chapter 6 (bones)

bone/ osseous tissue

hard, dense connective tissue that forms most of the adult skeleton, and is the supportive structure of the body

cartilage

found in the areas where bones move; semi-rigid connective tissue that provides flexibility and a smooth surface for movement

skeletal system

composed of bones and cartilage:

• supports body

• a point of attachment for tendons (muscle to bone) and ligaments (bone to bone)

• protect internal organs

• assist body movement 

• blood cell production

• store and releases minerals and fats(triglycerides/adipose cells)

bone marrow

softer connective tissue that fills interior of bone

yellow marrow

contains adipose tissue; triglycerides stored here can act as a source of energy; found in medullary cavity; red marrow → yellow marrow with age

red marrow

site of hematopoiesis (blood cell production) for RBC, WBC, and platelets

long bones

cylindrical shape, longer than wide (describes shape of the bone and not size)

• act as levels; move when muscles contract

• ex. humerus, femur, tibia, metacarpals, phalanges, etc.

short bones

cube-like shape; equal in length, width, and thickness

• provide stability and support; also limited motion

• ex. carpals and tarsals (only ones)

flat bones

thin and often curved (not necessarily actually flat)

• points of attachments for muscles and protect internal organs

• ex. cranial bones, scapulae, sternum, and ribs

irregular bones

does not have an easily categorized shape

• have more complex shapes

• ex. vertebrae and facial bones (like ones with sinuses)

sesamoid bones

small, round bone

• form in tendons where a lot of pressure is generated in a joint; help tendons deal with compression

• differ in number and placement person to person; usually found in relationship with feet, hands, and knees

• the patellae is the only common sesamoid bone found in every person

diaphysis

(part of a long bone) the tubular shaft that is between the proximal and distal ends of the bone; contains the medullary cavity; walls of the diaphysis are made of compact bone

medullary cavity

the hollow region in the diaphysis that contains yellow marrow

epiphysis

the wider parts on each end of the bone; filled with spongy bone and red bone marrow

metaphysis

the portion of the bone where the diaphysis and epiphysis bone; it also contains the epiphyseal plate/line

epiphyseal plate/line

plate: a layer of hyaline cartilage in growing bone

line: when bone stops growing (18-21 yrs.), the cartilage of the plate will be replaced by osseous tissue and this becomes the epiphyseal line 

nutrient foramen

small hole that allows nutrient supply and veins to go through into the medullary cavity and supports marrow up to the epiphyseal line

• other vessels serve epiphysis

• smaller vessels penetrate periosteum to serve compact bone

periosteum

fibrous membrane that covers the outer surface of the bone

• contains blood vessels, nerves, and lymphatic vessels that nourish compact bone

• point of attachment for tendons and ligaments

• covers the entire outer surface besides wither epiphyses meet other bones to form joints

• assists fracture repair

• contain osteogenic cells and osteoblasts that help the bone grow in length but not thickness

endosteum

membranous lining of the medullary cavity that has a role in bone growth, repair, and remodeling

• contains: osteogenic cells, osteoclasts, osteoblasts

articular cartilage

thin layer of cartilage (hyaline) that covers epiphysis; found where the bone forms an articular surface

• reduces friction and acts as a shock absorber

diploe (form of spongy bone)

flat bones (like in the cranium) have these that are lined on either side by a layer of compact bone

• protect internal organs (like brain)

articulation 

where two bone surfaces come together 

projection

an area of a bone that projects above the surface of the bone

• point of attachment for tendons and ligaments

hole

an opening or groove in bone that allows blood vessels and nerves to enter the bone

osteoblast

bone cell that forms new bone; found in growing centers of bone (like periosteum and endosteum

• do not divide

• synthesize and secrete collagen matrix and calcium salts

osteocytes

• formed by the matrix surrounding the osteoblast calcifies and becomes trapped in it

• it is the primary cell of mature bone and the most common type

• maintain mineral concentration of the matrix by secretion of enzymes

• do not divide (no miotic activity)

• located in lacuna and communicate by canaliculi 

lacuna

small spaces between lamellae that house osteocytes

canaliculi

long cytoplasmic processes that act as channels in the bone matrix that help osteocytes communicate with each other and receive nutrients (connect lacunae); gap junctions; waste removal

• connect with each other and eventually lead to central canal

osteogenic cell

undifferentiated with high miotic activity; only dividing bone cells

• immature osteogenic cells are found in deep layers of periosteum and marrow

• differentiate and develop into osteoblasts 

• help replenish old osteoblasts and osteocytes sine they cannot divide

osteoclast

responsible for resorption and breakdown

• found on bone surfaces

• multinucleated

• originate from monocytes and macrophages (white blood cells)

• osteoclasts are constantly breaking down (while osteoblasts are constantly forming new bone)

• responsible for subtle reshaping of bone

compact bone

dense and strong so it can help withstand compressive forces

• external layer of all bones

• found under periosteum and in the diaphysis

• provide support and protection

osteon (haversian system)

structural unit of compact bone

lamellae

concentric rings of calcified matrix that compose osteon

• run parallel to each other and diaphysis 

central canal (haversian canal)

run down the center of each osteon

• contain blood vessels, nerves, and lymphatic vessels (get in and out through foramina)

perforating canal

extend to periosteum and endosteum

outer circumferential lamellae 

encircle the bone beneath the periosteum 

inner circumferential lamellae

encircle the medullary cavity

spongy bone (cancellous bone)

contain open spaces and supports changing weigh distribution

• contain osteocytes housed in lacunae, but not in the centric circles of lamellae

• lacunae found in trabeculae

• spaces in spongy bone contain red bone marrow that is protected by trabeculae where hematopoiesis occurs 

trabeculae

in spongy bone, the lacunae is found in the lattice network of matric spikes called trabeculae 

• form along lines of strength and help bones resist stresses without breaking

• provide balance to dense and heavy compact bone by making bones lighter so muscles can move them

• spaces in spongy bone contain red bone marrow that is protected by trabeculae where hematopoiesis occurs 

ossification

formation of new bone

intramembranous ossification

compact and spongy bone develop from sheets of mesenchymal connective tissue

• flat bones on face, cranial bones, clavicle bones, and mandible bones form through this

• w/o going through cartilage stage

• many ossification centers

steps:

1. mesenchymal cells in embryonic skeleton gather together and differentiate —- - some will become capillaries; but others will become osteogenic cells which will become osteoblasts

2. development of ossification center: osteoblasts secrete organic and uncalcified extracellular matrix (osteoid)

3. calcification: calcium and other mineral salts are deposited and extracellular matrix calcifies (hardens) - traps osteoblasts → osteocytes - then new osteogenic cells → new osteoblasts

4. formation of trabeculae: extracellular matrix develops into trabeculae that fuse to form spongy bone - happens in osteoid secreted around capillaries - trabecular bone crowd nearby blood vessels, which condense into red marrow

5. development of periosteum: mesenchyme at the periphery (osteoblasts on the surface of spongy bone) develop to become periosteum - creates a protective layer of compact bone superficial to trabecular bone

ossification center

osteoblasts in a cluster that eventually spread out in the formation of bone tissue

osteoid

uncalcified and unmineralized matrix secreted by osteoblasts that calcify

endochondral ossification

how bone develops by replacing hyaline cartilage; cartilage acts as a template (takes longer than intramembranous ossification); forms long bones and bones at base of skull

1. development of cartilage model (6-8 weeks after conception): mesenchymal cells develop into chondroblasts (then chondrocytes) which form the model

2. growth of cartilage model: growth occurs by cell division of chondrocytes (soon perichondrium develops)                                                                                         - matrix produced → chondrocytes grow in size                                                   - matrix calcifies and then nutrients cant reach chondrocytes → death and disintegration                                                                                                           - blood vessels come into the resulting spaces → further enlarge spaces and bring in osteogenic cells (enlarging spaces eventually become medullary cavity)

3. development of primary ossification center (second or third fetal life): in this region of the diaphysis, bone tissue replaced most of the cartilage (osteoclasts breakdown for remodeling and calcium homeostasis)                                           - cartilage grows → capillaries penetration                                                           - this initiates the perichondrium → periosteum                                                   - this is where osteoblasts form a periosteal collar of compact bone around cartilage of diaphysis

4. development of medullary cavity: bone breakdown by osteoclasts forms the medullar y cavity

5. development of secondary ossification centers (after birth and has same steps as before): these occur in the epiphyses of bone

6. formation of articular cartilage and epiphyseal plate (which are the only cartilage remaining)” both structures consist of hyalin cartilage                           - bone length is compete by 18-21 yrs of age                                                       - bones continue to thicken and repair themselves throughout life

perichondrium

the membrane that covers the cartilage and eventually becomes periosteum

primary ossification center

a region deep in the periosteal collar where ossification begins (diaphysis region)

secondary ossification center 

occurs in epiphyseal regions

interstitial growth

growth in length that occurs in the epiphyseal plate ( a layer of hyaline cartilage)

1. chondrocyte proliferation on epiphyseal side of growth plate (cartilage formed)

2. cartilage replaced by bone on diaphyseal side of growth plate (cartilage ossified and diaphysis grows in length)

3. when cartilage becomes bone, growth plate closes and becomes epiphyseal line

epiphyseal plate

where bone growth in length occurs; has 4 zones

reserve zone (zone 1)

the region closes to the epiphyseal end of plate

• contains small chondrocytes within the matrix

• chondrocytes do not participate in growth but do secure the plate to the osseous tissue of epiphysis

proliferative zone (zone 2)

• contains stacks of slightly larger chondrocytes

• makes new chondrocytes to replace those that die at diaphyseal end of plate

• cellular division here and and maturation of cells in next zone cause growth in length

zone of maturation and hypertrophy (zone 3)

older and larger chondrocytes found here

zone of calcified matrix (zone 4)

zone closest to the diaphysis

• chondrocytes here are dead because the matrix around them has been calcified

• capillaries and osteoblasts from the diaphysis penetrate into this zone

• osteoblasts secrete the bone tissue on the calcified cartilage

• this connects the epiphyseal plate to diaphysis

epiphyseal line

the chondrocytes in epiphyseal plate have ceased proliferation and bone replaces cartilage; longitude growth stops

appositional growth

the growth diameter (can continue when growth in length has stopped)

1. osteoblasts in periosteum lay down new bone tissue (intramembranous ossification)

2. osteoclasts in endosteum destroy and resorb old bone that lines the medullary cavity which widens medullary cavity

modeling

increasing of the diameter of the medullary cavity; the process where matrix is resorbed on one surface of a bone and put onto another 

remodeling

in adult life; resorption of old or damaged bone takes place on the surface where osteoblasts lay new bone to replace which is resorbed

• injury and exercise can lead to remodeling

fracture hematoma (step 1 of bone repair)

results of blood vessels being torn by fracture, and the blood begins to clot which disrupts blood flow to the bone and results in the death of bone cells around fracture

internal callus is formed (step 2 of bone repair)

phagocytosis removes cellular debris; internal callus is formed from chondrocytes from the endosteum secreting a fibrocartilaginous matrix between the two ends of the broken bone

• fibroblasts deposit collagen to form a fibro-cartilaginous callus

external callus formed (step 3 of bone repair)

created by the periosteal chondrocytes and osteoblasts; it is made of hyaline cartilage and bone around outside of the break

• osteoclasts resorb dead bone and osteogenic cells become active to create osteoblasts

• osteoblasts form bony callus of spongy bone

• endochondral ossification

remodeling (step 4 of repair)

spongy bone is replaced by compact bone

hypocalcemia

abnormally low levels of calcium which can have adverse effects on body systems like circulation, muscles, nerves, and bone

hypercalcemia

abnormally high levels of calcium → nervous system is underactive and causes lethargy, sluggish reflexes, constipation, and confusion

calcium

needed to form calcium phosphate and calcium carbonate → form hydroxyapatite crystals that give bone its hardness

vitamin D

needed for calcium absorption in small intestine

vitamin K (and B12)

supports bone mineralization; has synergistic effect with vitamin D

magnesium

structural component

fluoride

structural component; increases density and helps stabilize bone

omega-3 fatty acids

reduces the inflammation that can interfere with osteoblast function

growth hormone

increases length of long bones, enhances mineralization, and improves bone density

thyroxine (thyroid hormone)

stimulates bone growth and promotes synthesis of bone matrix

insulin-like growth factor

stimulate osteoblasts, promote cell division at epiphyseal plate, and enhance protein synthesis

thyroid hormones and insulin

stimulate osteoblasts and protein synthesis

sex hormones

• promote osteoblastic activity and production of bone matrix

• responsible for growth spurts 

• promote conversion of epiphyseal plate to line

• promote widening of pelvis in female skeleton 

calcitriol (active form of vitamin D)

stimulates absorption of calcium and phosphate from digestive tract and produced by kidneys

parathyroid hormone

• stimulates osteoclasts proliferation and resorption of bone by osteoclasts

• promotes reabsorption of calcium by kidney tubules

• increases calcium absorption by small intestine

calcitonin

inhibits osteoclast activity and stimulates calcium uptake by bones