Bone growth, remodeling and repair

Process of Ossification

-6 weeks into development, skeleton is all cartilage -cartilage cells then replaced by bone cells

Calcitonin

-Secreted from C cells in the thyroid gland Responses: in bones- osteoclast activity inhibited; calcium deposited in bone matrix In intestines: calcium absorption decreased In kidneys:inhibits calcium reabsorption

Intramembranous Ossification

-begins when mensynchmal cells differentiate into osteoblasts -occurs in deeper layers of tissue -bones called dermal bones or membrane bones (i.e. roofing bones of skull, lower jaw, collar bone)

Endochondral Ossification

-initial skeleton of embryo formed of hyaline cartilage -cartilage replaced by bone through endochondral ossification -bones grow in diameter and length

Steps of Endochondral Ossification

1. Cartilage model enlarges -enlarge chondrocytes die and disintegrate 2. blood vessels grow around the edge of the cartilage model 3. Blood vessels penetrate cartilage and enter central region -begin spongy bone production at primary ossification center 4. Growth continues along with remodeling -medullary cavity created 5. Capillaries and osteoblasts migrate into the epiphysis -creates secondary ossification center 6. epiphysis fill with spongy bone -epiphyseal cartilage separates epiphysis from diaphysis

Fracture repair steps

1. Frature hematoma formation -large clot closes injured vessels 2. Callus Formation -internal: network of spongy bones that unites inner edges of fracture -external:composed of cartilage and bone that stabilizes outer edges of fracture 3. Spongy bone formation -cartilage of external callus replaced by spongy bone -bone fragments and dead bone are removed and replaced 4.Compact bone formation -spongy bone replaced by compact bone (remodeling)

Steps of Intramembranous Ossification

1. ossification center -mesenchymal cells secrete osteoid matrix; osteoid matrix becomes mineralized 2. Bones grow out in small struts (spicules) -osteoblasts get trapped in pockets and mature into osteocytes 3.blood vessels enter area -bones spicules meet and fuse; blood vessels trapped in developing bone 4.continue deposition of bone by osteoblast close to blood vessel 5. remodeling around blood vessels produces osteons of compact bone

Importance of calcium

99% deposited in skeleton functions: muscle contraction blood coagulation nerve impulse generation

General categories of fractures

Closed or simple - all internal (doesnt break in skin) Open or compound - project through skin -dangerous because infections and uncontrolled bleeding

Types of Ossification

Intramembranous Ossification -involved with the development of clavicle, mandible, skull and face Endochondral Ossification -involved with development of vertebrae, hips, and limbs

What factors increase blood calcium levels?

Parathyroid Hormone Calcitonin

Types of fractures

Transverse fracture -break shaft across long axis spiral fracture -produced by twisting stress -spread along length of bone displaced frature -produced new and abnormal bone arrangements -non displaced fractures retain normal alignment comminuted fracture -shatter affected area producing fragments

How doe a fracture heal?

as long as blood supply and cellular parts of periosteum and endosteum survive

Osteogenesis

bone formation

Calcification

deposition of calcium into the bone tissue

Maintaining calcium levels

intestines: absorb calcium and phosphate under hormonal control Bones: osteoclasts erode matrix and release calcium osteoblasts use calcium to deposit new matrix kidneys: varying levels of calcium and phosphate loss in urine

How are minerals and bone composition important for bones?

minerals contribute to the osmotic balance of body fluids 67% inorganic components 33% organic components 99% of bodys calcium is in bones

Parathyroid hormones

secreted from parathyroid glands responses: in bones- osteoclasts erodes matrix, releasing stored calcium in intestine- enhanced and calcium absorption increased in kidneys- calcium reabsorption

Aging and the skeletal system

when osteoclast activity is faster than osteoblasts activity, bones become porous -estrogen keeps osteoclasts activity under control -bones become porous (osteoporosis)