SN5
SHORT NOTE
5M
★ High-yield
Describe the types of ossification with examples. Add a note on the cells of bone.
Definition
Ossification (osteogenesis) is the process of bone formation, occurring by two mechanisms — intramembranous and endochondral.
Intramembranous Ossification
- No cartilage model — bone forms directly in mesenchyme
- Occurs in the skull vault, maxilla, most of the mandible, and the clavicle
- The membrane that persists on the surface becomes the periosteum
Endochondral Ossification
- Primary centres appear before birth (7th–12th week); secondary centres mostly after birth
- The lower femoral and upper tibial epiphyses appear at birth — used to determine whether a fetus was viable in medicolegal practice
- Growth in length is from the epiphyseal plate; in thickness from the periosteum (appositional)
Comparison
| Feature | Intramembranous | Endochondral |
|---|---|---|
| Precursor | Mesenchyme directly | Hyaline cartilage model |
| Bones formed | Skull vault, maxilla, mandible, clavicle | All long bones, vertebrae, base of skull |
| Growth plate | Absent | Present |
| Speed | Faster | Slower |
| Repair of fracture | Contributes | The chief mechanism |
Cells of Bone
| Cell | Origin | Function |
|---|---|---|
| Osteoprogenitor | Mesenchyme | Stem cell |
| Osteoblast | Osteoprogenitor | Forms osteoid and mineralises it; rich in alkaline phosphatase |
| Osteocyte | Trapped osteoblast | Maintains matrix; mechanosensor; lies in a lacuna, communicating through canaliculi |
| Osteoclast | Monocyte–macrophage lineage | Resorbs bone; multinucleate; lies in a Howship lacuna with a ruffled border |
Applied Aspects
- Achondroplasia — defective endochondral ossification gives short limbs with a normal trunk and a large skull vault, since the vault ossifies in membrane; the commonest cause of disproportionate dwarfism
- Osteogenesis imperfecta — a type I collagen defect; brittle bones and blue sclerae
- Cleidocranial dysostosis — defective membranous ossification; absent or hypoplastic clavicles (the shoulders can be approximated) and delayed closure of fontanelles
- Rickets — failure of mineralisation at the growth plate widens the metaphysis, giving cupping and fraying on radiographs
- Fracture healing recapitulates ossification — haematoma, soft callus (cartilage), hard callus (woven bone), then remodelling into lamellar bone
All questions in General Anatomy & Osteology
- Q1Classify bones with examples. Describe the parts of a long bone and its blood supply. Add a note on the types of epiphysis. ★
- Q2Classify joints with examples. Describe the structure of a synovial joint and add a note on Hilton's law. ★
- Q3Classify skeletal muscle according to shape and action. Describe its nerve supply and add a note on the functional groups of muscles. ★
- Q4Describe the types of blood vessels. Define anastomosis and end artery with examples, and add a note on portosystemic anastomoses. ★
- Q5Describe the structure of skin and fascia. Add a note on the lymphatic system and the thoracic duct. ★
- SN1Describe the epiphyseal plate with its zones. Add a note on the Salter–Harris classification. ★
- SN2Classify cartilage with its sites and features. Add a note on articular cartilage. ★
- SN3Describe the structure and types of a synovial joint. Add a note on synovial fluid. ★
- SN4Define end artery. Give its types with examples and the consequences of its occlusion. ★
- SN5Describe the types of ossification with examples. Add a note on the cells of bone. ★
- SN6Describe the deep fascia with its modifications and functions. Add a note on compartment syndrome. ★
- SN7Describe the anatomical position, the planes of the body and the terms of movement.