Ch. 6 - Bone Growth

skeletal cartilages

provide support and cushioning

appositional growth

adds to overall thickness

• only CT and cartilages

interstitial growth

adds to overall length

• only CT and cartilages

long bone

longer than it is wide

• humerus

irregular bone

doesn’t look like any other bones

• vertebra

flat bone

thin and light

• mostly in skull, sternum, and parts of the pelvic bone

short bone

equal in length and width; often where there’s lots of force

• talus

bone functions

• support

• protection: surrounds organs / tissues

• movement: skeletal muscles must cross joints to move

• mineral storage and balance: calcium phosphate, balancing pH

• blood cell production: in marrow

• fat storage: stored in yellow marrow of long bones

yellow bone marrow

fat

red bone marrow

blood producing

periosteum

membrane of bone that is very tightly attached

• pulls on bone and makes processes

• appositional growth

• dense irregular fibrous CT

perforating (sharpey’s) fibers

hold the periosteum to the surface of bone

diploë

spongy bone that is found between layers of flat bones

• contains red bone marrow

• helps distribute stress evenly and absorb shock because it rearranges faster

osteogenic cell

stem cells that are destined to become bone

• O₂% determines cell type

• lays down matrix; cells get trapped in matrix and harden

osteoblast

matrix-synthesizing cell responsible for bone growth

osteocyte

mature bone cell that maintains the bone matrix

• connected to neighbors which helps with vascularization and repairs (4-6 weeks)

osteoclast

bone - resorbing cell

• multi-nucleated

• ruffled border that is an extension of cell membrane (looks for things to dissolve w/ acid)

circumferential lamellae

the layers connect and go all the way around the bone

interstitial lamellae

fills in the gaps between circumferential lamellae

true osteons

only found in compact bone

key feature of compact bone

osteons

compact bone structure

tube within tube structure

lacuna

small, hollow cavity embedded within the mineralized extracellular matrix of bone that houses a single mature bone cell

osteon

the tiny, cylinder-shaped building block of compact bone

trabecualae

little beam / scaffolding of spongy bone

red lines

body weight stress

purple lines

stress from muscles pulling

hydroxyapatite crystals

main inorganic component of bone matrix

• bone matrix is 35% organic and 65% inorganic

parietal bone

brain pushing on fetal parietal bone stimulates growth

fetal skeleton

most is hyaline cartilage that ossifies

• major ossification centers at 12 weeks

intramembranous ossification

the direct conversion of embryonic connective tissue (mesenchyme) into bone without first forming a cartilage intermediate

• occurs mainly in flat bones of the skull and clavicle

intramembranous ossification

1. ossification centers appear in the fibrous connective tissue membrane

2. bone matrix (osteoid) is secreted within the fibrous membrane and calcifies

3. woven bone( network of trabeculae) and periosteum form

4. lamellar bone replaces woven bone, just deep to the periosteum; red marrow appears

endochondral ossification steps

1. bone collar forms around hyaline cartilage model

2. cartilage in the center of the diaphysis calcifies and then develops cavities

3. the periosteal bud invades the internal cavities and spongy bone begins to form

4. the diaphysis elongates and a medullary cavity forms as ossification continues; secondary ossification centers appear in the epiphyses

5. the epiphyses ossify; when completed, hyaline cartilage remains only in the epiphyseal plate and articular cartilages

bone grows

increases thickness by appositional growth

• adds more concentric lamellae

bone growth zones

• proliferation zone

• hypertrophic zone

• calcification zone

• ossification zone

proliferation zone

cartilage cells undergo mitosis

hypertrophic zone

older cartilage cells enlarge

calcification zone

matrix becomes calcified, cartilage cells die, and matrix begin deteriorating

ossification zone

new bone formation is occurring

parathyroid glands

release parathyroid hormone (PTH)

• stimulates osteoclasts

blood calcium homeostasis

9-11mg / 100ml

calcitriol

vitamin D

• increases blood levels of Ca⁺⁺

• encourages calcium absorption

• produced by melanocytes

PTH

increases blood levels of Ca⁺⁺

• important as we age

calcitonin

decreases blood levels of Ca⁺⁺ when levels are high

• stimulates osteoblasts

• more active when younger

hypercalcemia

higher than normal levels of calcium in the blood

• occurs when too much calcium enters the extracellular fluid or when the kidneys fail to excrete it normally

• excess PTH signals osteoclasts to break down bone tissue faster than it can rebuild → weakens bones over time

hypocalcemia

lower than normal level of calcium in the blood serum

• occurs due to disruptions in the hormone or mineral systems that regulate blood calcium (PTH and vitamin D)

• the body pulls stored calcium from the skeleton to maintain normal levels in the blood → weakens bones over time

growth hormone (GH)

• children: signals cartilage and bone cells to multiply and lengthen the epiphyseal plates, determining final height

• adults: metabolism regulation, body structure maintenance, triggers release of insulin like growth factor

GH hyposecretion

dwarfism

GH hypersecretion

• gigantism - before puberty

• acromegaly - adulthood

thyroid hormone (TH)

responsible for maintaining metabolic rate

sex hormones

responsible for eunuchs (growth spurts)

• testosterone

• estrogen

• progesterone

insulin like growth factors

produced primarily by the liver in response to growth hormone (GH), it drives bone, cartilage, and organ growth

rickets

a childhood bone disorder that causes the softening and weakening of growing bones, usually due to a prolonged deficiency in vitamin D, calcium, or phosphorus

• results in bowed legs

scurvy

a disease caused by a severe deficiency of vitamin C after prolonged inadequate dietary intake of fresh fruits and vegetables

• often causes teeth to fall out because collagen is broken down

wolff’s law

bone remodels in response to mechanical stress

• mechanical stress: physical force and load placed on bones during movement, gravity, and muscle contraction

bone repair

1. a hematoma forms

2. fibrocartilaginous callus forms

3. bony callus forms

4. bone remodeling occurs

hematoma

a collection of pooled, clotted blood that forms outside of blood vessels due to trauma, impact, or a bone fracture

comminuted fracture

bone fragments into three or more pieces

• common in the elderly and those with osteoporosis

compression fracture

bone is crushed

• common in porous bones (osteoporotic bones) subjected to extreme trauma like a fall

spiral fracture

ragged break occurs when excessive twisting forces are applied to a bone

• common in sports

epiphyseal fracture

epiphysis separates from the diaphysis alone the epiphyseal plate

• occurs where cartilage cells are dying and calcification of the matrix is occurring

• common in children

depressed fracture

broken bone portion is pressed inward

• common in the skull

greenstick fracture

bone breaks incompletely, only one side of the shaft breaks and the other side bends

• common in children whose bones have relatively more organic matrix and are more flexible

osteogenesis imperfecta

also known as brittle bone disease, is a rare genetic disorder present from birth that causes bone to break easily —often from little or no apparent trauma

• most cases are caused by mutations in the COL1A1 or COL1A2 genes, which disrupt the production or structure of type I collagen—the main protein that provides strength and flexibility to bones and connective tissues.

osteomyelitis

a painful infection and inflammation of the bone or bone marrow most commonly caused by bacteria such as Staphylococcus aureus

osteoporosis

a systemic bone disease where bones become weak, thin, and brittle due to a decrease in bone mineral density and the deterioration of bone tissue, greatly increasing the risk of fractures

caused by:

• inadequate nutrition

• lack of calcium

• too much PTH