Bone Structure and Core MSK Histology Flashcards
How does skeletal muscle appear?
Cells are striated, unbranched and multinucleate (forms a syncytium)
What shape are skeletal muscle fibres and how are they grouped?
Fibres are long cylinders 10-100 micrometers in diameter and 1000-200000 micrometers in length; grouped into bundles called fascicles (each muscle contains several fascicles)
Where are the nuclei of skeletal muscle cells located?
At the periphery of the fibre, just under the cell membrane (called the sarcolemma in muscle cells)
What is the epimysium?
Connective tissue that surrounds the muscle as a whole
What is the perimysium?
Connective tissue that surrounds a single fasicle
What is the endomysium?
Connective tissue that surrounds a single muscle fibre
How is muscle organised?
Each muscle fibre contains many myofibrils, with each myofibril consisting of many sarcomeres placed end to end
What are sarcomeres?
Unit of contraction of muscle cell, smallest contractile elements in the straited muscle cell
Where do sarcomeres extend to?
From one Z-line to the next
Why do striated fibres have a regular pattern of bands running across the fibres?
Sarcomeres in the myofibrils have alternating light and dark bands, and are held in registry with one another across the fibre
How are striations organised in a fibre?
They run across the fibre at right angles to the long axis
How do sarcomeres align with each other in a fibre?
The Z-disks in the sarcomere in one myofibril will be aligned with the Z-disks of the sarcomeres in the surrounding myofibrils
What does the motor unit of a skeletal muscle consist of?
One motor neuron and all of the muscle fibres that it innervates
What does having fewer muscle fibres attached to a motor neuron allow?
Finer control of movement
What is the neuromuscular junction?
Special type of synapse at the end of each branch of the motor neuron axon
How are the fibres of each motor unit organised in the skeletal muscle?
Scattered throughout the muscle and are all the same fibre type
What is type I skeletal muscle?
Relatively slow contracting fibres that depend on oxidative metabolism, abundant mitochondria and myoglobin, resistant to fatigue, produce less force, often called “red fibres”
What is type IIa skeletal muscle?
Relatively fast contracting but also reasonably resistant to fatigue, relatively uncommon
What is type IIb skeletal muscle?
Fast contracting fibres that depend on anaerobic metabolism, few mitochondria and less myoglobin, fatigue relatively easily, produce relatively greater force, often called “white fibres”
What are some features of cartilage?
Semi-rigid, deformable, permeable, avascular, cells nourished by diffusion through the extracellular matrix
What are some features of bone?
Rigid, not permeable, cells within the bone must be nourished by blood vessels that pervade the tissue
What are chondrocytes?
Cells found in cartilage (chondroblasts when immature), active cells that both secrete and maintain the extracellular matrix around them
Where are chondrocytes located in cartilage?
Live within a space in the extracellular matrix termed the lucuna
What is the confirmation of the extracellular matrix in hyaline cartilage?
75% water, 25% organic material
What is the organic material that makes up the extracellular matrix?
60% type II collagen = finer (15-45 micrometers), forms 3D network
40% proteoglycan aggregates = made up of GAGs (most commonly keratin sulphate and chondroitin suphate) bound to a core protein and often linked to hyaluronan
What are the types of cartilage?
Hyaline, elastic, fibrocartilage
What are some features of hyaline cartilage?
Most common form, found at articular surfaces, tracheal rings, costal cartilage, epiphyseal growth plates and in fetus as precursor to many bones, blue-white and translucent
What are some features of elastic cartilage?
Light yellow, addition of elastic fibres make it quite flexible
What are some features of fibrocartilage?
Hybrid between tendon and hyaline cartilage, bands of densely packed type I collagen interleaved with rows of chondrocytes surrounded by small amounts of cartilaginous ECM, appears white
What are some function of bone?
Support and lever for movement, protects internal organs, calcium store, haemopoiesis
How does bone act as a calcium store?
More than 95% of the total calcium in the body is stored in the bones and there is constant exchange of calcium between bone and the blood
Where does haemopoiesis begin, and where are the main sites in adults?
Begins in bone marrow before birth; by early twenties typically only the axial and limb girdle skeleton is involved in blood production
What is the composition of bone?
23% collagen, 65% bioapatite (form of calcium phosphate, mostly hydroxyapatite), 10% water, 2% non-collagen proteins
What kind of bone makes up the outer shell (diaphysis)?
Dense cortical bone
What kind of bone makes up the ends of the bones (epiphyses)?
Cancellous/trabecular bone (fine meshwork)
When does bone undergo remodelling?
Throughout life
What are some similarities between compact and trabecular bone?
Similar bone cells and matrix, both lamellar (made of layers)
Where are the marrow cavities in relation to trabecular bone?
Adjacent to the bone
How do blood vessels and nerves penetrate the bone?
Through small canals
What are the living cells found in bone called?
Osteocytes
What are cement lines?
Visible lines around osteons that have formed during remodelling
Does trabecular bone have Haversian canals?
No = struts are thin so osteocytes survive from contact with marrow spaces
What are the different types of bone cells?
Osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts
What are osteoprogenitor cells?
Serve as pool of reserve osteocytes, found on bone surfaces
What are osteoblasts?
Bone forming cells found on the surface of developing bone, have plentiful RER and prominent mitochondria
Where are osteocytes located?
Trapped within the bone matrix
What are osteoclasts?
Large (>40 micrometers) multinucleated cells, found on bone surface and are responsible for bone resorption, several cells fuse to form single giant cell, thought to be derived from macrophage lineage
How does bone remodelling begin?
Number of osteoclasts will congregate and begin to “drill” into the bone forming a tunnel
How are blood vessels involved in bone remodelling?
Blood vessels grow into the tunnel created by osteoclasts, bringing with them osteoblasts which then line the tunnel and begin laying down new lamellar bone
When does bone remodelling stop?
Until only the space of a Haversian canal remains
What is osteoid?
The collagen, glycosaminoglycans (GAGs), proteoglycans and other organic compounds secreted by osteoblasts
What happens to osteoid over time?
It becomes mineralised in the extracellular space
What is the main mineral of bone?
Calcium phosphate crystals, particularly hydroxyapatite (Ca10(PO4)6(OH)2)
How does woven bone differ from normal lamellar bone?
Collagen fibres are orientated haphazardly, isn’t as strong as lamellar bone, and is subsequently remodelled into lamellar bone by being broken down by osteoclasts and reformed by new osteoclasts
When is woven bone laid down?
During development of after a bone is broken
