Bone is an organ and part of the skeletal system of animals contributing to the shape, movement and providing support. It is a specialized tissue and exists primarily as cartilage in watery solution in the fetus and infants. The cartilage tissue undergoes changes (calcification) in time giving rise to mineralized bone. We will examine cartilage transformation to bone.
·
Cartilage: a type of connective tissue. Chondrocytes
that produce cartilage, ground matrix and fibers are housed in lacunae lacunae.
·
Cartilage
is large water, non-vascularized, lacks nerves and surrounded by perichondrrium
(a layers of irregular but vascularized connective tissue) which
provides resistance.
·
Types
of cartilage : hyaline, elastic and fibrocartilage
a.
Hyaline cartilage: glassy appearance, contains collagenous fibers; provide support,
flexibility and resilience. Abundant in
skeletal cartilage and major component of areola, respiratory, costal and nasal
cartilages.
b.
Elastic cartilage: contains abundant elastic and
collagen fibers.; more flexible than hyaline cartilages; support outer ear and
epiglottis.
c.
Fibrocartilage: highly compressed and
resistant to stretch; contains thick collagen fibers; found in vertebral discs
and knee joint cartilages
Cartilage grow in two ways: appositional
(outward) growth involves the addition of new materials or matrix to existing
tissue by cells in the perichondrium. Interstitial
(inward) growth involves mitotic expansion of the chondrocytes within the
lacunae.
·
There
are 206 bones in the human skeleton grouped into axial (skull, vertebral
column, and thoraxc bones) and appendicular system (bone of the upper and lower
limbs, pelvic bones, shoulder bones).
·
Bone
are classified on the basis of length and shape. There are long and short bones as well as flat, irregular,
compact or spongy bones.
·
Function
of bones
-
support (framework)
-
protection
(skull, thorax, vertebral column)
-
movement
(surfaces for muscle attachment)
-
mineral
depot (calcium, iron, phosphates)
-
RBC
formation (hematopoeisis)
A.
Long bones:
• Consist shaft
(diaphysis) and end (epiphysis) contain sheets cartilage and the
medullary cavity contain bone marrow. The end or epiphysis contains soft spongy
bone and an epiphyseal line (growth line).
• Membranes: Periosteum membrane covers the
diaphysis and is anchored to the bone surface by Sharpey’s fibers.
Endosteum (contains osteoblast and osteoclast cells) lines
the inside of the bone tissue.
Osteoblast are bone forming and osteoclasts demineralize bone.
• Nutrients are supplied thru
blood vessels that penetrate the osteons (Haversian cannals)
B. Short, Flat and Irregular bones
·
These
bones contain periosteum on the outside and endosteum inside. They lacl
epiphysis and are not cylindrical however, they contain bone marrow. The
internal layer of the spongy tissues is
called diploe
C. Structure of Bone tissue
· The basic structural unit of a bone tissue is the osteon (Haversian system or groups of ring
systems).
·
An osteon consist of concentric rings called
lamellae and a central canal with blood vessels and nerve supply.
·
Volkmann’s perforating cannals are
those that lie horizontal to the Haversian cannal.
·
Osteocytes (star-shaped mature bone cells)
occupy spaces called lacunae, which received nutrients and blood supply thru
cananiculi that connects all the lacunae in the osteon.
·
The chemical composition of bone is made up
of organic materials consisting of osteoid (organic materials composed
of ground matrix) and bone cells (osteocytes, ostroblasts and osteoclasts); and
inorganic component (hydroxyapatite:
calcium and phosphate salts.
·
Bone Osteogenesis
(Ossification)
The process begins in the 6th or 7th
of gestation and involve replacement of existing tissue with bone tissue. It
begins with ossification which contains osteoblasts, osteoids and mesenchymal
cells in fibrous connective tissue. The bone cells secrete bone matrix calcify
trapping bone cells into lacunae. Blood vessels infiltrate the tissue forming
lamellae or trabeculae (woven bone).
Two types of osteogenesis: intramembranous and endochondral
bone formation.
·
Intramembranous ossification:
- Primitive cells are transformed to osteoblast cells that deposit bone matrix around themselves and become engulfed in their shell or lacunae. Matured osteoblast are osteocytes.
-
Most
broad and flat bones including skull bones are formed by intramembranous
process. Osteoblast cells first deposit spongy bones within membranes that would
become bone and later overlay with periosteum and compact bone over the spongy
tissue.
- Osteoclast cells modify the final shape by remodeling the bone
(osteoclast cells remove bone materials).
· Endochondral ossification
-
The
process involves bone formation by replacing the hyaline cartilage. Most of the bones of the skeletal system are
formed by endochondral ossification process.
-
The
hyaline cartilage as the model tissue grow rapidly but degenerates. During this
change, osteoblasts infiltrate the tissue transforming into osteocytes in their
lacunae and gradually replaces the hyaline cartilage with spongy bone,
periosteum and compact bone.
·
Interstitial growth: growth within the lacunae; osteocytes undergo mitosis. This increase
in bone cells enlarges and also creates multiple lacunae entrapping the
increased number of osteocytes. In the long bones, growth in length occurs when
mitosis of cells above the end plate (spongy bones) pushes down cells of the
diaphysis resulting in extension of the diaphysis.
·
Appositional growth: Osteoblast cells build bones
by secreting bone matrix which thickens the periosteum and adds width or
lateral growth. Lateral (thickness) growth in bone is balances by activities of
osteoclast which chip bone materials away.
·
Growth
is influenced by growth hormone (GH) which control activities in infancy and
young age. Growth diminishes at adolescence. Excessive stimulation promotes
gigiantism and absence produces dwarfism (achondroplasia)
·
Bone
remodeling process (thickening or density and bone thinning or resorption)
occurs at the level of periosteum and endosteum in the presence of osteoblast
(bone forming) and osteoclast (bone thinning).
·
Activities
are regulated by Parathyroid (PTH) or Parathormone (PT) and Calcitonin (CT)
hormones.
·
When
blood Ca++ ions levels are high, calcitonin stimulates osteoblast
cells to take up calcium ions from blood and deposit them for bone
formation. When blood level Ca++
ions are low during stress, osteoporosis, etc, PT is secreted which stimulates
osteoclasts to degrade bone matrix thus releasing Ca++ ions into
blood
Bone Repair: The process involves hematoma, fibrocartilage,
callus and remodeling
·
Following
a bone fracture, blood escapes from ruptured vessels and fills up tissue are
producing edema and hematoma.
·
Blood
vessel infiltration allow WBC to clean up the area promoting secretion and
deposition of bone matrix forming fibrocartilage.
·
Bone
matrix deposition in discontinous fashion forms bony callus which replaces
fibrocartilage. Osteoclasts remove excess deposition of matrix during
remodeling process.
1.
Osteomalacia/rickets:
insufficiency of calcium ions or Vit. D especially during infancy resulting in
bending of bones.
2.
Osteoporosis:
bone thinning resulting in bone breaking. May be associated with hyperactivity
of PT hormone overstimulating osteoclast cells.
3.
Paget’s
disease: characterized by excessive and
abnormal bone remodeling