Musculoskeletal Histology Flashcards
3 types of muscle?
Skeletal (voluntary)
Cardiac
Smooth
3 features of skeletal muscle cells/fibres?
Striated
Unbranched
Multi-nucleate (forming a synctium) - nuclei are at the fibre periphery, under the membrane (called the sarcolemma)
How are muscle fibres grouped?
Into bundles called FASCICLES
Connective tissue in skeletal muscles?
If it surrounds the muscle as a whole - epimysium
Around a single fascicle - perimysium
Around a single muscle fibre - endomysium
What are sarcomeres?
Smallest functional contractile elements; these group together to form myofibrils which, in turn, group to form the muscle fibre
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Why are skeletal muscle fibres striated?
Sarcomeres in the myofibrils, with their alternating dark and light bands, are held in registry with one another
Z-lines in the sarcomere of one myofibril will be aligned with the Z-lines of the sarcomeres in other myofibrils, so it looks like something is running across the fibre
What is a motor unit?
An α-motor neurone and all of the muscle fibres that it innervates
How do muscle fibres in a motor unit vary?
The number of fibres can vary; fewer the no. of muscle fibres in a motor unit, the finer the control of movement
However, the fibres are all of the same type and are scattered in the muscle
Junction between the motor neuron and skeletal muscle?
Axon of the motor neuron branches as it nears its termination
Each branch ends in a NEUROMUSCULAR JUNCTION
3 types of skeletal muscle fibres?
Type I - relatively slowly contracting fibres that depend on oxidative metabolism
Type IIa (uncommon) - intermediate between the other two; relatively fast contracting but also reasonably resistant to fatigue
Type IIb (AKA white fibres) -fast-contracting fibres that depend upon anaerobic metabolism; few mitochondria, fatigue easily and produce relatively great force
Compare cartilage and bone?
Cartilage is semi-rigid and deformable, avascular and cell nourished by diffusion through the EC matrix
Bone is rigid and not permeable; cells within the bone must be nourished by blood vessels that pervade the tissue
Cartilage cells?
Chondrocytes - within the lacuna (space in the EC matrix); they are active cells which secrete and maintain the EC matrix
They are called chrondroblasts when immature
Components of the ECM in hyaline cartilage (most common)?
- 75% water
- 25% organic material:
• 60% type II collagen
• 40% proteoglycan aggregates (made up of GAGs) bound to a core protein and often linked to hyaluronan
Differences between type I and II collagen?
Type I is the most common form of collagen and aggregates into linear bundles
Type II is finer and forms a 3D network
Types of cartilage?
Hyaline cartilage (most common) - grossly, blue-white in colour and transclucent
Elastic cartilage - grossly, light yellow in colour; elastic fibres make it flexible
Fibrocartilage - grossly, appears white and is a hybrid of tendon and hyaline cartilage
Bands of densely packed type I collagen inter-weaved with rows of chondrocytes are surrounded cartilagenous ECM
Examples of sites where hyaline cartilage is found?
Articular surfaces
Tracheal rings
Costal cartilage
Epiphyseal growth plates
In fetus, precursor to many bones
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Functions of bone?
- Support
- Levers for effective movement
- Protection of internal organs
- Ca2+ store
- Haemopoiesis
Describe haemopoiesis
Blood cell production
- While in utero, blood is produced in the liver and spleen but it begins in the bone marrow before birth
- By time of birth, bone marrow is the site of haemopoiesis
- By early 20s, only the axial and limb girdle skeleton is involved in blood production
Composition of bone?
Collagen, minerals, water and non-collagen proteins
Morphology of bone?
Outer shell of dense cortical bone makes up the diaphysis
Cancellous/trabecular bone occupies the epiphyses (looks like inside of an aero bar)
One similarity and one difference between compact and trabecular bone?
Both are lamellar (made up of layers)
Adjacent to trabecular bone, there are marrow cavities (spaces)
Bone as a living tissue?
Penetrated by small canals for blood vessels and nerves
Contains osteocytes (living cells)
Morphology of cortical bone?
Cross-sectioning shows OSTEONS (look like “tree rings”)
These consist of concentric lamellae around a Haversian canal, which contains nerves and blood vessels that serve the bone
Difference between Haversian canal and Volkmann’s canal?
Haversian canal is the axis of the osteon and is longitudinal
Volkmann’s canal has a transverse orientation and interconnects Haversian canals
What are cement lines?
Lines that are often visible surrounding the osteon; only found in osteons that have formed during REMODELLING, not in original development
Describe the structure of trabecular bone
AKA spongy bone
Has osteocytes but lacks Haversian canals; osteocytes can survive from contact with the marrow spaces
4 types of cells of bone, their location and functions?
Osteoprogenitor cells - located on bone surfaces, e.g: under the periosteum; they form a pool of reserve osteoblasts
Osteoblasts - bone-forming cells found on the surface of developing bone; have plentiful RER and prominent mitochondria
Osteocytes - bone cell trapped within bone matrix
Osteoclasts - large multi-nucleated cells found on the surface of bone and responsible for bone resorption
Describe remodelling of bone
Osteoclasts congregate and begin to ‘drill’ into the bone, forming a tunnel through which a blood vessel will grow
This brings osteoblasts which line the tunnel and begin laying down new lamellar bone; continues until only the space of a Haversian canal remains
What is the basic multicellular unit (BMU)?
Collection of osteoclasts and osteoblasts that participate in bone remodelling
How does bone mineralisation occur?
Osteoblasts secrete collagen, GAGs, proteoglycans and other organic components of the matrix (together, these are called the OSTEOID)
It becomes mineralised over time in the EC space
What does bone mineral consist of?
Principally, calcium phosphate crystals, part. hydroxyapatite
Formation of osteoclasts?
Derived from macrophage lineage; several will fuse to form a single GIANT CELL
Difference between the function of osteoclasts and osteoblasts?
Osteoclasts - destroy bone
Osteoblasts - secrete matrix that will become bone
Consequences of remodelling bone?
Younger osteons often partially obliterate older osteons
When can woven bone be seen?
During development
Following a fracture
Compare lamellar and woven bone
Woven bone:
• Collagen fibres are laid down haphazardly
• Not as strong as lamellar bone
• Is subsequently remodelled into lamellar bone as it is broken down by osteoclasts and reformed by osteoblasts
Lamellar bone:
• Collagen fibres are orientated in one direction
