Bone tissue (osseous tissue)

Textus osseus

Also known as: osseous tissue, bone, classification of bones

A mineralised connective tissue of cells embedded in a matrix of type I collagen and hydroxyapatite, combining tensile and compressive strength with low weight. It forms every bone of the skeleton in two architectures — compact and spongy.

Long Bone — Structure

Overview

Mineral fraction (wet weight)≈60–70% (hydroxyapatite)

Organic fraction≈20–30% (≈90% type I collagen)

Water≈10%

Density of cortical bone≈1.9 g/cm³

Bone is a specialised connective tissue whose extracellular matrix is roughly two-thirds mineral and one-third organic by weight. Collagen fibres resist tension and bending (bone without collagen is brittle like chalk), while hydroxyapatite crystals deposited along the fibres resist compression (demineralised bone is rubbery). Each bone is an organ that also contains marrow, periosteum, endosteum, blood vessels, nerves and, at joints, articular cartilage.

Microscopically, mature bone is lamellar — collagen laid down in regular layers whose fibres alternate in direction from one lamella to the next, like plywood. Woven (primary) bone, with randomly arranged collagen, is formed rapidly during development, fracture repair and in pathological states (Paget disease, tumours) and is later replaced by lamellar bone. Lamellar bone is arranged as compact (cortical) bone and spongy (trabecular) bone.

Bones are classified by shape: long (longer than wide, with a shaft and two ends — femur, humerus, metacarpals, phalanges), short (cube-like — carpals, tarsals), flat (thin, often curved plates of compact bone enclosing spongy diploë — cranial vault, sternum, ribs, scapula), irregular (vertebrae, hip bone, sphenoid, ethmoid) and sesamoid (develop within tendons — patella, pisiform, sesamoids of the hallux and thumb). Pneumatised bones (frontal, maxilla, sphenoid, ethmoid, temporal) contain air sinuses.

Location: Throughout the skeleton.

Parts & subdivisions

Compact (cortical) bone
Dense outer layer built of osteons; ≈80% of skeletal mass.
Spongy (trabecular) bone
Lattice of trabeculae housing red marrow; ≈20% of mass but most of the surface area.
Bone matrix
Organic osteoid (type I collagen + ground substance) mineralised by hydroxyapatite.
Bone cells
Osteoprogenitor cells, osteoblasts, osteocytes (≈90–95% of bone cells) and osteoclasts.
Long bones
Diaphysis, metaphyses and epiphyses; act as levers — femur, tibia, humerus, metacarpals, phalanges.
Short bones
Roughly cuboidal, mostly spongy bone with a thin cortex — carpals and tarsals.
Flat bones
Two plates of compact bone (inner and outer tables) sandwiching spongy diploë — skull vault, sternum, ribs, scapula.
Irregular bones
Complex shapes — vertebrae, hip bones, sphenoid, ethmoid.
Sesamoid bones
Form within tendons where they cross joints, improving leverage and reducing friction — patella (largest), pisiform, hallux sesamoids.

Anatomy

Histology: Lamellar bone: lamellae 3–7 µm thick with alternating collagen orientation; osteocytes in lacunae linked by canaliculi. Woven bone: irregular collagen, more cells, less mineral — temporary.

Function

Provides a rigid yet slightly elastic framework for support and leverage
Stores calcium, phosphate, magnesium, sodium and carbonate
Houses haematopoietic tissue in its spongy bone
Self-repairs and adapts its architecture to mechanical load

Physiology: Continuously remodelled by basic multicellular units (osteoclasts + osteoblasts), ≈5–10% of the skeleton per year; remodelling is faster in trabecular than in cortical bone.

Clinical significance

Woven bone as a marker of disease

Persistence of woven bone in adults indicates rapid, disordered formation — fracture callus, Paget disease, fibrous dysplasia, osteosarcoma or hyperparathyroid bone disease.

Test yourself

A 24-year-old falls on an outstretched hand. X-rays are normal but there is tenderness in the anatomical snuffbox. Why is a missed fracture of this bone particularly serious?

Its blood supply enters distally, so the proximal pole may undergo avascular necrosis
It is the only carpal with no ligamentous attachments
It transmits the ulnar nerve through Guyon’s canal
Its fracture always dislocates the lunate into the carpal tunnel

Answer: Its blood supply enters distally, so the proximal pole may undergo avascular necrosis

The scaphoid receives its blood supply from branches of the radial artery entering the distal pole and dorsal ridge, flowing retrogradely. A waist fracture can devascularise the proximal pole, causing non-union and avascular necrosis — so suspected fractures are immobilised and re-imaged even when the first X-ray is normal.

Which cells are primarily responsible for sensing mechanical load and regulating bone formation through secretion of sclerostin?

Osteoblasts
Osteoclasts
Osteocytes
Osteoprogenitor cells

Answer: Osteocytes

Osteocytes, connected via canaliculi, are the mechanosensors of bone. Loading reduces their secretion of sclerostin (a Wnt antagonist), releasing osteoblast activity; disuse increases sclerostin and bone loss. Romosozumab is an anti-sclerostin antibody.

Parathyroid hormone increases osteoclastic bone resorption mainly by which mechanism?

Binding directly to PTH receptors on mature osteoclasts
Increasing osteoprotegerin secretion
Inhibiting calcitriol synthesis
Increasing RANKL expression by osteoblasts

Answer: Increasing RANKL expression by osteoblasts

Osteoclasts lack PTH receptors. PTH acts on osteoblasts (and osteocytes), raising RANKL and lowering OPG; RANKL binds RANK on osteoclast precursors, promoting their differentiation and activity. PTH also stimulates renal 1α-hydroxylase, increasing calcitriol.