Bone structure, growth + healing Flashcards
Diaphysis
tubular shaft running between proximal + distal ends
hollow region in diaphysis = medullary cavity (filled with yellow marrow)
walls of diaphysis = compact bone
Epiphysis
wider section at end of bone
filled with spongy bone
red marrow fills spaces in spongy bone
Metaphysis
where epiphysis and diaphysis meet
contains epiphyseal plate (growth plate)
Epiphyseal plate
growth plate
layer of hyaline cartilage in growing bone
cartilage replaced by osseous tissue in early adulthood
epiphyseal plate becomes epiphyseal line
Endosteum
membranous lining of medullary cavity
site of bone growth, repair + remodelling
Periosteum
fibrous outer membrane of bone
contains blood vessels, nerves and lymphatic vessels that supply compact bone
tendons + ligaments attach at periosteum
Osteoblast
bone-forming
does not divide/undergo mitosis
synthesise + secrete collagen matrix + calcium salts
as secreted matrix around osteoblast calcifies, osteoblast becomes trapped, changes structure and becomes osteocyte
(forms bone matrix)
Osteocyte
located in a lacuna
surrounded by bone tissue
maintain mineral concentration of matrix by secretion of enzymes
(maintains bone tissue)
Osteogenic cell
undifferentiated
high mitotic activity
only bone cells that divide
differentiate and develop into osteoblasts
Osteoclast
bone-resorbing
found on bone surfaces
originate from monocytes + macrophages (not osteogenic cells)
2 types of bone
cortical/compact bone
cancellous/trabecular/spongy bone
Compact bone
organised as parallel columns called osteons/haversian systems
lengthwise down axis of long bones
composed of lamellae (concentric rings of bone) surrounding haversian canal
haversian canal contains nerves, blood vessels + lymphatic system
vessels and nerves branch off at right angles through perforating/Volkmann’s canals to periosteum + endosteum
osteocytes in lacunae
Spongy (cancellous) bone
contains osteocytes in lacunae, but not in concentric circles
found in trabeculae (lattice-like network of matrix spikes)
trabeculae form along lines of stress to strengthen bone
make bones lighter - easier to move
contain red marrow - site of haematopoiesis
What is intramembranous ossification?
compact + spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue
begins in utero and continues into adolescence
eg. clavicles, face flat bone, most of cranial bones
Process of intramembranous ossification
mesenchymal cells in embryonic skeleton gather and begin to differentiate into specialised cells
some become capillaries, others become osteogenic cells and then osteoblasts
early osteoblasts occur in cluster = ossification centre
osteoblasts secrete osteoid (uncalcified matrix) which calcifies as minerals are deposited, trapping osteoblasts
trapped osteoblasts = osteocytes
surrounding osteogenic cells differentiate into new osteoblasts
osteoid around capillaries = trabecular matrix
osteoblasts on spongy bone surface = periosteum
periosteum creates layer of compact bone superficial to trabecular bone
crowded blood vessels condense into red marrow
What is endochondral ossification?
bone develops by replacing hyaline cartilage
cartilage = template to be replaced by new bone (cartilage does not become bone)
eg. long bones, bones at base of skull
Process of endochondral ossification
some mesenchymal cells differentiate into chondrocytes (cartilage cells) that form cartilaginous skeletal model
then the perichondrium (membrane that covers cartilage) appears
matrix is produced and calcifies, nutrients can no longer reach chondrocytes, causing death of surrounding cartilage
blood vessels invade spaces and bring osteogenic cells which become osteoblasts
enlarging space becomes medullary cavity
capillaries penetrate growing cartilage causing perichondrium to become periosteum
here the osteoblasts form a periosteal collar of compact bone around the cartilage of the diaphysis
primary ossification centre forms
chondrocytes + cartilage continue to grow at ends of bone
when fetal skeleton fully formed, only articular cartilage and cartilage of epiphyseal plate remain
epiphyseal regions = secondary ossification centres, where bones increase in length after birth
Bone remodelling
resorption of old/damaged bone on the same surface where osteoblasts lay new bone to replace that which is resorbed
mature osteoclasts secrete acid + proteases onto surface making a pit called a Howship’s Lacuna
osteoclast apoptosis
amount of bone synthesised matches amount resorbed
osteoblasts activated to replace resorbed bone
newly secreted matrix (osteoid) becomes mineralised and forms mature bone
remodelling ends when bone formation complete and osteoblasts in new bone matrix as osteocytes/become quiescent (inactive) bone lining cells
net result of each cycle = formation of new osteon
Describe the fracture healing process
rupture of blood vessels causes haematoma to form
disruption of blood flow to bone causes death of bone cells around fracture
within 48 hours, chondrocytes from endosteum have created internal callus (fibrocartilaginous matrix secreted between bone ends)
periosteal chondrocytes and osteoblasts create external callus of hyaline cartilage and bone called bony callus, stabilises fracture
over next few weeks, osteoclasts resorb dead bone, osteogenic cells become active and differentiate into osteoblasts
cartilage in calli replaced by trabecular bone by endochondral ossification
internal + external calli unite and compact bone replaces spongy bone at outer edges of fracture
What is osteonecrosis?
avascular necrosis
ischaemic necrosis of bone + bone marrow
caused by fracture, steroids, alcohol, sickle cell anaemia + the bends
How do bones increase in length?
bone cannot increase in size by interstitial growth, only apposition
osteoblastic apposition not sufficient for rapid longitudinal growth
cartilage undergoes interstitial growth and is replaced by bone
