The Skeletal System

osteon

basic structural unit of compact bone; an osteon is also known as the Haversian system

periosteal bud

collection of elements (including a nutrient artery and vein, nerve fibers, red marrow elements, osteogenic cells and osteoclasts) that invade the developing medullary cavity during endochondral ossification

hypercalcemia

condition in which blood calcium levels are above the normal range

hypocalcemia

condition in which blood calcium levels are below the normal range

lacuna

pocket of space in which chondrocytes or osteocytes reside

hydroxyapatite

primary mineral salt of the inorganic portion of bone matrix, containing mostly calcium phosphate

bone resorption

process of breaking down or digesting bone that is carried out by osteoclasts

reduction (of fracture)

realigning the bone, either manually (closed reduction) or surgically (open reduction)

hemopoiesis

red blood cell production

diaphysis

shaft of a long bone

osteomalacia

softening of the bones in adults, typically as a result of a calcium or vitamin D deficiency

diploë

spongy bone that is formed during intramembranous ossification

metaphysis

the part of the bone between the diaphysis and epiphysis; in children, the metaphysis is called the epiphyseal growth plate, but when longitudinal growth of the bone is completed in adults, the metaphysis becomes the epiphyseal line

periosteal bone collar

thick collar of compact bone that surrounds the central marrow-filled medullary cavity

Describe the location and function of the central (or haversian) canals.

The central canals lie in the core of each osteon. They contain the blood vessels and nerve fibers that supply each osteon.

osteoporosis

condition of having weak or fragile bones, as a result of losing too much bone, making too little bone, or both

bone remodeling

constant cycle of depositing and resorbing (i.e. breaking down) bone

rickets

softening of bones in adults, caused by vitamin D deficiency

What factors can affect bone growth, repair and remodeling?

-Mechanical stress -Nutrition -Hormonal regulation -Regulation of calcium homeostasis -Aging

Describe the process of endochondral ossification.

1) Cartilage model develops and grows 2) Primary ossification center develops in the center of the diaphysis 3) Periosteal bud invades the diaphysis 4) Secondary ossification centers develop in the epiphyses 5) Epiphyseal plates and articular cartilage form

Compare and contrast the microscopic structure of compact and spongy bone.

Compact bone has an organized system of tightly-packed osteons. Spongy bone has a honeycomb and almost haphazard appearance, due to the arrangement of trabeculae according to the lines of stress.

Which bones are sites of red blood cell production in children? in adults?

In children, red blood cells are produced in the medullary cavity of the diaphysis and all areas of spongy bone. In adults, they are produced in the heads of the femur and humerus.

What is the impact on the skeleton of rising levels of sex hormones at puberty?

Sex hormones cause growth spurts and skeletal differences between males and females. (For instance, higher estrogen levels cause females to grow shorter and develop wider pelvises than males.)

What is the function of Sharpey's fibers?

Sharpey's fibers attach the periosteum, ligaments and tendons to the underlying bone and distribute mechanical forces over the greater surface of the bone.

What bones make up the axial skeleton? How many bones are included in the axial skeleton?

Skull, vertebral column and rib cage; 80

endosteum

delicate, vascularized membrane that covers the trabeculae of spongy bone surrounding the medullary cavity and lines the canals and canaliculi in osteons of compact bone; the endosteum contains osteoblasts and osteoclasts

periosteum

double-layered, vascularized membrane that covers all but the articulating surfaces of long bone; the periosteum consists of an outer fibrous layer made of dense irregular CT and an inner osteogenic layer containing osteoblasts and osteoclasts

epiphysis

end of a long bone

alkaline phosphatase

enzyme secreted by osteoclasts that triggers calcification (i.e. the process of depositing calcium salts into the matrix)

interstitial lamella

incomplete lamella; interstitial lamella either fill the gap between forming osteons or represent remnants of an osteon

avascular

lacking blood vessels

circumferential lamella

lamella that are located deep to the periosteum and superficial to the endosteum; circumferential lamella help resist twisting of the long bone

epiphyseal line

line of junction remaining between the diaphysis and epiphysis when longitudinal growth of the bone is complete

osteogenesis

(i.e. ossification) process of creating new bone

ossification

(i.e. osteogenesis) process of creating new bone

Identify the primary hormones that regulate bone growth and development. What is the general function of each of the hormones identified?

1) Growth hormone (hGH) and insulin-like growth factors (IGFs): act as primary growth factors during childhood 2) Thyroid hormones: triggers cell metabolism, increases metabolic rate, and stimulates osteoblasts, thereby impacting skeletal development 3) Sex hormones: become more important in skeletal development after puberty; turn off mitosis to allow osteoblasts to overtake chondrocyte activity; also causes skeletal changes in males and females

Identify and describe the physiologic steps involved in the repair of bone fractures.

1) Hematoma formation: blood clot forms at the area of bone fracture; bone cells begin to die; inflammation is triggered by inflammatory chemicals, and macrophages, neutrophils and lymphocytes begin to infiltrate the area; macrophages digest tissue debris and release chemicals that inspire tissue repair 2) Formation of fibrocartilaginous callus: blood vessels begin to grow; fibroblasts and osteoblasts enter the site of fracture, though the osteoblasts are not immediately activated; some fibroblasts secrete collagen fibers that span between jagged edges of the bone, while others differentiate into chondroblasts to convert the pre-matrix into a fibrocartilaginous callus 3) Formation of bony callus: osteoblasts are activated and begin to cluster around blood vessels to secrete spongy bone matrix; osteoclasts resorb fibrocartilage, which is eventually replaced with bony matrix called the bony callus 4) Bone remodeling

Compare and contrast the three types of skeletal cartilage. Thinking about where each type of cartilage is found in the body, describe how the structure of each type supports its function.

1) Hyaline cartilage: primarily found as costal cartilage (connecting the ribs to the sternum), respiratory cartilage (in the larynx skeleton and respiratory passageways), articular cartilage (on the articulating surfaces of bones), and nasal cartilage; characterized by a glossy appearance, spherical chondrocytes and thin, imperceptible collagen fibers in the extracellular matrix; provides support with resilience and flexibility 2) Elastic cartilage: found only in the ears and epiglottis; contains more elastic than collagen fibers; able to recoil and retain its shape 3) Fibrocartilage: found in the pubic symphysis, knee joint, between vertebral bones, and other areas where the body experiences heavy compressive stress; extracellular matrix contains thick collagen fibers and roughly parallel rows of chondrocytes; toughest cartilage found in the body that provides great compressive strength

Identify and describe the two mechanisms of cartilage growth.

1) Interstitial growth: "growth from within;" chondrocytes undergo mitosis within their own lacunae and secrete extracellular matrix, therefore expanding the cartilage from within 2) Appositional growth: "growth from outside;" fibroblasts in the perichondrium differentiate into chondrocytes and secrete extracellular matrix on the external surface of the cartilage

Name, describe and give an example of each of the four categories of bone, classified by shape.

1) Long bone: longer than they are wide with a shaft and two ends (ex: humerus) 2) Short bone: roughly cube-shaped (ex: patella) 3) Flat bone: thin, flat and slightly curved (ex: sternum) 4) Irregular bone: doesn't fit in the other three categories of bone (ex: hip bone)

Identify and describe the four types of cells that form and maintain bone tissue.

1) Osteogenic cells: highly unspecialized stem cells for bone tissue that are located in the periosteum and endosteum; derive from mesenchyme 2) Osteoblasts: actively mitotic and immature bone cells that are responsible for producing the organic portion of the bone matrix 3) Osteocytes: mature bone cells that are inactively mitotic but function to maintain the bone matrix 4) Osteoclasts: huge, multinucleate cells with a characteristic ruffled border that are formed from the fusion of 50+ monocyte cells; perform bone resorption

Describe the process of intramembranous ossification.

1) Primary ossification center develops in the fibrous CT membrane 2) Osteoid is secreted by osteoblasts within the fibrous membrane 3) Woven bone and the periosteum begin to form 4) Periosteum develops, and woven bone is eventually replaced by lamellar bone

appositional growth

1) cartilaginous growth that occurs from outside; during appositional growth, fibroblasts from the perichondrium differentiate into immature chondrocytes that secrete matrix against the external face of the cartilage tissue 2) process of bone growth in thickness

What is/are the function(s) of bone remodeling?

Renew and heal bone, as well as maintain calcium homeostasis

Describe the zones into which chondrocytes are organized within the epiphyseal plate.

1) Quiescient zone: zone closest to the epiphysis side of the epiphyseal plate; chondrocytes are resting, and their main function is to anchor the epiphyseal plate to the epiphysis 2) Proliferation zone: chondrocytes are actively mitotic and pushing the epiphysis farther from the diaphysis 3) Hypertrophic zone: chondrocytes hypertrophy, meaning they enlarge, rupture and die 4) Calcification zone: site marked by spicules of calcified cartilage, as a result of the pH change caused by the hypertrophic zone 5) Osteogenic zone: site of new bone tissue formation located closest to the diaphysis side of the epiphyseal plate; osteoclasts partially erode calcified cartilage while osteoblasts lay down new bone tissue

Describe the general structure of long bone.

A long bone consists of a shaft and two ends, called the diaphysis and epiphyses respectively, that are separated by a metaphysis. The diaphysis is composed of a thick collar of compact bone surrounding a central marrow-containing medullary cavity, whereas the epiphyses are made primarily of spongy bone surrounded by an outer layer of compact bone. The long bone is covered externally by the periosteum and internally by the endosteum. The periosteum consists of an outer fibrous layer, made of dense irregular connective tissue, and an inner osteogenic layer that contains osteoblasts and osteoclasts. These cells can also be found in the endosteum. Sharpey's fibers help anchor the periosteum to the bone matrix, and the long bone's articulating surfaces are covered by articular cartilage.

Identify and describe the location of the hyaline cartilage structures that remain when secondary ossification is complete.

After secondary ossification is complete, hyaline cartilage is found only on epiphyseal surfaces (as the articular cartilage) and at the junction of the diaphysis and epiphysis (where it forms the epiphyseal plates).

What complementary processes are involved in bone remodeling?

Bone depositing and bone resorption

What are the four basic components of the skeletal system?

Bones, cartilage, ligaments, and joints

What is the function of the epiphyses? of the articular cartilage?

Both the epiphyses and articular cartilage function in shock absorption. Articular cartilage also particularly helps reduce friction between opposing bones.

What is the function of calcitonin? What are the target sites for the action of this hormone? What effect does calcitonin have at its target sites?

Calcitonin is released by the thyroid gland when blood calcium levels are higher than normal. CT targets the osteoclasts to inhibit bone resorption, as well as the kidneys to increase the rate of calcium excretion.

What are the primary minerals stored by the bones?

Calcium phosphate

Describe the general characteristics of cartilage, paying attention to the location of perichondrium, blood vessels & nerves, and chondrocytes as well as to the general composition of the extracellular matrix.

Cartilage is avascular and innervated (meaning it contains no blood vessels or nerves). It is surrounded by the perichondrium, a membrane of dense irregular CT that resists outward compression of the cartilage as well as nourishes the cartilage with nutrients via the blood vessels. Chondrocytes are encased in lacunae found in the extracellular matrix, which is firm, gel-like, and consisting of 80% water, cellular adhesion molecules (CAMs), glycoproteins, and proteoglycans.

What are two ways in which aging affects the skeleton? At what age (approximately) do the bones of the body typically achieve their greatest density?

Decreases rate of protein synthesis and causes demineralization (or loss of minerals from the bone matrix); bone reaches greatest density around the age of 30

What tissues make up the embryonic skeleton before the sixth week of development?

Fibrous connective tissue and hyaline cartilage

What happens to bones during prolonged periods of inactivity? Why do athletes have stronger bones than sedentary people?

Inactivity reduces osteoblast activity, which results in a net loss of bone mass. Athletes have stronger bones, because they increase the mechanical stress placed on their bones. Consequently, osteoblasts are stiumlated to secrete more matrix and therefore increase bone production.

Compare and contrast intramembranous and endochondral ossification.

Intramembranous ossification is the process by which bone forms from the fibrous CT membranes of the embryonic skeleton. The clavicle and cranial bones of the skull develop via intramembranous ossification, and the resulting bone is called a membrane bone. Endochondral ossification is the process by which bone forms from the hyaline cartilage of the embryonic skeleton. All other bones (except the clavicle) below the base of the skull develop via endochondral ossification, and the resulting bone is called a cartilage (or endochondral) bone.

How do long bones grow in length?

Long bones grow in length via interstitial growth in the epiphyseal plate, which is divided into five different zones: 1) Quiescient zone: located closest to the epiphysis side of the epiphyseal plate; chondrocytes are resting, and their main function is to join the epiphyseal plate to the epiphysis 2) Proliferating zone: chondrocytes are actively mitotic and consequently push rows of older chondrocytes deeper towards the hypertrophic zone 3) Hypertrophic zone: chondrocytes hypertrophy, meaning they enlarge, rupture and die; as a result, a pH change occurs that activates calcification in the calcification zone 4) Calcification zone: marked by spicules of calcified cartilage; site of calcification, during which calcium is deposited in bone tissue 5) Osteogenic zone: site of osteogenesis, during which osteoclasts partially erode calcified cartilage and osteoblasts lay down new bone tissue

Name and describe the process by which long bones grow in thickness/diameter.

Long bones growth in thickness/diameter via appositional growth: 1) Osteoblasts beneath the periosteum form the bone matrix in periosteal ridges that surround a periosteal blood vessel. 2) The periosteal ridges enlarge and completely surround the blood vessel to form the central canal of a new osteon. 3) The periosteum which lines the central canal becomes the endosteum. 4) The osteoblasts in the endosteum form new lamallae, and growth proceeds inward toward the central blood vessel. 5) Simultaenously, other osteoblasts under the periosteum are beginning the same process. 6) As bone tissue forms on the external surface of the bone, osteoclasts in the endosteum of the medullary cavity engage in bone resportion. Consequently, even though the bone diameter is growing, the medullary cavity is also increasing in size, so the bone does not become too heavy.

What is the function of nutrient foramen?

Nutrient foramen are holes through which the nerve fibers and blood vessels found in the periosteum pass through the shaft to enter the medullary cavity.

What cells secrete osteoid?

Osteoblasts

Describe the cellular structure of osteoclasts. What is the function of these cells?

Osteoclasts are giant multinucleate cells formed by the fusion of 50+ monocyte cells. They contain a distinctive ruffled border that serves to increase surface area and tightly seal the bone from surrounding area. The function of osteoclasts is to resorb bone.

How do osteoclasts break down bone matrix?

Osteoclasts first use their ruffled borders to cling to the bone and seal it from surrounding area. Then, they secrete lysosomal enzymes that digest the organize matrix and protons. The resulting acidic brew converts calcium salts in the bone into a soluble form that can pass through the osteoclast and be released into interstitial fluid and blood. Osteoclasts can also phagocytize dead osteocytes and demineralized bone matrix. After bone resorption is complete, osteoclasts undergo apoptosis.

In which zone of the epiphyseal plate does ossification occur? Which zone exhibits a high rate of chondrocyte cell division?

Osteogenic zone; proliferation zone

Name the organic portion of bony matrix.

Osteoid

What is the most important hormone regulating blood calcium levels and calcium storage in adults?

Parathyroid hormone (PTH)

What is the function of parathyroid hormone? What are the target sites for the action of this hormone? What effect does parathyroid hormone have at its target sites?

Parathyroid hormone is secreted by parathyroid glands when blood calcium levels fall below normal. PTH targets the osteoclasts to engage in bone resorption, as well as the kidneys to synthesize more Vitamin D and decrease the rate of calcium excretion.

In what part of bone does blood cell production occur?

Red marrow

What are the differences between red and yellow marrow?

Red marrow produces red blood cells, while yellow marrow contains mostly fat cells. However, the body can convert yellow marrow to red marrow when it needs more red blood cells.

What type of bone makes up the epiphyses?

Spongy bone (surrounded by an outer shell of compact bone)

What is the general function of spongy bone? of compact bone?

Spongy bone contains red marrow and helps the bone better resist stress. Compact bone makes the bone hard.

Identify the functions of the skeletal system.

Support, protect, allow for movement, store minerals and growth factors, produce blood cells (hemapoiesis)

Describe the microscopic structure of compact bone, paying attention to the location fo the lamellae, lacunae, osteocytes and canaliculi.

The basic structural unit of compact bone is the osteon (AKA Haversian system). Each osteon is a group of tightly-compacted lamellae, which form concentric rings that surround a central canal (Haversian canal) containing blood vessels and nerve fibers that serve the osteon. The osteocytes reside in lacunae at the junctions of the lamellae, and the canaliculi connect the lacunae to each other and to the central canal.

Which materials of the bone matrix are responsible for the tensile strength (i.e. ability to resist stretching forces between its end) of bone? for the compression strength of bone? Which materials make bone hard and heavy?

The collagen fibers give the bone matrix its tensile strength. The ground substance gives the bone its compression strength. The mineral salts make bone hard and heavy.

What is the function of the epiphyseal plate?

The epiphyseal growth plate is the source of longitudinal growth in bone.

What is the significance of epiphyseal plate closure?

The epiphyseal plate closure signals the completion of longitudinal growth of the bone.

Describe the medullary cavity (i.e. where is it located? what structures are found there?)

The medullary cavity is the hollow cylinder located inside the diaphysis and surrounded by a thin layer of spongy bone. In newborns, the medullary cavity is filled with red marrow; in adults, it is replaced by yellow marrow.

Where is the metaphysis? What is the functional significance of this part of the bone?

The metaphysis is located between the epiphysis and diaphysis. The metaphysis helps transfer weight from joint surfaces to the diaphysis, and specifically in children, it is the source of longitudinal growth of bone.

What materials make up the organic and the inorganic portions of the bone matrix?

The organic portion is made of the cells and osteoid, which consists of collagen fibers and ground substance (proteoglycans and glycoproteins). The inorganic portion is composed of mineral salts, especially hydroxyapatite.

Describe the location and function of the perforating (or Volkmann's) canals.

The perforating canals lie at right angles to the long angle of the bone and connect the blood and nerve supply of the medullary cavity to the central canals.

Describe the structure and function(s) of the periosteum.

The periosteum is the double-layered membrane that covers all but the articulating surfaces of the long bone. The outer fibrous layer is made of dense irregular CT, and the inner osteogenic layer contains osteoblasts and osteoclasts. The periosteum supplies the blood vessels and nerve fibers, which pass through the shaft and enter through the nutrient foramen of the medullary cavity. It also provides anchoring points for tendons and ligaments.

Where are the primary centers of ossification typically located? the secondary centers of ossification?

The pirmary ossification centers are located in the center of hte bone shaft. The secondary ossification centers are locatd in the epiphyses.

How do the trabeculae of spongy bone respond to changes in the patterns of stress to which the bone is exposed?

The trabeculae rearrange themselves according to the lines of stress.

Describe the general structure of short, flat and irregular bones.

They are thin plates of spongy bone surrounded by compact bone. Like long bone, they are lined externally by the periosteum and internally by the endosteum. Unlike long bone, they contain marrow but lack a diaphysis, epiphyses, or medullary cavity.

What type of bone makes up the diaphysis?

Thick collar of compact bone

What bones make up the appendicular skeleton? How many bones are included in the appendicular skeleton?

Upper and lower limbs, and the shoulder and pelvic girdles; 126

How does vitamin C influence bone structure?

Vitamin C is necessary for the synthesis of collagen fibers.

How does vitamin D influence bone structure?

Vitamin D acts as a hormone to promote calcium absorption.

osteoid

name referring to the organic portion of the bone matrix; osteoid consists of ground substance and collagen fibers