BIOL 251 Exam 2

Functions of Bones

Support, Protection, Attachment point, Storage, Blood cell formation, Hormone production

Functions of Bones: Support

Holds up the body, cradles organs

Functions of Bones: Protection

• Central nervous system

- The skull protects the brain

- Vertebrae wrap around spinal cord

• Visceral organs

- Rib cage wraps around organs in thorax & upper abdominal cavity

Functions of Bones: Attachment point

Skeletal muscle attaches to bone via tendons

Functions of Bones: Storage

minerals, fat (yellow marrow in bones of adults)

Functions of Bones: Blood cell formation

hematopoiesis

Functions of Bones: Hormone production

osteoclacin regulates insulin release, glucose homeostasis, & organ expenditure

Hematopoiesis

formation of blood cells in red bone marrow

Characteristics of all cartilage types

Strength & resilience (cartilage can be compressed & return to original shape), High water content (contributes to flexibility), no nerve supply & avascular, surrounded externally by fibrous connective tissue called PERICHRONDRIUM (which is vascularized)

3 types of cartilage

hyaline, elastic, fibrocartilage

Hyaline cartilage

most abundant type of cartilage, contains collagen fibers

Hyaline cartilage examples

Articular cartilage, costal cartilage, respiratory joints cartilage, nasal cartilage

Elastic cartilage

contains more elastic fibers

Elastic cartilage examples

external ear, epiglottis

FIbrocartilage

Contain rows of chondrocytes alternating with thick collagen bands, most compressible & great tensile fibrocartilage strength

FIbrocartilage examples

vertebral discs, knee, pubic symphysis

Types of cartilage growth

appositional and interstitial

Appositional cartilage growth

laying down new cartilage on old cartilage

Cells just under perichondrium deposit new matrix on top of “old” cartilage

Occurs at surface of cartilage tissue

Appositional growth causes cartilage to increase WIDTH

Interstitial cartilage growth

Cells divide and secrete matrix within pre-existing cartilage

Occurs deeper in cartilage tissue

Interstitial growth causes cartilage to increase LENGTH

Bones can be classified by

location and shape

Location of bone

Axial skeleton or appendicular skeleton

Axial skeleton

long axis of body

skull, vertebral column, & ribs

Appendicular skeleton

limbs, pectoral girdle, pelvic girdle

Shape of bones

long bones, short bones, flat bones, irregular bones

Long bones

longer than they are wide

Long bone examples

almost all arm & leg bones

Short bone

cube-shapes

Short bone examples

bones in wrists & ankles, sesamoid bones

Sesamoid bones

bone that forms in a tendon (e.g. patella)

Flat bones

thin, flat, curved bones

Flat bone example

sternum, scapulae, ribs, most cranial bones

Irregular bones

anything that does not fit in an above category

Irregular bones example

vertebrae, os coxa (hip bones)

Gross anatomy of bone

outer layer of compact bone and inner layer of spongy bone

Compact (lamellar) bone

looks smooth and solid, no space, external surface

Spongy (trabecular) bone

has open spaces with needle-like no space pieces of bone called trabeculae

Open space is filled with red marrow or yellow marrow

Trabeculae found in greatest concentration along lines of stress

Structure of flat, irregular, & short bone

Thin plate of spongy bone covered by compact bone

No large cavities for bone marrow

4 similar features of long bones

diaphysis, epiphysis, membranes (periosteum & endosteum), vascularization & innervation

Long bones: Diaphysis

bone shaft

• Composed of compact bone “collar” with internal medullary cavity

• Cavity contains bone marrow

Long bones: Epiphysis

bone ends

• Composed of compact bone externally & you spongy bone internally joint (covered with articular cartilage)

• Location where one bone articulates with another bone at a joint and/or allows for attachment of ligaments and tendons

Long bones: membranes

periosteum & endosteum

Periosteum

covers external bone surface except at epiphysis (because epiphysis is covered w/ articular cartilage)

• Very well vascularized and innervated

Endosteum

overs internal bone surfaces (trabeculae in spongy bone, cavities in compact bone)

• Also contains osteoprogenitor cells

Long bones: vascularization & innervation

• Nutrient artery and nutrient vein serve diaphysis

• Epiphyseal artery and epiphyseal vein serve epiphyses

• Nerves travel with blood vessels

Osteon

structural unit of compact bone, helps bone withstand pressure/stress

A single osteon is composed of…

several layers called lamella packed closely together

For a single lamella,

collagen fibers run in one direction

In adjacent lamella,

collagen fibers always run in opposite directions

Central canals

run through center of each osteon

• Contains nerve and blood vessels

Perforating canals

extending from central canal connect neighboring osteons and medullary cavity

Interstitial lamellae

incomplete lamellae found in between complete osteons

Interstitial lamellae function

fill gaps between osteons

Circumferential lamellae

found just deep to periosteum

• Extend completely around circumference of diaphysis

Circumferential lamellae function

resists twisting of long bone

Red bone marrow (hematopoietic tissue)

production of blood cells

Red bone marrow (hematopoietic tissue) in INFANTS

adolescents are filled with red bone marrow

Red bone marrow (hematopoietic tissue) in ADULTS

skull, ribs, hips, sternum, clavicles, scapula, vertebrae, heads of femur and humerus

Yellow marrow

still vascularized, contains more fat & less blood supply than red marrow, can be converted back to red marrow in life-saving conditions

Yellow marrow in ADULTS

medullary cavity of long bones

4 cell types responsible for bone growth

osteoprogenitor (osteogenic) cells, osteoblasts, osteocytes, osteoclasts

Osteoprogenitor (osteogenic) cells

stem cells, mitotically active cells, can remain as osteogenic cells or differentiate to form osteoblasts

Osteoblasts

bone-forming cells, secrete unmineralized matrix (osteoid) that forms bone tissue

Osteocytes

mature bone cell, respond to mechanical stress on bone and chemical signals, often have several projections of cell membrane surface

Osteoclasts

bone-degrading cells; maintains, repairs, and remodels bones; important function in blood calciium homeostasis

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Organic chemical composition of bone

cells and osteoid (ground substance and fibers)

Inorganic chemical composition of bone

mineral salts (mostly calcium phosphate packed around collagen fibers)

Osteomalacia (adults) & Ricket’s (children)

Less mineral salts deposited in bone than normal

Bone is weak/soft, bends more easily

Osteomalacia (adults) & Ricket’s (children) cause

insufficient calcium in diet and/or vitamin D deficiency

Endochondral ossification

Formation of ossified bone by replacement of cartilage with bone

Occurs in most bones below the skull

Hyaline cartilage used as a “blueprint” to form ossified bone

Endochondral ossification steps

formation of bone collar, cavity forms in diaphysis center, formation of intial spongy bone in diaphysis, formation of medullary cavity & elongation of diaphysis, secondary ossification continues in epiphyses

Endochondral Ossification: Step 1

Formation of a bone collar

Osteoblasts lay down bone matrix against cartilage surface to form a collar

What happens after bone collar formation?

Primary ossification center (POC)

POC is rigid & tough on outside, filled with cartilage on inside

Endochondral Ossification: Step 2

Cavity forms in diaphysis center, cells inside ossification center die off, cartilage inside POC breaks down, cartilage outside cavity continues to glow (elongates bone)

Endochondral Ossification: Step 3

Formation of initial spongy bone in diaphysis, periosteal bud invades cavity, osteoblasts secrete matrix around calcified cartilage of cavity formed

What does the periosteal bud contain?

nutrient artery/vein, nerve f fibers, red marrow elements, osteoprogenitor cells, osteoclasts

Endochondral Ossification: Step 4

Formation of medullary cavity & elongation of diaphysis (forms the MEDULLARY CAVITY)

Cartilage in remaining diaphysis is calcified, broken down, replaced with bone

Secondary ossification center forms in epiphyses

Endochondral Ossification: Step 5

Secondary ossification continues in epiphyses

Bone growth in length

Occurs at epiphyseal plate by interstitial growth

Process of growth in length

New cartilage laid down in epiphyseal plate, cartilage cells at center of epiphyseal plate enlarge & cartilage is calcified, calcified cartilage is broken down by osteoclasts & osteoblasts lay down new bone tissue

Bone growth in width

occurs at same time in bone lengthening by appositional growth

Process of growth in width

Osteoblasts secrete new matrix at the periosteum, osteoclasts break down bone tissue at the endosteum

Growth hormone

controls activity at the epiphyseal plate

Growth hormone is released by

anterior pituitary gland in brain

More active growth hormone leads to

gigantism

Less growth hormone leads to

shorter people, dwarfism

Sex hormones

estrogen & testosterone

Estrogen

causes growth spurt at puberty, causes “feminization” of certain parts of skeleton

High levels of estrogen induces

epiphyseal plate closure

Testosterone

causes “masculinization” of certain parts of skeleton

What is the typical age at which a male stops growing?

21

What is involved in bone remodeling?

bone deposition and resorption

Bone deposition

new bone tissue by osteoblasts

Bone resorption

breaking down old bone tissue with osteoclasts

Importance of bone remodeling

Maintenance of Ca²⁺ homeostasis, bone health

Maintenance of Ca²⁺ homeostasis

Ca²⁺ is essential for excitability of body cells (especially neurons and muscle cells)

Bone health

Mechanical/gravitational forces acting on bone tissue drive remodeling → strengthens bone exactly where it is needed

2 factors of control of deposition and resorption

Parathyroid hormone (PTH) and mechanical stress

Parathyroid hormone (PTH)

released in response to decreasing blood Ca²⁺ levels

Once blood Ca²⁺ returns to normal, PTH release decreases

Effects of increased PTH release

Number of osteoclasts at bone increases

Osteoclasts become more active in bone tissue