Bone Histology
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Direct answer
Bone is a connective tissue whose matrix of type I collagen is impregnated with hydroxyapatite, making it rigid yet slightly elastic. Compact bone is organised into Haversian systems (osteons) of concentric lamellae around central canals, interconnected by Volkmann canals; spongy bone is a trabecular meshwork holding marrow. Osteoblasts secrete osteoid, osteocytes maintain the matrix from lacunae, and multinucleate osteoclasts resorb it, the three remodelling bone throughout life.
What you must remember
- Osteon: four to twenty concentric lamellae around a Haversian canal with vessels and nerves; between osteons lie interstitial lamellae, remnants of old osteons, with circumferential lamellae under the periosteum and endosteum.
- Osteocytes: osteoblasts trapped in lacunae, their processes meeting in gap junctions within canaliculi, so nutrients diffuse along the canalicular network.
- Osteoblasts: cuboidal, basophilic, alkaline-phosphatase-rich cells on bone surfaces laying down osteoid; serum alkaline phosphatase rises in growth, fracture healing, rickets, osteosarcoma and Paget disease.
- Osteoclasts: multinucleate cells of monocyte lineage on resorbing surfaces, with ruffled borders and acid phosphatase, sitting in Howship lacunae; stimulated by parathyroid hormone, inhibited by calcitonin and bisphosphonates.
- Matrix: about one-third organic (type I collagen, osteocalcin) and two-thirds mineral; the unmineralised osteoid seam widens in rickets and osteomalacia.
- Woven versus lamellar: immature woven bone with irregular collagen forms the fetal skeleton and fracture callus; lamellar bone with parallel collagen replaces it.
- Ossification: intramembranous — bone forms directly in mesenchyme (skull vault, mandible, clavicle); endochondral — a cartilage model is replaced, a primary centre appearing in the diaphysis before birth and secondary centres later in the epiphyses.
- Clinical anchors: osteoporosis is reduced mass with normal mineralisation; osteopetrosis is failed osteoclast resorption; Paget disease is disorganised remodelling.
Common confusion
Haversian canals run longitudinally within osteons; Volkmann canals run obliquely, linking adjacent osteons to the periosteum, and lack concentric lamellae — a favourite spotter distinction. Osteoblasts build and osteoclasts resorb, with mirrored hormonal control: parathyroid hormone raises calcium through osteoclasts, calcitonin restrains them. Osteocytes are buried osteoblasts, not a separate lineage.
Exam-focused takeaway
Histology practicals require ground bone sections — osteons, lamellae, lacunae and canaliculi — and decalcified sections showing cells and osteoid. Theory questions ask for the structure of an osteon, the functions and origins of the three bone cells, woven versus lamellar bone, and the two ossification types with examples. In the viva, explain remodelling, its hormonal control and the histology of rickets.
Frequently asked questions
What is an osteon?
The structural unit of compact bone — a central Haversian canal with vessels surrounded by concentric lamellae, osteocytes in lacunae and canaliculi.
How do osteocytes obtain nutrition?
Through canaliculi: their processes meet in gap junctions, letting metabolites pass from the Haversian canal outwards through the impermeable calcified matrix.
Which cell resorbs bone and how?
The osteoclast, a multinucleate macrophage-lineage cell that seals to the surface with a ruffled border and secretes acid and lysosomal enzymes into Howship lacunae.
How do intramembranous and endochondral ossification differ?
Intramembranous ossification replaces mesenchyme directly, as in the skull vault and clavicle; endochondral ossification replaces a cartilage model, building all long bones through a growth plate.
What happens histologically in rickets?
Vitamin D deficiency leaves osteoid unmineralised and growth plate cartilage uncalcified, so the hypertrophic zone widens into a rachitic metaphysis — bowed legs, beaded ribs and widened wrists.