - long hollow shaft in center of bone, filled with yellow bone marrow in medullary cavity

What is the medullary cavity?

inside of diaphysis, has red and yellow bone marrow

What do red and yellow bone marrow do?

• red: produces stem cells capable of generating red and white blood cells

• yellow - produces stem cells that can generate fat, bone, cartilage, and muscle. can also store fat.

Where can red bone marrow be found?

diaphysis and epiphysis

What is the metaphysis?

area between the medullary cavity and epiphyseal plate; one on each end of bone

What is the epiphyseal plate?

region of cartilage between epiphysis and metaphysis that can lengthen diaphysis during ossification

What is cortical bone and what is it made of?

• dense, outer layer of bone that supports body weight (compact)

• made of osteons with cylindrical osteon layers called lamellae

• has haversian canals at the center of osteons with blood vessels for nutrient supply - run vertically

• has Volkmann's canals with blood vessels connecting Haversian systems to periosteum - run horizontaly

What are the haversian canals?

• tubes at the center of osteons with blood vessels for nutrient supply

• in cortical bone

• run vertically

What are Volkmann’s canals?

• channels with blood vessels connecting Haversian systems to periosteum and bone cells outside of cetner of osteons

• in cortical bone osteons

• run horizontally

What are the layers around the cortical bone?

• periosteum - external sheath surronding cortical bone that provides nutrients for growth/healing

• endosteum - internal sheath linin the internal surface of cortical bone

What provides nutrients to bone cells outside the center of osteons and periosteum?

volkmann’s canals

What lines the internal surface of cortical bone?

endosteum

What surronds cortical bone and provides nutrients? What brings it nutrients?

• periosteum

• nutrinets brought by volkmann’s canals

What is cancellous bone?

• spongy, inner layer of bones

• soaks up red bone marrow via trabeculae

What is trabeculae?

branching structures made of connective tissue that support the cancellous bone

What is bone remodeling?

• process of alternating between ossification (bone formation) and resorption (bone loss)

• old bone replaced with new bone

What are the cells involved in bone remodeling?

• osteoblasts - bone building

• osteocytes - bone maintenance

• osteoclasts - bone breakdown

What are osteoblasts?

• bone building cells

• secrete proteins and utilize calcium from blood

• mature into osteocytes

What are osteocytes?

• mature osteoblasts trapped within bony matrix of osteons

• maintain bone

What are osteoclasts?

• bone degrading cells

• eat and reabsorb bone by releasing enzymes and decreasing pH

• release Ca and phosphate back into blood

What are the organic and inorganic components of bones?

• organic - osteoid

• inorganic - hydroxyapatite

What is osteoid?

• bone’s organic component

• proteins, collagen fibers

• gives strength and flexibility

What is hydroxyapatite?

• bone’s inorganic component

• imbedded with osteoid

• gives hardness and density

When blood calcium is low, what happens to bones?

broken down (bone resorption) to release calcium

What hormone increases calcium in the blood?

parathyroid hormone

What does parathyroid hormone do?

• increase Ca in the bloodsteam

• stimulates osteoclasts to break down bone, increasing bone resorption and releasing ca

  • increases reabsorption of calcium in the kidney, decreasing Ca excretion in urine

How does PTH affect the kidney?

increase reabsorption of Ca at the kidney, decreasing Ca exertion in urine

What does calcitonin do?

• decreases ca in bloodstream

• inhibits osteoclast activity, decreasing bone resorption to allow ca to remain in blood

• decreases reabsorption of ca in the kidneys, increasing Ca exretion in the urine

• “tones down” calcium in blood

What gland causes secretion of calcitonin?

thyroid gland

What are the models of embryonic ossificaton?

intramembranous and endochondral

Describe intramembranous ossification

• direct bone formation from sheets of embryonic connective tissue without first forming cartilage

• osteoblasts develop osteoid, which calcifies and forms cortical bone

• occurs mostly in flat bones, like skull

Describe endochondral ossification

• when bone forms by replacing hyaline cartilage

• cartilage forms, then osteoid is deposited at ossification centers, gradually replacing cartilage at the diaphysis then epiphysis

• osteoid calcifies and forms cortical bone

• most bones use this, including long bones and ribs

Long bones are formed through what type of ossification

endochrondral

What is found in the epiphysis of a long bone?

spongy bone with red bone marrow