Chapter 6 Bones and Skeletal Tissue

Ossification (osteogenesis)

-process of bone formation; begins in embryonic period and continues through childhood with most bones completing the process by age 7 -postnatal bone growth until early adulthood -bone tissue replaces embryonic tissue to from a skeleton

intersitial growth (2nd method of cartilage growth)

-requires presence of epiphyseal cartilage Concurrent models of epiphyseal ends is to maintain proportion -the lacunae-bound chondrocytes divide and secrete new matrix, expanding the cartilage from within. Typically, cartilage growth ends during adolescence when the skeleton stops growing.

Wolff's Law

A bone grows or remodels in response to forces or demands placed upon it R of L handedness results in thicker and stronger bone of the upper limb (dominant hand) prevents buckling and injury of bone bones of fetus or bedridden human = featureless soft bone

Osteoporosis

A condition in which the body's bones become weak and break easily. refers to a group of diseases in which bone resorption outpaces bone deposit. The composition of the matrix remains normal but bone mass declines, and the bones become porous and light. The bones become so fragile that they can break from something as simple as stepping off a curb or a hearty sneeze.

Diaphysis

A tubular diaphysis or shaft, forms the long axis of the bone. It is constructed of a relatively thick collar of compact bone that surrounds a central medullary cavity or marrow cavity, that contains no bone tissue. of a long bone

Bone growth/repair

All throughout life

hyaline cartilage location

Articular cartilages (artic = joint, point of connection), which cover the ends of most bones at movable joints Costal cartilages, which connect the ribs to the sternum (breastbone) Respiratory cartilages, which form the skeleton of the larynx (voice box) and reinforce other respiratory passageways Nasal cartilages, which support the external nose

Bones don't begin with bone tissue. What do they begin with?

Bones begin as fibrous membranes or hyaline cartilages.

PTH (parathyroid hormone)

Control by PTH acts to preserve blood calcium homeostasis, not the skeleton's strength or well-being. As blood concentrations of calcium rise, the stimulus for PTH release ends. The decline of PTH reverses its effects and causes blood Ca2+ levels to fall.

2nd step : endochondral ossification

Except for the clavicles, essentially all bones below the base of the skull form by endochondral ossification. Beginning late in the second month of development, this pro-cess uses hyaline cartilage "bones" formed earlier as models, or patterns, for bone construction. It is more complex than intramembranous ossification because the hyaline cartilage must be broken down as ossification proceeds. most bones formed this way Cartilage model replaced by bone replacement begins in middle(dyaphysis) replacement follows in ends (epiphyses)

First step/method of bone growth : intramembranous ossification

Forms flat bones of skulls ; mandible, clavicle Stem cells differentiate to osteoblasts Produces spongy bone, then compact bone

Hormonal Regulation of Bone Growth

Growth hormone: most important hormone in stimulating epiphyseal plate activity in infancy and childhood Thyroid hormone: modulates activity of growth hormone, ensuring proper proportions Testosterone (males) and estrogens (females) at puberty: promote adolescent growth spurts End growth by inducing epiphyseal plate closure Excesses or deficits of any hormones cause abnormal skeletal growth

epiphyseal plate/closure

Growth plate, made of cartilage, gradually ossifies -bone lengthening starts to stop (requires presence of cartilage -bone of epiphysis and diaphysis fuses -females (about 18 years old) -males (about 21)

compact bone

Hard and dense, but not solid, bone tissue that is beneath the outer membrane of a bone.

Fibrocartilage

Highly compressible with great tensile strength, fibrocartilages consist of roughly parallel rows of chondrocytes alternating with thick collagen fibers -Fibrocartilages occur in sites that are subjected to both pressure and stretch, such as the padlike cartilages (menisci) of the knee and the discs between vertebrae

endochondral ossification

In endochondral ossification (endo = within, chondro = cartilage), a bone develops by replacing hyaline cartilage. The resulting bone is called an endochondral bone.

intramembranous ossification

In intramembranous ossification, a bone develops from a fibrous membrane and the bone is called a membranous bone.

irregular bones

Irregular bones have complicated shapes that fit none of the preceding classes. Examples include the vertebrae and the hip bones

Control remodeling of bone

Maintaining Ca2+ homeostasis-A hormonal negative feedback loop involving parathyroid hormone maintains Ca2+ homeostasis in the blood. Keeping bone strong-Mechanical and gravitational forces acting on a bone drive remodeling where it is required to strengthen that bone.

bone function

Mineral storage. Bone is a reservoir for minerals, most importantly calcium and phosphate. The stored minerals are released into the bloodstream in their ionic form as needed for distribution to all parts of the body. Indeed, "deposits" and "withdrawals" of minerals to and from the bones go on almost continuously. Blood cell formation. Most blood cell formation, or hematopoiesis, occurs in the red marrow of certain bones.

Aging and the Skeletal System

Physical activity decreases, bones lose mass, decrease protein synthesis (bones brittle, more likely to break), decreasing size and reiliency of intervertebral discs causes some height loss, Osteoporosis

axial skeleton

Portion of the skeletal system that consists of the skull, rib cage, and vertebral column the axial skeleton forms the long axis of the body and includes the bones of the skull, vertebral column, and rib cage. Generally speaking these bones protect, support, or carry other body parts.

Bone Homeostasis

Recycle 5-7% of bone mass each week Spongy bone replaced ~ every 3-4 years Compact bone replaced ~ every 10 years Older bone becomes more brittle Calcium salts crystallize Fractures more easily Consists of bone remodeling and bone repair

bone function

Support. Bones provide a framework that supports the body and cradles its soft organs. For example, bones of lower limbs act as pillars to support the body trunk when we stand, and the rib cage supports the thoracic wall. Protection. The fused bones of the skull protect the brain. The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs of the thorax. Anchorage. Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts. As a result, we can walk, grasp objects, and breathe. The design of joints determines the types of movement possible.

How many bones are in the human body?

The 206 named bones of the human skeleton are divided into two groups: axial and appendicular. T

When describing endochondral ossification, some say "bone chases cartilage." What does that mean?

The cartilage model grows, then breaks down and is replaced by bone.

hyaline cartilage

The most abundant cartilage type in the body; provides firm support with some pliability

appendicular skeleton

The portion of the skeleton that attaches to the axial skeleton and has the limbs attached to it the appendicular skeleton consists of the bones of the upper and lower limbs and the girdles (shoulder bones and hip bones) that attach the limbs to the axial skeleton. Bones of the limbs help us move from place to place (locomotion) and manipulate our environment.

Osteocytes

The spidery osteocytes are mature bone cells that occupy spaces (lacunae) that conform to their shape. Osteocytes monitor and maintain the bone matrix. They also act as stress or strain "sensors" and respond to mechanical stimuli (bone loading, bone deformation, weightlessness). Osteocytes communicate this information to the cells responsible for bone remodeling (osteoblasts and osteoclasts) so that bone matrix can be made or degraded as mechanical stresses dictate. Osteocytes can also trigger bone remodeling to maintain calcium homeostasis

bone function

Triglyceride (fat) storage. Fat, a source of energy for the body, is stored as yellow marrow in the cavities of long bones. Hormone production. Bones produce osteocalcin, a hormone that helps to regulate insulin secretion, glucose homeostasis, and energy expenditure

Rickets

Vitamin D deficiency(osteomalacia) in children bowed legs, bone deformity

Paget's disease

a bone disease of unknown cause characterized by the excessive breakdown of bone tissue, followed by abnormal bone formation; abnormally high spongy to compact bone ratio This, along with reduced mineralization, causes a spotty weakening of the bone

growth balance

balance must exists between osteoclasts and osteoblasts if too much new tissue if formed, it will become heavy excessive loss of calcium weakens bones, osteoporosis or become too flexible; rickets or osteomalacia

Osteoblasts

bone forming cells; new born formation

short bones

bones of the wrist and ankles ; roughly cube shaped

mineral storage of bones

bones store calcium -99% of body calcium up to 2k of calcium hormones control/release - PTH, calcitrol release bone calcium calcitonin stores bone calcium blood levels kept constant 9-11 mg/dl

long bones

bones that are longer than they are wide -All limb bones except the patella (kneecap) and the wrist and ankle bones are long bones. Notice that these bones are named for their elongated shape, not their overall size. The three bones in each of your fingers are long bones, even though they are small.

1. proliferation zone

cartilage cells undergo mitosis 1 Proliferation zone: The cells at the "top" (epiphysis-facing) side of the stack next to the resting zone comprise the proliferation or growth zone. These cells divide quickly, pushing the epiphysis away from the diaphysis and lengthening the entire long bone.

elastic cartilage

cartilage with abundant elastic fibers; more flexible than hyaline cartilage - external ear and epiglottis

Cartilage grows in two days - appositional growth

cartilage-forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue.

Osteoclasts are

cells that break down bone matrix; dissolve and remodel bone

bone modeling

determines bone shape, begins in the womb, continues until early adulthood continuous breakdown and reforming of bone tissue shape reflects applied loads - reshapes bones based on stress mineral turnover

osteomalacia

disease marked by softening of the bone caused by calcium and vitamin D deficiency in adults pain when weight is put on the bones

Flat bone

flat bones are thin, flattened, and usually a bit curved. The sternum (breastbone), scapulae (shoulder blades), ribs, and most cranial bones of the skull are flat bones

Intramembranous bone formation

forms the cranial bones of the skull (frontal, parietal, occipital, and temporal bones) and the clavicles. Most bones formed by this process are flat bones. At about week 8 of development, ossification begins within fibrous connective tissue membranes formed by mesenchymal cells. This process involves four major steps Development of the ossification center Calcification Formation of trabeculae Development of the periosteum

bone injury and repair steps

fracture - crack or break in bone steps in repair: fracture hematoma (swelling of blood to stop bleeding) mitoses in periosteum, endosteum (bone cells dividing and migrating to area of FX) internal/external callus (stabalizing area and forming cartilage that leads to new bone bone remodeling

3. Calcification zone

matrix calcifies; cartilage cells die; matrix begins deteriorating; blood vessels invade cavity 3 Calcification zone: The surrounding cartilage matrix calcifies, the chondrocytes die, and the matrix begins to deteriorate, allowing blood vessels to invade. This leaves long, slender spicules of calcified cartilage at the epiphysis-diaphysis junctions, which look like stalactites hanging from the roof of a cave.

spongy bone (cancellous bone)

mesh-like bone tissue found in the interior of bones, and surrounding the medullary cavity

normal bone growth requirements

minerals ; calcium/phosphate vitamins ; D3(absorption of calcium), C(essential for collagen production), A (bone formation) Hormones ; growth/sex/thyroid etc

4. ossification zone

new bone formation is occurring 4 Ossification zone: The calcified spicules are invaded by marrow elements from the medullary cavity. Osteoclasts partly erode the cartilage spicules, then osteoblasts cover them with new bone. Ultimately spongy bone replaces them. Eventually, as osteoclasts digest the spicule tips, the medullary cavity also lengthens.

2. Hypertrophic zone

older cartilage cells enlarge 2 Hypertrophic zone: The older chondrocytes in the stack, which are closer to the diaphysis, hypertrophy (enlarge). Their lacunae erode and enlarge, leaving large interconnecting spaces.

5 Zones of Interstitial long Bone Growth

resting zone ( proliferation zone (growth) hypertrophic zone calcification zone ossification zone (osteogenic)

Epiphysis

the end part of a long bone, initially growing separately from the shaft. bone ends ; An outer shell of compact bone forms the epiphysis exterior and the interior contains spongy bone. A thin layer of articular (hyaline) cartilage covers the joint surface of each epiphysis, cushioning the opposing bone ends during movement and absorbing stress.

osteopenia

thinner than average bone density starts before age 40 women ; lose 8% per decade men ; lose 3% per decade spongy bone most affected

bone marking

trochanter; large rough projection spine - pointed process condyle; smooth rounded articuar process fossa; shallow depression foramen; rounded passageway process; projection of bone