Flashcards in Bone (new) Deck (13):
What is bone composed of?
- 65% mineral: calcium hydroxyaparite crystals
- 23% collagen: type I
- 10% water
- 2% non-collagen proteins
What are the 2 main types of bone tissue?
1. Cancellous bone: network of fine bony columns/plates to combine strength and lightness. Spaces are filled by bone marrow.
2. Compact bone: forms the external surfaces of bone (comprises 80% of body's skeletal mass).
Describe the 3 main types of bone cell.
1. Osteoblasts: make bone by moving over bone matrix and depositing osteoid proteins (mainly collagen) followed by minerals (inc. calcium).
2. Osteoclasts: large multinucleated cells produced by fusion of several smaller cells. Mediate bone resorption by secreting acids and enzymes to disintegrate bone matrix.
3. Osteocytes: derived from osteoblasts trapped in bone matrix and encased in lacunae. Sensory cells involved in signalling processes.
Describe the structure of compact bone.
- Composed of cylindrical structures called osteons containing a central Haversian canal with branching Volkmann's canals (carry blood/lymph vessels and nerves).
- Osteocytes are entombed in lacunae between bone lamellae, but have very slender cytoplasmic processes that reach out to those of adjacent osteocytes via canaliculi (connect with central Haversian canal). These processes connect via gap junctions such that nutrients can be passed between osteocytes.
Describe the structure of cancellous bone.
- Consists of trabeculae containing numerous osteocytes embedded within irregular lamellae of bone.
- Adipose and haemopoietic cells lie in the cavities.
- No Haversian or Volkmann's canals.
How does cancellous bone change over time?
Gets remodelled from immature to mature.
- Immature: osteocytes are fairly randomly arranged.
- Mature: osteocytes arranged in concentric lamellae of osteons, with resorption canals running parallel with osteons' long axes.
What is the periosteum?
Dense layer of vascular CT enveloping the bones (except at the surfaces of joints)
What is the endosteum?
Thin layer of CT in the middle of bone tissue and bone marrow
What are the 2 types of bone formation?
1. endochondral ossification
2. intramembranous ossification
How are endochondral and intramembranous ossification different?
Endochondral ossification = mineralisation of a pre-existing hyaline cartilage template to form bone.
- long bone dev.
- continued lengthening of long bones by further ossification at epiphyseal plates
Intramembranous ossification = condensation of mesenchymal tissue to form bone.
- flat bone dev.
- increasing long bone girth
Describe the process of endochondral ossification.
1. Embryo (6-8 wks): collar of periosteal (compact) bone appears in shaft of initial cartilage model.
2. Foetus (8-12 wks): central cartilage calcifies. Nutrient artery penetrates, supplying bone-depositing osteogenic cells: primary ossification centre forms.
3. Postnatal: bone medulla becomes cancellous bone although some cartilage forms epiphyseal growth plates.
Periosteal ossification increases bone girth.
Epiphyses develop secondary centres of ossification.
4. Prepubertal: epiphyses ossify. Growth plates continue to move apart, lengthening the bone.
5. Mature adult: epiphyseal growth plates replaced by bone. Hyaline cartilage persists at ends.
Describe the process of intramembranous ossification.
1. Small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells - a nidus. MSCs become osteoprogenitor cells (each develop more Golgi and rER).
2. Osteoprogenitor cells become osteoblasts and lay down an ECM containing type I collagen - osteoid.
3. Osteoid mineralises to form rudimentary bone tissue spicules surrounded by osteoblasts and containing osteocytes.
4. Spicules join to form trabeculae... merge to form woven bone... replaced by lamellae of mature compact bone.