Histology Flashcards
Where are the nuclei in skeletal muscle?
At the periphery of the fibre just under the sarcolemma
How are muscle fibres grouped?
grouped into bundles called fascicles
What is epimysium?
connective tissue that surrounds the whole muscle
What is perimysium?
connective tissue surrounding a single fascicle
What is endomysium?
connective tissue surrounding a single muscle fibre
What are sarcomeres?
the ‘unit of contraction’ of the muscle cell
What forms a myofibril?
thousands of sarcomeres are placed end-to-end to form a myofibril
What produces the striations in skeletal muscles?
The alternating dark and light bands of the sarcomeres
What does a motor unit consist of?
consists of one motor neurone and all of the muscle fibres that it innervates
What determines the number of muscle fibres in a motor unit?
- The fewer the number of muscle fibres in a motor unit, the finer the control of movement
- Each motor neuron will innervate a small number of muscle fibers, enabling many nuances of movement of the entire muscle
Describe Type I fibres.
- Relatively slowly conducting fibres that depend on oxidative metabolism
- Have abundant mitochondria and myoglobin (will stain darker)
- Are resistant to fatigue
- Produce relatively less force
- Often called ‘red’ fibres
Describe Type IIa fibres?
- This is the intermediate between the other two
- They are relatively fast contracting, but are also reasonably resistant to fatigue
- They are relatively uncommon
Describe Type IIb fibres?
- Fast contracting fibres that depend on anaerobic metabolism
- They have few mitochondria and less myoglobin than type I fibres (will stain lighter)
- They fatigue relatively easily and produce relatively great force
- Often called ‘white’ fibres
What is cartilage?
semi-rigid and deformable, permeable, avascular, cells nourished by diffusion through the extracellular matrix
What are chondrocytes?
the cells found in cartilage (chondroblasts when immature)
Where do chondrocytes live?
live in a space in the extracellular matrix termed a lacuna
What is the function of chondrocytes?
are active cells which secrete and maintain the extracellular matrix
What does hyaline cartilage look like?
blue-white in colour, translucent, most common form of cartilage
Where is hyaline cartilage found?
Found in e.g. articular surfaces, tracheal rings, costal cartilage
What does elastic cartilage look like?
light yellow in colour, the addition of elastic fibres (replace lots of ECM) make it quite flexible
Where is elastic cartilage found?
Found in e.g. auricle of ear, most of epiglottis
What does fibrocartilage look like?
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
Where is fibrocartilage found?
Found in e.g. IV discs
Describe the composition of the extra-cellular matrix in hyaline cartilage?
- 75% water
- 25% organic material
- 60% Type II collagen - finer than Type I, forms a 3D meshwork
- 40% proteoglycan aggregates - made up of GAGs (most commonly keratan sulfate and chondroitin sulphate) bound to a core protein and often linked to a hyaluronan
What is bone?
rigid, not permeable, cells within the bone must be nourished by blood vessels that pervade the tissue
What are the functions of bone?
- Support
- Levers for effective movement
- Protection of internal organs (e.g. cranium, ribs)
- Calcium store - more than 95% of the total calcium found in the body is found in bone and their is constant change of calcium between bone and the blood
- Haemopoiesis (blood cell production) - although in utero blood is produced by the liver and the spleen, it begins in the bone marrow well before birth and by birth bone marrow is the primary site of haemopoieses
- By the early 20s typically only the axial and limb girdle skeleton is involved in blood production
What is intramembranous ossification?
- the process of bone development from fibrous membranes
- Involved in the formation of the flat bones of the skull, mandible, and the clavicles
What is endochondral ossification?
- is the process of bone development from hyaline cartilage
- Involved in the rest of the bones of the body
How do cartilaginous bones ossify?
first ossify within the diaphysis, then secondarily ossify within the epiphysis
What is bone length increased by?
increased by cartilage proliferation at growth plates between the metaphysis and epiphysis; the cartilage then ossifies
What is bone girth increased by?
increased by cells from the periosteum, which lay down circumferential new bone on the periphery of the existing cortex
What is the diaphysis?
long shaft of bone
What is the epiphysis?
ends of bone
What is epiphyseal plate?
growth plate
What is the metaphysis?
between epiphysis and diaphysis
What is the periosteum?
- membrane that covers the outer surface of all bones, except at the articular surfaces
- Outer fibrous layer, inner ‘osteogenic’ layer which contains progenitor cells that develop into osteoblasts
What is the medullary cavity?
hollow chamber in bone
What is the composition of bone?
- 65% bioapatite (a form of calcium phosphate, mostly hydroxyapatite)
- 23% collagen
- 10% water
- 2% non-collagen proteins
What kind of bone make up the shaft?
Outer shell of dense cortical bone
What kind of bone makes up the epiphysis?
Cancellous or trabecular bone
What is the difference between the compact and trabecular bone?
the main difference is the presence of spaces (marrow cavities) adjacent to trabecular bone
What does lamellar mean?
(made up of layers)
How does bone get nutrients?
it is penetrated by small canals for blood vessels and nerves, and contains living cells called osteocytes
What is the morphology of cortical bone?
- The lamellae are organised into concentric circles, which surround a vertical Haversian canal (transmits small neurovascular and lymphatic vessels)
- This entire structure is called an osteon and is the functional unit of bone
- Osteocytes are located between the lamellae, within lacunae
- The lacunae are interconnected by a series of interconnecting tunnels, called canaliculi
What is the morphology of trabecular bone?
- Trabecular bone has osteocytes and is lamellar in nature, but generally lacks Haversian canals
- Because the struts are thin, the osteocytes can survive from contact with the marrow spaces
What happens to bone strength around 50 years?
decrease in trabecular thickness which is more pronounced for non load-bearing horizontal trabeculae
What happens to bone strength around 80 years?
decrease in number of connections between vertical trabeculae and decrease in trabecular strength
What are osteoprogenitor cells?
located on bone surfaces (e.g. under the periosteum), serve as a pool of reserve osteoblasts
What are osteoblasts?
- bone forming cells found on the surface of developing bone
- Derived from mesenchymal cells in the bone marrow stroma
- They are mononuclear and have plentiful RER and prominent mitochondria
What are osteocytes?
- primary cell of mature bone and most common type of bone cell; derived from osteoblasts which become buried in bone matrix
- Thought to sense mechanical strain on the skeleton
What are osteoclasts?
- large (up to 40µm or more) multinucleated cells, found on the surface of bone and are responsible for bone reabsorption
- Believed to be derived from the macrophage lineage of cells (haematopoietic progenitors)
- Several osteoclasts will fuse and form a giant cell
How does bone remodelling occur?
- Bone undergoes a continual remodelling cycle at distinct sites
- This contributes to calcium homeostasis and also to skeletal repair
- ~10% of the adult skeleton is remodelled each year
- In bone remodeling, a number of osteoclasts will congregate and begin to ‘drill’ into the bone, forming a tunnel
- A blood vessel will grow into the tunnel bringing with it osteoblasts which line the tunnel and begin laying down new lamellar bone
- This process continues until only the space of a Haversian canal remains
- The collection of osteoclasts and osteoblasts that participate in this process at a site is called the basic multicellular unit (BMU)
What is the function of an osteoblast?
secrete collagen, glycosaminoglycans, proteoglycans and other organic compounds of the matrix which is collectively termed osteoid
What is the mineral of the bone made up of?
calcium phosphate crystals, particularly hydroxyapatite
What is woven bone?
first bone laid down during development or following a break
How does woven bone differ from lamellar bone?
It differs from normal lamellar bone in that the collagen fibres are laid down in a haphazard fashion (rather than being orientated all in one direction)
How is woven bone remodelled into lamellar bone?
Woven bone is not as strong as lamellar bone and is subsequently remodeled into lamellar bone by being broken down by osteoclasts and reformed by new osteoblasts
