Bone Cells &
Bone Development
Cells of Bone Tissue
Let’s meet Bob and Claude!
Osteoprogenitor (Osteogenic) Cells
 Embryonic cells that divide to produce
osteoblasts
 Are located in inner periosteum &
endosteum
 Assist in fracture repair
Osteoblasts
 Immature bone producing
cells that secrete matrix
compounds
 not yet calcified to form
bone
 Osteoblasts surrounded
by osseous tissue become
osteocytes
Osteocytes
 Mature bone cells that
maintain the bone matrix
 Do not divide
Osteoclasts
 Breaks down bone
 Dissolve bone matrix and release stored
minerals which is taken up by the blood
 Causes osteoporosis (loss of bone tissue) if
bone is unable to repair
Bone Metabolism
 Bone stores 99% of total calcium in body
 Calcium ions involved with many body
systems
 nerve & muscle cell function
 blood clotting
 enzyme function in many biochemical
reactions
Bone Homeostasis and
Metabolism
Bone Metabolism
(Background)
 The thyroid gland is located in the front of
the neck and wraps around the trachea;
has a shape that is similar to a butterfly
formed by two wings (lobes) and helps to
regulate metabolism
Bone Metabolism: Hormones
 Parathyroid hormone (PTH) is secreted if
Ca2+ levels falls
 PTH gene is turned on and more PTH is
secreted from gland, osteoclast activity
increased
 Calcitriol is a hormone that works with PTH &
promotes absorption of calcium in digestive
tract & blood, to prevent it from being
released in the urine.
Bone Metabolism: Hormones
 Calcitonin is a hormone that produces the
opposite effects:
 if calcium level gets too high, there is less
absorption in the digestive tract, increased
excretion by the kidneys, and movement of
calcium from the blood into the bones
 inhibits osteoclast activity
 increases bone formation by osteoblasts
Bone Remodeling
 Ongoing process
 Osteoclasts secrete enzymes and acids
to break down bone
 This releases calcium and other minerals of
bone for resorption by blood
 Osteoblasts rebuild osteons
Bone Tissue and Exercise
 Pull on bone by skeletal muscle and gravity is
mechanical stress
 Lack of mechanical stress results in bone loss
 reduced activity while in a cast or bedridden person
 Weight-bearing exercises build
bone mass
Osteoporosis Prevention
 Calcium supplements, weight-bearing
exercise, & hormone replacement therapy
(including a form of calcitonin)
 behavior when young may be most important
factor!
Disorders of the Skeletal System
 Failure of bone calcification
 Rickets
 Growing children
 Osteomalacia
 adults
Herniated Disc
 Ligaments of the intervertebral discs
become weak or are injured, the
surrounding fibrocartilage ruptures, and
the material inside protrudes
Bunion
 Inflammation of the joint, bone spurs,
calluses
 Caused by wearing tight fitting shoes
Abnormal Spinal Curvatures
Fractures
 A fracture is break in a bone
Fracture Repair
 A fracture is break in a bone
 Stage 1 (Inflammation)
 Repair starts with formation of fracture
hematoma damaged blood vessels produce
clot, bone cells die, new capillaries grow into
damaged area
 Stage 2 (Repair)
 Callus (healing tissue) formation- fibroblasts
lay down collagen fibers, chondroblasts
produce cartilage to span the broken areas of
the bone
Fracture Repair
 Bone Formation:
 osteoblasts secrete spongy bone that joins
the broken areas of bone
 lasts 3-4 months
 Bone remodeling:
 compact bone replaces the spongy in the
callus
 surface is remodeled back to normal shape
Bone Formation
 Osteogenesis:
 bone formation
 Ossification:
 the process of replacing
other tissues with bone
Ossification
 1) Intramembranous ossification (dermal
ossification)
 occurs in the dermis
 Produces bones such as mandible and clavicle
 2) Endochondral ossification
 Bone replaces cartilage
 Most bones formed this way
 Observed easily in long bones
Ossification
1. Chondrocytes in the center of hyaline
cartilage enlarge, calcify, and die, leaving
cavities in cartilage
Ossification
2. Blood vessels grow around the edges of the cartilage
and osteogenic cells begin to change to osteoblasts.
Produces layer of superficial bone around shaft
which becomes compact bone
Ossification
3. Blood vessels enter the cartilage. Spongy bone
develops at the primary ossification center in the
shaft where bone tissue replaces cartilage. A
marrow cavity is created.
Ossification
4. Capillaries and osteoblasts enter the
epiphyses creating secondary ossification
centers
Ossification
5. Epiphyses fill with spongy bone (no cavity in
this region of the bone). On the ends, hyaline
cartilage that remains is articular cartilage.
Epiphyseal Plate
 At the Epiphyseal plate the bone grows
lengthwise as bone tissue replaces
cartilage
Epiphyseal Line
 When long bone stops growing after
puberty, epiphyseal cartilage disappears at
the growing epiphyseal plate and an
epiphyseal line is visible on X-rays.
Bone Growth
 New cartilage is formed continuously on
external face of articular cartilage and on
the surface of the epiphyseal plate that
faces the bone end (further away from
medullary cavity)
 Old cartilage on the internal face is broken
down and replaced by bone
Bone Growth
Bone Growth
 Bone increases in diameter through
appositional growth
 Appositional growth is controlled by
hormones
 Osteoblasts in periosteum add bone tissue
to external face of diaphysis
 Osteoclasts in endosteum remove bone
from inner face of diaphysis wall