Chapter 7
Bone Development and Growth
Types of Bone Development
• Intramembraneous Bones
– Form by replacing existing connective tissue
– Originate within sheetlike layers of connective tissues
– Ex. – Skull bones
• Endochondral Bones
– Form by replacing existing connective tissue
– Begin as masses of hyaline cartilage that are later
replaced by bone tissue
– Ex. – most skeletal bones
The process of forming bones is called osteogenesis
Intramembraneous Bones
• Development (intramembranous ossification)
1.
Membrane-like layers of relatively undifferentiated connective tissues
appear
•
2.
3.
Tissue cells enlarge and differentiate into osteoblasts (bone-forming
cells)
Osteoblasts deposit bony matrix around themselves, forming spongy
bone in all directions along the blood vessels within the layers of
connective tissues
•
4.
Spongy bone may later become compact bone through spaces being filled
with bone matrix
As development continues:
1.
2.
5.
These tissue layers are supplied by dense networks of blood vessels
Osteoblasts may become completely surrounded by extracellular matrix and
secluded within lacunae (and are now called osteocytes)
Extracellular matrix encloses cellular processes of the osteoblasts to form
canaliculi
Cells of the connective tissue that are outside the developing bone
give rise to the periosteum.
6. Osteoblasts on the inside of the periosteum form a layer of compact
bones over the surface of the newly formed spongy bone
Endochondral Bones
• Development (endochondral ossification)
– Masses of hyaline cartilage shaped like future bones
develops
– Cartilage cells enlarge, lacunae grow, and surrounding matrix
breaks down, leading to the death and degeneration of
cartilage cells
– As cartilage decomposes, a periosteum forms from
connective tissue surrounding the structure
– Blood vessels and partially differentiated connective tissue
cells invade the disintegrating cartilage tissue
– Some of the invading cells further differentiate into
osteoblasts and begin to form spongy bone in the spaces
previously occupied by cartilage
– Osteoblasts beneath the periosteum deposit compact bones
around the spongy bone
Osteogenesis in Long Bones
• Bony tissue begins to replace hyaline cartilage in
the center of the diaphysis (called the primary
ossification center)
– Bone then develops from the center out while
osteoblasts in the periosteum deposit a thin layer of
compact bone around the primary ossification center
• The epiphyses remain cartilage until secondary
ossification centers form in the epiphyses and
spongy bone begins to be deposited in all directions
– As this secondary process occurs, a band of cartilage
called the epiphyseal plate separates the two ossification
centers
Osteogenesis in Long Bones
Osteogenesis in Long Bones
• Growth at the epiphyseal plate:
– The cartilage cells of the epiphyseal plate form four layers:
• 1st layer
– Closest to the end of the epiphysis
– Composed of resting cells that do not actively participate in growth
– Anchors the epiphyseal plate to the bpny tissue of the epiphysis
• 2nd layer
– Include rows of young cells going through mitosis
– As new cells and extracellular matrix forms, the cartilage plate thickens
• 3rd Layer
– Consists of older cells that are left behind when new cells appear
– This enlarges and thicken the epiphyseal plate causing the whole bone to
lengthen
– Invading osteoblasts (which secrete calcium salts) accumulate in the
extracellular matrix of this layer adjacent to the oldest cartilage cells,
causing the matrix to calcify and cells to die
• 4th layer
– Consists of dead cells and calcified extracellular matrix
– Very thin
Osteogenesis in Long Bones
Growth at the epiphyseal plate:
Osteogenesis in Long Bones
Growth at the epiphyseal plate:
Osteogenesis in Long Bones
• Growth at the epiphyseal plate:
– After calcification, large, multi-nucleated cells called
osteoclasts break down the calcified matrix by:
• Secreting an acid that dissolves the inorganic component of
the calcified matrix
• Digestion of organic components of the calcified matrix by
lysosomal enzymes
• Phagocytosis
Osteoclasts originate from the fusion of single-nucleated white blood
cells called monocytes
– After osteoclasts remove the extracellular matrix,
osteoblasts invade the region and deposit bone tissue in
place of the calcified cartilage
Osteogenesis in Long Bones
• Growth at the epiphyseal plate:
– As long as the cartilaginous cells of the epiphyseal plate
are active, a long bone will continue to lengthen
– However, once the ossification centers of the diaphysis
and epiphysis meet and the epiphysial plates ossify, the
bone can no longer lengthen on that end
– Bones will thicken as compact bone is deposited on the
outside , just beneath the periosteum
• As this happens, osteoclasts erode bone tissue on the inside of
the bone, forming the medullary cavity
Homeostasis of Bone Tissue
• Bone remodeling
– Occurs throughout life as osteoclasts resorb bone
tissue and osteoblasts replace bone tissue
• 3% to 5% of bone calcium is exchanged each year in
adult skeletons
• However, total mass of bone tissue remains nearly
constant
Factors Affecting Bone Development,
Growth, and Repair
• Vitamin D
– Necessary for proper absorption of calcium in the
small intestine
– Lack of proper absorption leads to a lack of calcium in
bone matrix resulting in softening of the bone (called
rickets in children and osteomalacia in adults)
– Vitamin D is uncommon in foods but is present in
fortified dairy products
– Vitamin D also forms from dehydrocholesterol (which
is produced by cells in the digestive tract or obtained
from food) that is carried by blood to the skin where
exposure to UV light converts it to Vitamin D
Factors Affecting Bone Development,
Growth, and Repair
• Vitamin A
– Necessary for osteoblast and osteoclast activity
during normal development
– Deficiency of Vitamin A may impede growth
development
Factors Affecting Bone Development,
Growth, and Repair
• Vitamin C
– Required for collagen synthesis
– When Vitamin C is lacking, osteoblasts cannot
produce enough collagen in the extracellular
matrix of the bone tissue, resulting in abnormally
slender and fragile bones
Factors Affecting Bone Development,
Growth, and Repair
• Growth hormone
– Secreted by the pituitary gland
– Stimulates the division of cartilage cells in the epiphyseal plates
– In the absence of growth hormone, the long bones of the limbs
fail to develop normally, a condition called pituitary dwarfism
(resulting in a child who is very short but with normal body
proportions)
– If excess growth hormone is released prior to the ossification of
the epiphyseal plates, pituitary gigantism results (height in these
individuals may exceed 8 feet)
– In adults, secretion of excess growth hormone results in the
enlargement of the hands, feet, and jaw (called acromegaly)
Factors Affecting Bone Development,
Growth, and Repair
• Thyroxine
– Hormone produced by the thyroid
– Stimulates replacement of cartilage in the
epiphyseal plate of long bones
– Can halt bone growth by causing premature
ossification of the epiphyseal plate or by not
stimulating the pituitary gland to produce enough
growth hormone
Factors Affecting Bone Development,
Growth, and Repair
• Parathyroid
– Hormone that stimulated an increase in the
number and activity level of osteoclasts, which
break down bone
Factors Affecting Bone Development,
Growth, and Repair
• Estrogens and Testosterone
– Promote the formation of bone tissue
• More abundant during puberty
– Stimulate ossification of the epiphyseal plates
which stops bone lengthening
– The effect of estrogens is stronger than the effect
of testosterone, causing girls to reach maximum
heights at an earlier age than boys
Factors Affecting Bone Development,
Growth, and Repair
• Physical Stress
– Stimulates bone growth
• Stress applied to bones by contracting muscles causes
bone to thicken and strengthen (hyperatrophy) at
attachment sites
– Lack of exercise causes bones to atrophy (become
thinner and weaker)