Bone Formation,
Growth, &
Maintenance
Types of Bone Cells
 Osteocytes
 Mature bone cells
 Osteoblasts
 Bone-forming cells
 Osteoclasts
 Bone-destroying cells
 Break down bone matrix for remodeling and
release of calcium
 Bone remodeling is a process by both
osteoblasts and osteoclasts
Bone Formation
• Ossification - the process of bone
formation
• Begins ~ 2 months fetal age
• Ends in late puberty ~ age 20 (slightly
earlier in females)
Bone Formation
• Long, short, and irregular bones develop
from hyaline cartilage
• Flat bones develop from fibrous
membranes - bone develops within sheets
of connective tissue
• Bones ossify from the middle of the bone
outward. All new tissue is spongy at first
but then is broken down and rearranged
into compact bone.
Long Bone Formation and Growth
Figure 5.4a
Changes in the Human Skeleton
 In embryos, the skeleton is primarily hyaline
cartilage
 During development, much of this cartilage
is replaced by bone
 Cartilage remains in isolated areas
 Bridge of the nose
 Parts of ribs
 Joints
X-Ray of Newborn
QuickTime™ and a
decompressor
are needed to see this picture.
Bone Formation
• Osteoblasts (bone-forming cells) cover the
cartilage with bone matrix. Cartilage cells die as
nutrition is cut off because the hard matrix does
not allow for diffusion. Cartilage membranes
(perichondrium) convert to periosteum and send
osteocytes (bone cells) into lacunae (hollow
spaces within the matrix where cells reside). {The
cartilage cells are killed off and bone cells take
over their “home”} The bone cells are able to
survive within a solid matrix because they have
the Haversian system to ensure a blood supply.
Bone Growth - Length
• Growth in length occurs at the ends
(epiphyses). New cartilage is formed and
gradually transformed into bone. The
epiphyseal plate is a band of cartilage located
in the epiphysis. As long as that plate is
present, bone growth can continue. Cartilage
is replaced by bone but new cartilage is
produced in the epiphyseal plate, allowing
bone growth to continue.
Bone Growth - Length
• Growth of long bones is controlled by
hormones (growth hormones & sex hormones,
which are produced at sexual maturity). The
sex hormones cause the osteoblasts (bone
forming cells) to divide more rapidly, causing a
growth spurt. However, these hormones
cause bone cells to replace cartilage faster
than cartilage can form so the epiphyseal
plates disappear and bone growth stops.
Long Bone Formation and Growth
Figure 5.4b
Bone Growth - Diameter
• Growth in diameter occurs as osteoblasts add
bone tissue to the outer diaphysis and
osteoclasts (bone-destroying cells) break down
tissue in the middle, hollowing out a larger
marrow cavity. This allows bones to grow
without adding too much weight.
Bone Maintenance
• 98% of all bone growth is complete by age 20
(slightly earlier in females)
• After cartilage cells have died, the boneforming cells are mainly involved in repair and
replacement of cells
• The balance between bone growth and bone
destruction is achieved at midlife. After ~50
years, bone mass gradually declines, but at a
faster rate in women.
Bone Density:
• AGE
Young: bone is made faster than it’s destroyed
Maturity: there is a balance
Old age: less bone is deposited
Bone Density:
• Hormones - influence the rate of mineral
deposition in bone; absorption and secretion of
calcium and and phospate.
• Genetic & Environmental Factors - family
history, diet (calcium & Vitamin D), etc.
Bone Density:
• Applied Stress
Weight bearing exercise = thicker, stronger
bones
Lightly stressed bones are thinner & weaker
Bone Density
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Hormones & the Skeleton
• Thyroid Gland:
Secretes calcitonin, which maintains the
homeostatic level of calcium in the blood.
It acts to lower blood calcium levels by
inhibiting the reabsorption of calcium from
bones
Hormones & the Skeleton
• Parathyroid gland:
Secretes parathormone (PTH), which
regulates calcium and phosphate levels in
the blood. It acts to increase levels of
calcium and phosphate ions in the blood
by promoting absorption of calcium from
the intestines and kidney tubules and also
by stimulating osteoclasts to break down
bone.
Hormones & Vitamin D
• The sun is the normal source of Vitamin D
but you can also get it in your diet. If
Vitamin D is present in adequate amounts,
bone structure can be properly maintained.
If Vitamin D is not present in adequate
amounts, bones do not develop in children
(Rickets) and adult bones are
demineralized and become soft and pliable
(Osteomalacia)
End