Ultrastructure of Bone

Question 1

list the numerous functions of bone

Answer 1

• Haematopoiesis
• Lipid and mineral storage – bone is a reservoir holding adipose tissue within the bone marrow and calcium within the hydroxyapatite crystals.
• Support
• Protection

Question 2

what is haematopoiesis?

Answer 2

– the formation of blood cells from haematopoietic stem cells found in the bone marrow.

Question 3

what are the three cellular components of bone?

Answer 3

osteoblasts
osteocytes
osteoclasts

Question 4

describe the function of osteoblasts

Answer 4

Synthesise uncalcified/unmineralised extracellular matrix called osteoid. This will later become calcified/mineralised to form bone.

Question 5

describe the function of osteocytes

Answer 5

As the osteoid mineralises, the osteoblasts become entombed between lamellae in lacunae where they mature into osteocytes. They then monitor the minerals and proteins to regulate bone mass.

Question 6

describe the function of osteoclasts

Answer 6

Derived from monocytes and resorb bone by releasing H+ ions and lysosomal enzymes. They are large and multinucleated cells.

Question 7

describe the extracellular matrix (ECM)

Answer 7

The ECM of bone is highly specialised. In addition to collagen and the associated proteins usually found in connective tissue, bone is impregnated with mineral salts, in particular calcium hydroxyapatite crystals. These crystals associate with the collagen fibres, making bone hard and strong. This matrix is organised into numerous thin layers, known as lamellae.

Question 8

what is the external connective tissue layer found on bones called?

Answer 8

periosteum

Question 9

which connective tissue layer lines the cavities of bone?

Answer 9

endosteum

Question 10

describe compact bone

Answer 10

Compact bone forms the outer ‘shell’ of bone. In this type of bone, the lamellae are organised into concentric circles, which surround a vertical Haversian canal (which transmits small neurovascular and lymphatic vessels). This entire structure is called an osteon and is the functional unit of bone.

Question 11

where are osteocytes located in compact bone?

Answer 11

Osteocytes are located between the lamellae, within small cavities (known as lacunae). The lacunae are interconnected by a series of interconnecting tunnels, called canaliculi.

Question 12

describe spongy bone

Answer 12

Spongy bone makes up the interior of most bones and is located deep to the compact bone. It contains many large spaces – this gives it a honeycombed appearance.

The bony matrix consists of a 3D network of fine columns, which crosslink to form irregular trabeculae.
The spaces between trabeculae are often filled with bone marrow.

Question 13

what is yellow bone marrow composed of?

Answer 13

adipocytes

Question 14

what is red bone marrow composed of?

Answer 14

haematopoietic stem cells

Question 14

what is red bone marrow composed of?

Answer 14

haematopoietic stem cells

Question 15

what is ossification?

Answer 15

Ossification is the process of producing new bone. It occurs via one of two mechanisms:

Endochondral ossification.
Intramembranous ossification

Question 16

what is endochondral ossification? and give an example of a bone that undergoes this

Answer 16

– Where hyaline cartilage is replaced by osteoblasts secreting osteoid. The femur is an example of a bone that undergoes endochondral ossification.

Question 17

what is intramembranous ossification? give an example of bones that undergo this

Answer 17

– Where mesenchymal (embryonic) tissue is condensed into bone. This type of ossification forms flat bones such as the temporal bone and the scapula.

Question 18

describe bone remodelling

Answer 18

This is the process whereby mature bone tissue is reabsorbed, and new bone tissue is formed. It is carried out by the cellular component of bone.
Osteoclasts break down bone via a cutting cone. The nutrients are reabsorbed, and osteoblasts lay down new osteoid. Remodelling occurs primarily at sites of stress and damage, strengthening the areas affected.

Question 19

What is interstitial bone growth?

Answer 19

In interstitial growth, chondrocytes secrete new matrix within the cartilage and this causes it to grow in length. eg. epiphyseal plate of femur

Question 20

what is appositional bone growth?

Answer 20

Appositional growth occurs when chondroblasts secrete new matrix along existing surfaces and this causes the cartilage to expand and widen.