Biology 211
Anatomy & Physiology I
Dr. Thompson
Histology of Bone
Recall:
SYSTEMS are composed of one or more
organs, all serving a common function
ORGANS are composed of one or more types
of tissues, all serving a common function
TISSUES are composed of one or more types
of cells and their products, all serving a
common function
Organs of skeletal system = Bones
(e.g. femur, ulna, vertebra, mandible)
Like all organs, these contain four types of tissue:
Epithelium
Connective tissue
Nervous tissue
Muscular tissue
However: Two types of specialized
connective tissues predominate:
Cartilage and Bone Tissue
each of which is surrounded by
dense irregular connective tissue:
Perichondrium and Periosteum
Cartilage: Function = Flexible Support
Found in skeletal system:
Costal cartilages
Intervertebral disks
Articular cartilages at
ends of bones
Nasal septum
Also found in
External ear
Larynx
Trachea & bronchi
Cartilage: Function = Flexible Support
In the embryo, forms the "model" for many bones and:
a) Remains on ends of long bones throughout life
b) Remains at growth plates (epiphyseal plates) of
immature growing bones.
Cartilage:
Young cells = chondroblasts
Actively forming new
extracellular matrix
Mature cells = chondrocytes
Maintain and repair
extracellular matrix
Extracellular Matrix
Ground substance = very
firm gel
Fibers = collagen, elastic
Chondrocytes located in
lacunae
Cartilage: Three types:
Hyaline
Elastic
All three types:
Cells are the same
Differences due to fibers
Fibrous
Hyaline Cartilage:
Extracellular matrix appears smooth,
No fibers evident by light microscopy
Cells (in lacunae) form isogenous groups or nests
Extracellular matrix often stains more darkly around cells
Elastic Cartilage:
Many elastic fibers visible in extracellular matrix
Cells (in lacunae) form isogenous groups or nests
Extracellular matrix often stains more darkly around cells
Fibrous Cartilage:
Many large collagen fibers visible in extracellular matrix
Cells (in lacunae) usually individual (no isogenous groups)
Extracellular matrix usually stains more darkly around cells
Bone:
Young cells = osteoblasts
Actively forming new
extracellular matrix
Mature cells = osteocytes
Maintain and repair
extracellular matrix
Located in lacunae
Third cell type = osteoclast
Reabsorb extracellular
matrix
Bone:
Extracellular matrix:
Fibers: Collagen
Ground substance contains
precipitated calcium
phosphate called
calcium hydroxyapatite
Bone tissue constantly remodeling itself:
Old extracellular matrix removed by osteclasts
New extracellular matrix produced by osteocytes
Two forms of bone tissue:
Trabecular ("woven", "spongy")
Shelves or ridges ("trabeculae")
of extracellular bone matrix with
osteocytes embedded within it.
Lamellar ("compact", "Haversian")
Concentric rings ("lamellae") of
extracellular matrix and ostocytes,
surrounding a central canal
containing nerves and capillaries
Lamellar Bone:
Lamellar Bone:
Osteocytes
in lacunae,
between
lamellae
Both trabecular and lamellar bone:
Osteocytes have lost ability to divide
so
Osteoblasts at edges
Growth can occur only by
differentiation of new
osteoblasts, which then form
extracellular matrix around
themselves
As the osteoblasts become
trapped in their matrix, they
mature into osteocytes
located in lacunae
Both trabecular and lamellar bone:
Osteocytes have long cellular
extensions to retain connections
with each other.
These extensions of osteocytes
run through little canals called
canaliculi in the rigid extracellular
matrix
Two types of bone formation ("osteogenesis") for
growth, remodeling, and repair:
- Endochondral osteogenesis:
Model of cartilage formed first.
Cartilage then replaced by bone
.
- Intramembranous osteogenesis:
Bone formed within pre-existing
ordinary connective tissue, often
at edge of existing bone.
Bones (organs) grow by both intramembranous and
endochondral osteogenesis:
Average age (years) at completion of ossification:
Scapula - 18-20
Clavicle - 23-31
Os coxa - 18-23
Vertebrae - 25
Sacrum - 23-25
Sternum - body - 23-25
- manubrium - 30-35
Humerus, radius, ulna - 17-20
Femur, tibia, fibula - 18-22
Bones repair fractures by both intramembranous and
endochondral osteogenesis:
Bone tissue is constantly remodelling itself to adjust to
different physical and biochemical demands placed
upon it. Old bone is destroyed by osteoclasts and new
bone is produced by osteoblasts and osteocytes.
Example:
If you have a cast on your arm,
the bones will become smaller
since they are not being
stressed and more bone tissue
is removed than is
synthesized.
But:
When the cast is removed and the bones again
subjected to stress, they will become larger again.
Bone remodelling regulated by many hormones:
Growth hormone stimulated osteoblast activity & collagen
synthesis
Thyroid hormone stimulates osteoblast activity & collagen
synthesis; stimulates formation of ossification centers
Testosterone stimulates osteoblast activity & bone growth
Progesterone stimulates osteoclast activity & bone loss
Estrogen stimulates osteoblast activity & bone growth
Two hormones primarily responsible for day-to-day
remodelling of bone to regulate concentration of
calcium in blood:
Calcitonin Secreted by thyroid gland
Stimulates osteoblasts to produce more matrix
Inhibits osteoclasts from breaking down matrix
Thus: Calcium removed from blood & stored in bone
Parathyroid Hormone Secreted by parathyroid glands
Inhibits osteoblasts from producing more matrix
Stimulates osteoclasts to break down matrix
Thus: Calcium released from bone into blood
Blood Ca++ level
returns to normal
Ca++ moved from
blood to bone
Blood Ca++ level
continues to decrease
Parathyroid
glands
secrete PTH
Thyroid
secretes
calcitonin
Ca++ moved from
bone to blood
Blood Ca++ level
continues to increase
Blood Ca++ level
returns to normal