HISTOLOGY 1.10: OSTEOGENESIS
Bone develops by a process of transformation from an
existing connective tissue.
Types of ossification:
On the basis of tissues involved:
1. Direct:
connective tissue
2. Indirect:
connective tissue
cartilage
On the basis of the procedure:
1. Desmal/intramembranous
direct
1. Chondral:
perichondral
endochondral
indirect
bone
bone
Intramembranous/desmal ossification
The bones of the calvarium develop by this ossification.
The bone forms directly from a layer of mesenchyme in the embryo.
Steps:
1.
2.
3.
4.
5.
Capillaries invade the area
Mesenchymal cells become spherical, form osteoprogenitor cells
Osteoprogenitor cells give rise to osteoblasts, the bone forming cells
Osteoblast synthesize and secrete osteoid, contribute to mineralization
Osteoblast surrounded by the osteoid transform into osteocytes
Formations:
1.
2.
3.
4.
Ossification centers
Bony spicules, trabeculae
Trabecular, spongy or cancellous bone – immature
Replaced by mature lamellar bone
Intramembranous/desmal ossification in light micrographs
Low power micrograph showing the „intramembranous” character
2
1
1
3
2
1: mesenchyme
2: primary bone trabecule
3: osteoclast
Arrows show osteoblasts
Endochondral ossification
Bones of the extremities,vertebral column, pelvis, base of skull are formed
by this type of ossification.
The bone is formed initially of hyaline cartilage models that are
replaced by bone.
Steps:
1. The cartilage model grows more quickly at the end of the long bone
2. The cartilage cells in the midsection mature and enlarge, release
matrix vesicles, matrix calcifies
3. Perichondrial mesenchymal cells produce a bony collar around the
diaphysis causing further trouble in the metabolism of the chondrocytes
(direct/perichondral ossification)
4. A capillary loop accompanied by mesenchymal cells perforates
the bony collar and enters the hypertrophic cartilage
5. Chondroclasts phagocyte degenerating chondrocytes leaving
pin-shaped calcified remnants of the matrix
6. Mesenchymal cells differentiate into osteoprogenitor cells which give rise
to osteoblasts laying down the first layers of osteoid onto the surface of
calcified remnants of cartilage matrix
I.
Formation of the bony collar:
A.1. Bony collar formed around the diaphysis
of the degenerating cartilage
A.2. Intact hyaline cartilage at the epiphysis
A.3. Degenerating hyaline cartilage with
hypertrophised chondrocytes and
calcified ground substance
B.1. Ground substance of hyaline cartilage
B.2. Newly divided flattened chondrocytes
B.3. Zone of proliferation
B.4. Zone of degeneration
B.5. Calcium carobonate deposits in
the ground substance
II. Penetration of the capillary
loop at the diaphysis
A.1. Periosteum around the bony
collar
A.2. Capillary loop emerging from
the periosteum
B.1. Osteoblasts on the surface of
the bony collar
B.2. Osteoclast perforating the bony
collar
B.3. Bony collar with osteocytes in
the lacunae of osteoid
B.4. Capillary loop
B.5. Multinucleated modified endothel
B.6. Mesenchymal cells accompanying
the capillary loop
III. Formation of the primary
ossification center
A.1. Cavity formed by chondroclasts
A.2. Blood vessel
A.3. Perforation of the bony collar
B.1. Bony collar
B.2. Hyaline cartilage
B.3. Chondrocytes
B.4. Periosteum
B.5. Capillaries of the periosteum
B.6. Perforation site on the bony collar
B.7. Blood vessel
B.8. Mesenchymal cells
B.9. Capillary loop inside the primary
bone marrow cavity
B.10. Degenerated chondrocyte
B.11. Howship’s lacuna
B.12. Chondroclast
IV. Zones of the endochondral
ossification
A.1. Ossification line
A.2. Secondary ossification centers
B.1. Chondrocyte in mitotic division
B.2. Chondroclast phagociting degenerated
chondrocytes
B.3. Ossification line
B.4. Mineralized pin-shaped cartilage
ground substance
B.5. Osteoblasts on the surface of primary
bone trabecule
3 B.6. First layer of osteoid
2 B.7. A primary bone trabecule
B.8. Newly embedded osteocyte
B.9. A branch of the capillary loop
B.I.
B.II.
B.III.
B.IV.
B.V.
Zone of resting hyaline cartilage
Zone of proliferation
Zone of maturation
Zone of degeneration
Zone of ossification
Endochondral ossification – light microscopy
V.
IV.
III.
II.
I.
Ossification line
Summary of the formation of a long bone with the secondary ossification centers
Growth of the bone:
The newly formed bone is transformed into compact or lamellar bone:
concurrent periosteal growth and endosteal resorption enlarges the marrow
cavity and increases the width of the diaphysis.
Epiphyseal plate: disk-shaped intact layer of hyaline cartilage ensure the
longitudinal growth of the long bones until puberty.
Bone modeling:
Primitive woven bone is replaced by lamellar bone.
The changes in size and shape during the growth process, it is continuous
until adult shape is obtained.
Drift: eccentrical growing upon biomechanical demands.
Bone remodeling:
Does not involve anatomical surfaces, cyclic, and slower procedure.
During a lifetime the material of the whole skeleton turn over several times.