Skeletal System

Prolonged exposure to reduced gravity environments can cause

bone loss, increased loss of bone minerals, increased chances for renal stones and is a factor in possible post mission bone fractures.

Bone Function

o Provides support.

o Protects the internal organs.

o Assists body movements (in conjunction with muscles)

o Mineral Homeostasis

o Participates in blood cell production in the epiphysis (hemopoiesis).

o Stores triglycerides in adipose cells of yellow marrow (within shaft).

mineral homeostasis

stores and releases calcium and phosphorous on demand

§ Bones are reservoir for these two things.

triglycerides in adipose cells of yellow marrow serve as a

potential energy reserve

(seen in long bone)

epiphysis

o Proximal and distal ends of long bones.

o Where spongy bone is.

o Where articulation of other bones occur.

diaphysis

o The long, cylindrical, main portion of the bone

o Shaft or body

o Where it is hollow inside surrounded by dense compact bone.

metaphyses

o The regions between the diaphysis and the epiphyses.

Epiphysial Growth Plate vs Epiphysial Line

Epiphysial Growth Plate: a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length

Epiphysial Line: When a bone ceases to grow in length at about ages 14-24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure

medullary cavity

o A hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels.

o This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed.

endosteum

o A thin membrane that lines the medullary cavity and the internal spaces of spongy bone. It contains a single layer of osteoprogenitor cells and a small amount of connective tissue.

periosteum

o Tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of osteoprogenitor cells.

- outside layer of the bone

articular cartilage

covers the surfaces of bones where they come together to form joints

- allows smoother motion and not bone on bone

bone contains

an abundant extracellular matrix that surrounds widely separated cells.

what percentage of water makes up the ECM?

15%

what percentage of collagen makes up the ECM?

30%

what percentage of crystalized minerals makes up the ECM?

55%

collagen

·Makes bone more flexible since the crystalized mineral salts itself are hard and brittle.

o Allows tensile pulling.

bone contains 4 types of cells:

osteoprogenitor cells

osteoblasts osteocytes

osteoclasts

osteoprogenitor cells

bone stem cells able to differentiate into the other types of cells.

osteoblasts

bone-building cells that secrete matrix.

§ Build matrix.

- pulls calcium and phosphate out of blood vessels

osteocytes

mature bone cells

§ Maintain and monitor extracellular matrix.

osteoclasts

o remodel bones and cause them to release calcium.

§ Carve out/break down.

§ Come from white blood cell line.

· About 50 monocytes.

§ Line the endosteum.

§ Have finger like projections

why do osteoclasts have hair like projections?

increases surface area of the cell which helps osteoclasts release acids to break down bone.

compact bone

o Protection and support.

§ Weight bearing (Ex. Femur)

o Surrounds medullary cavity.

spongy (cancellous) bone

o Lightweight

o Tissue support

o Found in epiphysial ends of long bones.

design of the Eiffel Tower was based on

an analysis of how the femur bone supports the weight of the body.

trabeculae of spongy bone

o Provides support while remaining lightweight.

o Consists of bone lamellae that are arranged in an irregular pattern of thin columns.

o These macroscopic spaces are lined by endosteum and filled with red bone marrow in bones that produce blood cells, and yellow bone marrow (adipose tissue) in other bones.

red and yellow bone marrow contain

numerous small blood vessels that provide nourishment to the osteocytes.

why is it dangerous when the brain bleeds?

skull is a closed box that does not articulate with other bones besides at the atlanto-occipital joint. when an individual is hemorrhaging they do not realize until it's severe and symptoms show up. at this time the pressure herniates the brain through the foramen magnum. only way to stop the pressure from building is to make a hole in the skull.

osteon

mature bone unit

perforating (sharpey's) fibers

secure the periosteum to underlying bone

nutrient arteries

the main blood vessel that supplies nutrients to a bone.

canals and canaliculi

§ Central (Haversian) canal runs through core of osteon.

· Contains blood vessels and nerve fibers.

- up and down (vertical)

Perforating (Volkmann's) canals

§ Lined with endosteum at right angles to central canal.

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

- side to side (horizontal)

lacunae

o Small cavities contain osteocytes.

§ Between the concentric bone lamellae are small spaces.

canalicuili

Hairlike canals, connect lacunae to each other and central canal.

periosteum pt.2

two parts: Outer fibrous layer/ Inner osteogenic layer

lamellae

Concentric rings made up of groups of hollow tubes of bone matrix

o Circular plates of mineralized extracellular matrix of increasing diameter, surrounding a small network of blood vessels and nerves located in the osteonic canal.

- made of collagen that runs in different directions

- increase surface area

why do the osteons run parallel to each other?

makes it highly resistant to twisting forces

Osteon (Haversian) Canal Structures

o Artery with capillaries, vein, and nerve fiber.

where are osteoblasts and osteoclasts found?

outside in the trabeculae/along the trabeculae

Blood and Nerve Supply of Bone

o Maintain tissue health.

o Live tissue needs blood for nutrients.

o Nerves to send messages/sensory nerve fibers

osteogenesis

new bone formation

ossification

the process of replacing other tissue with bone.

calcification

the process of depositing calcium salts (occurs during ossification).

resorption

when osteoclasts break down and remove bone.

when does osteogenesis begin?

Begins in the 2nd month of development.

osteogenesis: postnatal bone growth

§ Until early adulthood (age 25)

· Interstitial

· Appositional

interstitial

the cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix.

appositional

growth of the cartilage in thickness due to more extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium.

- the process by which bones increase in diameter, or width, through the addition of new bone tissue at their surface

osteogenesis: bone remodeling and repair

§ Lifelong

· Regulated mechanically and hormonally.

endochondral ossification

§ Formation of bone on a temporary cartilage/scaffold.

§ Hyalin cartilage -> endochondral (cartilage) bones.

§ Forms most of skeleton.

§ 2nd month of development.

intramembranous ossification

§ Formation of bone directly onto fibrous connective tissue.

§ Fibrous membrane -> membrane bones

§ Forms flat bones (ex. Clavicles and cranial bones)

· Ribs

· Pelvic girdle bones.

Intramembranous ossification occurs in flat bones when

- a connective tissue membrane is replaced by bone.

o Adolescent growth spurt.

Intramembranous ossification 1: ossification center

development of ossification center; osteoblasts secrete organic extracellular matrix.

Intramembranous ossification 2: calcification

calcium + other mineral salts deposited, + ECM calcifies (hardens)

Intramembranous ossification 3: formation of trabeculae

ECM develops into trabeculae that fuse to form spongy bone.

Intramembranous ossification 4: development of the periosteum

mesenchyme at the periphery of the bone develops into periosteum.

Endochondral ossification replaces

- cartilage with bone in the developing embryo and fetus.

endochondral ossification 1: Development of cartilage model

- Mesenchymal (undifferentiated) cells develop into chondroblasts, which form the cartilage model.

endochondral ossification 2: Growth of cartilage model

- Growth occurs by cells division of chondrocytes.

endochondral ossification 3: Development of primary ossification center

- In this region of the diaphysis, bone tissue replaces most of the cartilage.

endochondral ossification 4: Development of the medullary (marrow) cavity

- Bone breakdown by osteoclasts forms the medullary cavity.

endochondral ossification 5: Development of secondary ossification centers

- These occur in the epiphysis of the bone.

- Makes spongy bone

endochondral ossification 6: Formation of articular cartilage and epiphyseal plate

- Both structures consist of hyalin cartilage.

similarities between primary and secondary ossification

o In endochondral bone (bone that starts off as cartilage)

o Triggered by penetrating blood supply (nutrient artery)

interstitial growth

- Lengthening

o Postnatal long bone elongation.

§ Requires epiphyseal cartilage.

§ Epiphyseal plate maintains constant thickness.

· Rate of cartilage growth on one side balanced by bone replacement on other.

§ Concurrent remodeling of epiphyseal ends maintain proportion.

epiphyseal plate

o Site of Post-natal Long Bone Elongation

o Cartilage replaced by bone in growing skeleton.

interstitial growth step 1: resting zone of epiphyseal plate

anchors growth plate to bone of epiphysis

- when chondrocytes are present

- proximal end

interstitial growth step 2: proliferation zone

§ Cartilage cells undergo mitosis.

interstitial growth step 3: hypertrophic zone

§ Older cartilage cells enlarge.

interstitial growth step 4: calcification zone

§ Matrix calcifies.

§ Cartilage cells die.

§ Matrix begins deteriorating.

§ Blood vessels invade cavity.

interstitial growth step 5: ossification zone

new bone forms

- distal end

interstitial growth pt 2

o Near end of adolescence chondroblasts divide less often.

o Epiphyseal plate thins then is replaced by bone.

o Bone epiphysis lengthening ceases.

- creates less cartilage to ossify

when does interstitial growth cease for females and why?

§ around 18 years old

· Estrogen slows down chondrocyte formation.

· Estrogen highest with ovulation.

when does interstitial growth cease for males?

around 21 years old

growth arrest lines

o Bone growth ceases for a short amount of time.

§ Malnutrition

§ Not synthesizing enough Vitamin D.

§ Traumatic injury.

iliozarov frame

o External fixator/fixation

o Holds and stabilize bone.

§ Mostly for younger individuals to encourage bone growth.

who do physicians use internal fixation for as treatment?

would be used on someone whos bones are done growing.

appositional growth

bones thicken thanks to the cooperative action of osteoblasts and osteoclasts.

- more osteoblasts

- widening in diameter

appositional growth 1

Ridges in the periosteum create groove for periosteal blood vessel.

appositional growth 2

§ Periosteal ridges fuse, forming an endosteum-lined tunnel.

appositional growth 3

§ Osteoblasts in endosteum build new concentric lamellae inward toward center of tunnel forming a new osteon.

appositional growth 4

§ Bone grows outward as osteoblasts in periosteum build new circumferential lamellae. Osteon formation repeats as new periosteal ridges fold over blood vessels.

appositional growth continued

- Widening

o Throughout life

o Osteoblasts beneath periosteum secrete bone matrix on external bone.

o Osteoclasts remove bone on endosteal surface.

o Growth > resorption

Bone Homeostasis: Bone Remodeling

o Consists of bone deposition (osteoblasts) and bone resorption (osteoclasts).

o Occurs at surfaces of periosteum and endosteum.

o Remodeling units

§ Adjacent osteoblasts and osteoclasts.

As osteoclasts deposit bone on the outer surface...

osteoclasts widen the medullary cavity from within.

bone homeostasis

o Recycle 5-7% of bone mass each week.

- consists of bone remodeling and bone repair

Spongy bone replaced around every

3-4 years

Compact bone replaced around every

10 years

older bone becomes more brittle because

§ Calcium salts crystalize.

§ Fractures more easily.

nutrient effects

§ Calcium, phosphorous, magnesium, fluoride, manganese, vitamin D.

hormone controls

§ Whenever and when

· Insulin-like growth factors, GH, T3/T4 (thyroid hormones), insulin

· Sex hormones

mechanical stress

where on the bones?

- more mechanical stress the more wear on the bone

Glandular cells secrete hormone into bloodstream ->

target cells (cells with hormone receptors) respond to hormone + hormones have no effect on other cells.

growth hormone pt 1.

stimulates osteoblast activity and synthesis of bone matrix.

thyroxin (thyroid)

stimulates osteoblast activity and synthesis of bone matrix.

sex hormones (estrogen/androgen)

stimulates osteoblast activity and synthesis of bone matrix.

parathyroid

Stimulates osteoclast, elevates Ca2+ (calcium) concentrations.

calcitonin (thyroid)

Inhibits osteoclast activity, reduces Ca2+ (calcium) concentrations