Biology 2401 Chapter 6 Bones and Skeletal Tissues

Human skeletons have

206 named bones.

Calcium ion blood homeostasis -

9-11 mg/deciliter (100ml).

Red marrow (hematopoietic tissue) -

Found within the trabecular cavities of spongy bone in long bones and in inner cavities of flat bones. These cavities are sometimes called red marrow cavities.

Hypertrophic zone (diaphysis):

The older chondrocytes in the stack, which are closer to the diaphysis, hypertrophy, and their lacunae erode and enlarge, leaving large interconnecting spaces.

Calcification zone:

The surrounding cartilage matrix calcifies and these chondrocytes die and deteriorate, producing the calcification zone.

Ossification zone:

This leaves long slender spicules of calcified cartilage at the epiphysis-diaphysis junction. These calcified spicules ultimately become part of the ossification or osteogenic zone, and are invaded by marrow elements from the medullary cavity. Osteoclasts partly erode the cartilage spicules, then osteoblasts quickly cover them with new bone. Ultimately spongy bone replaces them. Eventually as osteoclasts digest the spicule tips, the medullary cavity also lengthens.

Membrane bone -

a bone developed by replacing fibrous membrane.

Endochondral bone -

a bone developed by replacing hyaline cartilage.

Perichondrium -

a compression resistant layer of dense irregular connective tissue that surrounds skeletal cartilage and prevents expansion. Contains blood vessels for nutrient diffusion through cartilage matrices.

Endosteum -

a delicate connective tissue membrane covering the internal bone surfaces. Also lines the canals that pass through the compact bone. Contains osteogenic stem cells that can differentiate into other bone cells.

Lamella -

a matrix tube of compact bone (lamellar bones) that envelops osteons. Collagen fibers run parallel in each ring, and rings always alternate in different direction of parallel. Bone salt crystals run along collagen fibers.

Condyle - (Bone markings)

a rounded projection found at bone joints.

Calcified bone cannot

accommodate mitosis.

Sesamoid bones - (Short bones)

act to alter the direction of pull of a tendon. Ex. the patella.

Osteoclasts -

actively reabsorbing bone in shallow depressions called resorption bays and exhibit a distinctive ruffled border that contacts the bone. Giant multinucleate cells located at sites of bone resorption. Born from hematopoietic stem cells.

Serotonin -

adjusts osteoblast activity to match calcium blood levels with calcium ingestion. Made in the intestines and released during eating.

Process - (Bone markings)

any bony prominence. Site for muscle and ligament attachment.

Epiphyseal plate closure -

bone of the epiphysis and diaphysis fuse. Age 18 for females, and 21 for males.

Head - (Bone markings)

bony expansion carried on a narrow neck found in bone joints.

Fractures -

breaks in bones. Classifications: Position of the bone ends after fracture: In nondisplaced fractures, the bone ends retain their normal position. In displaced fractures, the bone ends are out of normal alignment. Completeness of the break: If the bone is broken through, the fracture is a complete fracture. If not, it is an incomplete fracture. Whether the bone ends penetrate the skin: If so, the fracture is an open (compound) fracture. If not, it is a closed (simple) fracture. Named by the location, appearance, and nature of the break.

Depressed fracture -

broken bone portion is pressed inward. Typical of skull fractures.

Bone deposits -

calcification is triggered by debated mechanical signals involving local concentrations of calcium and phosphate ions in the endosteal cavity. When Ca+2 x Pi reaction products reach a specific level, tiny crystals of hydroxyapatite form spontaneously and catalyze further crystallization of calcium salts in the area. Matrix proteins bind and concentrate calcium. Enzyme alkaline phosphatase is essential for mineralization.

Meatus - (Bone markings)

canal-like passageway.

Skeletons start off as

cartilage and fibrous membranes. Typically growth ends during adolescence.

Interstitial growth -

cartilage matrix formation that starts in the division of lacunae-bound chondrocytes expanding the cartilage from within.

Appositional growth -

cartilage matrix formation that starts in the surrounding perichondrium and is secreted against the external face of the existing cartilage tissue.

Skeletal cartilage -

cartilage tissue molded into location for specific function. Mostly water, and is surrounded by a layer of dense irregular connective tissue (perichondrium).

Sinus - (Bone markings)

cavity within a bone, filled with air and lined with mucous membrane.

Proliferation zone:

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 legnthening the entire long bone.

Irregular bones -

complicated shapes that fit none of the other classes. Ex. vertebrae and the hip bones.

Fibrocartilages -

compressible and has great tensile strength. Roughly parallel rows of chondrocytes alternating with thick collagen fibers. Found at sites that are subject to both pressure and stretch such as the menisci (cartilages of the knee) and the discs between vertebrae.

Long bones -

considerably longer than they are wide. A shaft with two ends which are often expanded. Includes all limb bones except the patella (kneecap) and the wrist/ankle bones.

Periosteum -

double-layered membrane covering the external surface of the entire bone except the joint surfaces. Outer fibrous layer is dense irregular connective tissue. The inner osteogenic layer consists of primitive stem cells. These osteogenic cells give rise to all bone cells except bone-destroying cells. Supplied with nerve fibers and blood vessels passing through the shaft to the marrow cavity via nutrient foramen. Secured to underlying bone by sharpey's fibers extending from fibrous layers into the bone matrix.

Parathyroid hormones are inhibited by

electrical fields.

Rickets -

epiphyseal plates cannot calcify, continue to widen, and visible enlargement of long bones. Produced by a poor diet (insufficient calcium or vitamin D).

Epiphyseal fracture -

epiphysis separates from the diaphysis along the epiphyseal plate. Tends to occur where cartilage cells are dying and calcification of the matrix is occurring.

Paget's disease -

excessive and haphazard bone deposits and resorption. Pagetic bones are made hastily with an abnormally high ratio of spongy bone to compact bone. Bone ratio and reduced mineralization causes spotty weakening of the bones. Late in the disease, osteoblasts continue while osteoclasts decline leading to irregular bone thickenings or fillings of marrow cavities with pagetic bone. Causes deformations and pain in most the spine, pelvis, femur, and skull. Affects about 3% of NA elderly people. Therapies include calcitonin and bisphosphonates.

Interstitial lamella -

fill gaps between forming osteons or are remnants of osteons that have been cut by bone remodeling. Inbetweens of osteons and incomplete lamellae.

Fracture reduction -

first step in fracture treatment where bone ends are realigned by closed (external) use of the physician's hands in positioning or open (internal) use of pins and/or wires to secure positioning.

Intramembranous ossification -

forms cranial bones (frontal, parietal, occipital, and temporal bones) and clavicles.

Inward bone markings (depressions) -

fossae, sinuses, foramina, and grooves. Allow nerves and blood vessels to pass.

Groove - (Bone markings)

furrow. Used for passage of blood vessels and nerves.

Osteoporosis -

group of diseases where bone resorption outpaces bone deposition. Spongy bone of the spine is most vulnerable. Also the femur's neck is vulnerable. 30% of women between the ages of 60 and 70 have osteoporosis, and 70% of women have it by age 80. Can develop purely due to immobility. Sex hormones (androgens and estrogens) help maintain the health and normal density of the skeleton by restraining osteoclasts. Other factors: Petite body form. Insufficient exercise to stress the bones. A diet poor in calcium and protein. Abnormal vitamin D receptors. Smoking tobacco (reduces estrogen levels). Hormone-related conditions such as hyperthyroidism, low blood levels of thyroid-stimulating hormone, and diabetes mellitus.

Outward bone markings (projections) -

heads, trochanters, spines, and others. Most indicate the stresses created by muscles attached to and pulling on bone markings or are modified surfaces where bones meet and form joints.

Trabeculae - (Spongy bone)

help bone resist stress by aligning along lines of stress. Small flat pieces inside the diploë that separates bone marrow. Contain irregularly arrange lamellae and osteocytes connected by canaliculi. Devoid of osteons.

Bone lining cells -

help maintain the matrix with osteocytes. Flat cells found on bone surfaces devoid of remodeling. Periosteal cells are found on the external bone surface, and endosteal cells are found on the interior bone surface.

Hypercalcemia -

high ionic calcium levels causes nonresponsiveness and inability to function. Sustained symptoms include the formation of kidney stones or deposits of calcium slats in other organs reducing their function.

When secondary ossification is complete,

hyaline cartilage exists only on epiphyseal surfaces as articular cartilages, and at junctions of the diaphysis and epiphysis as epiphyseal plates.

Gigantism -

hypersecretion of growth hormones in children.

5-7% of our bone mass

is recycled every week. Some half gram of calcium a day. Spongy bone every 3-4 years. Compact bone ever 10 or more years.

Inorganic matrix composition:

is: ~85% Hydroxyapatite, ~10% calcium carbonate, and ~5% trace minerals (flouride, potassium, and magnesium).

Circumferential lamellae -

just deep to the periosteum , resist twisting of long bones by extending around the circumference of the diaphysis.

Tuberosity - (Bone markings)

large rounded projection; may be roughened. Site for muscle and ligament attachment.

Endochondral ossification -

late into the second month of development, hyaline cartilage is replaced by bone constructs. Hyaline cartilage must be broken down for ossification to proceed.

Excessive intake of carbonated or alcoholic beverages

leaches minerals from bone and decreases bone density.

Axial skeleton -

long axis of the body and includes the bones of the skull, vertebral column, and rib cage. Protect and support other body parts.

Hypocalcemia -

low ionic calcium levels causes hyperexcitability.

Genetics plays the

major role in determining how much a person's bone density will change over their lifetime. (Gene coding for vitamin D's docking site).

Medullary cavity -

marrow cavity containing fat (yellow marrow).

Hydroxyapatites -

mineral salts make up 65% of bone tissue's mass. Largely calcium phosphate tightly packed around collagen fibers in the EM. Healthy bone is a strong a steel in resisting tension, and half as strong at resisting compression.

Osteogenic cells -

mitotically active stem cells found in the membranous periosteum and endosteum. In growing bones, they are squamous cells. When stimulated, they differentiate into osteoblasts or bone lining cells. (Osteoprogenitor cells).

Osteocytes -

monitor and maintain the bone matrix. Mature bone cells occupying lacunaes conforming to their shapes. If they die, the surrounding matrix is absorbed. Relay mechanical stimuli to bone remodeling cells (osteoblasts and osteoclasts) to prevent calcium homeostasis.

Line - (Bone markings)

narrow ridge of bone; less prominent than a crest. Site for muscle and ligament attachment.

Crest - (Bone markings)

narrow ridge of bone; usually prominent. Site for muscle and ligament attachment.

Fissure - (Bone markings)

narrow, slitlike opening. Used for passage of blood vessels and nerves.

By age 25

nearly all bones have completely ossified and skeletal growth ceases.

Canaliculi -

nourishing canal connecting lacunae to each other and to the central canal. Purpose is to link all osteoblast/osteocytes together through gap junctions.

Most adults have red marrow

only in the heads of their femurs and humeruses.

Nutrient foramen -

openings between bone shafts and marrow cavities.

Compresses and stressed regions are

oppositely charged. Deforming bone produces an electrical current, and possibly used for directing bone remodeling.

Osteoid -

organic part of the matrix. 1/3rd of the matrix, including ground substance (proteoglycans and gylcoproteins) and collagen fibers.

Epiphyseal line -

place between diaphysis and epiphysis of an adult long bone. Remnant of this are found where hyaline cartilage grows in lengthening the bone. Sometimes called the metaphysis or simply physis.

Osteomalacia (soft bones) -

poor calcium salt concentrations in osteoids. Produced by a poor diet (insufficient calcium or vitamin D).

Ossification (osteogenesis) -

process of bone formation. Embryos are formed this way. Children are grown this way. Adult rebuild this way.

Spiral fracture -

ragged break occurs when excessive twisting forces are applied to a bone. Common sports fracture.

Epicondyle - (Bone markings)

raised area on or above a condyle. Site for muscle and ligament attachment.

Parathyroid hormone (PTH) -

raises ionic calcium levels by stimulating osteoclasts to reabsorb bone. Osteoclasts will break down matrices regardless of their age, but will target areas of low mechanical stress.

Leptin -

regulates bone density by inhibiting osteoblasts through a hypothalamus pathway and sympathetic bone nerves. Released by adipose tissue.

Osteocalcin -

regulates insulin secretion, glucose homeostasis, and energy expenditures.

Diseases of the human skeleton

result from bone deposits and bone resorption imbalances.

Yellow marrow can

revert to red marrow if needed.

Perforating canal -

right angles to the long axis of the bone connecting the blood and nerve supply of the medullary cavity to the central cavity. (Volkmann's canal).

Short bones -

roughly cube shaped. Ex. wrist and ankle.

Foramen - (Bone markings)

round or oval opening through a bone. Used for passage of blood vessels and nerves.

Bones work by use of

sacrificial bonds between collagen molecules. One broken (easily) energy is dissipated and fracture forces are prevented. Most sacrificial bonds re-form when given time.

Irregular bones develop from

several distinct ossification centers.

Fossa - (Bone markings)

shallow, basinlike depression in a bone, often serving as an articulate surface.

Spine - (Bone markings)

sharp, slender, often pointed projection. Site for muscle and ligament attachment.

Bone markings -

sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels and nerves.

Tubercle - (Bone markings)

small rounded projection or process. Site for muscle and ligament attachment.

Facet - (Bone markings)

smooth, nearly flat articular surface found at bone joints.

Diploë -

spongy bone intermediate between outer and inner surface of the cortical bone.

Osteon -

structural unit of compact bone. Groups of hollow collagen matrix tubes ring around blood vessels and nerve fibers. (Haversian system).

Elastic cartilages -

supports flexibility and resilience more than hyaline. Found in two location (the external ear and the epiglottis). Contain more stretchy elastic fibers than hyaline cartilages.

Hyaline cartilage -

supports flexibility and resilience. Most abundant skeletal carilages. Spherical chondrocytes mixed with fine collagen fibers inside their matrices. Found in articular cartilages (cover the ends of most bones and movable joints); costal cartilages (connect the ribs to the sternum); respiratory cartilages (form the skeleton of the larynx and reinforce other respiratory passageways); and nasal cartilages (support the external nose).

Over the hill begins

the decrease in bone mass with age (except the skull). More osteons remain incompletely formed, mineralization is less complete, and the amount of nonviable bone increases.

Calcium is absorbed in

the intestine under the control of vitamin D metabolites.

Ionic calcium drives

the physiological processes of nerve impulses, muscle contractions, blood coagulation, secretion by glands and nerve cells, and cell division.

Primary ossification center -

the starting place of endochondral ossification. First, perichondrium is converted to periosteum. Secondly, mesenchymal cells specialize into osteoblasts. Lastly, ossification begins.

Flat bones -

thin, flattened, and a bit curved. Ex, the sternum, scapulae, ribs, and most skull bones.

Growth hormone -

thyroid hormones that modulate the growth of skeletal regions during infancy/childhood. Later, induce epiphyseal closure.

Calcification front -

transition between the osteoid seam and older mineralized bone.

Diaphysis -

tubular shaft that forms the long axis of long bones. Constructed of a relatively thick collar of compact bone surrounding a central medullary cavity.

Osteoid seam -

unmineralized band of bone matrix 10-12 micrometers wide marking new matrix deposits of osteoblasts. Matures (calcifies) in about a week.

Appendicular skeleton -

upper and lower limbs and the shoulder/hip bones attached to the limbs. Help locomotion and manipulate our environment.

Trochanter - (Bone markings)

very large, blunt, irregularly shaped process. Site for muscle and ligament attachment.

Calcitonin -

very little effect on lowering blood calcium homeostasis. Produced by parafollicular cells (C cells) of the thyroid gland.

Excessive weight gains can

widen bones through appositional growth.

Bone growth

widens and thickens simultaneously.

More than 99% of the body's calcium

is present as bone minerals.

Stages of ossification:

are as follows: A bone collar forms around the diaphysis of the hyaline cartilage model. Osteoblasts of the newly converted periosteum secrete osteoid against the hyaline cartilage diaphysis, encasing it in a collar of bone called the periosteal bone collar. Cartilage in the center of the diaphysis calcifies and then develops cavities. As the bone collar forms, chondrocytes within the shaft hypertrophy (enlarge) and signal the surrounding cartilage matrix to calcify. Then, because calcified cartilage matrix is impermeable to diffusing nutrients, the chondrocytes die and the matrix begin to deteriorate. This deterioration open up cavities, but the bone collar stabilizes the hyaline cartilage model. Elsewhere, the cartilage remains healthy and continues to grow briskly, causing the cartilage model to elongate. The periosteal bud invades the internal cavities and spongy bone forms. In month 3, the forming cavities are invaded by a collection of elements called the periosteal bud, which contains a nutrient artery and vein, nerve fibers, red marrow elements, osteogenic cells, and osteoclasts. The entering osteoclasts partially erode the calcified cartilage matrix, and the osteogenic cells become osteoblasts and secrete osteoid around the remaining calcified fragments of hyaline cartilage. In this way, bone-covered cartilage trabeculae, the earliest version of spongy bone, is formed. The diaphysis elongates and a medullary cavity forms. As the primary ossification center enlarges, osteoclasts break down the newly formed spongy bone and open up a medullary cavity in the center of the diaphysis. Throughout the fetal period (week 9 until birth), the rapidly growing epiphyses consist only of cartilage, and the hyaline cartilage models continue to elongate by division of viable cartilage formation along the length of the shaft as cartilage calcifies, erodes, and then is replaced by little bony spikes on the epiphyseal surfaces facing the medullary cavity. At birth, most of our long bones have a bony diaphysis surrounding remnants of spongy bone, a widening medullary cavity, and two cartilaginous epiphyses. Shortly before or after birth, secondary ossification centers appear in one or both epiphyses, and the epiphyses gain bony tissue (long bones typically have both ends with secondary while short bones have only one end). The cartilage in the center of the epiphysis calcifies and deteriorates, opening up cavities that allow a periosteal bud to enter. Bone trabeculae appear, just as they did earlier in the primary ossification center. The epiphyses ossify. Bone trabeculae appear, just as they did earlier in the primary ossifiacation center.

Treatments for osteoporosis:

are: Hormone replacement therapy (HRT) slows bone loss. Bisphosphonates decreases osteoclast activity and number (partially reversing bone loss). Selective estrogen receptor modulators (SERMs) mimic estrogen bone-sparing properties. Monoclonal antibody denosumab reduces fractures in men with prostate cancer and improves bone density.

Function of bones

are: Support; provide a framework the body uses for soft organ placement. Protection; bones fuse to form the skull, vertebrae surround the spinal cord, and the rib cage protects the thoracic cavity. Anchorage; provides leverage for skeletal muscles to move body parts (joints determine the types of possible movement). Mineral and growth factor storage; mineral reservoir for calcium and phosphate to be released into the blood as ions. Stores growth factors in bone matrices. Blood cell formation; most hemoglobin occurs in the red marrow cavities of certain bones. Triglyceride storage; stored in bone cavities. Hormone production; osteocalcin (regulates insulin secretion, glucose homeostasis, and energy expenditures) is produced.

Ramus - (Bone markings)

armlike bar of bone found in bone joints.

Sex hormones -

at puberty, release in increasing amounts to promote growth spurts typical of adolescence. Also masculinizes/feminizes parts of the skeleton.

Chondrocytes -

basic components found in all three types of cartilage (hyaline, elastic, and fibrocartilage). Usually encased in lacuane (small cavities) within the extracellular matrix containing ground substances and fibers.

Greenstick fracture -

bone breaks incompletely. Only one side of the shaft breaks; the other side bends. Common in children (relatively more organic matrix than adults).

Bone remodeling -

bone deposit and bone resorption of the periosteum and endosteum surfaces.

Epiphyses -

bone ends broader than the diaphysis. Outer shell forms epiphysis exterior and interior contains spongy bone. Articular cartilages thinly covers the joint surface of each epiphysis at their ends.

In young adults

bone formation and resorption are in balance.

Osteoblasts -

bone forming cells that secrete the bone matrix. Actively mitotic, secreting unmineralized bone matrix including collagen and calcium-binding proteins that make up the initial unmineralized bone (osteoid). Cube shaped when depositing matrix, and squamous when inactive. Turn to osteocytes when completely surrounded by secreted matrix.

Comminuted fracture -

bone fragments into three or more pieces. Particularly common in elderly (brittle).

Wolff's law -

bone grows or remodels in response to the demands placed on it. Other factors: Handedness results in the bones of one upper limb being thicker than other of the less-used limb. Vigorous exercise of the most-used limb leads to large increases in bone strength. Curved bones are thickest where they are most likely to buckle. The trabeculae of spongy bone form trusses, or struts, along lines of compression. Large, bony projections occur where heavy, active muscles attach. The bones of weight lifters have enormous thickenings at the attachment sites of the most-used muscles.

Compression fracture -

bone is crushed. Common in porous bones (osteoporotic bones) subjected to extreme trauma.

Bone restoration -

consumes brittle bone salts and extracellular matrices recycling them. Osteoclasts move along a bone's surface digging grooves by breaking down the bone matrix. Ruffled borders of the osteoclast clings tightly to the bone, sealing off the area of bone destruction and secreting protons (H+) and lysosomal enzymes that digest the organic matrix. The resorption bay converts the calcium salts into soluble forms that pass easily into a solution. Osteoclasts phagocytize the demineralized matrix and dead osteocytes. The digested matrix, growth factors, and dissolved minerals are endocytosed, transported across the osteoclast (transcytosis), and released at the opposite side (interstitial fluid and the blood). Eventually resportion is completed and osteoclasts undergo apoptosis.

Central canal -

core of each osteon containing small blood vessels and nerve fibers. (Haversian canal).

Dwarfism -

deficits in growth hormones or thyroid hormones.

Compact bone -

dense outer layer that is smooth and solid. Covered by connective tissue membranes.

Fracture repair -

immobilization of fractured bone through cast or traction. Takes usually 6-8 weeks for small to medium sized bones. Repair takes longer in large bones and in elderly people. Steps: A hematoma forms. Blood vessels in the bone and periosteum are torn and hemorrhage. A mass of clotted blood forms at the fracture site. Bone cells deprived of nutrition die, and the tissue at the site becomes swollen, painful, and inflamed. Fibrocartilaginous callus forms. Soft granulation tissue (soft callus) forms within a few days. Capillary growth into the hematoma and phagocytic cells invasion begins clean up of debris. Fibroblasts, cartilage, and osteogenic cells invade the facture site from the nearby periosteum and endosteum and begin to reconstruct the bone. Fibroblasts produce collagen fibers that span the break and connect the broken bone ends. Some precursor cells differentiate into chondroblasts that secrete cartilage matrices. Within this mass of repair tissue, osteoblasts begin forming spongy bone. The cartilage cells farthest from the capillaries secrete an externally bulging cartilaginous matrices that later calcify. This entire mass of repair tissue, now called the fibrocartilaginous callus, splint the broken bone. Bony callus forms. Within a week, new bone trabeculae appear in the fibrocartilaginous callus and gradually convert it to a bony (hard) callus of spongy bone. Bony callus formation continues until a firm union forms about two months later. This process generally repeats the events of endochondral ossification. Bone remodeling occurs. Beginning during bony callus formation and continuing for several months after, the bony callus is remodeled. The excess material on the diaphysis exterior and within the medullary cavity is removed, and compact bone is laid down to reconstruct the shaft walls. The final structure of the remodeled area resembles the original unbroken bony region because it responds to the same set of mechanical stressors.

Nose bones and lower jaw bones grow

imperceptibly throughout life.

Notch - (Bone markings)

indentation at the edge of a structure. Used for passage of blood vessels and nerves.

Spongy bone -

internal honeycomb that holds red and yellow bone marrow.

Whether and when of bone remodeling

is determined by hormonal controls in response to changing blood calcium levels.

Where of bone remodeling

is determined by mechanical stresses. Least stressed areas are most sensitive to PTH stimulation.