Chapter 6- bones and bone tissue

Epiphyseal plate- zones of cells

(from top to bottom) 1. Zone of ossification (has calcified chondrocytes and osteoblasts that build bone) 2. Zone of calcification (contains dead chondrocytes some of which are calcified) 3. Zone of hypertrophy and maturation (contains mature chondrocytes) 4. Zone of proliferation (actively dividing chondrocytes in the lacunae) 5. Zone of reserve cartilage (zone closest to the epiphysis- cells not directly involved in bone growth but can divide of needed) (not actively involved in longitudinal growth)

How does the skeletal system maintain homeostasis?

-multiple functions in maintaining homeostasis -the bones store minerals that help maintain an acid-base and electrolyte balance in the blood and bones. (calcium, phosphorous, magnesium) -Triglycerides in stored in the yellow marrow can be broken down into fatty acids and used as energy in cells -Blood cell production called "hematapoiesis" takes place in the red marrow of the bone

Inorganic matrix

1. 65% of total bone weight 2. predominant ingredient is calcium salts and a good amount of phosphorous 3. Most of the calcium and phosphorous exit the body as large molecules called hydroxyapatite crystals 4. this composition makes bone one of the hardest substances in the body, gives bones the strength and ability to resist compression 5. other minerals in the matrix include salts of bicarbonate, potassium, magnesium, and sodium

Why are both the inorganic and organic matrices required for bone tissue to function normally? what would happen if either of the matrices were defective?

1. Both are required for bone tissue to function because both work together to make the bone strong. 2. If only the inorganic matrix was intact then the bone would be brittle and shatter. 3. If only the organic matrix was intact it would be flexible and resist compression

Hormones in bone growth

1. Cells of the endocrine glands secrete hormones into the blood 2. Primary example is growth hormone- produced by the anterior pituitary gland; highest amounts produced at childhood and infancy 3. growth hormone enhances protein synthesis. aids in appositional and longitudinal growth by: - an increase in the rate of mitosis of chondrocytes in the epiphyseal plate -an increase in the activity of the osteogenic cells, including their activity in the zone of ossification -direct stimulation of osteoblasts in the periosteum, triggering appositional growth

Calcitonin

1. Decline in PTH secretion 2. Secretion of the hormone calcitonin is triggered; produced by cells of the thyroid gland and has opposite effect of PTH 3. decreases both the activity of osteoclasts and the formation of new osteoclasts 4. most critical in times of active bone remodeling, as occurs in longitudinal bone growth 5. In adults calcitonin is not as potent of a blood calcium ion concentration regulator compared to PTH

What are the three hormones that impact bone growth?

1. Human growth hormone- aids in longitudinal and appositional growth; produced in highest amounts in infancy and childhood; enhances protein synthesis and cell division in nearly all tissues 2. Testosterone- aids in the appositional growth of men; men then haven more calcium salts and thicker bones than females; accelerates the closure of epiphyseal plates but is apart of the process of "growth spurts" because it increases the rate of mitosis at the epiphyseal plate 3. Estrogen- increases rate of longitudinal bone growth and inhibits osteoclasts; also accelerates the closure of the epiphyseal plate; has a much more potent affect in closing females epiphyseal plates and is the reason why women are shorter.

Classification of bones by shape

1. Long bones 2. Short bones 3. Flat bones 4. Irregular bones 5. Sesamoid bones

Osteocyte

1. Matured osteoblast 2. Osteoblasts become surrounded and eventually trapped by secreted bone matrix in a small cavity known as lacuna ->osteocytes 3. Relatively inactive 4. Secrete chemicals required to maintain ECM 5. recruit osteoblasts to build up areas of the bone under tension

Bone resorption- bone remodeling

1. Old bone destroyed by this process 2. Osteoclasts secrete hydrogen ions from their ruffles borders onto bone ECM 3. hydrogen ions make pH more acidic, which breaks down the pH- sensitive hydroxyapatite crystals in the inorganic matrix 4. liberated minerals can then be used in the body 5. osteoclasts secrete enzymes to catalyze reactions that degrade proteoglycans, glycoaminoglycans, and glycoproteins. 6. Broken down products taken into the osteoclast for possible reuse

Functions of skeletal system

1. Protection 2. mineral storage and acid-base homeostasis 3. Blood cell formation 4. Fat storage 5. Movement 6. Support

Parathyroid hormone

1. Receptor: parathyroid gland cells detect a low blood calcium ion level 2. Control center: parathyroid gland cells release parathyroid hormone into the blood 3. Effector/response: Parathyroid hormone stimulates effects that increase the blood calcium ion level 4. Homeostasis and negative feedback: the calcium ion concentration returns to normal homeostasis range and negative feedback loop decreases the parathyroid gland cell secretion of PTH.

Endochondral ossification process

1. The chondroblasts in the perichondrium differentiate into osteoblasts 2. The bone begins to ossify from outside: -2a: osteoblasts build the bone collar on the external surface of bone -2b: simultaneously, the internal cartilage begins to calcify and the chondrocytes die 3. In the primary ossification center, osteoblasts replace calcified cartilage with early spongy bone; in secondary ossification centers and medullary cavity develop 4. As the medullary cavity enlarges, the remaining cartilage is replaced by bone; the epiphyses finish ossifying

What happens when bone resorption is greater than bone deposition? What happens in the opposite case?

1. When bone resorption is greater than bone deposition then the hormone calcitonin is released to stop the secretion of PTH. Calcitonin is released by the thyroid to decrease the activity and formation of osteoclasts. This is due to an increase in calcium ions. 2. When bone deposition is greater than resorption the body will secrete PTH from the parathyroid glad because calcium ion levels had decreased. PTH will stimulate effects that will increase the blood calcium ion level.

Secondary ossification center

1. also known as lamellar bone 2. Fully formed lamellae w/ regularly arranged collagen bundles that are parallel to one another- makes it much stronger than primary bone 3. Has higher percentage of inorganic matrix which contributes to its strength

Medullary cavity

1. also known as the marrow cavity- much of the bone marrow of the long bone is housed here 2. marrow can be yellow or red depending on the bone and age of the person

Growth in width

1. appositional growth- osteoblasts between the periosteum and the bone surfaces lay down new bone 2. does not initially result in the formation of new osteons; new circumferential lamellae are formed 3. primarily responsible for thickening the compact bone of the diaphysis- marrow cavities enlarge due to the digestion of the inner circumferential lamellae by osteoclasts

Endochondral ossification

1. built on a model made of hyaline cartilage 2. essential during fetal development 3. Cartilage is present during this process

Intramembranous ossification

1. built on a model or starting material made of a membrane of embryonic connective tissue 2. bones formed this way are primary bones 3. Ex: bones of the skull, clavicles 4. form during fetal development 5. occurs in mesenchymal membrane 6. inner spongy bone forms before outer compact bone- "sandwich" 7. large bones often have more than one primary ossification center that take time to fuse together; example: the soft spots of an infants brain

Bone deposition

1. carried out by osteoblasts in the periosteum and endosteum 2. Osteoblasts secrete different proteoglycans and glycoproteins that bind to calcium ions, ATP, and enzymes 3. These vesicles bind to collagen fibers, and their calcium ions eventually crystallize, which ruptures the vesicle 4. this begins the process of calcification 5. Forms new bone

Process of longitudinal growth

1. chondrocytes divide in the zone of proliferation 2. Chondrocytes that reach the next zone enlarge and mature 3. Chondrocytes die and they matrix calcifies 4. Calcified cartilage is replaced with bone

Osteon

1. compact bone resembles a forrest of small trees- each individual "tree" represents an osteon 2. "Haversian system" 3. the rings of the mini tree are very thin bone called lamellae

Irregular bones

1. do not fit into the other classes due to their irregular shapes 2. ex: vertebrae and certain skull bones (vertebrae)

Epiphysis

1. enlarged rounded ends of bone 2. covered with thin layer of hyaline cartilage (articular cartilage)- allows for the bones to rub together with reduced friction on the joints

Bone callus

1. fracture ends are linked by a soft callus 2. The hard callus stage lasts until the fragments are firmly united by new bone (3-4months) 3. Bone callus growth begins at the periphery of the fracture site, where the strain is lowest. The production of this bone reduces the strain more centrally, which in turn forms bony callus. 4. Thus, hard callus formation starts peripherally and progressively moves towards the center of the fracture and the fracture gap. 5. The initial bony bridge is formed externally or within the medullary canal, away from the original cortex. 6. Then, by endochondral ossification, the soft tissue in the gap is replaced by woven bone that eventually joins the original cortex.

End of longitudinal growth

1. growth continues at the epiphyseal plate as long as mitosis is happening in the zone of proliferation 2. 12-15 yrs old mitosis slows 3. Epiphyseal plate gradually shrinks, zone of proliferation progressively taken over by the zones of calcification and ossification 4. 18-21 yrs old zone of proliferation completely ossifies; epiphyseal plate said to be closed 5. closed epiphyseal plate leaves calcified remnant called the epiphyseal line

compact bone

1. hard dense outer bone 2. enables to resist most stresses placed on it (linear compression and twisting)

Primary ossification center

1. immature bone also known as woven bone 2. irregularly arranged collagen bundles, abundant osteocytes, little inorganic matrix 3. in most locations of the bod, primary bone is reabsorbed by osteoclasts and then replaced by mature secondary bone

Osteoblast

1. immature cell 2. cuboidal and columnar cells found in inner periosteum and endosteum 3. derived from flattened cells called osteogenic cells, differentiate into osteoblasts when stimulated by certain chemical signals 4.Bone- building cells 5. bone deposition- they build up the bone ECM 6. secrete organic matrix and aid in formation of inorganic matrix

Testosterone- hormones in bone growth

1. increases appositional bone growth 2. cause bones in males to become much thicker and have more calcium salt deposits than females 3. Increases the rate of mitosis at the epiphyseal plate 4. Accelerates the closure of the epiphyseal plates

Endosteum

1. inner bone membranes lined with this 2. contain different bone cells that help maintain bone homeostasis 3. thinner and lacks fibrous outer layer compared to periosteum

spongy bone

1. inner honeycomb-like bone or cancellous bone 2. forms framework of bony struts that allows it to resist many forces and provide place for bone marrow to reside

Compound fracture

1. involve damage around the fracture 2. "open" fracture

organic matrix

1. known as osteoid 2. consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone specific proteins 3. predominant protein fibers in bone are collagen fibers, which form cross links with one another and help bone to resist torsion (twisting) and tensile (pulling or stretching) forces. 4. collagen fibers are one of the most important components of bone ECM in terms of strength- if the fibers become inadequate or ineffective then the bone can weaken 5. Support functions- large molecules such as glycosaminoglycans and proteoglycans create a gradient that draws water out of the blood vessels and cells by osmosis- traps water w/in ECM and helps tissue resist compression 6. glycoproteins act as molecular glue- connect hydroxyapatite and bone cells

Osteon structure

1. lamellae- osteons contain 4-20 lamellae (concentric lamellae); greatly enhances strength; lamellae's rings run in separate directions which allows the osteon to resist twisting and bending forces in multiple directions 2. central canal- center of each osteon has a hole; contains blood vessels and nerves that supply the cells of the osteon; canal lined with endosteum 3. lacunae- osteoblasts are surrounded by the ECM that they secrete to become osteocytes; small cavities that are filled with extracellular fluid located between lamellae; about 20,000-30,000 osteocytes and lacunae are found in each cubic millimeter of bone 4. canaliculi- lacunae connected by tiny canals; osteocytes have long thin arms that extend through canaliculi to contact arms of other osteocytes; gap junctions located at these connections that allow for small molecules to pass from cell to cell; allow oxygen and nutrients from blood to reach every osteocyte

Periosteum

1. membrane covering surface of bone 2. composed of dense irregular collagenous connective tissue 3. richly supplied with blood vessels and nerves 4. perforating fibers attach it to underlying bone (Sharpey's fibers) `

Fracture

1. most dramatic bone injury 2. also referred to as a broken bone 3. two types to classify: simple and compound

Long Bones

1. named for overall shape; longer than they are wide 2. Ex: most bones of the arms and legs, as well as, bones of the hands, feet, fingers, and toes (not named for size) (humerus)

Short bones

1. named for shape rather than size; as long as they are wide, roughly cube shaped 2. Ex: carpals, tarsals (trapezium)

events of intramembranous ossification

1. osteoblasts develop in primary ossification center 2. osteoblasts secrete organic matrix which calcifies 3. early spongy bone is formed 4. early compact bone is formed

Osseous (bone) tissue

1. primary tissue type (bone tissue) 2. specialized connective tissue 3. bone mainly consists of extracellular matrix

Osteogenesis

1. process of bone formation -> also known as ossification 2. begins during embryonic period and for some bones through childhood -> most bones ossified by age 7 3. two types of ossification: intramembranous and endochondrial ossification

Bone resorption

1. secrete hydrogen ions and enzymes from ruffle border 2. hydrogen ions create acidic environment that dissolves the inorganic matrix, and the enzymes breakdown the organic matrix 3. The liberated minerals, amino acids, and sugars then enter the blood foe reuse in the body or excreted as waste

Osteoclast

1. somewhat resemble jellyfish 2. large multinucleated cells derived from the fashion of cells formed in bone marrow 3. reside in shallow depressions on the internal or external surfaces of bone 4. responsible for the process of of bone resorption- break down the bone ECM. Accomplish bone resorption by secreting hydrogen ions and enzymes from region of cell called ruffle border

Sesamoid bone

1. specialized bones located within tendons 2. generally small, relatively flat, and oval shaped 3. give tendon mechanical advantage- better leverage for muscles, reduce wear and tear of tendon 4. Ex: knee cap

Simple fracture

1. the skin and tissue around the fracture remain intact 2. "closed" fracture

Flat bones

1. thin and broad 2. Ex: most of the bones of the skull, clavicles, ribs, sternum, and bones of the pelvis (sternum)

Trabeculae

1.Branching ribs of bones 2. covered with endosteum and usually do not contain osteons 3. contain concemtric lamellae, in which you find canaliculi and lacunae housing osteocytes. 4. no central or perforating canals are present 5. obtain oxygen and nutrients from blood vessels in bone marrow

Process of bone remodeling

Constant process of formation and loss 1. Bone deposition 2. Bone resorption

What type of cells are responsible for longitudinal growth in a bone? Where are these cells located?

Longitudinal growth occurs from the division oh chondrocytes. These cells are located in the epiphyseal plate. The epiphyseal plate contains five different zones of cells, each with distinct appearances. ( zone of ossification, calcification, hypertrophy and maturation, proliferation, reserve cartilage)

What role do osteoblasts play in intramembranous ossification?

Mesenchymal cells develop into osteoblasts and then osteoblasts secrete organic matrix of bone. Then after some time calcium salts and other components of inorganic matrix are deposited in trabaculae, process called calcification. Trapped osteoblasts become osteocytes.

What takes place during appositional growth?

Osteoblasts between the periosteum and the bone surface lay down new bone. Primarily responsible for thickening the bone of the diaphysis. Medullary cavity enlarges due to the digestion of the inner circumferential lamellae by osteoclasts. Bone growth in width may continue after bone growth in length ceases depending on things like hormones and things like nutrition.

What are primary and secondary bone? How do they differ in structure and function?

Primary bone is immature bone known as woven bone that is irregularly arranged and in most locations it is reabsorbed by mature secondary bone. Primary bones go through intramembranous ossification. Secondary bone is also known as lamellar bone and is much stronger than primary bone due to having more inorganic matrix. Secondary bone forms from hyaline cartilage.

What role does the bone collar play in endochondral ossification?

The bone collar forms when the osteoblasts secrete organic bone ECM deep into the periosteum. The bone collar is a ring of early compact bone. When the bone collar calcifies ECM surrounding the internal chondrocytes calcifies. This cuts off blood supply causing chondrocytes to die. The death leaves cavities surrounded by calcified cartilage. Then the osteoclasts etch a hole in the bone collar that allows a group of blood vessels and bone cells to enter primary ossification center.

Does bone remodeling occur in healthy bone? Why or why not?

Yes, bone goes through a constant process of bone formation and loss. The bone does this because of a few reasons; maintenance of calcium ion homeostasis, bone repair, replacement of primary bone with secondary bone, replacement of older brittle bone with new bone, and bone adaption to tension and stress.

Red bone marrow

consists of loose connective tissue supporting islands of blood forming **In infants and young children, most bone marrow is red because of their rapid rate of growth requires a constant supply of new blood cells

Diaphysis

long shaft of the bone; unique to long bones

How do bones grow in length?

longitudinal growth; occurs by the division of chondrocytes in the epiphyseal plate

Yellow bone marrow

stores triglycerides, consists mostly of blood vessels and adipocytes **at about age 5, yellow bone marrow begins to replace some of the red bone marrow