16.0.2 POJA-L7202 LOCOMOTOR SYSTEM: BONE
INTRODUCTION-2: TYPES OF BONE AND BONE FORMATION
Types of bone The production of bone (ossification) results in primary bone that will be replaced by mature secondary bone. Bone production occurs by:
INTRODUCTION-2: TYPES OF BONE AND BONE FORMATION
Types of bone The production of bone (ossification) results in primary bone that will be replaced by mature secondary bone. Bone production occurs by:
Ossification
- Intramembranous ossification. Embryonic tissue (=mesenchyme) condenses into bone as found in those bones derived from the neural crest e.g. frontal, parietal bones of the skull.
- Endochondral ossification. The previous formed predecessor hyaline cartilage is converted into bone by endochondral ossification e.g. in long bones such as the femur, or in other bones of the skull derived from the paraxial mesoderm e.g. sphenoid and occipital bones.
Anatomical classification
- Long bones: mostly compacted bone with little marrow (femur, humerus, fingers).
- Flat bones: thin and curved, consisting of two outer layers of compact bone and an inner layer of spongy bone (cranial bones, sternum, scapulae, ribs)
- Short bones: cube shaped (wrist, ankle, joints).
- Irregular bones: (sacrum, vertebrae).
- Sesamoid bones: embedded in tendons (hand, knees and feet).
Tissue classification (based on texture)
- Cortical: compact bone with a hard dense and strong outer layer.
- Cancellous: spongy or trabecular bone consisting of a network of trabeculae, and being lighter, less dense, and more flexible than compact bone.
Histological classification (based on matrix arrangement)
- Plexiform or primary bone.
- Lamellar or secondary bone.
Primary or woven bone is characterised by the irregular arrangement of collagen fibres, large cell numbers, and reduced mineral content. It is a temporary structure firstly appearing during embryonic development.
During repair of fractures, woven bone is also the initial start-up type of bone formation.
In adults woven bone is present in e.g. petrosal bone, alveolar sockets at the insertions of tendons into bones (oral cavity).
Usually the plexiform tissue is gradually replaced by the lamellar bone.
Secondary or lamellar bone is the type of bone in the adult skeleton and is characterized by regular, parallel arranged collagen fibres within the lamels.
Most bones have an outer cortex of compact bone and an interior of trabecular, spongy bone tissue.
Primary or woven bone is characterised by the irregular arrangement of collagen fibres, large cell numbers, and reduced mineral content. It is a temporary structure firstly appearing during embryonic development.
During repair of fractures, woven bone is also the initial start-up type of bone formation.
In adults woven bone is present in e.g. petrosal bone, alveolar sockets at the insertions of tendons into bones (oral cavity).
Usually the plexiform tissue is gradually replaced by the lamellar bone.
Secondary or lamellar bone is the type of bone in the adult skeleton and is characterized by regular, parallel arranged collagen fibres within the lamels.
Most bones have an outer cortex of compact bone and an interior of trabecular, spongy bone tissue.
- Compact bone or cortical bone: homogeneous, dense cortical layer of lamellar bone. The bulk consists of the main components the osteons consisting of concentric bony lamellar structures with a central so-called Haversian canal for the blood supply.
- Spongy bone or cancellous bone or trabecular bone: thin bony stake-like projections are located on the inside of the lamellar bone. These trabeculae are aligned along compressive and tensile forces (trajectory direction). In the interior spaces between the network of the trabeculae bone bone marrow is present, providing the source of stem cells for the blood cell formation (haematopoiesis).