Histology · Connective tissue, cartilage and bone
Ossification, growth plate and bone remodeling !
- Process
How bone forms in the embryo, how long bones grow in length at the growth plate, and how adult bone is renewed and repaired.
Updated 2026-11-01
See it in 3D: Femur · Tibia · Humerus
What it is
Bone forms in two ways: directly inside mesenchyme (intramembranous ossification, in flat bones of the skull and most of the clavicle) or by replacing a hyaline cartilage model (endochondral ossification, in long bones, vertebrae and the skull base). Long bones grow in length at the epiphyseal (growth) plate until it closes after puberty. Throughout life osteoclasts and osteoblasts work in pairs to remodel bone and repair fractures.
Key facts
- Remodeling unit: osteoclasts dig a tunnel or pit, then osteoblasts fill it; in cortical bone this produces a new osteon.
- Osteoblasts express RANKL, which activates osteoclast precursors through RANK; osteoprotegerin (OPG) is a decoy that blocks it.
- Parathyroid hormone raises blood calcium by stimulating resorption indirectly through osteoblasts; calcitonin inhibits osteoclasts.
- Estrogen restrains osteoclasts; its fall after menopause speeds bone loss.
- Fracture repair: hematoma, soft (cartilage) callus, hard (bony) callus, then remodeling into lamellar bone.
Step by step
- Mesenchymal cells condense and become chondrocytes, forming a hyaline cartilage model of the future bone, wrapped in perichondrium.
- In the middle of the shaft, chondrocytes enlarge and the matrix calcifies; the cells die, and the perichondrium becomes periosteum with a collar of bone.
- Vessels invade the calcified cartilage, bringing osteoprogenitor cells: the primary ossification centre forms in the diaphysis.
- Osteoblasts lay bone on the calcified cartilage remnants while osteoclasts hollow out the marrow cavity.
- After birth, secondary ossification centres appear in the epiphyses.
- Cartilage remains only as articular cartilage and as the epiphyseal plate between epiphysis and diaphysis.
- At the end of growth the plate is replaced by bone, leaving the epiphyseal line.
Zones of the epiphyseal plate, from epiphysis to diaphysis
| Zone | What you see | What happens |
|---|---|---|
| Reserve (resting) | Scattered small chondrocytes | Anchors the plate; stem-like cells |
| Proliferation | Stacks of flat chondrocytes in columns, like coins | Cells divide; this is where length is added |
| Hypertrophy (maturation) | Large swollen chondrocytes with pale cytoplasm | Cells enlarge and prepare the matrix to calcify |
| Calcification | Dark calcified matrix, dying chondrocytes | Matrix mineralizes; vessels invade |
| Ossification | Osteoblasts on spicules of calcified cartilage | New bone is laid on the cartilage scaffold |
Clinical notes
- Rickets (children): poor mineralization at the growth plate leaves a wide, disorganized zone of hypertrophic cartilage, bowed legs and widened wrists.
- Osteomalacia (adults): unmineralized osteoid seams widen; bones soften and ache.
- Achondroplasia: an activating FGFR3 mutation slows proliferation at the growth plate, shortening long bones while intramembranous bones are spared.
- Growth plate (Salter-Harris) fractures in children can stop or deform growth of the limb.
- Osteoporosis: resorption outpaces formation in each remodeling cycle.
Test yourself
Think of the answer, then tap the question to see it.
Which bones form by intramembranous ossification?
The flat bones of the skull vault, most of the clavicle and parts of the mandible; they form directly from mesenchyme without a cartilage model.
In which zone of the growth plate do chondrocytes form stacked columns and divide?
The zone of proliferation.
How do osteoblasts control osteoclast activity?
They express RANKL, which binds RANK on osteoclast precursors and drives their maturation; they also secrete osteoprotegerin, a decoy that blocks RANKL.
What is the order of events in fracture healing?
Hematoma and inflammation, a soft cartilage callus, a hard bony callus formed by endochondral ossification, then remodeling into lamellar bone.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Bone tissue: cells, compact and spongy bone
- Cartilage: hyaline, elastic, fibrocartilage
- Hematopoiesis and bone marrow
- Cell adaptations: hypertrophy, hyperplasia, atrophy, metaplasia
Related conditions
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 6.4 Bone Formation and Development (OpenStax, 2022); 6.5 Fractures: Bone Repair (OpenStax, 2022); Osteoporosis (MedlinePlus, U.S. National Library of Medicine, 2025).
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Representative model, for educational purposes only.