chapter 7: bone tissue

osteology

osteology

the study of bone

the skeletal system is composed of

• bones

• cartilages

• ligaments

• tendons

bones

• complex living organs

• Made of osseous tissue, blood, bone marrow, cartilage, adipose, nervous tissue, fibrous connective tissue​

• Connective tissue matrix is hardened and mineralized (calcified)​

ligaments connect

bone to bone

tendons connect

muscle to bone

functions of the skeleton

• support​

• protection- brain, spinal cord, organs​

• movement- move limbs, breathing​

• Electrolyte balance​---- Particularly calcium (Ca2+) and phosphate (PO42-)​

  released into tissue and blood from bone ​

• Blood formation – bone marrow produces blood cells​

compact (cortical) bone- osseous tissue

dense outer layer of osseous tissue

spongy (trebacular) bone- osseous tissue

consists of lattice of trebaculae and spaces filled with bone marrow

long bones

• Longer than wide​

• Produced primarily by endochondral ossification​

• Act as rigid levers to allow movement​

• Bones of the limbs, hands, feet, fingers, toes

flat bones

ex: cranial bones, sternum, scapula, ribs, portions of the hip bones

• formed like a sandwich - compact bone on outside and spongy bone in the middle

• made of crossbars called trabeculae with “spaces” filled with bone marrow

short bones

• about the same in length and width

• often slide against each other to produce gliding movements

• ex: wrist and ankle bones, patells

irregular bones

• do not fit into any of these other categories

• ex: vertebrae, ossicles of the ear

articular cartilage- gross anatomy of long bones

• hyaline cartilage covering both ends

• reduces friction between bones

diaphysis- gross anatomy of long bones

• shaft

• compact bone covering spongy bone

• contains medullary cavity --- marrow cavity (yellow or red marrow)

epiphyses- gross anatomy of long bones

• ends of long bones

• outer compact bone and interior spongy bone

• adults have an epiphyseal line --- remnant of epiphyseal plate

• epiphyseal plate - “growth plate” during childhood

periosteum- gross anatomy of long bones

2 layer membrane

• outer - fibrous layer (dense irr CT)

• inner - osteogenic layer→ contains osteogenic cells that differentiate into bone cells

• provides attachment for tendons and ligaments

• rich supply of nerve fibers and blood vessels → pass through diaphysis to medullary cavity via nutrient foramina

endosteum - gross anatomy of long bones

internal surface membrane

• dense irregular CT

• contains osteogenic cells that differentiate into bone cells

• covers trabeculae of spongy bone

gross anatomy- flat bones

• spongy bone covered by compact bone

• has periosteum and endosteum

• no diaphysis or epiphyses

• ex: skull bones, sternum, scapula, and ribs

gross anatomy - short bones

• about equal in length and width

• spongy bone covered by compact bone

• has periosteum and endosteum

• no diaphysis, epiphyses, or growth plates

• ex: carpals (wrist) and tarsals (ankle)

sesamoid bones

short bone that forms in tendon (kneecap)

gross anatomy - irregular bones

• spongy bone covered by compact bone

• has periosteum and endosteum

• no diaphysis, epiphyses, growth plates

• ex: vertebrae and coxal (hip) bones

osteon - compact bone

fundamental unit (haversion system)

lamella - compact bone

bone matrix around osteon

concentric lamellae

around osteons

circumferential lamellae

perimeter of bone around diaphysis

lacuna - compact bone

space containing the osteocyte

central canal - compact bone

contains blood vessels and nerves (haversion canal)

perforating (volkmann’s) canal - compact bone

transverse canals & link to central canals

canaliculi - compact bone

microscopic canals between lacunae

anatomy of spongy bone

• consists of spicules and trabeculae

• hardened lamella containing osteocytes

• spaces filled with bone marrow

• trabeculae NOT randomly arranged along stress lines

bone marrow (soft tissue)

occupies

1. medullary (marrow) cavity of long bones

2. spaces between trabeculae of spongy bone

red bone marrow

site of hematopoiesis (blood cell formation)

AKA myeloid tissue or hemopoietic tissue

yellow bone marrow

fat storage

red bone marrow found in

infants - dominates all marrow cavities

adults - found in skull, vertebrae, ribs, sternum, and proximal heads of humerus and femur

yellow marrow is found

adults - medullary cavvity of long bone diaphyses

osteogenic cells (unipotent)

give rise to most other bone cells

• found in the osteogenic layer of periosteum and endosteum

• multiply continually

• some differentiate into osteoblasts

osteoblasts

bone secreting cells

• secrete protein mixture (osteoid) that hardens and becomes the bony matrix

• NONMITOTIC---- formed from osteogenic cells in response to mechanical stress

osteocytes

mature bone cells (osteoblasts that have trapped themselves in the matrix they secreted)

• reside in spaces called lacunae which are connected by canaliculi

• function to maintain bony matrix

• also play a role in sensing stressors and influence bone remodeling

what do canaliculi allows osteocytes to do?

they allow them to connect and communicate via gap junctions

osteoclasts

bone dissolving cells

• perform osteolysis (opposite of osteogenesis)

• do NOT develop osteogenic cells

• formed from the fusion of stem cells

• ruffled border present in reabsorption bays-----involved in bone remodeling and blood calcium homeostasis

the matrix of osseous tissue (organic material)

• about 35% of bone tissue

• called OSTEOID

• provides flexibility and tensile strength

• secreted by osteoblasts---- mixture of collagen, proteoglycans, and glycoproteins

• collagen molecules have sacrificial bonds

the matrix of osseous tissue (inorganic material)

• about 65% of bone tissue

• provides rigidness and compression resistance

• 85% = hydroxyapatite (crystalized calcium phosphate salt)

  10 % = calcium carbonate

  5% = magnesium, sodium, potassium, fluoride,

  sulfates, carbonates, and hydroxide ions

ossification or osteogenesis =

bone formation

3 major processes of bone development

1. bone formation = embryos through early childhood

2. bone growth = embryos through early adulthood (early 20’s)

3. bone remodeling = lifelong (i.e. only type of bone development in adults)

before week 8, the embryo skeleton consists of

fibrous membranes and hyaline cartilage

endochondrial ossification

• replacing hyaline cartilage with bone

• most bones of the body formed this way (limbs, vertebrae, ribs, scapula, and pelvis)

intramembranous ossification

• bone develops from fibrous membranes

• forms the flat bones of the skull and parts of the clavicle

1st step of intramembranous ossification

• formation of membrane from mesenchymal cells

• membrane is invaded by blood vessels

• change in nutrition leads to the differentation of mesenchymal cells to osteoblasts

2nd step of intramembranous ossification

• osteoblasts secrete osteoid and mineralization of the matrix occurs

• blood vessels are trapped in smaller spaces

• osteoblasts surround the blood vessels, lay matrix, get trapped, and become osteocytes

3rd step of intramembranous ossification

• more condensation of mesenchyme occurs on both sides of the original membrane = FORMS THE PERIOSTEUM

• again, osteoblasts form IN the periosteum and lay down bony matrix completing the spongy bone

4th step of intramembranous ossification

continued action of the osteoblasts from the periosteum results in the production of compact bone layers

1st part of endochondral ossification

1-8 week embryo - mesenchyme develops into hyaline cartilage covered with perichondrium in place of future bones

• chondrocytes produce cartilage to increase thickness

2nd part of endochondral ossification

primary ossification center forms when chondrocytes die and degenerate forming medullary cavity

• perichondrium starts producing osteoblasts and becomes periosteum

• osteoblasts secrete a bony collar around diaphysis

3rd step of endochondral ossification

vascular tissue invades marrow cavity, delivers osteoclasts which hollow out the marrow cavity and osteoblasts which create new bone

• cartilage degenerates and is replaced by osseous tissues by osteoblasts

• cartilage degeneration region = metaphysis

• secondary ossification center forms in epiphysis(ses)

4th step of endochondral ossification

at birth - osteoclasts break down newly formed spongy bone creating medullary cavity

• cartilage in epiphysis (ses) keep growing

• secondary marrow cavity forms

5th step of endochondral ossification

infancy and childhood - epiphyses fill with spongy bone

• cartilage is only articular (covering epiphyses) or in the epiphyseal plate (growth plate)

• epiphyseal plate allows for bone elongnation via interstitial growth

6th step of endochondral ossification

adulthood - cartilage in epiphyseal plate is depleted

growth plates are “closed”

• bones continuing thickening and remodeling but no longer grow in length

1.

interstitial growth in the metaphysis- interstitial growth@the epiphyseal plates

mitosis of chondroblasts in the zone of proliferation and subsequent growth of those chondroblasts in the zone of hypertrophy pushes the zone of reserve cartilage towards the ends of bones

---this causes bones to get longer

2.

interstitial growth in the metaphysis- interstitial growth @the epiphyseal plates

towards the end of adolescence, the chondroblasts of the epiphyseal plate divide less often

---this results in the “thinning” of the epiphyseal plate

3.

interstitial growth in the metaphysis- interstitial growth@the epiphyseal plates

eventually the bone of the epiphysis fuses with bone of the diaphysis

• called epiphyseal plate closure

• usually occurs around 18 years of age for females and 21 years of age for males

zone of reserve cartilage - interstitial growth at the epiphyseal plates

typical histology of resting hyaline cartilage

zone of cell proliferation - interstitial growth at the epiphyseal plates

chondrocytes multiplying and lining up in rows of small flattened lacunae

zone of cell hypertrophy - interstitial growth at the epiphyseal plates

cessation of mitosis; enlargement of chondrocytes and thinning of lacuna walls

zone of calcification - interstitial growth at the epiphyseal plates

temporary calcification of cartilage matrix between columns of lacunae

zone of bone deposition - interstitial growth at the epiphyseal plates

breakdown of lacuna walls, leaving open channels; death of chondrocytes; bone deposition by osteoblasts, forming trabeculae of spongy bone

appositonal growth (bone thickening)

same process as intramembranous ossification

• osteoblasts beneath the periosteum lay new bone, creating the circumferential lamellae

• osteoclasts on endosteum remove bone

• leads to increase in overall thickness of the bone while keeping bone weight low

bone remodeling - bone depostion and reabsorption

occur at the surface of the periosteum (osteogenic layer) and endosteum

involves remodeling units

• osteoblasts and osteoclasts clustered together

• directed by stress sensing osteocytes

in a healthy adult:

bone mass stays constant

• amount of reabsorption = amount of deposition

as you age, reabsorption > deposition

• can lead to osteoporosis

wolff’s law

• remodel in response to mechanical stress

• trabeculae align with the direction of mechanical stress

effects of wolff’s law

• Bones in one limb are thicker than those of the less used limb​

• Curved bones are thickest where they are most likely to fracture ​

• Trabeculae form along lines of compression​

• Large bony projections occur at sites of strong, active muscles ​

• Fetus bones are featureless​

• Bedridden people (and astronauts) have net bone loss​

• "text neck" can cause permanent changes to your vertebrae

mineral deposition

osteoblasts create organic osteoid (mostly collagen fibers) as a template for osseous tissue

minerals crystallize on the osteoid

• calcium, phosphate, and other ions from the blood are deposited as hydroxyapatite crystals​

• Only occurs when a critical concentration of calcium and phosphate are available (i.e. homeostatic)​

Osteoblasts must also “neutralize” inhibitors that prevent mineralization ​

• Inhibitors prevent the formation of calculus (calcified masses) in the wrong places (ectopic ossification)​

• arteioclerosis  – calcification (i.e. hardening) of the arteries ​→leads to high blood pressure, kidney failure, heart failure ​

mineral reabsorption

bone reabsorption due to osteoclasts

• Detect falling levels of calcium in the tissue fluid​

• Secrete lysozymes - enzymes break down collagen in the bone​

• Secrete hydrocloric acid - breaks down inorganic components (hydroxyapatite)​

• Liberates the minerals and ions (e.g. calcium) back into the bloodstream

calcium homeostasis

bone remodeling maintains blood calcium levels

• calcium is required for muscle contraction, nervous system physiology, and other things

hypocalcemia

deficiency in blood calcium levels

• leads to excessive excitability of neurons and muscle tetany

• trosseau’s sign occurs when blood pressure cuff presses the brachial nerve

• can cause rickets (chronic hypocalcemia)

hypercalcemia

too much blood calcium (less common)

• decrease neuron excitability and muscle function

• sluggish reflexes, depression of the nervous system, emotional disturbances, cardiac arrest

calcium homeostasis depends on

• balance between dietary intake and urinary/fecal loss of calcium

• calcium exchange with osseous tissue (i.e. bone remodeling)

regulated by hormones

calcitrol, calcitonin, and parathyroid hormones

calcitrol (form of vitamin D)

calcitrol raises blood calcium levels by

• increasing calcium absorption by the small intestine

• reducing calcium excretion in feces

• increasing bone reabsorption by stimulating osteoclast activity

calcitonin

• secreted by the thyroid gland in response to elevated blood calcium levels

reduces blood calcium levels by…

• reducing osteoclast (destroys bone) activity

• stimulating osteoblasts (builds bone) activity

parathyroid hormone

• released by parathyroid gland in response to low blood calcium levels

raises blood calcium by…

• increasing osteoclast production

• inhibition of osteoclast ability to secrete collagen

• increases calcium reabsorption by kidneys

• increases phosphate excretion by kidneys (prevents bone mineralization)

fractures - breaking a bone

• stress fractures - purely mechanical damage

• pathological fractures - imbalances in bone physiology

classifications:

1. nondisplaced vs displaced

2. complete vs incomplete

3. compound (skin open) vs simple (skin closed)

fracture treatment and repair - realign the broken bones

closed (external) reduction - bones aligned “by hand” (cast only)

open (internal) reduction - bones secured with surgical pins and wires

fracture treatment and repair - immobilization

• time to heal varies with age (longer if older), severity, and age

• hard callus takes 4-6 weeks to form (time in cast)

bone healing step 1

1. hematoma formation

the hematoma is converted to granulation tissue by invasion of cells and blood capillaries

bone healing step 2

2. soft callus formation

deposition of collagen and fibrocartilage converts granulation tissue to soft callus

bone healing step 3

3. hard callus formation

osteoblasts deposit in a temporary bony collar around the fracture to unite the broken pieces while ossification occurs

bone healing step 4

4. bone remodeling

small bone fragments are removed by osteoclasts, while osteoblasts deposit spongy bone and then convert it to compact bone

osteomalacia (adults)

• Osteoid is produced but adequate calcium is not mineralized​

• Pain in back and joints, muscle weakness, trouble walking, spinal deformity

rickets (children)

• leads to bowed legs, deformitites of pelvis, skull, and rib cage

• not common in the U.S.

osteomalacia and rickets are caused by

insufficient calcium in diet or vitamin D deficiency

• drink milk (or almond milk) fortified with vitamin D or get out in the sun with no sunscreen on for 15 minutes, 3 days per week

osteoporosis

• bone reabsorption is significantly greater than deposition

• bones become fragile - particularly the spine and the head of the femur

risk factors for osteoporosis

• being older, particularly postmenopausal women

• lack of exercise

• diet low in calcium, protein, vitamin D

• smoking and sedentary lifestyle

• hyperthyroidism, low blood levels of TSH, diabetes mellitus

• genes: european and asian ancestry

treating osteoporosis

• calcium and vitamin D supplements

• weight- bearing exercise

• new drugs that reduce osteoclast activity or mimic estrogen

prevention of osteoporosis

increase bone density when young

• weight-bearing exercise

• proper diet

• don’t smole

• moderation with alcohol

• don’t drink sodas

• active lifestyle