Ch. 6 Bones & Skeletal Tissue

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Ossification

(Osteogenisis) the process of bone formation; formation of the bony skeleton in embryos and bone growth goes in until early adulthood as body continues to increase in size

Membrane Bone

bone developed from fibrous membranes in a process called intramembranous ossification

Endochondral Bone

bone developed by replacing hyaline cartilage in process called Endochondral Ossification

Intremembranous Ossification

results in formation of cranial bones of the skull and the clavicles, flat bones mostly formed by this process, begins on fibrous connective tissue membranes formed by mesenchymal cells. 4 Steps

Endochondral Ossification

all bones (except clavicles) of the skeleton below base of skull form by this process; begins in 2nd month of development and uses hyaline cartilage as models for bone construction

Primary Ossification Center

region of formation of long bone begins in center of hyaline cartilage shaft

Periosteal Bud

contains a nutrient artery and vein, lymphatic vessels, nerve fibers, red marrow elements, osteoblasts, and osteoclasts

Interstitial Growth

length of long bones

Appositional Growth

thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces

Growth in Length of Long Bones

Epiphyseal plate cartilage organizes into four important functional zones: Proliferation (growth), Hypertrophic, Calcification, Ossification (osteogenic)

Proliferation (growth)

these cells divide quickly, pushing the epiphyisis away from the diaphysis, causing entire bone to lengthen

Hypertrophic

older chondrocytes in stack that are closer to diaphysis, and lacunae enlarge

Calcification

surrounding cartilage matrix calcifies and chondrocytes die

Ossification

new bone formation occurs when calcified spicules erode

Growth Hormone

most important stimulus of epiphyseal plate activity released by the anterior pituitary gland

Thyroid Hormone

modulates the activity of growth hormone , ensuring the skeleton has proper proportions as it grows

Testosterone and Estrogen

promote adolescent growth spurts, end growth by inducing epiphyseal plate closure

Bone Remodeling

bone deposit and bone resorption

Bone Deposit

Occurs where bone is injured or added strength is needed; Requires a diet rich in protein; vitamins C, D, and A; calcium; phosphorus; magnesium; and manganese

1. osteoblasts secrete unmineralized matrix at the osteoid seam.
2. ~1 wk later, osteoid calcifies calcium phosphate salt deposits and for hydroxyapatite crystals

Bone Resorption

(removal) osteoclasts secrete lysosomal enzymes (digest organic matrix) and acids (convert calcium salts into soluble forms). Dissolved Matrix is transcytosed across osteoclast, enters interstitial fluid and then blood

Osteoclast moves along bone, digging grooves as it dissolves matrix. Ruffled edge forms tight seal against matrix to limit area of resorption. Secretes lysosomal enzymes (acid hydrolases) to break down organic matrix and hydrochloric acid to dissolve calcium

Control of Remodeling

controlled by Hormonal mechanisms that maintain calcium homeostasis in the blood, and Mechanical and gravitational forces.

Normal (constitutive) bone remodeling regulated by:
1) Parathyroid Hormone (PTH) -- To maintain blood calcium homeostasis
2) Mechanical Stress -- To strengthen bone where most needed

Hormonal Control of Blood Ca2+

Calcium is necessary for Transmission of nerve impulses, Muscle contraction, Blood coagulation, Secretion by glands and nerve cells, Cell division

Parathyroid Hormone (PTH)

primarily involved in hormonal control; released when blood levels of ionic calcium decline, increased PTH stimulates osteoclasts to reabsorb bone, releasing calcium to blood. Osteoclast break down old and new matrix. As blood concentration of blood rises the stimulus for PTH release end.

Leptin

also been known to influence density by inhibiting osteoblast

Response to Mechanical Stress

second set of controls regulating bone remodeling; Wolffs Law

Two type of mech stress: use (muscle pull) and gravity

Wolff's law

A bone grows or remodels in response to forces or demands placed upon it; Observations supporting Wolff's law:
(1) Handedness (right or left handed) results in bone of one upper limb being thicker and stronger
(2)Curved bones are thickest where they are most likely to buckle
(3)Trabeculae form along lines of stress
(4)Large, bony projections occur where heavy, active muscles attach

Bone Anatomy and Bending Stress

body weight transmitted to the head of the femur threatens to bend the bone along the indicated arc, compressing it on one side (converging arrows on the right) and stretching it on the other side (diverging arrows on left). Because these two forced cancel each other internally, much less bone material is needed internally then superficially

Fractures

breaks; excessive intake of vitamin A appears to increase fracture risk in some; Classifications: (1) Position of the bone ends after fracture [nondisplaced fractures - the bone ends retain their normal position, displaced fractures - bone ends are out of normal alignment], (2) Completeness of Break [ complete - broken all the way through, incomplete - not broken all the way through], (3) Orientation of the break to the long axis of the bone [linear - break parallels long axis, transverse - break perpendicular to bones long axis, (4) Whether bone ends penterate the skin [Open - bone ends penetrate the skin, Simple - bone ends do not penetrate the skin

Fractures (2)

all fractures can be describes in terms of location, external appearance, nature of the break

Fractures (3)

Reduction

treatment for bone fracture involving realignment of the broken bone ends

Closed (external) Reduction

bone ends are coaxed into position by the physicians hands

Open (Internal) Reduction

bone ends are secured together surgically with pins or wires

Stages in healing of Fracture

Hematoma Forms, Fibrocartilaginous Callus Forms, Bony Callus Forms, Bone Remodeling Occurs

Hematoma Forms

torn blood vessels tear and hemorrhage, blood, clots called hematoma form, and sites become swollen painful, and inflamed

Fribrocartilaginous Callus Forms

formation of granulation tissue or soft callus, capillaries grow into hematoma and phagocytic cels invade area and clean up debris, Osteoblast and and fibroblast begin to reconstruct bone. Osteoblasts begin forming spongy bone within 1 week, Fibroblasts secrete collagen fibers to connect bone ends; This Mass of repair tissue now called fibrocartilaginous callus spints broken bone

Bony Callus Forms

New trabeculae form a bony (hard) callus, Bony callus formation continues until firm union is formed in ~2 months

Bone Remodeling Occurs

In response to mechanical stressors over several months, Final structure resembles original

Osteomalacia

includes a number of disorders in which the bones are inadequatly mineralized. Osteoid is produced, but calcium salts are not deposited, so bones soften and weaken. caused by insufficient calcium in diet and by vitamin D deficiancy.

Rickets

analogous disease in children. Bowed legs and deformities of the pelvis, skull, and rib cage are common. Because epiphyseal plates cant be calcified they continue to widen and the ends of long bones become visibly enlarged and abnormally long. caused by insufficient calcium in diet and by vitamin D deficiency.

Osteoporosis

refers to a group of diseases in which bone resorption outpaces bone deposit, normal matrix composition but reduced density. Spongy bone of spine and neck of femur become most susceptible to fracture;

Risk factors: age; being female; petite body size; lack of exercise; lack of estrogen, calcium, vitamin D, or protein; low diabetes mellitus. Osteopenia = pre-osteoporosis.

Osteoporosis Treatment

Calcium, vitamin D, and fluoride supplements;
Weight-bearing exercise throughout life;
Hormone (estrogen) replacement therapy slows bone loss;
Bisphosphonates - inhibit osteoclast activity;
Raloxifene - selectively mimics estrogen in bone

Paget's Disease

Excessive and haphazard bone formation and breakdown, usually in spine, pelvis, femur, or skull; Excessive and haphazard bone formation and breakdown, usually in spine, pelvis, femur, or skull

Pagetic bone has very high ratio of spongy to compact bone and reduced mineralization, Unknown cause (possibly viral), Treatment includes calcitonin and biphosphonates

Developmental Aspects of Bones

Embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms, At birth, most long bones are well ossified (except epiphyses); Nearly all bones completely ossified by age 25, Bone mass decreases with age beginning in 4th decade, Rate of loss determined by genetics and environmental factors , In old age, bone resorption predominates

Skeletal Cartilage

made of variety of cartilage tissue, which consists primarily of water. Contains no blood vessels and is surrounded by a layer of dense irregular connective tissue, perichondrium. Contains all 3 types of cartilage: Hyaline, Elastic, and Fibrocartilage

Resilience

ability to spring back to original shape after being compressed

Perichondrium

(around the cartilage) acts like a girdle to resist outward expansion when the cartilage is compressed. Contains blood vessels from which nutrients diffuse through matrix to reach the cartilage cells

Hyaline Cartilage

provide support with flexibility and resilience. Are the most abundant of skeletal cartilages.

Include: (1) articular cartilages: cover ends of most bones at movable joints, (2) costal cartilages: connect ribs to sternum (breastbone), (3) respiratory cartilages: form skeleton of larynx (voicebox), (4) nasal cartilages: support external nose

Elastic Cartilages

contain more stretchy elastic fibers, able to stand up to repeated bending. Found in external ear and the epiglottis (flap that bends to cover the opening of the larynx each time we swallow)

Fibrocartilages

highly compressible and have great tensile strength, consist of chondrocytes and thick collagen fibers. In sites with heavy pressure and strength, such as padlike cartilages of knee and discs between vertebrae

Appositional Growth

(1st type of cartilage growth) cartilage forming cells in surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue.

Interstitial Growth

(2nd type of cartilage growth) lacunae-bound chondrocytes divide and secrete new matrix, expanding the cartilage from within.

Axial Skeleton

form long axis of body and includes bones of the skull, vertebral column, and rib cage. In general these bones are involved in protecting, supporting, or carrying other body parts.

Appendicular Skeleton

consists of bones of the upper and lower limbs and girdles (shoulder bones and hip bones) that attach the limbs to the axial skeleton.

Classification of Bones

1. Long Bones
2. Short Bones
3. Flat Bones
4. Irregular Bones

Long Bones

longer then they are wide, has a shaft plus 2 ends [All limb bones except the patella (kneecap) and the wrist and ankle bones are long bones]

Short Bones

roughly cube-shaped (bones of wrsit and ankle are examples)

Sesamoid Bone

(shaped like a sesame seed) are special type of short bones that form in tendons . Vary in sizes and numbers in individuals. Some alter directions of pull of tendon other their functions are unknown

Flat Bones

thin, flattened and usually a bit curved (sternum, scapulae, ribs, and most skull bones are flat bones)

Irregular Bones

complicated shapes that fit none of the preceding classes (include vertebrae and hip bones)

Functions of Bones

1. Support (for body and soft organs), 2. Protection (for brain, spinal cord and vital organs, 3. Movement (levers for muscle action), 4. Storage (minerals [calcium and phosphorus] and growth factors), 5. Blood Cell Formation (occurs in marrow cavities of certain bones), 6. Triglyceride (Fat) Storage (fat is stored in bone cavities and represents a source of stored energy for the body)

Bone Markings

bones display projections, depressions, and openings that serve as sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels

Projections

are bulges that grow outward from the bone surface include head, trochanters, spines, and others.

Depression

cavities that indent the bone, usually serve to allow passage of nerves and blood vessels

Openings

usually serve to allow passage of nerves and blood vessles

Compact Bone

smooth and solid outer layer of every bone

Spongy Bone

internal to compact bone, a honeycomb of small needle-like or flat pieces called trabeculae

Diaphysis

shaft of bone, forms long axis of bone, constructed of thick compact bone that surrounds meduallar cavity (contains fat/ yellow marrow)

Epiphyses

the bone ends, exterior is made of compact bone and their interior contains spongy bone. Joint surface of each epiphysis is covered with thin layer of articular (hyaline) cartilage [cushions opposing bone ends during joint movement and absorbs stress]

Epiphyseal Line/Plate

between diaphysis and epiphysis of an adult bone. A disc of hyaline cartilage that grows during childhood to lengthen bone. sometimes called metaphysis

Periosteum

white glistening, double layered external surface of the entire bone except the joint surface. Has out fibrous layer (dense irregular connective tissue) and an inner osteogenic layer (consists of osteoblasts)

Osteoblasts

secrete bone matrix elements, and osteoclasts

Osteoclasts

bone destroying cells

Osteogenic Cells

primitive stem cells that give rise to the osteoblasts

Osteocytes

maintain the bone matrix, also act as stress or strain sensors in cases of bone deformation.

Nutrient Foramina

nerve fibers, lymphatic vessels, and blood vessels from periosteum enter the diaphysis from this opening

Perforating (Sharpey's) Fibers

tufts of collagen fibers that extend from its fibrous layer into bone matrix, which secure periosteum to underlying bone

Endosteum

delicate connective tissue membrane that covers internal bone surfaces

Diploe

another name for spogny bone is flat bones

bone marrow

Flexible tissue found in the interior of bones; 2 types: red and yellow marrow.

Red Marrow

(Hematopoitec Tissue) typically found within the trabecular cavities of spongy bone of long bones and in the diploe of flat bones

Red Marrow Cavities

trabecular cavities of spongy bone of long bones and diploe of flat bones

yellow marrow

nutrient (fat) storage; can revert to red marrow if needed

Osteon/ Haversian System

the structural unit of compact bone that is riddled with passageways that serve as conduits for nerves, blood vessels, and lymphatic vessels. Each osteon is an elongated cylinder oriented parallel to long axis of bone

Lamella

column like matrix tube of an osteon (compact bone often times called lamellar bone)

Central Canal/ Haversian Canal

runs through the core of each osteon and contain small blood vessels and nerve fibers that serve need of the osteons cells

Perforating Canals/ Volkmann's Canal

second type of canals, lie at right angles to the long axis of the bone and connect the blood and nerve supply of periosteum to those in the central canals and medullary cavity

Lacunae

occupied by spider-shaped osteocytes at junctions of the lamelle

Canaliculi

hairlike canals that connect lacunae to each other and to the central canal

concentric lamellae

surrounding a central canal, part of intact osteons

Interstitial Lamellae

incomplete lamellae that lie between intact osteons, either fill gaps between forming osteons or are remnants of osteons that have been cut through by bone remodeling

Circumferential Lamellae

located deep to periosteum and superficial to endosteum and extend around entire circumference of the diaphysis and resist twisting of long bone

Trabeculae

in spongy bone, align along lines of stress, no osteons, Contain irregularly arranged lamellae, osteocytes, and canaliculi, Capillaries in endosteum supply nutrients

Chemical Composition of Bone (Osseous tissue)

organic and inorganic components; matrix, proper combination allows bones to be durable and strong without being brittle

Organic Components

include cells (osteogenic, osteoblasts, osteocytes, and osteoclasts) and osteoid (organic part of the matrix)

Osteoid

makes up 1/3 of matrix, includes ground substances (composed of proteoglycans and glycoprotiens) and collagen (provide tensile strength and flexibility) fibers, secreted by osteoblasts

Hydroxyapatites

mineral salts, 65% of bone by mass, Mainly calcium phosphate crystals, Responsible for hardness and resistance to compression

osteogenesis imperfecta

genetic deficiency in amount or structure of collagen; leads to weak bones and other tissues and short stature

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