the skeletal system:bone tissuebone marrow (called hematopoiesis) •energy storage in yellow bone...
TRANSCRIPT
6-1
The Skeletal System:Bone Tissue
• Dynamic and ever-changing throughout life
• Skeleton composed of many different tissues
– cartilage, bone tissue, epithelium, nerve, blood forming tissue, adipose, and dense connective tissue
6-2
Functions of Bone
• Supporting & protecting soft tissues
• Attachment site for muscles making
movement possible
• Storage of the minerals, calcium &
phosphate -- mineral homeostasis
• Blood cell production occurs in red
bone marrow (called hematopoiesis)
• Energy storage in yellow bone marrow
6-3
Anatomy of a Long Bone• Diaphysis = shaft
• Epiphysis = one end of a long bone, expanded portion, forms a joint (articulates) with another bone
• Metaphysis = growth plate region
• Articular cartilage covers joint surfaces, acts as friction & shock absorber
• Medullary cavity = marrow cavity
• Endosteum = lining of marrow cavity
• Periosteum = tough membrane covering entire bone, except for epiphyses but not the cartilage
• Projections of bones (called processes) provide sites for the attachment of ligaments and tendons
• Grooves and openings in the bone serve as passageways for blood vessels and nerves
6-4
Histology of Bone
• A type of connective
tissue as seen by widely
spaced cells separated by
matrix
• Matrix of 25% water,
25% collagen fibers &
50% crystalized mineral
salts (hydroxy apatite)
• 4 types of cells in bone
tissue
6-5
Cell Types of Bone
• Osteoprogenitor cells ---- undifferentiated cells (stem cells)
– can divide to replace themselves & can become osteoblasts
– found in inner layer of periosteum and endosteum
• Osteoblasts--form matrix & collagen fibers but can’t divide (build
new bone)
• Osteocytes ---mature cells that no longer secrete matrix
• Osteoclasts---- huge multinucleated cells
– function in bone resorption at surfaces such as endosteum (break down bone)
6-6
Matrix of Bone• Inorganic mineral salts provide bone’s hardness and make it
resistant to crushing
– hydroxyapatite (calcium phosphate) & calcium carbonate
• Organic collagen fibers provide bone’s strength and
flexibility
– their tensile strength resists being stretched or torn
– remove minerals with acid & rubbery structure results
• Calcification is hardening of tissue when mineral crystals
deposit around collagen fibers
• Bone is not completely solid since it has small spaces for
vessels and red bone marrow
– spongy bone has many such spaces
– compact bone has very few
6-7
Types of Bone in a Long Bone
• The wall of the diaphysis of a long bone is
composed of tightly packed tissue called
compact bone.
• The epiphyses are composed of spongy
bone with thin layers of compact bone on
the surface.
6-8
Histology of Compact Bone• Osteon consists of concentric rings (lamellae) of calcified matrix
surrounding a vertically oriented blood vessel in a passage called the osteonic canal . Osteons give bone its substance.
• Osteocytes found in spaces called lacunae.
• Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next cell
• Interstitial lamellae represent older osteons that have been partially removed during tissue remodeling
6-9
The Trabeculae of Spongy Bone • Latticework of thin plates of bone called trabeculae oriented
along lines of stress
• Spaces in between these struts are filled with red marrow where
blood cells develop
• Found in ends of long bones and inside flat bones such as the
hipbones, sternum, sides of skull, and ribs.
No true Osteons.
6-10
Bone Formation or Ossification
• All embryonic connective tissue begins as
mesenchyme.
• Intramembranous bone formation = formation
of bone directly from mesenchymal cells.
• Endochondral ossification = formation of bone
from hyaline cartilage.
6-11
Intramembranous Bone Formation
• Mesenchymal cells become osteoprogenitor cells then osteoblasts.
• Osteoblasts surround themselves with matrix to become osteocytes.
• Matrix calcifies into trabeculae with spaces holding red bone marrow.
• Mesenchyme condenses as periosteum at the bone surface.
• Superficial layers of spongy bone are replaced with compact bone.
6-12
Endochondral Bone Formation (1)
• Development of Cartilage model
– Mesenchymal cells form a cartilage
model of the bone during development
• Growth of Cartilage model
– in length by chondrocyte cell division
and matrix formation
– in width by formation of new matrix
on the periphery by new chondroblasts
from the perichondrium
– cells in midregion burst and change
pH triggering calcification and
chondrocyte death
6-13
Endochondral Bone Formation (2)
• Development of Primary Ossification Center– nutrient artery penetrates center of
cartilage model
– perichondrium changes into periosteum
– osteoblasts and osteoclasts migrate to center of cartilage model
– osteoblasts deposit bone matrix forming spongy bone trabeculae
– Osteoclasts destroy trabeculae forming medullary cavity. Later compact bone forms
6-14
Endochondral Bone Formation (3)
• Development of Secondary Ossification Center
– blood vessels enter the epiphyses around time of birth
– spongy bone is formed but no medullary cavity
• Articular Cartilage
– cartilage on ends of bone remains as articular cartilage.
6-15
Bone Growth in Length
• Epiphyseal plate or cartilage growth plate
(exists between primary and secondary
ossification centers)
– Are bands of cartilage. As these cartilage
cells grow and divide, the bone grows in
length.
– cartilage cells are produced by mitosis on
epiphyseal plate
– cartilage cells are destroyed and replaced
gradually by bone
– Between ages 18 to 25, epiphyseal plates
close( bone replaces the cartilage, epiphyseal
line) and bone growth stops.
• Growth in length stops at age 25
6-16
Bone Growth in Width
• Only by appositional growth at the bone’s surface
• Periosteal cells differentiate into osteoblasts and form bony ridges
and then a tunnel around periosteal blood vessel.
• Concentric lamellae fill in the tunnel to form an osteon.
6-17
Factors Affecting Bone Growth
• Nutrition
– adequate levels of minerals and vitamins
• calcium and phosphorus for bone growth
• vitamin C for collagen formation
• vitamins K and B12 for protein synthesis
• vitamin D for calcium absorption
• Sufficient levels of specific hormones
– during childhood need insulinlike growth factor
• promotes cell division at epiphyseal plate
• need hGH (growth), thyroid (T3 &T4) and insulin
– sex steroids at puberty
• growth spurt and closure of the epiphyseal growth plate
• estrogens promote female changes -- wider pelvis
6-18
Types of Bone Marrow
1. Red marrow: forms red blood cells, white blood cells,and platelets. Blood cell formation is called hematopoiesis.
• Is red due to the presence of the oxygen carrying pigment, hemoglobin
• Is most numerous in infants. As infants get older, it is replaced by yellow marrow.
2. Yellow marrow: functions in fat storage
6-19
Hormonal Abnormalities
• Oversecretion of hGH during childhood
produces gigantism
• Undersecretion of hGH or thyroid hormone
during childhood produces short stature
• Both men or women that lack estrogen
receptors on cells grow taller than normal
– estrogen responsible for closure of growth plate
6-20
Bone Remodeling
• Ongoing since osteoclasts carve out small tunnels and osteoblasts rebuild osteons.
– Osteoclasts dissolve bone matrix and release calcium and phosphorus into interstitial fluid(bone resorption)
– osteoblasts take over bone rebuilding (bone formation)
• Continual redistribution of bone matrix along lines of mechanical stress
– distal end of femur is fully remodeled every 4 months
6-21
Calcium Homeostasis & Bone Tissue
• Skeleton is reservoir of Calcium & Phosphate
• Calcium ions involved with many body systems
– nerve & muscle cell function
– blood clotting
– enzyme function in many biochemical reactions
• Small changes in blood levels of Ca+2 can be deadly (plasma level maintained 9-11mg/100mL)
– cardiac arrest if too high. If too high, osteoblasts use excess to build new bone.
– respiratory arrest if too low. If too low, osteoclasts break down bone to replace blood levels of calcium.
6-22
Hormonal Influences
• Parathyroid hormone (PTH) is secreted if Ca+2 levels falls
– PTH gene is turned on & more PTH is secreted from gland
– osteoclast activity increased, kidney retains Ca+2.
• Calcitonin hormone is secreted from thyroid if Ca+2 blood levels get too high
– inhibits osteoclast activity
– increases bone formation by osteoblasts
6-23
Exercise & Bone Tissue
• Pull on bone by skeletal muscle and gravity is
mechanical stress .
• Stress increases deposition of mineral salts &
production of collagen (calcitonin prevents bone
loss)
• Lack of mechanical stress results in bone loss
– reduced activity while in a cast
– astronauts in weightlessness
– bedridden person
• Weight-bearing exercises build bone mass (walking or weight-lifting)
6-24
Osteoporosis
• Decreased bone mass resulting in porous bones
• Those at risk
– white, thin menopausal, smoking, drinking female with
family history
– athletes who are not menstruating due to decreased body
fat & decreased estrogen levels
– people allergic to milk or with eating disorders whose
intake of calcium is too low
• Prevention or decrease in severity
– adequate diet, weight-bearing exercise, & estrogen
replacement therapy (for menopausal women)
– behavior when young may be most important factor