Anatomy and Physiology - Lecture 5

What does the human skeleton initially consist of?

• it initially consists of cartilage, which is later replaced by bone except in areas that require flexibility

What is skeletal cartilage made of?

• its made of highly resilient, molded cartilage tissue that consists primarily of water, providing flexibility and cushioning

Does cartilage contain blood vessels or nerves?

• no, cartilage contains no blood vessels or nerves

Perichondrium

• a layer of dense connective tissue that surrounds cartilage like a girdle

• it helps cartilage resist outward expansion and contains blood vessels for nutrient delivery

Chondrocytes

• cartilage cells that are encased in small cavities called lacunae within a jelly-like extracellular matrix

Three main types of cartilage in the body

• hyaline cartilage

• elastic cartilage

• fibrocartilage

Hyaline Cartilage

• it provide support, flexibility, and resilience

• it is the most abundant cartilage type and its matrix contains only collagen fibers

• it is found in articular joints, costal (ribs), respiratory (larynx, trachea), and nasal cartilages (tip of nose)

Elastic Cartilage

• contains elastic fibers in addition to collagen, making it more flexible

• found in only two places, the external ear and the epiglottis (the flap that covers the larynx when swallowing)

Fibrocartilage

• made of thick collagen fibers that give it great tensile strength

• found in the knee and the intervertebral discs between the vertebrae

Two ways cartilage grows

• appositional growth

• interstitial growth

Appositional Growth

• cartilage-forming cells in the perichondrium secrete new matrix against the external face of existing cartilage, adding layers to the surface

Interstitial Growth

• chondrocytes divide within lacunae and secrete new matrix from inside the cartilage, expanding it from within

What happens to cartilage as a person ages?

• cartilage growth slows and often becomes calcified or replaced by bone tissue in adulthood

Why is cartilage important for skeletal development?

• it serves as the template for bone formation and provides flexibility and cushioning in areas like joints and the rib cage

7 major functions of bones

1. support → form the framework that supports the body and cradles soft organs like muscles and tissues

2. protection → they enclose and safeguard vital organs such as the brain (skull), spinal cord (vertebrae) and heart and lungs (rib cage)

3. movement/anchorage → bones act as levers that muscles attach to; when muscles contract, bones move, enabling locomotion and manipulation of the environment

4. mineral and growth-factor storage →they serve as reservoirs for minerals, especially calcium and phosphorus, that can be released into the bloodstream to maintain mineral balance

5. blood-cell formation → In red bone marrow cavities of certain bones through a process called hematopoiesis.

6. triglyceride (fat) storage → Fat is stored in yellow bone marrow as an energy reserve for the body.

7. hormone production → osteocalcin is the hormone produced and it regulates insulin secretion, glucose levels, and overall metabolism; it also influences fat storage and energy use

How many names bones are in the human skeleton?

206

How is the skeleton divided based on location?

divided into two groups:

• the axial skeleton

• the appendicular skeleton

Axial Skeleton

• the long axis of the body

• includes the skill, vertebral column, and rib cage

• it supports, protects, and carries other body parts such as the head, neck, and trunk

Appendicular Skeleton

• the bones if the upper and lower limbs, plus the girdles (shoulder and hip) that attach them to the axial skeleton

• it allows movement and manipulation of the enviroment

How are bones classified by shape?

Into four types:

• long bones

• short bones

• flat bones

• irregular bones

Long Bones

• bones that are longer than they are wide

• with a shaft and two ends

• they are mostly limb bones (femur, humerus)

• they act as levers for movement and support the weight of the bodt

Short Bones

• cube-shaped bones found in the wrist (carpals) and ankle (tarsals); they provide stability and some movement

Sesamoid bones

• special short bones that form within tendons to reduce friction and modify pressure (e.g the patella)

• their number and size can vary among different peopl

Flat Bones

• thin, flat, and slightly curved bones that protect internal organs and provide large surfaces for muscle attachment (e.g., sternum, ribs, scapulae, skull bone)

• they protect internal organs and provide surfaces for muscle attachment

Bones that are part of both the axial and appendicular skeletons

• the clavicle (appendicular) connects to the sternum (axial) linking the two skeleton regions

Three levels of bone structure

• gross anatomy (visible structure)

• microscopic anatomy (cellular level)

• chemical composition (molecular level)

Two types of bone structure

• compact bone

• spongy bone

Compact Bone

• the dense, outer layer of bone that looks smooth and solid

• it provides strength and protection

Spongy Bone (cancellous bone)

• a honeycomb-like network of small, needle-shaped or flat pieces called trabeculae, with open space filled with red or yellow bone marrow

Structure of short, irregular, and flat bones

• they consist of thin plates of spongy bones covered by compact bone

• periosteum on the outside and endosteum on the inside

• bone marrow is scattered throughout the spongy bone rather than confined to a cavity

What covers the area of bone that forms a movable joint?

• hyaline cartilage, which reduces friction and absorbs shock

Diaphysis of a Long Bone

• the shaft or long axis of the bones

• its made of bone surrounding a medullary (marrow) cavity

What does the medullary cavity contain?

• in adults, it contains yellow bone marrow (fat)

Epiphyses

• the ends of long bones, made of compact bone externally and spongy bone internally

Epiphyseal Line

• a remnant of the epiphyseal plate (growth plate) in adults where bone growth occurred during childhood

Periosteum

• its a white, double-layered membrane covering the external surface of bones (except at joints)

• it protects the bone, nourishes it, and serves as an attachment site for tendons and ligaments

two layers:

• an outer fibrous layer of dense irregular connective tissue

• an inner osteogenic layer containing stem cells

Sharpey’s Fibers (perforating fibers)

• collagen fibers that secure the periosteum to the underlying bone matrix

Endosteum

• a delicate connective tissue membrane lining the internal bone surfaces, including trabeculae and canals in compact bones

• osteogenic cells that can differentiate into other bone cells, such as osteoblasts and osteoclasts are found here

Bone Markings

• features on bone surfaces that serve as attachment sites for muscles, tendons, and ligaments

3 categories

• projections (outward bulges)

• depressions (grooves or indentation)

• openings (holes or canals)

Hematopoietic tissue in bones

• red bone marrow is found within trabecular cavities of spongy bone and diploe of flat bones, such as sternum

• in newborns, medullary cavities and all spongy bone contain red bone marrow

• in adults, red bone marrow is located in head of femur and humerus, but most active areas of hematopoiesis are flat bone diploe and some irregular bones (such as the hip bone)

• yellow bone marrow can convert to red, if person becomes anemic

Osteogenic Cells

• stem cells in the periosteum and endosteum that divide and differentiate into osteoblasts or bone-lining cells

Osteoblasts

• bone-forming cells that secrete unmineralized bone matrix called osteoid, composed of collagen and calcium-binding proteins

• they are important because they build new bone tissue and are actively mitotic, playing a key role in bone growth and remodelling

Osteocytes

• mature bone cells found in lacunae that maintain the bone matrix and act as stress sensors

• they detect mechanical stress or strain and signal osteoblasts and osteoclasts to remodel bone accordingly

Bone-lining Cells

• flat bone cells on bone surfaces that help maintain the bone matrix; on the external surface they’re called periosteal cells, and on the internal surface, endosteal cells

Osteoclasts

• large, multinucleate cells that break down bone tissue (bone absorption) and are derived from the same stem cells as macrophages

• they are active in depressions called resorption bays; they gave ruffled borders that increase surface area for dissolving bone

Compact Bone

• also called lamellar bone

• consists of:

→ osteon (Haversian system)

→ canals anf canaliculi

→ interstitial and circumferential lamellae

Osteon (Haversian system)

• the structural unit of compact bone

• it’s an elongated cylinder that runs parallel to the long axis of the bone, acting as a weight-bearing pillar

Lamellae

• rings of bone matrix within an osteon that contain collagen fibers running in alternating directions to resists twisting forces

Central (Haversian) canal

• the canal running through the core of an osteon, containing blood vessels and nerve fibers

Perforating (Volkmann’s) canals

• canals that run perpendicular to the central canal, connecting blood vessels and nerves between periosteum, medullary cavity, and central canal

Lacunae

• small cavities within bone that house osteocytes

Canaliculi

• tiny channels that connect lacunae to each other and to the central canal, allowing nutrient and waste exchange between osteocytes

Interstitial Lamellae

• remnants of old osteons that fill spaces between newer osteons

Circumferential Lamellae

• layers of bone matrix that extend around the entire surface of the diaphysis, just deep to the periosteum, helping the bone resist twistin

Spongy Bone - Anatomy

• appears poorly organized but is actually organized along lines of stress to help bone resist any stress

• trabeculae, like cables on a suspension bridge, confer strength to bone

• no osteons are present, but trabeculae do contain irregularly arranged lamellae and osteocytes interconnected by canaliculi

• capillaries in endosteum supply nutrients

What two component make up bone?

• organic and inorganic components

Organic components of bone

• cells (osteogenic, osteoblasts, osteocytes, bone-lining cells, osteoclasts)

• osteoid (the unmineralized organic matrix)

What is osteoid made of?

• ground substance and collagen fibers, which provide flexibility and tensile strength

Why is collagen important in bone?

• it resists stretching and provides flexibility, preventing bones from breaking under stress

What are sacrificial bonds in collagen?

• small connections between collagen molecules that break under stress to absorbs energy and prevent fractures, then reform when the stress is removed

Inorganic components of bone

• hydrocyapatite’s (mineral salts), mainly tiny crystals of calcium phosphate embedded in the collagen matrix

• about 65% of bone madd is inorganic

What gives bone its hardness and strength?

• the inorganic mineral salts, primarily calcium phosphate, provide hardness and resistance to compression

Ossification (osteogenesis)

• the process of bone tissue formation that begins in the embryo and continues throughout life as bone grows, remodel, and repair

What are the three stages of bone development across life?

1. formation of the bony skeleton in embryos

2. posnatal bone growth until earl adulthood

3. bone remodelling and repair throughout life

Two main type of ossification?

• endochondral ossification

• intramembranous ossification

Endochondral ossification

• the process by which bone forms by replacing hyaline cartilage models

• nearly all bones bones below the skull except the clavicles are formed by thus

• it begins in the late second month of fetal development

What must occur before ossification can begin in cartilage?

• the hyaline cartilage must be broken down to make way for bone formation

What is the primary ossification center?

• the region in the center of the diaphysis where bone formation first begins

What triggers the transformation of perochondrium into periosteium?

• blood vessels infiltrate the perchondrium, supplying nutrients and converting it into periosteum

What do mesenchymal cells in the periosteum differentiate into?

• osteoblasts, which begin forming bone

Five main steps of endochondral ossification

1. bone collar forms around the diaphysis of the cartilage model

2. central cartilage in diaphysis calcifies and forms cavities

3. periosteal bud invades cavities, forming spongy bone

4. diaphysis elongates, and a medullary cavity forms

5. epiphyses ossify, leaving only articular cartilage and apuphyseal plates

What does periosteal bud consist of?

• blood vessels, nerves, red marrow, osteogenic cells, and osteoclasts that invade the cartilage model

Intramembranous ossification

• bone development from a fibrous connective tissue membrane, rather than from cartilage

Which bones form by intramembranous ossification

• flat bones of the skull (frontal, parietal, occipital, temporal) and the clavicles

What type of cells form the fibrous membrane?

• mesenchymal cells

What are the four major steps in intramembranous ossification

1. ossification center form as mesenchymal cells cluster and become osteoblasts

2. osteoid is secreted by osteoblasts then calcified

3. woven (immature) bone and periosteum form

4. woven bone is remodelled into lamellar bone, and red marrow appears

What is woven bone?

• the initial, unorganized bone formed during ossification

• it is later remodelled into mature lamellar bone

Postnatal Bone Growth

• Long bones grow lengthwise by interstitial (longitudinal) growth of epiphyseal plate

• Bones increase thickness through appositional growth

• Bones stop growing during adolescence

• some facial bones continue to grow slowly through life

Growth in Length of Long Bones

Interstitial growth requires presence of epiphyseal cartilage in the epiphyseal plate

Epiphyseal plate maintains constant thickness

– Rate of cartilage growth on one side balanced by bone replacement on other

• Epiphyseal plate consists of five zones:

1. Resting (quiescent) zone

2. Proliferation (growth) zone

3. Hypertrophic zone

4. Calcification zone

5. Ossification (osteogenic) zon

• Near end of adolescence, chondroblasts divide less often

• Epiphyseal plate thins, then is replaced by bone

• Epiphyseal plate closure occurs when epiphysis and diaphysis fuse

• Bone lengthening ceases

– Females: occurs around 18 years of age

– Males: occurs around 21 years of age

Resting (quiescent) zone

• area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive

Proliferation (growth) zone

• area of cartilage on diaphysis side of epiphyseal plate that is rapidly dividing

• new cells formed move upward, pushing epiphysis away from diaphysis, causing lengthening

Hypertrophic Zone

• area with older chondrocytes closer to diaphysis

• cartilage lacunae enlarge and erode, forming interconnecting spaces

Calcification Zone

• surrounding cartilage matrix calcifies; chondrocytes die and deteriorate

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Ossification Zine

• Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction

• Spicules are then eroded by osteoclasts and are covered with new bone by osteoblasts

• Ultimately replaced with spongy bone

• Medullary cavity enlarges as spicules are eroded

Growth in Width (Thickness)

• Growing bones widen as they lengthen through appositional growth

– Can occur throughout life

• Bones thicken in response to increased stress from muscle activity or added weight

• Osteoblasts beneath periosteum secrete bone matrix on external bone

• Osteoclasts remove bone on endosteal surface

• Usually more building up than breaking down which leads to thicker, stronger bone

that is not too heavy

Hormonal Regulation of Bone Growth

• Growth hormone: most important hormone inb 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

Bone Remodeling

• About 5–10% of our skeleton is replaced every year

– Spongy bone entirely replaced ~ every 3-4 years

– Compact bone entirely replaced ~ every 10 years

• Bone remodeling consists of both bone deposit and bone resorption

– Occurs at surfaces of both periosteum and endosteum

– Remodeling units: packets of adjacent osteoblasts and osteoclasts coordinate

 emodeling process

Bone Resorption

• Resorption is function of osteoclasts

– Dig depressions or grooves as they break down matrix

– Secrete lysosomal enzymes and protons (H+) that digest matrix

– Acidity converts calcium salts to soluble forms

• Osteoclasts also phagocytize demineralized matrix and dead osteocytes

– Digested products are transcytosed across cell and released into interstitial fluid

and then into blood

– Once resorption is complete, osteoclasts undergo apoptosis

• Osteoclast activation involves PTH (parathyroid hormone) and immune T cell proteins

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Bone Deposit

• New bone matrix is deposited by osteoblasts

• Osteoid seam: band of unmineralized bone matrix that marks area of new matrix

• Calcification front: abrupt transition zone between osteoid seam and older

mineralized bone

Control of Remodeling

Hormonal controls

– Parathyroid hormone (PTH): produced by parathyroid glands in response to low

blood calcium levels

 Stimulates osteoclasts to resorb bone

 Calcium is released into blood, raising levels

 PTH secretion stops when homeostatic calcium levels are reached

– Calcitonin: produced by parafollicular cells of thyroid gland in response to high

levels of blood calcium levels

 Effects are negligible, but at high pharmacological doses it can lower blood

calcium levels temporarily

Response to mechanical stress

– Bones reflect stresses they encounter

 Bones are stressed when weight bears on them or muscles pull on them

– Wolf’s law states that bones grow or remodel in response to demands placed

on them

 Stress is usually off center, so bones tend to bend

 Bending compresses one side, stretches other side

– Diaphysis is thickest where bending stresses are greatest

– Bone can be hollow because compression and tension cancel each other out in center of bone

Bone Repair

Fractures are breaks

– During youth, most fractures result from trauma

– In old age, most result from weakness of bone due to bone thinning

Fracture Classification

Three “either/or” fracture classifications

– Position of bone ends after fracture

 Nondisplaced: ends retain normal position

 Displaced: ends are out of normal alignment

– Completeness of break

 Complete: broken all the way through

 Incomplete: not broken all the way through

– Whether skin is penetrated

 Open (compound): skin is penetrated

 Closed (simple): skin is not penetrated

• Can also be described by location of fracture, external appearance, and nature of break

Fracture Treatment and Repair

• Treatment involves reduction, the realignment of broken bone ends

– Closed reduction: physician manipulates to correct position

– Open reduction: surgical pins or wires secure ends

– Immobilization of bone by cast or traction is needed for healing

 Time needed for repair depends on break severity, bone broken, and age

of patient

Repair involves four major stages:

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

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Hematoma formation

– Torn blood vessels hemorrhage, forming mass of clotted blood called a

hematoma

– Site is swollen, painful, and inflamed

Fibrocartilaginous callus formation

– Capillaries grow into hematoma

– Phagocytic cells clear debris

– Fibroblasts secrete collagen fibers to span break and connect broken ends

– Fibroblasts, cartilage, and osteogenic cells begin reconstruction of bone

 Create cartilage matrix of repair tissue

 Osteoblasts form spongy bone within matrix

– This mass of repair tissue is called fibrocartilaginous callus

Bony callus formation

– Within one week, new trabeculae appear in fibrocartilaginous callus

– Callus is converted to bony (hard) callus of spongy bone

– Bony callus formation continues for about 2 months until firm union forms

Bone remodeling

– Begins during bony callus formation and continues for several months

– Excess material on diaphysis exterior and within medullary cavity is removed

– Compact bone is laid down to reconstruct shaft walls

– Final structure resembles original structure

 Responds to same mechanical stressors

Bone Disorders

• Imbalances between bone deposit and bone resorption underlie nearly every disease

that affects the human skeleton.

• Three major bone diseases:

– Osteomalacia and rickets

– Osteoporosis

Osteomalacia and Rickets

• Osteomalacia

– Bones are poorly mineralized

– Osteoid is produced, but calcium salts not adequately deposited

– Results in soft, weak bones

– Pain upon bearing weight

• Rickets (osteomalacia of children)

– Results in bowed legs and other bone deformities because bones ends

are enlarged and abnormally long

– Cause: vitamin D deficiency or insufficient dietary calcium

Osteoporosis

• is a group of diseases in which bone resorption exceeds deposit

• Matrix remains normal, but bone mass declines

– Spongy bone of spine and neck of femur most susceptible

 Vertebral and hip fractures common

Risk factors f

– Most often aged, postmenopausal women

 Affects 30% of women aged 60–70 years and 70% by age 80

 Estrogen plays a role in bone density, so when levels drop at menopause,

women run higher risk

– Men are less prone due to protection by the effects of testosterone

– Insufficient exercise to stress bones

– Diet poor in calcium and protein

– Smoking

– Genetics

– Hormone-related conditions

 Hyperthyroidism

 Diabetes mellitus

– Consumption of alcohol or certain medications