Bone Flashcards
How do long bones develop?
Endochondral ossification
How do flat bones develop and what do they develop from?
Intramembranous ossification, directly from mesenchymal tissue
Where does intramembranous ossification take place?
Within condensations of mesenchymal tissue - NOT by replacement of a pre-existing hyaline cartilage template
What are some examples of flat bones?
Skull, clavicle, scapula, pelvic bones
Does intramembranous ossification contribute to lengthening of long bones?
No, it contributes to thickening of long bones at periosteal surfaces (appositional growth)
Outline the stages of intramembranous ossification
A small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells (a nidus).
The MSCs become osteoprogenitor cells (each developing more Golgi apparatus and rough endoplasmic reticulum).
The osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing Type I collagen (osteoid).
The osteoid mineralises to form rudimentary bone tissue spicules, which are surrounded by osteoblasts, and contain osteocytes.
The spicules join to form trabeculae, which merge to form woven bone, which is finally replaced by the lamellae of mature compact bone.
What do spicules connect to form?
Trabeculae
Name 3 types of cell which remodel bone
Osteoblasts, osteocytes, osteoclasts
What is the main unction of osteoblasts
To deposit new bone
What is the main function of osteoclasts?
To resorb bone (and they hence lie in a depression)
What are the 2 types of bone?
Compact (cortical) and Cancellous (trabecular)
Where is compact (cortical) bone found?
Compact bone forms the external surfaces of bones and comprises ca. 80% of the body’s skeletal mass.
Describe cancellous (trabecular) bone
Cancellous bone forms a network of fine bony columns or plates to combine strength with lightness. The spaces are filled by bone marrow.
What do Haversian and Volkmann’s canals contain?
Blood vessels, lymph vessels, and nerves
What is an osteon
The fundamental functional unit of much of compact bone. Roughly cylindrical structures.
How do the canals lie in relation to the osteon?
Haversian - through the osteon (parallel)
Volkmann - across osteon (perpendicularly)
What are osteocytes?
Osteoblasts which have become embedded in the material they have secreted
How are osteocytes arranged?
Immature bone - randomly arranged
Mature bone - concentric lamellae of osteon s
Where are resorption canals found?
Parallel with the osteons long axis
How are nutrients passed between osteocytes?
The osteocytes have very slender cytoplasmic processes, which reach out to those of adjacent osteocytes, via canaliculi. These processes connect via gap juctions such that nutrients can be passed between osteocytes.
Describe the structure of cancellous (trabecular) bone
The internal histological structure of the trabeculae is similar to that of compact bone, with the presence of osteocytes lying between lamellae. There are however no Haversian or Volkmann’s canals.
Which cells are found in cancellous (trabecular) bone and where are they found?
Each trabeculum consists of numerous osteocytes embedded within irregular lamellae of bone. Osteoblast and osteoclasts on their surfaces act to remodel them.
What lies in the lacunae between trabeculae
Adipose and haemopoietic cells
Briefly describe how bone is resorbed
A cutting cone is boring a tunnel through the bone by the action of osteoclasts, which release H+ ions and lysosomal enzymes.